rev 58768 : 8238358: Implementation of JEP 371: Hidden Classes
Reviewed-by: alanb, cjplummer, coleenp, dholmes, dlong, forax, jlahoda, psandoz, plevart, vromero
Contributed-by: mandy.chung@oracle.com, lois.foltan@oracle.com, david.holmes@oracle.com, harold.seigel@oracle.com, serguei.spitsyn@oracle.com, alex.buckley@oracle.com, jamsheed.c.m@oracle.com, jan.lahoda@oracle.com, amy.lu@oracle.com
rev 58769 : [mq]: type-descriptor-name

   1 /*
   2  * Copyright (c) 2008, 2020, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.invoke;
  27 
  28 import jdk.internal.access.JavaLangAccess;
  29 import jdk.internal.access.SharedSecrets;
  30 import jdk.internal.misc.VM;
  31 import jdk.internal.module.IllegalAccessLogger;
  32 import jdk.internal.org.objectweb.asm.ClassReader;
  33 import jdk.internal.org.objectweb.asm.Opcodes;
  34 import jdk.internal.reflect.CallerSensitive;
  35 import jdk.internal.reflect.Reflection;
  36 import jdk.internal.vm.annotation.ForceInline;
  37 import sun.invoke.util.ValueConversions;
  38 import sun.invoke.util.VerifyAccess;
  39 import sun.invoke.util.Wrapper;
  40 import sun.reflect.misc.ReflectUtil;
  41 import sun.security.util.SecurityConstants;
  42 
  43 import java.lang.invoke.LambdaForm.BasicType;
  44 import java.lang.reflect.Constructor;
  45 import java.lang.reflect.Field;
  46 import java.lang.reflect.Member;
  47 import java.lang.reflect.Method;
  48 import java.lang.reflect.Modifier;
  49 import java.lang.reflect.ReflectPermission;
  50 import java.nio.ByteOrder;
  51 import java.security.ProtectionDomain;
  52 import java.util.ArrayList;
  53 import java.util.Arrays;
  54 import java.util.BitSet;
  55 import java.util.Iterator;
  56 import java.util.List;
  57 import java.util.Objects;
  58 import java.util.Set;
  59 import java.util.concurrent.ConcurrentHashMap;
  60 import java.util.stream.Collectors;
  61 import java.util.stream.Stream;
  62 
  63 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  64 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  65 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  66 import static java.lang.invoke.MethodType.methodType;
  67 
  68 /**
  69  * This class consists exclusively of static methods that operate on or return
  70  * method handles. They fall into several categories:
  71  * <ul>
  72  * <li>Lookup methods which help create method handles for methods and fields.
  73  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  74  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  75  * </ul>
  76  * A lookup, combinator, or factory method will fail and throw an
  77  * {@code IllegalArgumentException} if the created method handle's type
  78  * would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
  79  *
  80  * @author John Rose, JSR 292 EG
  81  * @since 1.7
  82  */
  83 public class MethodHandles {
  84 
  85     private MethodHandles() { }  // do not instantiate
  86 
  87     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  88 
  89     // See IMPL_LOOKUP below.
  90 
  91     //// Method handle creation from ordinary methods.
  92 
  93     /**
  94      * Returns a {@link Lookup lookup object} with
  95      * full capabilities to emulate all supported bytecode behaviors of the caller.
  96      * These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
  97      * Factory methods on the lookup object can create
  98      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  99      * for any member that the caller has access to via bytecodes,
 100      * including protected and private fields and methods.
 101      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 102      * Do not store it in place where untrusted code can access it.
 103      * <p>
 104      * This method is caller sensitive, which means that it may return different
 105      * values to different callers.
 106      * @return a lookup object for the caller of this method, with
 107      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
 108      */
 109     @CallerSensitive
 110     @ForceInline // to ensure Reflection.getCallerClass optimization
 111     public static Lookup lookup() {
 112         return new Lookup(Reflection.getCallerClass());
 113     }
 114 
 115     /**
 116      * This reflected$lookup method is the alternate implementation of
 117      * the lookup method when being invoked by reflection.
 118      */
 119     @CallerSensitive
 120     private static Lookup reflected$lookup() {
 121         Class<?> caller = Reflection.getCallerClass();
 122         if (caller.getClassLoader() == null) {
 123             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 124         }
 125         return new Lookup(caller);
 126     }
 127 
 128     /**
 129      * Returns a {@link Lookup lookup object} which is trusted minimally.
 130      * The lookup has the {@code UNCONDITIONAL} mode.
 131      * It can only be used to create method handles to public members of
 132      * public classes in packages that are exported unconditionally.
 133      * <p>
 134      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 135      * of this lookup object will be {@link java.lang.Object}.
 136      *
 137      * @apiNote The use of Object is conventional, and because the lookup modes are
 138      * limited, there is no special access provided to the internals of Object, its package
 139      * or its module.  This public lookup object or other lookup object with
 140      * {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
 141      * is not used to determine the lookup context.
 142      *
 143      * <p style="font-size:smaller;">
 144      * <em>Discussion:</em>
 145      * The lookup class can be changed to any other class {@code C} using an expression of the form
 146      * {@link Lookup#in publicLookup().in(C.class)}.
 147      * A public lookup object is always subject to
 148      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 149      * Also, it cannot access
 150      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 151      * @return a lookup object which is trusted minimally
 152      *
 153      * @revised 9
 154      * @spec JPMS
 155      */
 156     public static Lookup publicLookup() {
 157         return Lookup.PUBLIC_LOOKUP;
 158     }
 159 
 160     /**
 161      * Returns a {@link Lookup lookup} object on a target class to emulate all supported
 162      * bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 163      * The returned lookup object can provide access to classes in modules and packages,
 164      * and members of those classes, outside the normal rules of Java access control,
 165      * instead conforming to the more permissive rules for modular <em>deep reflection</em>.
 166      * <p>
 167      * A caller, specified as a {@code Lookup} object, in module {@code M1} is
 168      * allowed to do deep reflection on module {@code M2} and package of the target class
 169      * if and only if all of the following conditions are {@code true}:
 170      * <ul>
 171      * <li>If there is a security manager, its {@code checkPermission} method is
 172      * called to check {@code ReflectPermission("suppressAccessChecks")} and
 173      * that must return normally.
 174      * <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
 175      * full privilege access}.  Specifically:
 176      *   <ul>
 177      *     <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
 178      *         (This is because otherwise there would be no way to ensure the original lookup
 179      *         creator was a member of any particular module, and so any subsequent checks
 180      *         for readability and qualified exports would become ineffective.)
 181      *     <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
 182      *         (This is because an application intending to share intra-module access
 183      *         using {@link Lookup#MODULE MODULE} alone will inadvertently also share
 184      *         deep reflection to its own module.)
 185      *   </ul>
 186      * <li>The target class must be a proper class, not a primitive or array class.
 187      * (Thus, {@code M2} is well-defined.)
 188      * <li>If the caller module {@code M1} differs from
 189      * the target module {@code M2} then both of the following must be true:
 190      *   <ul>
 191      *     <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
 192      *     <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
 193      *         containing the target class to at least {@code M1}.</li>
 194      *   </ul>
 195      * </ul>
 196      * <p>
 197      * If any of the above checks is violated, this method fails with an
 198      * exception.
 199      * <p>
 200      * Otherwise, if {@code M1} and {@code M2} are the same module, this method
 201      * returns a {@code Lookup} on {@code targetClass} with
 202      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} and
 203      * {@code null} previous lookup class.
 204      * <p>
 205      * Otherwise, {@code M1} and {@code M2} are two different modules.  This method
 206      * returns a {@code Lookup} on {@code targetClass} that records
 207      * the lookup class of the caller as the new previous lookup class and
 208      * drops {@code MODULE} access from the full privilege access.
 209      *
 210      * @param targetClass the target class
 211      * @param caller the caller lookup object
 212      * @return a lookup object for the target class, with private access
 213      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
 214      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 215      * @throws SecurityException if denied by the security manager
 216      * @throws IllegalAccessException if any of the other access checks specified above fails
 217      * @since 9
 218      * @spec JPMS
 219      * @see Lookup#dropLookupMode
 220      * @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
 221      */
 222     public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
 223         if (caller.allowedModes == Lookup.TRUSTED) {
 224             return new Lookup(targetClass);
 225         }
 226 
 227         SecurityManager sm = System.getSecurityManager();
 228         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 229         if (targetClass.isPrimitive())
 230             throw new IllegalArgumentException(targetClass + " is a primitive class");
 231         if (targetClass.isArray())
 232             throw new IllegalArgumentException(targetClass + " is an array class");
 233         // Ensure that we can reason accurately about private and module access.
 234         if (!caller.hasFullPrivilegeAccess())
 235             throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
 236 
 237         // previous lookup class is never set if it has MODULE access
 238         assert caller.previousLookupClass() == null;
 239 
 240         Class<?> callerClass = caller.lookupClass();
 241         Module callerModule = callerClass.getModule();  // M1
 242         Module targetModule = targetClass.getModule();  // M2
 243         Class<?> newPreviousClass = null;
 244         int newModes = Lookup.FULL_POWER_MODES;
 245 
 246         if (targetModule != callerModule) {
 247             if (!callerModule.canRead(targetModule))
 248                 throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 249             if (targetModule.isNamed()) {
 250                 String pn = targetClass.getPackageName();
 251                 assert !pn.isEmpty() : "unnamed package cannot be in named module";
 252                 if (!targetModule.isOpen(pn, callerModule))
 253                     throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 254             }
 255 
 256             // M2 != M1, set previous lookup class to M1 and drop MODULE access
 257             newPreviousClass = callerClass;
 258             newModes &= ~Lookup.MODULE;
 259         }
 260 
 261         if (!callerModule.isNamed() && targetModule.isNamed()) {
 262             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 263             if (logger != null) {
 264                 logger.logIfOpenedForIllegalAccess(caller, targetClass);
 265             }
 266         }
 267         return Lookup.newLookup(targetClass, newPreviousClass, newModes);
 268     }
 269 
 270     /**
 271      * Returns the <em>class data</em> associated with the lookup class
 272      * of the specified {@code Lookup} object, or {@code null}.
 273      *
 274      * <p> Classes can be created with class data by calling
 275      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, Lookup.ClassOption...)
 276      * Lookup::defineHiddenClassWithClassData}.
 277      * A hidden class with a class data behaves as if the hidden class
 278      * has a private static final unnamed field pre-initialized with
 279      * the class data and this method is equivalent as if calling
 280      * {@link ConstantBootstraps#getStaticFinal(Lookup, String, Class)} to
 281      * obtain the value of such field corresponding to the class data.
 282      *
 283      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 284      * must have {@link Lookup#ORIGINAL ORIGINAL} access in order to retrieve
 285      * the class data.
 286      *
 287      * @apiNote
 288      * This method can be called as a bootstrap method for a dynamically computed
 289      * constant.  A framework can create a hidden class with class data, for
 290      * example that can be {@code List.of(o1, o2, o3....)} containing more than
 291      * one live object.  The class data is accessible only to the lookup object
 292      * created by the original caller but inaccessible to other members
 293      * in the same nest.  If a framework passes security sensitive live objects
 294      * to a hidden class via class data, it is recommended to load the value
 295      * of class data as a dynamically computed constant instead of storing
 296      * the live objects in private fields which are accessible to other
 297      * nestmates.
 298      *
 299      * @param <T> the type to cast the class data object to
 300      * @param caller the lookup context describing the class performing the
 301      * operation (normally stacked by the JVM)
 302      * @param name ignored
 303      * @param type the type of the class data
 304      * @return the value of the class data if present in the lookup class;
 305      * otherwise {@code null}
 306      * @throws IllegalAccessException if the lookup context does not have
 307      * original caller access
 308      * @throws ClassCastException if the class data cannot be converted to
 309      * the specified {@code type}
 310      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, Lookup.ClassOption...)
 311      * @since 15
 312      */
 313     static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
 314         if (!caller.hasFullPrivilegeAccess()) {
 315             throw new IllegalAccessException(caller + " does not have full privilege access");
 316         }
 317         Object classData = MethodHandleNatives.classData(caller.lookupClass);
 318         return type.cast(classData);
 319     }
 320 
 321     /**
 322      * Performs an unchecked "crack" of a
 323      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 324      * The result is as if the user had obtained a lookup object capable enough
 325      * to crack the target method handle, called
 326      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 327      * on the target to obtain its symbolic reference, and then called
 328      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 329      * to resolve the symbolic reference to a member.
 330      * <p>
 331      * If there is a security manager, its {@code checkPermission} method
 332      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 333      * @param <T> the desired type of the result, either {@link Member} or a subtype
 334      * @param target a direct method handle to crack into symbolic reference components
 335      * @param expected a class object representing the desired result type {@code T}
 336      * @return a reference to the method, constructor, or field object
 337      * @throws    SecurityException if the caller is not privileged to call {@code setAccessible}
 338      * @throws    NullPointerException if either argument is {@code null}
 339      * @throws    IllegalArgumentException if the target is not a direct method handle
 340      * @throws    ClassCastException if the member is not of the expected type
 341      * @since 1.8
 342      */
 343     public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
 344         SecurityManager smgr = System.getSecurityManager();
 345         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 346         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 347         return lookup.revealDirect(target).reflectAs(expected, lookup);
 348     }
 349     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 350     private static final java.security.Permission ACCESS_PERMISSION =
 351         new ReflectPermission("suppressAccessChecks");
 352 
 353     /**
 354      * A <em>lookup object</em> is a factory for creating method handles,
 355      * when the creation requires access checking.
 356      * Method handles do not perform
 357      * access checks when they are called, but rather when they are created.
 358      * Therefore, method handle access
 359      * restrictions must be enforced when a method handle is created.
 360      * The caller class against which those restrictions are enforced
 361      * is known as the {@linkplain #lookupClass() lookup class}.
 362      * <p>
 363      * A lookup class which needs to create method handles will call
 364      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 365      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 366      * determined, and securely stored in the {@code Lookup} object.
 367      * The lookup class (or its delegates) may then use factory methods
 368      * on the {@code Lookup} object to create method handles for access-checked members.
 369      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 370      * even private ones.
 371      *
 372      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 373      * The factory methods on a {@code Lookup} object correspond to all major
 374      * use cases for methods, constructors, and fields.
 375      * Each method handle created by a factory method is the functional
 376      * equivalent of a particular <em>bytecode behavior</em>.
 377      * (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
 378      * the Java Virtual Machine Specification.)
 379      * Here is a summary of the correspondence between these factory methods and
 380      * the behavior of the resulting method handles:
 381      * <table class="striped">
 382      * <caption style="display:none">lookup method behaviors</caption>
 383      * <thead>
 384      * <tr>
 385      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 386      *     <th scope="col">member</th>
 387      *     <th scope="col">bytecode behavior</th>
 388      * </tr>
 389      * </thead>
 390      * <tbody>
 391      * <tr>
 392      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 393      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 394      * </tr>
 395      * <tr>
 396      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 397      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
 398      * </tr>
 399      * <tr>
 400      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 401      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 402      * </tr>
 403      * <tr>
 404      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 405      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 406      * </tr>
 407      * <tr>
 408      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 409      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 410      * </tr>
 411      * <tr>
 412      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 413      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 414      * </tr>
 415      * <tr>
 416      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 417      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 418      * </tr>
 419      * <tr>
 420      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 421      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 422      * </tr>
 423      * <tr>
 424      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 425      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 426      * </tr>
 427      * <tr>
 428      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 429      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 430      * </tr>
 431      * <tr>
 432      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 433      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 434      * </tr>
 435      * <tr>
 436      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 437      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 438      * </tr>
 439      * <tr>
 440      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
 441      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 442      * </tr>
 443      * <tr>
 444      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 445      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 446      * </tr>
 447      * </tbody>
 448      * </table>
 449      *
 450      * Here, the type {@code C} is the class or interface being searched for a member,
 451      * documented as a parameter named {@code refc} in the lookup methods.
 452      * The method type {@code MT} is composed from the return type {@code T}
 453      * and the sequence of argument types {@code A*}.
 454      * The constructor also has a sequence of argument types {@code A*} and
 455      * is deemed to return the newly-created object of type {@code C}.
 456      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 457      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 458      * if it is present, it is always the leading argument to the method handle invocation.
 459      * (In the case of some {@code protected} members, {@code this} may be
 460      * restricted in type to the lookup class; see below.)
 461      * The name {@code arg} stands for all the other method handle arguments.
 462      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 463      * stands for a null reference if the accessed method or field is static,
 464      * and {@code this} otherwise.
 465      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 466      * for reflective objects corresponding to the given members declared in type {@code C}.
 467      * <p>
 468      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 469      * as if by {@code ldc CONSTANT_Class}.
 470      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 471      * <p>
 472      * In cases where the given member is of variable arity (i.e., a method or constructor)
 473      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 474      * In all other cases, the returned method handle will be of fixed arity.
 475      * <p style="font-size:smaller;">
 476      * <em>Discussion:</em>
 477      * The equivalence between looked-up method handles and underlying
 478      * class members and bytecode behaviors
 479      * can break down in a few ways:
 480      * <ul style="font-size:smaller;">
 481      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 482      * the lookup can still succeed, even when there is no equivalent
 483      * Java expression or bytecoded constant.
 484      * <li>Likewise, if {@code T} or {@code MT}
 485      * is not symbolically accessible from the lookup class's loader,
 486      * the lookup can still succeed.
 487      * For example, lookups for {@code MethodHandle.invokeExact} and
 488      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 489      * <li>If there is a security manager installed, it can forbid the lookup
 490      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 491      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 492      * constant is not subject to security manager checks.
 493      * <li>If the looked-up method has a
 494      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 495      * the method handle creation may fail with an
 496      * {@code IllegalArgumentException}, due to the method handle type having
 497      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 498      * </ul>
 499      *
 500      * <h2><a id="access"></a>Access checking</h2>
 501      * Access checks are applied in the factory methods of {@code Lookup},
 502      * when a method handle is created.
 503      * This is a key difference from the Core Reflection API, since
 504      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 505      * performs access checking against every caller, on every call.
 506      * <p>
 507      * All access checks start from a {@code Lookup} object, which
 508      * compares its recorded lookup class against all requests to
 509      * create method handles.
 510      * A single {@code Lookup} object can be used to create any number
 511      * of access-checked method handles, all checked against a single
 512      * lookup class.
 513      * <p>
 514      * A {@code Lookup} object can be shared with other trusted code,
 515      * such as a metaobject protocol.
 516      * A shared {@code Lookup} object delegates the capability
 517      * to create method handles on private members of the lookup class.
 518      * Even if privileged code uses the {@code Lookup} object,
 519      * the access checking is confined to the privileges of the
 520      * original lookup class.
 521      * <p>
 522      * A lookup can fail, because
 523      * the containing class is not accessible to the lookup class, or
 524      * because the desired class member is missing, or because the
 525      * desired class member is not accessible to the lookup class, or
 526      * because the lookup object is not trusted enough to access the member.
 527      * In the case of a field setter function on a {@code final} field,
 528      * finality enforcement is treated as a kind of access control,
 529      * and the lookup will fail, except in special cases of
 530      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 531      * In any of these cases, a {@code ReflectiveOperationException} will be
 532      * thrown from the attempted lookup.  The exact class will be one of
 533      * the following:
 534      * <ul>
 535      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 536      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 537      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 538      * </ul>
 539      * <p>
 540      * In general, the conditions under which a method handle may be
 541      * looked up for a method {@code M} are no more restrictive than the conditions
 542      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 543      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 544      * a method handle lookup will generally raise a corresponding
 545      * checked exception, such as {@code NoSuchMethodException}.
 546      * And the effect of invoking the method handle resulting from the lookup
 547      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 548      * to executing the compiled, verified, and resolved call to {@code M}.
 549      * The same point is true of fields and constructors.
 550      * <p style="font-size:smaller;">
 551      * <em>Discussion:</em>
 552      * Access checks only apply to named and reflected methods,
 553      * constructors, and fields.
 554      * Other method handle creation methods, such as
 555      * {@link MethodHandle#asType MethodHandle.asType},
 556      * do not require any access checks, and are used
 557      * independently of any {@code Lookup} object.
 558      * <p>
 559      * If the desired member is {@code protected}, the usual JVM rules apply,
 560      * including the requirement that the lookup class must either be in the
 561      * same package as the desired member, or must inherit that member.
 562      * (See the Java Virtual Machine Specification, sections {@jvms
 563      * 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
 564      * In addition, if the desired member is a non-static field or method
 565      * in a different package, the resulting method handle may only be applied
 566      * to objects of the lookup class or one of its subclasses.
 567      * This requirement is enforced by narrowing the type of the leading
 568      * {@code this} parameter from {@code C}
 569      * (which will necessarily be a superclass of the lookup class)
 570      * to the lookup class itself.
 571      * <p>
 572      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 573      * that the receiver argument must match both the resolved method <em>and</em>
 574      * the current class.  Again, this requirement is enforced by narrowing the
 575      * type of the leading parameter to the resulting method handle.
 576      * (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
 577      * <p>
 578      * The JVM represents constructors and static initializer blocks as internal methods
 579      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 580      * The internal syntax of invocation instructions allows them to refer to such internal
 581      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 582      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 583      * <p>
 584      * If the relationship between nested types is expressed directly through the
 585      * {@code NestHost} and {@code NestMembers} attributes
 586      * (see the Java Virtual Machine Specification, sections {@jvms
 587      * 4.7.28} and {@jvms 4.7.29}),
 588      * then the associated {@code Lookup} object provides direct access to
 589      * the lookup class and all of its nestmates
 590      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 591      * Otherwise, access between nested classes is obtained by the Java compiler creating
 592      * a wrapper method to access a private method of another class in the same nest.
 593      * For example, a nested class {@code C.D}
 594      * can access private members within other related classes such as
 595      * {@code C}, {@code C.D.E}, or {@code C.B},
 596      * but the Java compiler may need to generate wrapper methods in
 597      * those related classes.  In such cases, a {@code Lookup} object on
 598      * {@code C.E} would be unable to access those private members.
 599      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 600      * which can transform a lookup on {@code C.E} into one on any of those other
 601      * classes, without special elevation of privilege.
 602      * <p>
 603      * The accesses permitted to a given lookup object may be limited,
 604      * according to its set of {@link #lookupModes lookupModes},
 605      * to a subset of members normally accessible to the lookup class.
 606      * For example, the {@link MethodHandles#publicLookup publicLookup}
 607      * method produces a lookup object which is only allowed to access
 608      * public members in public classes of exported packages.
 609      * The caller sensitive method {@link MethodHandles#lookup lookup}
 610      * produces a lookup object with full capabilities relative to
 611      * its caller class, to emulate all supported bytecode behaviors.
 612      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 613      * with fewer access modes than the original lookup object.
 614      *
 615      * <p style="font-size:smaller;">
 616      * <a id="privacc"></a>
 617      * <em>Discussion of private and module access:</em>
 618      * We say that a lookup has <em>private access</em>
 619      * if its {@linkplain #lookupModes lookup modes}
 620      * include the possibility of accessing {@code private} members
 621      * (which includes the private members of nestmates).
 622      * As documented in the relevant methods elsewhere,
 623      * only lookups with private access possess the following capabilities:
 624      * <ul style="font-size:smaller;">
 625      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 626      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 627      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 628      *     for classes accessible to the lookup class
 629      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 630      *     within the same package member
 631      * </ul>
 632      * <p style="font-size:smaller;">
 633      * Similarly, a lookup with module access ensures that the original lookup creator was
 634      * a member in the same module as the lookup class.
 635      * <p style="font-size:smaller;">
 636      * Private and module access are independently determined modes; a lookup may have
 637      * either or both or neither.  A lookup which possesses both access modes is said to
 638      * possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.  Such a lookup has
 639      * the following additional capability:
 640      * <ul style="font-size:smaller;">
 641      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 642      *     such as {@code Class.forName}
 643      * </ul>
 644      * <p style="font-size:smaller;">
 645      * Each of these permissions is a consequence of the fact that a lookup object
 646      * with private access can be securely traced back to an originating class,
 647      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 648      * can be reliably determined and emulated by method handles.
 649      *
 650      * <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
 651      * When a lookup class in one module {@code M1} accesses a class in another module
 652      * {@code M2}, extra access checking is performed beyond the access mode bits.
 653      * A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
 654      * can access public types in {@code M2} when {@code M2} is readable to {@code M1}
 655      * and when the type is in a package of {@code M2} that is exported to
 656      * at least {@code M1}.
 657      * <p>
 658      * A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
 659      * via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
 660      * MethodHandles.privateLookupIn} methods.
 661      * Teleporting across modules will always record the original lookup class as
 662      * the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
 663      * and drops {@link Lookup#MODULE MODULE} access.
 664      * If the target class is in the same module as the lookup class {@code C},
 665      * then the target class becomes the new lookup class
 666      * and there is no change to the previous lookup class.
 667      * If the target class is in a different module from {@code M1} ({@code C}'s module),
 668      * {@code C} becomes the new previous lookup class
 669      * and the target class becomes the new lookup class.
 670      * In that case, if there was already a previous lookup class in {@code M0},
 671      * and it differs from {@code M1} and {@code M2}, then the resulting lookup
 672      * drops all privileges.
 673      * For example,
 674      * <blockquote><pre>
 675      * {@code
 676      * Lookup lookup = MethodHandles.lookup();   // in class C
 677      * Lookup lookup2 = lookup.in(D.class);
 678      * MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
 679      * }</pre></blockquote>
 680      * <p>
 681      * The {@link #lookup()} factory method produces a {@code Lookup} object
 682      * with {@code null} previous lookup class.
 683      * {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
 684      * to class {@code D} without elevation of privileges.
 685      * If {@code C} and {@code D} are in the same module,
 686      * {@code lookup2} records {@code D} as the new lookup class and keeps the
 687      * same previous lookup class as the original {@code lookup}, or
 688      * {@code null} if not present.
 689      * <p>
 690      * When a {@code Lookup} teleports from a class
 691      * in one nest to another nest, {@code PRIVATE} access is dropped.
 692      * When a {@code Lookup} teleports from a class in one package to
 693      * another package, {@code PACKAGE} access is dropped.
 694      * When a {@code Lookup} teleports from a class in one module to another module,
 695      * {@code MODULE} access is dropped.
 696      * Teleporting across modules drops the ability to access non-exported classes
 697      * in both the module of the new lookup class and the module of the old lookup class
 698      * and the resulting {@code Lookup} remains only {@code PUBLIC} access.
 699      * A {@code Lookup} can teleport back and forth to a class in the module of
 700      * the lookup class and the module of the previous class lookup.
 701      * Teleporting across modules can only decrease access but cannot increase it.
 702      * Teleporting to some third module drops all accesses.
 703      * <p>
 704      * In the above example, if {@code C} and {@code D} are in different modules,
 705      * {@code lookup2} records {@code D} as its lookup class and
 706      * {@code C} as its previous lookup class and {@code lookup2} has only
 707      * {@code PUBLIC} access. {@code lookup2} can teleport to other class in
 708      * {@code C}'s module and {@code D}'s module.
 709      * If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
 710      * a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
 711      * class {@code D} is recorded as its previous lookup class.
 712      * <p>
 713      * Teleporting across modules restricts access to the public types that
 714      * both the lookup class and the previous lookup class can equally access
 715      * (see below).
 716      * <p>
 717      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
 718      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
 719      * and create a new {@code Lookup} with <a href="#privacc">private access</a>
 720      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
 721      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
 722      * to call {@code privateLookupIn}.
 723      * A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
 724      * on all classes in {@code M1}.  If {@code T} is in {@code M1}, {@code privateLookupIn}
 725      * produces a new {@code Lookup} on {@code T} with full capabilities.
 726      * A {@code lookup} on {@code C} is also allowed
 727      * to do deep reflection on {@code T} in another module {@code M2} if
 728      * {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
 729      * the package containing {@code T} to at least {@code M1}.
 730      * {@code T} becomes the new lookup class and {@code C} becomes the new previous
 731      * lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
 732      * The resulting {@code Lookup} can be used to do member lookup or teleport
 733      * to another lookup class by calling {@link #in Lookup::in}.  But
 734      * it cannot be used to obtain another private {@code Lookup} by calling
 735      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
 736      * because it has no {@code MODULE} access.
 737      *
 738      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
 739      *
 740      * A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
 741      * allows cross-module access. The access checking is performed with respect
 742      * to both the lookup class and the previous lookup class if present.
 743      * <p>
 744      * A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
 745      * in all modules when the type is in a package that is {@linkplain Module#isExported(String)
 746      * exported unconditionally}.
 747      * <p>
 748      * If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
 749      * the lookup with {@link #PUBLIC} mode can access all public types in modules
 750      * that are readable to {@code M1} and the type is in a package that is exported
 751      * at least to {@code M1}.
 752      * <p>
 753      * If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
 754      * {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
 755      * the intersection of all public types that are accessible to {@code M1}
 756      * with all public types that are accessible to {@code M0}. {@code M0}
 757      * reads {@code M1} and hence the set of accessible types includes:
 758      *
 759      * <table class="striped">
 760      * <caption style="display:none">
 761      * Public types in the following packages are accessible to the
 762      * lookup class and the previous lookup class.
 763      * </caption>
 764      * <thead>
 765      * <tr>
 766      * <th scope="col">Equally accessible types to {@code M0} and {@code M1}</th>
 767      * </tr>
 768      * </thead>
 769      * <tbody>
 770      * <tr>
 771      * <th scope="row" style="text-align:left">unconditional-exported packages from {@code M1}</th>
 772      * </tr>
 773      * <tr>
 774      * <th scope="row" style="text-align:left">unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</th>
 775      * </tr>
 776      * <tr>
 777      * <th scope="row" style="text-align:left">unconditional-exported packages from a third module {@code M2}
 778      * if both {@code M0} and {@code M1} read {@code M2}</th>
 779      * </tr>
 780      * <tr>
 781      * <th scope="row" style="text-align:left">qualified-exported packages from {@code M1} to {@code M0}</th>
 782      * </tr>
 783      * <tr>
 784      * <th scope="row" style="text-align:left">qualified-exported packages from {@code M0} to {@code M1}
 785      * if {@code M1} reads {@code M0}</th>
 786      * </tr>
 787      * <tr>
 788      * <th scope="row" style="text-align:left">qualified-exported packages from a third module {@code M2} to
 789      * both {@code M0} and {@code M1} if both {@code M0} and {@code M1} read {@code M2}</th>
 790      * </tr>
 791      * </tbody>
 792      * </table>
 793      *
 794      * <h2><a id="access-modes"></a>Access modes</h2>
 795      *
 796      * The table below shows the access modes of a {@code Lookup} produced by
 797      * any of the following factory or transformation methods:
 798      * <ul>
 799      * <li>{@link #lookup() MethodHandles::lookup}</li>
 800      * <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
 801      * <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
 802      * <li>{@link Lookup#in Lookup::in}</li>
 803      * <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
 804      * </ul>
 805      *
 806      * <table class="striped">
 807      * <caption style="display:none">
 808      * Access mode summary
 809      * </caption>
 810      * <thead>
 811      * <tr>
 812      * <th scope="col">Lookup object</th>
 813      * <th style="text-align:center">protected</th>
 814      * <th style="text-align:center">private</th>
 815      * <th style="text-align:center">package</th>
 816      * <th style="text-align:center">module</th>
 817      * <th style="text-align:center">public</th>
 818      * </tr>
 819      * </thead>
 820      * <tbody>
 821      * <tr>
 822      * <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
 823      * <td style="text-align:center">PRO</td>
 824      * <td style="text-align:center">PRI</td>
 825      * <td style="text-align:center">PAC</td>
 826      * <td style="text-align:center">MOD</td>
 827      * <td style="text-align:center">1R</td>
 828      * </tr>
 829      * <tr>
 830      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
 831      * <td></td>
 832      * <td></td>
 833      * <td style="text-align:center">PAC</td>
 834      * <td style="text-align:center">MOD</td>
 835      * <td style="text-align:center">1R</td>
 836      * </tr>
 837      * <tr>
 838      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
 839      * <td></td>
 840      * <td></td>
 841      * <td></td>
 842      * <td style="text-align:center">MOD</td>
 843      * <td style="text-align:center">1R</td>
 844      * </tr>
 845      * <tr>
 846      * <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
 847      * <td></td>
 848      * <td></td>
 849      * <td></td>
 850      * <td></td>
 851      * <td style="text-align:center">2R</td>
 852      * </tr>
 853      * <tr>
 854      * <td>{@code CL.in(D).in(C)} hop back to module</td>
 855      * <td></td>
 856      * <td></td>
 857      * <td></td>
 858      * <td></td>
 859      * <td style="text-align:center">2R</td>
 860      * </tr>
 861      * <tr>
 862      * <td>{@code PRI1 = privateLookupIn(C1,CL)}</td>
 863      * <td style="text-align:center">PRO</td>
 864      * <td style="text-align:center">PRI</td>
 865      * <td style="text-align:center">PAC</td>
 866      * <td style="text-align:center">MOD</td>
 867      * <td style="text-align:center">1R</td>
 868      * </tr>
 869      * <tr>
 870      * <td>{@code PRI1a = privateLookupIn(C,PRI1)}</td>
 871      * <td style="text-align:center">PRO</td>
 872      * <td style="text-align:center">PRI</td>
 873      * <td style="text-align:center">PAC</td>
 874      * <td style="text-align:center">MOD</td>
 875      * <td style="text-align:center">1R</td>
 876      * </tr>
 877      * <tr>
 878      * <td>{@code PRI1.in(C1)} same package</td>
 879      * <td></td>
 880      * <td></td>
 881      * <td style="text-align:center">PAC</td>
 882      * <td style="text-align:center">MOD</td>
 883      * <td style="text-align:center">1R</td>
 884      * </tr>
 885      * <tr>
 886      * <td>{@code PRI1.in(C1)} different package</td>
 887      * <td></td>
 888      * <td></td>
 889      * <td></td>
 890      * <td style="text-align:center">MOD</td>
 891      * <td style="text-align:center">1R</td>
 892      * </tr>
 893      * <tr>
 894      * <td>{@code PRI1.in(D)} different module</td>
 895      * <td></td>
 896      * <td></td>
 897      * <td></td>
 898      * <td></td>
 899      * <td style="text-align:center">2R</td>
 900      * </tr>
 901      * <tr>
 902      * <td>{@code PRI1.dropLookupMode(PROTECTED)}</td>
 903      * <td></td>
 904      * <td style="text-align:center">PRI</td>
 905      * <td style="text-align:center">PAC</td>
 906      * <td style="text-align:center">MOD</td>
 907      * <td style="text-align:center">1R</td>
 908      * </tr>
 909      * <tr>
 910      * <td>{@code PRI1.dropLookupMode(PRIVATE)}</td>
 911      * <td></td>
 912      * <td></td>
 913      * <td style="text-align:center">PAC</td>
 914      * <td style="text-align:center">MOD</td>
 915      * <td style="text-align:center">1R</td>
 916      * </tr>
 917      * <tr>
 918      * <td>{@code PRI1.dropLookupMode(PACKAGE)}</td>
 919      * <td></td>
 920      * <td></td>
 921      * <td></td>
 922      * <td style="text-align:center">MOD</td>
 923      * <td style="text-align:center">1R</td>
 924      * </tr>
 925      * <tr>
 926      * <td>{@code PRI1.dropLookupMode(MODULE)}</td>
 927      * <td></td>
 928      * <td></td>
 929      * <td></td>
 930      * <td></td>
 931      * <td style="text-align:center">1R</td>
 932      * </tr>
 933      * <tr>
 934      * <td>{@code PRI1.dropLookupMode(PUBLIC)}</td>
 935      * <td></td>
 936      * <td></td>
 937      * <td></td>
 938      * <td></td>
 939      * <td style="text-align:center">none</td>
 940      * <tr>
 941      * <td>{@code PRI2 = privateLookupIn(D,CL)}</td>
 942      * <td style="text-align:center">PRO</td>
 943      * <td style="text-align:center">PRI</td>
 944      * <td style="text-align:center">PAC</td>
 945      * <td></td>
 946      * <td style="text-align:center">2R</td>
 947      * </tr>
 948      * <tr>
 949      * <td>{@code privateLookupIn(D,PRI1)}</td>
 950      * <td style="text-align:center">PRO</td>
 951      * <td style="text-align:center">PRI</td>
 952      * <td style="text-align:center">PAC</td>
 953      * <td></td>
 954      * <td style="text-align:center">2R</td>
 955      * </tr>
 956      * <tr>
 957      * <td>{@code privateLookupIn(C,PRI2)} fails</td>
 958      * <td></td>
 959      * <td></td>
 960      * <td></td>
 961      * <td></td>
 962      * <td style="text-align:center">IAE</td>
 963      * </tr>
 964      * <tr>
 965      * <td>{@code PRI2.in(D2)} same package</td>
 966      * <td></td>
 967      * <td></td>
 968      * <td style="text-align:center">PAC</td>
 969      * <td></td>
 970      * <td style="text-align:center">2R</td>
 971      * </tr>
 972      * <tr>
 973      * <td>{@code PRI2.in(D2)} different package</td>
 974      * <td></td>
 975      * <td></td>
 976      * <td></td>
 977      * <td></td>
 978      * <td style="text-align:center">2R</td>
 979      * </tr>
 980      * <tr>
 981      * <td>{@code PRI2.in(C1)} hop back to module</td>
 982      * <td></td>
 983      * <td></td>
 984      * <td></td>
 985      * <td></td>
 986      * <td style="text-align:center">2R</td>
 987      * </tr>
 988      * <tr>
 989      * <td>{@code PRI2.in(E)} hop to third module</td>
 990      * <td></td>
 991      * <td></td>
 992      * <td></td>
 993      * <td></td>
 994      * <td style="text-align:center">none</td>
 995      * </tr>
 996      * <tr>
 997      * <td>{@code PRI2.dropLookupMode(PROTECTED)}</td>
 998      * <td></td>
 999      * <td style="text-align:center">PRI</td>
1000      * <td style="text-align:center">PAC</td>
1001      * <td></td>
1002      * <td style="text-align:center">2R</td>
1003      * </tr>
1004      * <tr>
1005      * <td>{@code PRI2.dropLookupMode(PRIVATE)}</td>
1006      * <td></td>
1007      * <td></td>
1008      * <td style="text-align:center">PAC</td>
1009      * <td></td>
1010      * <td style="text-align:center">2R</td>
1011      * </tr>
1012      * <tr>
1013      * <td>{@code PRI2.dropLookupMode(PACKAGE)}</td>
1014      * <td></td>
1015      * <td></td>
1016      * <td></td>
1017      * <td></td>
1018      * <td style="text-align:center">2R</td>
1019      * </tr>
1020      * <tr>
1021      * <td>{@code PRI2.dropLookupMode(MODULE)}</td>
1022      * <td></td>
1023      * <td></td>
1024      * <td></td>
1025      * <td></td>
1026      * <td style="text-align:center">2R</td>
1027      * </tr>
1028      * <tr>
1029      * <td>{@code PRI2.dropLookupMode(PUBLIC)}</td>
1030      * <td></td>
1031      * <td></td>
1032      * <td></td>
1033      * <td></td>
1034      * <td style="text-align:center">none</td>
1035      * </tr>
1036      * <tr>
1037      * <td>{@code CL.dropLookupMode(PROTECTED)}</td>
1038      * <td></td>
1039      * <td style="text-align:center">PRI</td>
1040      * <td style="text-align:center">PAC</td>
1041      * <td style="text-align:center">MOD</td>
1042      * <td style="text-align:center">1R</td>
1043      * </tr>
1044      * <tr>
1045      * <td>{@code CL.dropLookupMode(PRIVATE)}</td>
1046      * <td></td>
1047      * <td></td>
1048      * <td style="text-align:center">PAC</td>
1049      * <td style="text-align:center">MOD</td>
1050      * <td style="text-align:center">1R</td>
1051      * </tr>
1052      * <tr>
1053      * <td>{@code CL.dropLookupMode(PACKAGE)}</td>
1054      * <td></td>
1055      * <td></td>
1056      * <td></td>
1057      * <td style="text-align:center">MOD</td>
1058      * <td style="text-align:center">1R</td>
1059      * </tr>
1060      * <tr>
1061      * <td>{@code CL.dropLookupMode(MODULE)}</td>
1062      * <td></td>
1063      * <td></td>
1064      * <td></td>
1065      * <td></td>
1066      * <td style="text-align:center">1R</td>
1067      * </tr>
1068      * <tr>
1069      * <td>{@code CL.dropLookupMode(PUBLIC)}</td>
1070      * <td></td>
1071      * <td></td>
1072      * <td></td>
1073      * <td></td>
1074      * <td style="text-align:center">none</td>
1075      * </tr>
1076      * <tr>
1077      * <td>{@code PUB = publicLookup()}</td>
1078      * <td></td>
1079      * <td></td>
1080      * <td></td>
1081      * <td></td>
1082      * <td style="text-align:center">U</td>
1083      * </tr>
1084      * <tr>
1085      * <td>{@code PUB.in(D)} different module</td>
1086      * <td></td>
1087      * <td></td>
1088      * <td></td>
1089      * <td></td>
1090      * <td style="text-align:center">U</td>
1091      * </tr>
1092      * <tr>
1093      * <td>{@code PUB.in(D).in(E)} third module</td>
1094      * <td></td>
1095      * <td></td>
1096      * <td></td>
1097      * <td></td>
1098      * <td style="text-align:center">U</td>
1099      * </tr>
1100      * <tr>
1101      * <td>{@code PUB.dropLookupMode(UNCONDITIONAL)}</td>
1102      * <td></td>
1103      * <td></td>
1104      * <td></td>
1105      * <td></td>
1106      * <td style="text-align:center">none</td>
1107      * </tr>
1108      * <tr>
1109      * <td>{@code privateLookupIn(C1,PUB)} fails</td>
1110      * <td></td>
1111      * <td></td>
1112      * <td></td>
1113      * <td></td>
1114      * <td style="text-align:center">IAE</td>
1115      * </tr>
1116      * <tr>
1117      * <td>{@code ANY.in(X)}, for inaccessible {@code X}</td>
1118      * <td></td>
1119      * <td></td>
1120      * <td></td>
1121      * <td></td>
1122      * <td style="text-align:center">none</td>
1123      * </tr>
1124      * </tbody>
1125      * </table>
1126      *
1127      * <p>
1128      * Notes:
1129      * <ul>
1130      * <li>Class {@code C} and class {@code C1} are in module {@code M1},
1131      *     but {@code D} and {@code D2} are in module {@code M2}, and {@code E}
1132      *     is in module {@code M3}. {@code X} stands for class which is inaccessible
1133      *     to the lookup. {@code ANY} stands for any of the example lookups.</li>
1134      * <li>{@code PRO} indicates {@link #PROTECTED} bit set,
1135      *     {@code PRI} indicates {@link #PRIVATE} bit set,
1136      *     {@code PAC} indicates {@link #PACKAGE} bit set,
1137      *     {@code MOD} indicates {@link #MODULE} bit set,
1138      *     {@code 1R} and {@code 2R} indicate {@link #PUBLIC} bit set,
1139      *     {@code U} indicates {@link #UNCONDITIONAL} bit set,
1140      *     {@code IAE} indicates {@code IllegalAccessException} thrown.</li>
1141      * <li>Public access comes in three kinds:
1142      * <ul>
1143      * <li>unconditional ({@code U}): the lookup assumes readability.
1144      *     The lookup has {@code null} previous lookup class.
1145      * <li>one-module-reads ({@code 1R}): the module access checking is
1146      *     performed with respect to the lookup class.  The lookup has {@code null}
1147      *     previous lookup class.
1148      * <li>two-module-reads ({@code 2R}): the module access checking is
1149      *     performed with respect to the lookup class and the previous lookup class.
1150      *     The lookup has a non-null previous lookup class which is in a
1151      *     different module from the current lookup class.
1152      * </ul>
1153      * <li>Any attempt to reach a third module loses all access.</li>
1154      * <li>If a target class {@code X} is not accessible to {@code Lookup::in}
1155      * all access modes are dropped.</li>
1156      * </ul>
1157      *
1158      * <h2><a id="secmgr"></a>Security manager interactions</h2>
1159      * Although bytecode instructions can only refer to classes in
1160      * a related class loader, this API can search for methods in any
1161      * class, as long as a reference to its {@code Class} object is
1162      * available.  Such cross-loader references are also possible with the
1163      * Core Reflection API, and are impossible to bytecode instructions
1164      * such as {@code invokestatic} or {@code getfield}.
1165      * There is a {@linkplain java.lang.SecurityManager security manager API}
1166      * to allow applications to check such cross-loader references.
1167      * These checks apply to both the {@code MethodHandles.Lookup} API
1168      * and the Core Reflection API
1169      * (as found on {@link java.lang.Class Class}).
1170      * <p>
1171      * If a security manager is present, member and class lookups are subject to
1172      * additional checks.
1173      * From one to three calls are made to the security manager.
1174      * Any of these calls can refuse access by throwing a
1175      * {@link java.lang.SecurityException SecurityException}.
1176      * Define {@code smgr} as the security manager,
1177      * {@code lookc} as the lookup class of the current lookup object,
1178      * {@code refc} as the containing class in which the member
1179      * is being sought, and {@code defc} as the class in which the
1180      * member is actually defined.
1181      * (If a class or other type is being accessed,
1182      * the {@code refc} and {@code defc} values are the class itself.)
1183      * The value {@code lookc} is defined as <em>not present</em>
1184      * if the current lookup object does not have
1185      * {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1186      * The calls are made according to the following rules:
1187      * <ul>
1188      * <li><b>Step 1:</b>
1189      *     If {@code lookc} is not present, or if its class loader is not
1190      *     the same as or an ancestor of the class loader of {@code refc},
1191      *     then {@link SecurityManager#checkPackageAccess
1192      *     smgr.checkPackageAccess(refcPkg)} is called,
1193      *     where {@code refcPkg} is the package of {@code refc}.
1194      * <li><b>Step 2a:</b>
1195      *     If the retrieved member is not public and
1196      *     {@code lookc} is not present, then
1197      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1198      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
1199      * <li><b>Step 2b:</b>
1200      *     If the retrieved class has a {@code null} class loader,
1201      *     and {@code lookc} is not present, then
1202      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1203      *     with {@code RuntimePermission("getClassLoader")} is called.
1204      * <li><b>Step 3:</b>
1205      *     If the retrieved member is not public,
1206      *     and if {@code lookc} is not present,
1207      *     and if {@code defc} and {@code refc} are different,
1208      *     then {@link SecurityManager#checkPackageAccess
1209      *     smgr.checkPackageAccess(defcPkg)} is called,
1210      *     where {@code defcPkg} is the package of {@code defc}.
1211      * </ul>
1212      * Security checks are performed after other access checks have passed.
1213      * Therefore, the above rules presuppose a member or class that is public,
1214      * or else that is being accessed from a lookup class that has
1215      * rights to access the member or class.
1216      * <p>
1217      * If a security manager is present and the current lookup object does not have
1218      * {@linkplain #hasFullPrivilegeAccess() full privilege access}, then
1219      * {@link #defineClass(byte[]) defineClass}
1220      * calls {@link SecurityManager#checkPermission smgr.checkPermission}
1221      * with {@code RuntimePermission("defineClass")}.
1222      *
1223      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
1224      * A small number of Java methods have a special property called caller sensitivity.
1225      * A <em>caller-sensitive</em> method can behave differently depending on the
1226      * identity of its immediate caller.
1227      * <p>
1228      * If a method handle for a caller-sensitive method is requested,
1229      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
1230      * but they take account of the lookup class in a special way.
1231      * The resulting method handle behaves as if it were called
1232      * from an instruction contained in the lookup class,
1233      * so that the caller-sensitive method detects the lookup class.
1234      * (By contrast, the invoker of the method handle is disregarded.)
1235      * Thus, in the case of caller-sensitive methods,
1236      * different lookup classes may give rise to
1237      * differently behaving method handles.
1238      * <p>
1239      * In cases where the lookup object is
1240      * {@link MethodHandles#publicLookup() publicLookup()},
1241      * or some other lookup object without the
1242      * {@linkplain #hasFullPrivilegeAccess() full privilege access},
1243      * the lookup class is disregarded.
1244      * In such cases, no caller-sensitive method handle can be created,
1245      * access is forbidden, and the lookup fails with an
1246      * {@code IllegalAccessException}.
1247      * <p style="font-size:smaller;">
1248      * <em>Discussion:</em>
1249      * For example, the caller-sensitive method
1250      * {@link java.lang.Class#forName(String) Class.forName(x)}
1251      * can return varying classes or throw varying exceptions,
1252      * depending on the class loader of the class that calls it.
1253      * A public lookup of {@code Class.forName} will fail, because
1254      * there is no reasonable way to determine its bytecode behavior.
1255      * <p style="font-size:smaller;">
1256      * If an application caches method handles for broad sharing,
1257      * it should use {@code publicLookup()} to create them.
1258      * If there is a lookup of {@code Class.forName}, it will fail,
1259      * and the application must take appropriate action in that case.
1260      * It may be that a later lookup, perhaps during the invocation of a
1261      * bootstrap method, can incorporate the specific identity
1262      * of the caller, making the method accessible.
1263      * <p style="font-size:smaller;">
1264      * The function {@code MethodHandles.lookup} is caller sensitive
1265      * so that there can be a secure foundation for lookups.
1266      * Nearly all other methods in the JSR 292 API rely on lookup
1267      * objects to check access requests.
1268      *
1269      * @revised 9
1270      */
1271     public static final
1272     class Lookup {
1273         /** The class on behalf of whom the lookup is being performed. */
1274         private final Class<?> lookupClass;
1275 
1276         /** previous lookup class */
1277         private final Class<?> prevLookupClass;
1278 
1279         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
1280         private final int allowedModes;
1281 
1282         static {
1283             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
1284         }
1285 
1286         /** A single-bit mask representing {@code public} access,
1287          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1288          *  The value, {@code 0x01}, happens to be the same as the value of the
1289          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
1290          *  <p>
1291          *  A {@code Lookup} with this lookup mode performs cross-module access check
1292          *  with respect to the {@linkplain #lookupClass() lookup class} and
1293          *  {@linkplain #previousLookupClass() previous lookup class} if present.
1294          */
1295         public static final int PUBLIC = Modifier.PUBLIC;
1296 
1297         /** A single-bit mask representing {@code private} access,
1298          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1299          *  The value, {@code 0x02}, happens to be the same as the value of the
1300          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
1301          */
1302         public static final int PRIVATE = Modifier.PRIVATE;
1303 
1304         /** A single-bit mask representing {@code protected} access,
1305          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1306          *  The value, {@code 0x04}, happens to be the same as the value of the
1307          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
1308          */
1309         public static final int PROTECTED = Modifier.PROTECTED;
1310 
1311         /** A single-bit mask representing {@code package} access (default access),
1312          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1313          *  The value is {@code 0x08}, which does not correspond meaningfully to
1314          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1315          */
1316         public static final int PACKAGE = Modifier.STATIC;
1317 
1318         /** A single-bit mask representing {@code module} access,
1319          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1320          *  The value is {@code 0x10}, which does not correspond meaningfully to
1321          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1322          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
1323          *  with this lookup mode can access all public types in the module of the
1324          *  lookup class and public types in packages exported by other modules
1325          *  to the module of the lookup class.
1326          *  <p>
1327          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1328          *  previous lookup class} is always {@code null}.
1329          *
1330          *  @since 9
1331          *  @spec JPMS
1332          */
1333         public static final int MODULE = PACKAGE << 1;
1334 
1335         /** A single-bit mask representing {@code unconditional} access
1336          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1337          *  The value is {@code 0x20}, which does not correspond meaningfully to
1338          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1339          *  A {@code Lookup} with this lookup mode assumes {@linkplain
1340          *  java.lang.Module#canRead(java.lang.Module) readability}.
1341          *  This lookup mode can access all public members of public types
1342          *  of all modules when the type is in a package that is {@link
1343          *  java.lang.Module#isExported(String) exported unconditionally}.
1344          *
1345          *  <p>
1346          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1347          *  previous lookup class} is always {@code null}.
1348          *
1349          *  @since 9
1350          *  @spec JPMS
1351          *  @see #publicLookup()
1352          */
1353         public static final int UNCONDITIONAL = PACKAGE << 2;
1354 
1355         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL);
1356         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);
1357         private static final int TRUSTED   = -1;
1358 
1359         /*
1360          * Adjust PUBLIC => PUBLIC|MODULE|UNCONDITIONAL
1361          * Adjust 0 => PACKAGE
1362          */
1363         private static int fixmods(int mods) {
1364             mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL);
1365             if (Modifier.isPublic(mods))
1366                 mods |= UNCONDITIONAL;
1367             return (mods != 0) ? mods : PACKAGE;
1368         }
1369 
1370         /** Tells which class is performing the lookup.  It is this class against
1371          *  which checks are performed for visibility and access permissions.
1372          *  <p>
1373          *  If this lookup object has a {@linkplain #previousLookupClass() previous lookup class},
1374          *  access checks are performed against both the lookup class and the previous lookup class.
1375          *  <p>
1376          *  The class implies a maximum level of access permission,
1377          *  but the permissions may be additionally limited by the bitmask
1378          *  {@link #lookupModes lookupModes}, which controls whether non-public members
1379          *  can be accessed.
1380          *  @return the lookup class, on behalf of which this lookup object finds members
1381          *  @see <a href="#cross-module-lookup">Cross-module lookups</a>
1382          */
1383         public Class<?> lookupClass() {
1384             return lookupClass;
1385         }
1386 
1387         /** Reports a lookup class in another module that this lookup object
1388          * was previously teleported from, or {@code null}.
1389          * <p>
1390          * A {@code Lookup} object produced by the factory methods, such as the
1391          * {@link #lookup() lookup()} and {@link #publicLookup() publicLookup()} method,
1392          * has {@code null} previous lookup class.
1393          * A {@code Lookup} object has a non-null previous lookup class
1394          * when this lookup was teleported from an old lookup class
1395          * in one module to a new lookup class in another module.
1396          *
1397          * @return the lookup class in another module that this lookup object was
1398          *         previously teleported from, or {@code null}
1399          * @since 14
1400          * @see #in(Class)
1401          * @see MethodHandles#privateLookupIn(Class, Lookup)
1402          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1403          */
1404         public Class<?> previousLookupClass() {
1405             return prevLookupClass;
1406         }
1407 
1408         // This is just for calling out to MethodHandleImpl.
1409         private Class<?> lookupClassOrNull() {
1410             if (allowedModes == TRUSTED) {
1411                 return null;
1412             }
1413             if (allowedModes == UNCONDITIONAL) {
1414                 // use Object as the caller to pass to VM doing resolution
1415                 return Object.class;
1416             }
1417             return lookupClass;
1418         }
1419 
1420         /** Tells which access-protection classes of members this lookup object can produce.
1421          *  The result is a bit-mask of the bits
1422          *  {@linkplain #PUBLIC PUBLIC (0x01)},
1423          *  {@linkplain #PRIVATE PRIVATE (0x02)},
1424          *  {@linkplain #PROTECTED PROTECTED (0x04)},
1425          *  {@linkplain #PACKAGE PACKAGE (0x08)},
1426          *  {@linkplain #MODULE MODULE (0x10)},
1427          *  and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}.
1428          *  <p>
1429          *  A freshly-created lookup object
1430          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
1431          *  all possible bits set, except {@code UNCONDITIONAL}.
1432          *  A lookup object on a new lookup class
1433          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
1434          *  may have some mode bits set to zero.
1435          *  Mode bits can also be
1436          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
1437          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
1438          *  The purpose of this is to restrict access via the new lookup object,
1439          *  so that it can access only names which can be reached by the original
1440          *  lookup object, and also by the new lookup class.
1441          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
1442          *  @see #in
1443          *  @see #dropLookupMode
1444          *
1445          *  @revised 9
1446          *  @spec JPMS
1447          */
1448         public int lookupModes() {
1449             return allowedModes & ALL_MODES;
1450         }
1451 
1452         /** Embody the current class (the lookupClass) as a lookup class
1453          * for method handle creation.
1454          * Must be called by from a method in this package,
1455          * which in turn is called by a method not in this package.
1456          */
1457         Lookup(Class<?> lookupClass) {
1458             this(lookupClass, null, FULL_POWER_MODES);
1459         }
1460 
1461         private Lookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1462             assert prevLookupClass == null || ((allowedModes & MODULE) == 0
1463                     && prevLookupClass.getModule() != lookupClass.getModule());
1464             assert !lookupClass.isArray() && !lookupClass.isPrimitive();
1465             this.lookupClass = lookupClass;
1466             this.prevLookupClass = prevLookupClass;
1467             this.allowedModes = allowedModes;
1468         }
1469 
1470         private static Lookup newLookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1471             // make sure we haven't accidentally picked up a privileged class:
1472             checkUnprivilegedlookupClass(lookupClass);
1473             return new Lookup(lookupClass, prevLookupClass, allowedModes);
1474         }
1475 
1476         /**
1477          * Creates a lookup on the specified new lookup class.
1478          * The resulting object will report the specified
1479          * class as its own {@link #lookupClass() lookupClass}.
1480          *
1481          * <p>
1482          * However, the resulting {@code Lookup} object is guaranteed
1483          * to have no more access capabilities than the original.
1484          * In particular, access capabilities can be lost as follows:<ul>
1485          * <li>If the new lookup class is in a different module from the old one,
1486          * i.e. {@link #MODULE MODULE} access is lost.
1487          * <li>If the new lookup class is in a different package
1488          * than the old one, protected and default (package) members will not be accessible,
1489          * i.e. {@link #PROTECTED PROTECTED} and {@link #PACKAGE PACKAGE} access are lost.
1490          * <li>If the new lookup class is not within the same package member
1491          * as the old one, private members will not be accessible, and protected members
1492          * will not be accessible by virtue of inheritance,
1493          * i.e. {@link #PRIVATE PRIVATE} access is lost.
1494          * (Protected members may continue to be accessible because of package sharing.)
1495          * <li>If the new lookup class is not
1496          * {@linkplain #accessClass(Class) accessible} to this lookup,
1497          * then no members, not even public members, will be accessible
1498          * i.e. all access modes are lost.
1499          * <li>If the new lookup class, the old lookup class and the previous lookup class
1500          * are all in different modules i.e. teleporting to a third module,
1501          * all access modes are lost.
1502          * </ul>
1503          * <p>
1504          * The new previous lookup class is chosen as follows:
1505          * <ul>
1506          * <li>If the new lookup object has {@link #UNCONDITIONAL UNCONDITIONAL} bit,
1507          * the new previous lookup class is {@code null}.
1508          * <li>If the new lookup class is in the same module as the old lookup class,
1509          * the new previous lookup class is the old previous lookup class.
1510          * <li>If the new lookup class is in a different module from the old lookup class,
1511          * the new previous lookup class is the old lookup class.
1512          *</ul>
1513          * <p>
1514          * The resulting lookup's capabilities for loading classes
1515          * (used during {@link #findClass} invocations)
1516          * are determined by the lookup class' loader,
1517          * which may change due to this operation.
1518          * <p>
1519          * @param requestedLookupClass the desired lookup class for the new lookup object
1520          * @return a lookup object which reports the desired lookup class, or the same object
1521          * if there is no change
1522          * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class
1523          * @throws NullPointerException if the argument is null
1524          *
1525          * @revised 9
1526          * @spec JPMS
1527          * @see #accessClass(Class)
1528          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1529          */
1530         public Lookup in(Class<?> requestedLookupClass) {
1531             Objects.requireNonNull(requestedLookupClass);
1532             if (requestedLookupClass.isPrimitive())
1533                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
1534             if (requestedLookupClass.isArray())
1535                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
1536 
1537             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
1538                 return new Lookup(requestedLookupClass, null, FULL_POWER_MODES);
1539             if (requestedLookupClass == this.lookupClass)
1540                 return this;  // keep same capabilities
1541             int newModes = (allowedModes & FULL_POWER_MODES);
1542             Module fromModule = this.lookupClass.getModule();
1543             Module targetModule = requestedLookupClass.getModule();
1544             Class<?> plc = this.previousLookupClass();
1545             if ((this.allowedModes & UNCONDITIONAL) != 0) {
1546                 assert plc == null;
1547                 newModes = UNCONDITIONAL;
1548             } else if (fromModule != targetModule) {
1549                 if (plc != null && !VerifyAccess.isSameModule(plc, requestedLookupClass)) {
1550                     // allow hopping back and forth between fromModule and plc's module
1551                     // but not the third module
1552                     newModes = 0;
1553                 }
1554                 // drop MODULE access
1555                 newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
1556                 // teleport from this lookup class
1557                 plc = this.lookupClass;
1558             }
1559             if ((newModes & PACKAGE) != 0
1560                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
1561                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
1562             }
1563             // Allow nestmate lookups to be created without special privilege:
1564             if ((newModes & PRIVATE) != 0
1565                     && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
1566                 newModes &= ~(PRIVATE|PROTECTED);
1567             }
1568             if ((newModes & (PUBLIC|UNCONDITIONAL)) != 0
1569                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, this.prevLookupClass, allowedModes)) {
1570                 // The requested class it not accessible from the lookup class.
1571                 // No permissions.
1572                 newModes = 0;
1573             }
1574             return newLookup(requestedLookupClass, plc, newModes);
1575         }
1576 
1577         /**
1578          * Creates a lookup on the same lookup class which this lookup object
1579          * finds members, but with a lookup mode that has lost the given lookup mode.
1580          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
1581          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED},
1582          * {@link #PRIVATE PRIVATE}, or {@link #UNCONDITIONAL UNCONDITIONAL}.
1583          *
1584          * <p> If this lookup is a {@linkplain MethodHandles#publicLookup() public lookup},
1585          * this lookup has {@code UNCONDITIONAL} mode set and it has no other mode set.
1586          * When dropping {@code UNCONDITIONAL} on a public lookup then the resulting
1587          * lookup has no access.
1588          *
1589          * <p> If this lookup is not a public lookup, then the following applies
1590          * regardless of its {@linkplain #lookupModes() lookup modes}.
1591          * {@link #PROTECTED PROTECTED} is always dropped and so the resulting lookup
1592          * mode will never have this access capability. When dropping {@code PACKAGE}
1593          * then the resulting lookup will not have {@code PACKAGE} or {@code PRIVATE}
1594          * access. When dropping {@code MODULE} then the resulting lookup will not
1595          * have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access.
1596          * When dropping {@code PUBLIC} then the resulting lookup has no access.
1597          *
1598          * @apiNote
1599          * A lookup with {@code PACKAGE} but not {@code PRIVATE} mode can safely
1600          * delegate non-public access within the package of the lookup class without
1601          * conferring  <a href="MethodHandles.Lookup.html#privacc">private access</a>.
1602          * A lookup with {@code MODULE} but not
1603          * {@code PACKAGE} mode can safely delegate {@code PUBLIC} access within
1604          * the module of the lookup class without conferring package access.
1605          * A lookup with a {@linkplain #previousLookupClass() previous lookup class}
1606          * (and {@code PUBLIC} but not {@code MODULE} mode) can safely delegate access
1607          * to public classes accessible to both the module of the lookup class
1608          * and the module of the previous lookup class.
1609          *
1610          * @param modeToDrop the lookup mode to drop
1611          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
1612          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
1613          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
1614          * @see MethodHandles#privateLookupIn
1615          * @since 9
1616          */
1617         public Lookup dropLookupMode(int modeToDrop) {
1618             int oldModes = lookupModes();
1619             int newModes = oldModes & ~(modeToDrop | PROTECTED);
1620             switch (modeToDrop) {
1621                 case PUBLIC: newModes &= ~(FULL_POWER_MODES); break;
1622                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
1623                 case PACKAGE: newModes &= ~(PRIVATE); break;
1624                 case PROTECTED:
1625                 case PRIVATE:
1626                 case UNCONDITIONAL: break;
1627                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
1628             }
1629             if (newModes == oldModes) return this;  // return self if no change
1630             return newLookup(lookupClass(), previousLookupClass(), newModes);
1631         }
1632 
1633         /**
1634          * Creates a class or interface from {@code bytes}
1635          * with the same class loader and in the same runtime package and
1636          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
1637          * {@linkplain #lookupClass() lookup class} as if calling
1638          * {@link ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1639          * ClassLoader::defineClass}.
1640          *
1641          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
1642          * {@link #PACKAGE PACKAGE} access as default (package) members will be
1643          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
1644          * that the lookup object was created by a caller in the runtime package (or derived
1645          * from a lookup originally created by suitably privileged code to a target class in
1646          * the runtime package). </p>
1647          *
1648          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
1649          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
1650          * same package as the lookup class. </p>
1651          *
1652          * <p> This method does not run the class initializer. The class initializer may
1653          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
1654          * Specification</em>. </p>
1655          *
1656          * <p> If there is a security manager and this lookup does not have {@linkplain
1657          * #hasFullPrivilegeAccess() full privilege access}, its {@code checkPermission} method
1658          * is first called to check {@code RuntimePermission("defineClass")}. </p>
1659          *
1660          * @param bytes the class bytes
1661          * @return the {@code Class} object for the class
1662          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1663          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1664          * @throws IllegalArgumentException the bytes are for a class in a different package
1665          * to the lookup class
1666          * @throws VerifyError if the newly created class cannot be verified
1667          * @throws LinkageError if the newly created class cannot be linked for any other reason
1668          * @throws SecurityException if a security manager is present and it
1669          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1670          * @throws NullPointerException if {@code bytes} is {@code null}
1671          * @since 9
1672          * @spec JPMS
1673          * @see Lookup#privateLookupIn
1674          * @see Lookup#dropLookupMode
1675          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1676          */
1677         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1678             ensureDefineClassPermission();
1679             if ((lookupModes() & PACKAGE) == 0)
1680                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1681             return makeClassDefiner(bytes.clone()).defineClass(false);
1682         }
1683 
1684         private void ensureDefineClassPermission() {
1685             if (allowedModes == TRUSTED)  return;
1686 
1687             if (!hasFullPrivilegeAccess()) {
1688                 SecurityManager sm = System.getSecurityManager();
1689                 if (sm != null)
1690                     sm.checkPermission(new RuntimePermission("defineClass"));
1691             }
1692         }
1693 
1694         /**
1695          * The set of class options that specify whether a hidden class created by
1696          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1697          * Lookup::defineHiddenClass} method is dynamically added as a new member
1698          * to the nest of a lookup class and/or whether a hidden class has
1699          * a strong relationship with the class loader marked as its defining loader.
1700          *
1701          * @since 15
1702          */
1703         public enum ClassOption {
1704             /**
1705              * Specifies that a hidden class be added to {@linkplain Class#getNestHost nest}
1706              * of a lookup class as a nestmate.
1707              *
1708              * <p> A hidden nestmate class has access to the private members of all
1709              * classes and interfaces in the same nest.
1710              *
1711              * @see Class#getNestHost()
1712              */
1713             NESTMATE(NESTMATE_CLASS),
1714 
1715             /**
1716              * Specifies that a hidden class has a <em>strong</em>
1717              * relationship with the class loader marked as its defining loader,
1718              * as a normal class or interface has with its own defining loader.
1719              * This means that the hidden class may be unloaded if and only if
1720              * its defining loader is not reachable and thus may be reclaimed
1721              * by a garbage collector (JLS 12.7).
1722              *
1723              * <p> By default, a hidden class or interface may be unloaded
1724              * even if the class loader that is marked as its defining loader is
1725              * <a href="../ref/package.html#reachability">reachable</a>.
1726 
1727              *
1728              * @jls 12.7 Unloading of Classes and Interfaces
1729              */
1730             STRONG(STRONG_LOADER_LINK);
1731 
1732             /* the flag value is used by VM at define class time */
1733             private final int flag;
1734             ClassOption(int flag) {
1735                 this.flag = flag;
1736             }
1737 
1738             static int optionsToFlag(Set<ClassOption> options) {
1739                 int flags = 0;
1740                 for (ClassOption cp : options) {
1741                     flags |= cp.flag;
1742                 }
1743                 return flags;
1744             }
1745         }
1746 
1747         /**
1748          * Creates a <em>hidden</em> class or interface from {@code bytes},
1749          * returning a {@code Lookup} on the newly created class or interface.
1750          *
1751          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1752          * which either defines {@code C} directly or delegates to another class loader.
1753          * A class loader defines {@code C} directly by invoking
1754          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1755          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1756          * to derive {@code C} from a purported representation in {@code class} file format.
1757          * In situations where use of a class loader is undesirable, a class or interface
1758          * {@code C} can be created by this method instead. This method is capable of
1759          * defining {@code C}, and thereby creating it, without invoking
1760          * {@code ClassLoader::defineClass}.
1761          * Instead, this method defines {@code C} as if by arranging for
1762          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1763          * from a purported representation in {@code class} file format
1764          * using the following rules:
1765          *
1766          * <ol>
1767          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1768          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1769          * This level of access is needed to create {@code C} in the module
1770          * of the lookup class of this {@code Lookup}.</li>
1771          *
1772          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1773          * structure of a supported major and minor version. The major and minor version
1774          * may differ from the {@code class} file version of the lookup class of this
1775          * {@code Lookup}.</li>
1776          *
1777          * <li> The value of {@code this_class} must be a valid index in the
1778          * {@code constant_pool} table, and the entry at that index must be a valid
1779          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1780          * encoded in internal form that is specified by this structure. {@code N} must
1781          * denote a class or interface in the same package as the lookup class.</li>
1782          *
1783          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1784          * where {@code <suffix>} is an unqualified name.
1785          *
1786          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1787          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1788          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1789          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1790          * refers to the new {@code CONSTANT_Utf8_info} structure.
1791          *
1792          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1793          *
1794          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1795          * purported representation {@code newBytes} as if by the rules of JVMS {@jvms 5.3.5},
1796          * with the following adjustments:
1797          * <ul>
1798          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1799          * that includes a single {@code "."} character, even though this is not a valid
1800          * binary class or interface name in internal form.</li>
1801          *
1802          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1803          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1804          *
1805          * <li> {@code C} is considered to have the same runtime
1806          * {@linkplain Class#getPackage() package}, {@linkplain Class#getModule() module}
1807          * and {@linkplain java.security.ProtectionDomain protection domain}
1808          * as the lookup class of this {@code Lookup}.
1809          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1810          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1811          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C},
1812          * {@link Class#getName()} returns the string {@code GN + "/" + <suffix>}, even though
1813          * this is not a valid binary class or interface name.</li>







1814          * </ul>
1815          * </li>
1816          * </ol>
1817          *
1818          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
1819          * Linkage occurs as specified in JVMS {@jvms 5.4.3}, with the following adjustments:
1820          * <ul>
1821          * <li> During verification, whenever it is necessary to load the class named
1822          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
1823          * made of any class loader.</li>
1824          *
1825          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
1826          * by {@code this_class}, the symbolic reference is considered to be resolved to
1827          * {@code C} and resolution always succeeds immediately.</li>
1828          * </ul>
1829          *
1830          * <p> If the {@code initialize} parameter is {@code true},
1831          * then {@code C} is initialized by the Java Virtual Machine.
1832          *
1833          * <p> The newly created class or interface {@code C} serves as the
1834          * {@linkplain #lookupClass() lookup class} of the {@code Lookup} object
1835          * returned by this method. {@code C} is <em>hidden</em> in the sense that
1836          * no other class or interface can refer to {@code C} via a constant pool entry.
1837          * That is, a hidden class or interface cannot be named as a supertype, a field type,
1838          * a method parameter type, or a method return type by any other class.
1839          * This is because a hidden class or interface does not have a binary name, so
1840          * there is no internal form available to record in any class's constant pool.
1841          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
1842          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
1843          * is not {@linkplain java.lang.instrument.Instrumentation#isModifiableClass(Class)
1844          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
1845          * JVM Tool Interface</a>.
1846          *
1847          * <p> A class or interface created by
1848          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1849          * a class loader} has a strong relationship with that class loader.
1850          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
1851          * that {@linkplain Class#getClassLoader() defined it}.
1852          * This means that a class created by a class loader may be unloaded if and
1853          * only if its defining loader is not reachable and thus may be reclaimed
1854          * by a garbage collector (JLS 12.7).
1855          *
1856          * By default, however, a hidden class or interface may be unloaded even if
1857          * the class loader that is marked as its defining loader is
1858          * <a href="../ref/package.html#reachability">reachable</a>.
1859          * This behavior is useful when a hidden class or interface serves multiple
1860          * classes defined by arbitrary class loaders.  In other cases, a hidden
1861          * class or interface may be linked to a single class (or a small number of classes)
1862          * with the same defining loader as the hidden class or interface.
1863          * In such cases, where the hidden class or interface must be coterminous
1864          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
1865          * option may be passed in {@code options}.
1866          * This arranges for a hidden class to have the same strong relationship
1867          * with the class loader marked as its defining loader,
1868          * as a normal class or interface has with its own defining loader.
1869          *
1870          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
1871          * may still prevent a hidden class or interface from being
1872          * unloaded by ensuring that the {@code Class} object is reachable.
1873          *
1874          * <p> The unloading characteristics are set for each hidden class when it is
1875          * defined, and cannot be changed later.  An advantage of allowing hidden classes
1876          * to be unloaded independently of the class loader marked as their defining loader
1877          * is that a very large number of hidden classes may be created by an application.
1878          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
1879          * just as if normal classes were created by class loaders.
1880          *
1881          * <p> Classes and interfaces in a nest are allowed to have mutual access to
1882          * their private members.  The nest relationship is determined by
1883          * the {@code NestHost} attribute (JVMS {@jvms 4.7.28}) and
1884          * the {@code NestMembers} attribute (JVMS {@jvms 4.7.29}) in a {@code class} file.
1885          * By default, a hidden class belongs to a nest consisting only of itself
1886          * because a hidden class has no binary name.
1887          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
1888          * to create a hidden class or interface {@code C} as a member of a nest.
1889          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
1890          * in the {@code ClassFile} structure from which {@code C} was derived.
1891          * Instead, the following rules determine the nest host of {@code C}:
1892          * <ul>
1893          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
1894          *     been determined, then let {@code H} be the nest host of the lookup class.
1895          *     Otherwise, the nest host of the lookup class is determined using the
1896          *     algorithm in JVMS {@jvms 5.4.4}, yielding {@code H}.</li>
1897          * <li>The nest host of {@code C} is determined to be {@code H},
1898          *     the nest host of the lookup class.</li>
1899          * </ul>
1900          *
1901          * <p> A hidden class or interface may be serializable, but this requires a custom
1902          * serialization mechanism in order to ensure that instances are properly serialized
1903          * and deserialized. The default serialization mechanism supports only classes and
1904          * interfaces that are discoverable by their class name.
1905          *
1906          * @param bytes the bytes that make up the class data,
1907          * in the format of a valid {@code class} file as defined by
1908          * <cite>The Java Virtual Machine Specification</cite>.
1909          * @param initialize if {@code true} the class will be initialized.
1910          * @param options {@linkplain ClassOption class options}
1911          * @return the {@code Lookup} object on the hidden class
1912          *
1913          * @throws IllegalAccessException if this {@code Lookup} does not have
1914          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1915          * @throws SecurityException if a security manager is present and it
1916          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1917          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1918          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1919          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
1920          * {@bytes} denotes a class in a different package than the lookup class
1921          * @throws IncompatibleClassChangeError if the class or interface named as
1922          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1923          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1924          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
1925          * {@code C} is {@code C} itself
1926          * @throws VerifyError if the newly created class cannot be verified
1927          * @throws LinkageError if the newly created class cannot be linked for any other reason
1928          * @throws NullPointerException if any parameter is {@code null}
1929          *
1930          * @since 15
1931          * @see Class#isHidden()
1932          * @jvms 4.2.1 Binary Class and Interface Names
1933          * @jvms 4.2.2 Unqualified Names
1934          * @jvms 4.7.28 The {@code NestHost} Attribute
1935          * @jvms 4.7.29 The {@code NestMembers} Attribute
1936          * @jvms 5.4.3.1 Class and Interface Resolution
1937          * @jvms 5.4.4 Access Control
1938          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
1939          * @jvms 5.4 Linking
1940          * @jvms 5.5 Initialization
1941          * @jls 12.7 Unloading of Classes and Interfaces
1942          */
1943         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
1944                 throws IllegalAccessException
1945         {
1946             Objects.requireNonNull(bytes);
1947             Objects.requireNonNull(options);
1948 
1949             ensureDefineClassPermission();
1950             if (!hasFullPrivilegeAccess()) {
1951                 throw new IllegalAccessException(this + " does not have full privilege access");
1952             }
1953 
1954             return makeHiddenClassDefiner(bytes.clone(), Set.of(options), false).defineClassAsLookup(initialize);
1955         }
1956 
1957         /**
1958          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
1959          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
1960          * returning a {@code Lookup} on the newly created class or interface.
1961          *
1962          * <p> This method is equivalent to calling
1963          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, true, options)}
1964          * as if the hidden class has a private static final unnamed field whose value
1965          * is initialized to {@code classData} right before the class initializer is
1966          * executed.  The newly created class is linked and initialized by the Java
1967          * Virtual Machine.
1968          *
1969          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
1970          * method can be used to retrieve the {@code classData}.
1971          *
1972          * @param bytes     the class bytes
1973          * @param classData pre-initialized class data
1974          * @param options   {@linkplain ClassOption class options}
1975          * @return the {@code Lookup} object on the hidden class
1976          *
1977          * @throws IllegalAccessException if this {@code Lookup} does not have
1978          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1979          * @throws SecurityException if a security manager is present and it
1980          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1981          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1982          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1983          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
1984          * {@bytes} denotes a class in a different package than the lookup class
1985          * @throws IncompatibleClassChangeError if the class or interface named as
1986          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1987          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1988          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
1989          * {@code C} is {@code C} itself
1990          * @throws VerifyError if the newly created class cannot be verified
1991          * @throws LinkageError if the newly created class cannot be linked for any other reason
1992          * @throws NullPointerException if any parameter is {@code null}
1993          *
1994          * @since 15
1995          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1996          * @see Class#isHidden()
1997          */
1998         /* package-private */ Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, ClassOption... options)
1999                 throws IllegalAccessException
2000         {
2001             Objects.requireNonNull(bytes);
2002             Objects.requireNonNull(classData);
2003             Objects.requireNonNull(options);
2004 
2005             ensureDefineClassPermission();
2006             if (!hasFullPrivilegeAccess()) {
2007                 throw new IllegalAccessException(this + " does not have full privilege access");
2008             }
2009 
2010             return makeHiddenClassDefiner(bytes.clone(), Set.of(options), false)
2011                        .defineClassAsLookup(true, classData);
2012         }
2013 
2014         /*
2015          * Validates the given bytes to be a class or interface and the class name
2016          * is in the same package as the lookup class.
2017          *
2018          * This method returns the class name.
2019          */
2020         private String validateAndGetClassName(byte[] bytes) {
2021             try {
2022                 ClassReader reader = new ClassReader(bytes);
2023                 if ((reader.getAccess() & Opcodes.ACC_MODULE) != 0) {
2024                     throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2025                 }
2026                 String name = reader.getClassName().replace('/', '.');
2027                 int index = name.lastIndexOf('.');
2028                 String pn = (index == -1) ? "" : name.substring(0, index);
2029                 if (!pn.equals(lookupClass.getPackageName())) {
2030                     throw newIllegalArgumentException(name + " not in same package as lookup class: " +
2031                             lookupClass.getName());
2032                 }
2033                 return name;
2034             } catch (IllegalArgumentException e) {
2035                 throw e;
2036             } catch (RuntimeException e) {
2037                 // ASM exceptions are poorly specified
2038                 ClassFormatError cfe = new ClassFormatError();
2039                 cfe.initCause(e);
2040                 throw cfe;
2041             }
2042         }
2043 
2044 
2045         /*
2046          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2047          * from the given bytes.
2048          *
2049          * Caller should make a defensive copy of the arguments if needed
2050          * before calling this factory method.
2051          *
2052          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2053          * {@bytes} denotes a class in a different package than the lookup class
2054          */
2055         private ClassDefiner makeClassDefiner(byte[] bytes) {
2056             return new ClassDefiner(this, validateAndGetClassName(bytes), bytes, STRONG_LOADER_LINK);
2057         }
2058 
2059         /**
2060          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2061          * from the given bytes.  The name must be in the same package as the lookup class.
2062          *
2063          * Caller should make a defensive copy of the arguments if needed
2064          * before calling this factory method.
2065          *
2066          * @param bytes   class bytes
2067          * @return ClassDefiner that defines a hidden class of the given bytes.
2068          *
2069          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2070          * {@bytes} denotes a class in a different package than the lookup class
2071          */
2072         ClassDefiner makeHiddenClassDefiner(byte[] bytes) {
2073             return makeHiddenClassDefiner(validateAndGetClassName(bytes), bytes, Set.of(), false);
2074         }
2075 
2076         /**
2077          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2078          * from the given bytes and options.
2079          * The name must be in the same package as the lookup class.
2080          *
2081          * Caller should make a defensive copy of the arguments if needed
2082          * before calling this factory method.
2083          *
2084          * @param bytes   class bytes
2085          * @param options class options
2086          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2087          * @return ClassDefiner that defines a hidden class of the given bytes and options
2088          *
2089          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2090          * {@bytes} denotes a class in a different package than the lookup class
2091          */
2092         ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2093                                             Set<ClassOption> options,
2094                                             boolean accessVmAnnotations) {
2095             return makeHiddenClassDefiner(validateAndGetClassName(bytes), bytes, options, accessVmAnnotations);
2096         }
2097 
2098         /**
2099          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2100          * from the given bytes.  No package name check on the given name.
2101          *
2102          * @param name    fully-qualified name that specifies the prefix of the hidden class
2103          * @param bytes   class bytes
2104          * @return ClassDefiner that defines a hidden class of the given bytes.
2105          */
2106         ClassDefiner makeHiddenClassDefiner(String name, byte[] bytes) {
2107             return makeHiddenClassDefiner(name, bytes, Set.of(), false);
2108         }
2109 
2110         /**
2111          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2112          * from the given bytes and options.  No package name check on the given name.
2113          *
2114          * @param name the name of the class and the name in the class bytes is ignored.
2115          * @param bytes class bytes
2116          * @param options class options
2117          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2118          */
2119         ClassDefiner makeHiddenClassDefiner(String name,
2120                                             byte[] bytes,
2121                                             Set<ClassOption> options,
2122                                             boolean accessVmAnnotations) {
2123             int flags = HIDDEN_CLASS | ClassOption.optionsToFlag(options);
2124             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2125                 // jdk.internal.vm.annotations are permitted for classes
2126                 // defined to boot loader and platform loader
2127                 flags |= ACCESS_VM_ANNOTATIONS;
2128             }
2129 
2130             return new ClassDefiner(this, name, bytes, flags);
2131         }
2132 
2133         static class ClassDefiner {
2134             private final Lookup lookup;
2135             private final String name;
2136             private final byte[] bytes;
2137             private final int classFlags;
2138 
2139             private ClassDefiner(Lookup lookup, String name, byte[] bytes, int flags) {
2140                 assert ((flags & HIDDEN_CLASS) != 0 || (flags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2141                 this.lookup = lookup;
2142                 this.bytes = bytes;
2143                 this.classFlags = flags;
2144                 this.name = name;
2145             }
2146 
2147             String className() {
2148                 return name;
2149             }
2150 
2151             Class<?> defineClass(boolean initialize) {
2152                 return defineClass(initialize, null);
2153             }
2154 
2155             Lookup defineClassAsLookup(boolean initialize) {
2156                 Class<?> c = defineClass(initialize, null);
2157                 return new Lookup(c, null, FULL_POWER_MODES);
2158             }
2159 
2160             /**
2161              * Defines the class of the given bytes and the given classData.
2162              * If {@code initialize} parameter is true, then the class will be initialized.
2163              *
2164              * @param initialize true if the class to be initialized
2165              * @param classData classData or null
2166              * @return the class
2167              *
2168              * @throws LinkageError linkage error
2169              */
2170             Class<?> defineClass(boolean initialize, Object classData) {
2171                 Class<?> lookupClass = lookup.lookupClass();
2172                 ClassLoader loader = lookupClass.getClassLoader();
2173                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2174                 Class<?> c = JLA.defineClass(loader, lookupClass, name, bytes, pd, initialize, classFlags, classData);
2175                 assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2176                 return c;
2177             }
2178 
2179             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2180                 // initialize must be true if classData is non-null
2181                 assert classData == null || initialize == true;
2182                 Class<?> c = defineClass(initialize, classData);
2183                 return new Lookup(c, null, FULL_POWER_MODES);
2184             }
2185 
2186             private boolean isNestmate() {
2187                 return (classFlags & NESTMATE_CLASS) != 0;
2188             }
2189         }
2190 
2191         private ProtectionDomain lookupClassProtectionDomain() {
2192             ProtectionDomain pd = cachedProtectionDomain;
2193             if (pd == null) {
2194                 cachedProtectionDomain = pd = JLA.protectionDomain(lookupClass);
2195             }
2196             return pd;
2197         }
2198 
2199         // cached protection domain
2200         private volatile ProtectionDomain cachedProtectionDomain;
2201 
2202         // Make sure outer class is initialized first.
2203         static { IMPL_NAMES.getClass(); }
2204 
2205         /** Package-private version of lookup which is trusted. */
2206         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2207 
2208         /** Version of lookup which is trusted minimally.
2209          *  It can only be used to create method handles to publicly accessible
2210          *  members in packages that are exported unconditionally.
2211          */
2212         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2213 
2214         static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
2215 
2216         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2217             String name = lookupClass.getName();
2218             if (name.startsWith("java.lang.invoke."))
2219                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2220         }
2221 
2222         /**
2223          * Displays the name of the class from which lookups are to be made.
2224          * followed with "/" and the name of the {@linkplain #previousLookupClass()
2225          * previous lookup class} if present.
2226          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2227          * If there are restrictions on the access permitted to this lookup,
2228          * this is indicated by adding a suffix to the class name, consisting
2229          * of a slash and a keyword.  The keyword represents the strongest
2230          * allowed access, and is chosen as follows:
2231          * <ul>
2232          * <li>If no access is allowed, the suffix is "/noaccess".
2233          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2234          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2235          * <li>If only public and module access are allowed, the suffix is "/module".
2236          * <li>If public and package access are allowed, the suffix is "/package".
2237          * <li>If public, package, and private access are allowed, the suffix is "/private".
2238          * </ul>
2239          * If none of the above cases apply, it is the case that full access
2240          * (public, module, package, private, and protected) is allowed.
2241          * In this case, no suffix is added.
2242          * This is true only of an object obtained originally from
2243          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2244          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2245          * always have restricted access, and will display a suffix.
2246          * <p>
2247          * (It may seem strange that protected access should be
2248          * stronger than private access.  Viewed independently from
2249          * package access, protected access is the first to be lost,
2250          * because it requires a direct subclass relationship between
2251          * caller and callee.)
2252          * @see #in
2253          *
2254          * @revised 9
2255          * @spec JPMS
2256          */
2257         @Override
2258         public String toString() {
2259             String cname = lookupClass.getName();
2260             if (prevLookupClass != null)
2261                 cname += "/" + prevLookupClass.getName();
2262             switch (allowedModes) {
2263             case 0:  // no privileges
2264                 return cname + "/noaccess";
2265             case UNCONDITIONAL:
2266                 return cname + "/publicLookup";
2267             case PUBLIC:
2268                 return cname + "/public";
2269             case PUBLIC|MODULE:
2270                 return cname + "/module";
2271             case PUBLIC|PACKAGE:
2272             case PUBLIC|MODULE|PACKAGE:
2273                 return cname + "/package";
2274             case FULL_POWER_MODES & (~PROTECTED):
2275             case FULL_POWER_MODES & ~(PROTECTED|MODULE):
2276                 return cname + "/private";
2277             case FULL_POWER_MODES:
2278             case FULL_POWER_MODES & (~MODULE):
2279                 return cname;
2280             case TRUSTED:
2281                 return "/trusted";  // internal only; not exported
2282             default:  // Should not happen, but it's a bitfield...
2283                 cname = cname + "/" + Integer.toHexString(allowedModes);
2284                 assert(false) : cname;
2285                 return cname;
2286             }
2287         }
2288 
2289         /**
2290          * Produces a method handle for a static method.
2291          * The type of the method handle will be that of the method.
2292          * (Since static methods do not take receivers, there is no
2293          * additional receiver argument inserted into the method handle type,
2294          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2295          * The method and all its argument types must be accessible to the lookup object.
2296          * <p>
2297          * The returned method handle will have
2298          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2299          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2300          * <p>
2301          * If the returned method handle is invoked, the method's class will
2302          * be initialized, if it has not already been initialized.
2303          * <p><b>Example:</b>
2304          * <blockquote><pre>{@code
2305 import static java.lang.invoke.MethodHandles.*;
2306 import static java.lang.invoke.MethodType.*;
2307 ...
2308 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2309   "asList", methodType(List.class, Object[].class));
2310 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2311          * }</pre></blockquote>
2312          * @param refc the class from which the method is accessed
2313          * @param name the name of the method
2314          * @param type the type of the method
2315          * @return the desired method handle
2316          * @throws NoSuchMethodException if the method does not exist
2317          * @throws IllegalAccessException if access checking fails,
2318          *                                or if the method is not {@code static},
2319          *                                or if the method's variable arity modifier bit
2320          *                                is set and {@code asVarargsCollector} fails
2321          * @throws    SecurityException if a security manager is present and it
2322          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2323          * @throws NullPointerException if any argument is null
2324          */
2325         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2326             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2327             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2328         }
2329 
2330         /**
2331          * Produces a method handle for a virtual method.
2332          * The type of the method handle will be that of the method,
2333          * with the receiver type (usually {@code refc}) prepended.
2334          * The method and all its argument types must be accessible to the lookup object.
2335          * <p>
2336          * When called, the handle will treat the first argument as a receiver
2337          * and, for non-private methods, dispatch on the receiver's type to determine which method
2338          * implementation to enter.
2339          * For private methods the named method in {@code refc} will be invoked on the receiver.
2340          * (The dispatching action is identical with that performed by an
2341          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2342          * <p>
2343          * The first argument will be of type {@code refc} if the lookup
2344          * class has full privileges to access the member.  Otherwise
2345          * the member must be {@code protected} and the first argument
2346          * will be restricted in type to the lookup class.
2347          * <p>
2348          * The returned method handle will have
2349          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2350          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2351          * <p>
2352          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2353          * instructions and method handles produced by {@code findVirtual},
2354          * if the class is {@code MethodHandle} and the name string is
2355          * {@code invokeExact} or {@code invoke}, the resulting
2356          * method handle is equivalent to one produced by
2357          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2358          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2359          * with the same {@code type} argument.
2360          * <p>
2361          * If the class is {@code VarHandle} and the name string corresponds to
2362          * the name of a signature-polymorphic access mode method, the resulting
2363          * method handle is equivalent to one produced by
2364          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2365          * the access mode corresponding to the name string and with the same
2366          * {@code type} arguments.
2367          * <p>
2368          * <b>Example:</b>
2369          * <blockquote><pre>{@code
2370 import static java.lang.invoke.MethodHandles.*;
2371 import static java.lang.invoke.MethodType.*;
2372 ...
2373 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2374   "concat", methodType(String.class, String.class));
2375 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2376   "hashCode", methodType(int.class));
2377 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2378   "hashCode", methodType(int.class));
2379 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2380 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2381 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2382 // interface method:
2383 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2384   "subSequence", methodType(CharSequence.class, int.class, int.class));
2385 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2386 // constructor "internal method" must be accessed differently:
2387 MethodType MT_newString = methodType(void.class); //()V for new String()
2388 try { assertEquals("impossible", lookup()
2389         .findVirtual(String.class, "<init>", MT_newString));
2390  } catch (NoSuchMethodException ex) { } // OK
2391 MethodHandle MH_newString = publicLookup()
2392   .findConstructor(String.class, MT_newString);
2393 assertEquals("", (String) MH_newString.invokeExact());
2394          * }</pre></blockquote>
2395          *
2396          * @param refc the class or interface from which the method is accessed
2397          * @param name the name of the method
2398          * @param type the type of the method, with the receiver argument omitted
2399          * @return the desired method handle
2400          * @throws NoSuchMethodException if the method does not exist
2401          * @throws IllegalAccessException if access checking fails,
2402          *                                or if the method is {@code static},
2403          *                                or if the method's variable arity modifier bit
2404          *                                is set and {@code asVarargsCollector} fails
2405          * @throws    SecurityException if a security manager is present and it
2406          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2407          * @throws NullPointerException if any argument is null
2408          */
2409         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2410             if (refc == MethodHandle.class) {
2411                 MethodHandle mh = findVirtualForMH(name, type);
2412                 if (mh != null)  return mh;
2413             } else if (refc == VarHandle.class) {
2414                 MethodHandle mh = findVirtualForVH(name, type);
2415                 if (mh != null)  return mh;
2416             }
2417             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2418             MemberName method = resolveOrFail(refKind, refc, name, type);
2419             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2420         }
2421         private MethodHandle findVirtualForMH(String name, MethodType type) {
2422             // these names require special lookups because of the implicit MethodType argument
2423             if ("invoke".equals(name))
2424                 return invoker(type);
2425             if ("invokeExact".equals(name))
2426                 return exactInvoker(type);
2427             assert(!MemberName.isMethodHandleInvokeName(name));
2428             return null;
2429         }
2430         private MethodHandle findVirtualForVH(String name, MethodType type) {
2431             try {
2432                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2433             } catch (IllegalArgumentException e) {
2434                 return null;
2435             }
2436         }
2437 
2438         /**
2439          * Produces a method handle which creates an object and initializes it, using
2440          * the constructor of the specified type.
2441          * The parameter types of the method handle will be those of the constructor,
2442          * while the return type will be a reference to the constructor's class.
2443          * The constructor and all its argument types must be accessible to the lookup object.
2444          * <p>
2445          * The requested type must have a return type of {@code void}.
2446          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2447          * <p>
2448          * The returned method handle will have
2449          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2450          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2451          * <p>
2452          * If the returned method handle is invoked, the constructor's class will
2453          * be initialized, if it has not already been initialized.
2454          * <p><b>Example:</b>
2455          * <blockquote><pre>{@code
2456 import static java.lang.invoke.MethodHandles.*;
2457 import static java.lang.invoke.MethodType.*;
2458 ...
2459 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2460   ArrayList.class, methodType(void.class, Collection.class));
2461 Collection orig = Arrays.asList("x", "y");
2462 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2463 assert(orig != copy);
2464 assertEquals(orig, copy);
2465 // a variable-arity constructor:
2466 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2467   ProcessBuilder.class, methodType(void.class, String[].class));
2468 ProcessBuilder pb = (ProcessBuilder)
2469   MH_newProcessBuilder.invoke("x", "y", "z");
2470 assertEquals("[x, y, z]", pb.command().toString());
2471          * }</pre></blockquote>
2472          * @param refc the class or interface from which the method is accessed
2473          * @param type the type of the method, with the receiver argument omitted, and a void return type
2474          * @return the desired method handle
2475          * @throws NoSuchMethodException if the constructor does not exist
2476          * @throws IllegalAccessException if access checking fails
2477          *                                or if the method's variable arity modifier bit
2478          *                                is set and {@code asVarargsCollector} fails
2479          * @throws    SecurityException if a security manager is present and it
2480          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2481          * @throws NullPointerException if any argument is null
2482          */
2483         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2484             if (refc.isArray()) {
2485                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2486             }
2487             String name = "<init>";
2488             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2489             return getDirectConstructor(refc, ctor);
2490         }
2491 
2492         /**
2493          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2494          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2495          * Such a resolution, as specified in JVMS 5.4.3.1 section, attempts to locate and load the class,
2496          * and then determines whether the class is accessible to this lookup object.
2497          * <p>
2498          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2499          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2500          *
2501          * @param targetName the fully qualified name of the class to be looked up.
2502          * @return the requested class.
2503          * @throws SecurityException if a security manager is present and it
2504          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2505          * @throws LinkageError if the linkage fails
2506          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2507          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2508          * modes.
2509          * @since 9
2510          * @jvms 5.4.3.1 Class and Interface Resolution
2511          */
2512         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2513             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2514             return accessClass(targetClass);
2515         }
2516 
2517         /**
2518          * Determines if a class can be accessed from the lookup context defined by
2519          * this {@code Lookup} object. The static initializer of the class is not run.
2520          * <p>
2521          * If the {@code targetClass} is in the same module as the lookup class,
2522          * the lookup class is {@code LC} in module {@code M1} and
2523          * the previous lookup class is in module {@code M0} or
2524          * {@code null} if not present,
2525          * {@code targetClass} is accessible if and only if one of the following is true:
2526          * <ul>
2527          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2528          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2529          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2530          *     in the same runtime package of {@code LC}.</li>
2531          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2532          *     a public type in {@code M1}.</li>
2533          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2534          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2535          *     if the previous lookup class is present; otherwise, {@code targetClass}
2536          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2537          * </ul>
2538          *
2539          * <p>
2540          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2541          * can access public types in all modules when the type is in a package
2542          * that is exported unconditionally.
2543          * <p>
2544          * Otherwise, the target class is in a different module from {@code lookupClass},
2545          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2546          * is inaccessible.
2547          * <p>
2548          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2549          * {@code M1} is the module containing {@code lookupClass} and
2550          * {@code M2} is the module containing {@code targetClass},
2551          * then {@code targetClass} is accessible if and only if
2552          * <ul>
2553          * <li>{@code M1} reads {@code M2}, and
2554          * <li>{@code targetClass} is public and in a package exported by
2555          *     {@code M2} at least to {@code M1}.
2556          * </ul>
2557          * <p>
2558          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2559          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2560          * containing the previous lookup class, then {@code targetClass} is accessible
2561          * if and only if one of the following is true:
2562          * <ul>
2563          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2564          *     {@linkplain Module#reads reads} {@code M0} and the type is
2565          *     in a package that is exported to at least {@code M1}.
2566          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2567          *     {@linkplain Module#reads reads} {@code M1} and the type is
2568          *     in a package that is exported to at least {@code M0}.
2569          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2570          *     and {@code M1} reads {@code M2} and the type is in a package
2571          *     that is exported to at least both {@code M0} and {@code M2}.
2572          * </ul>
2573          * <p>
2574          * Otherwise, {@code targetClass} is not accessible.
2575          *
2576          * @param targetClass the class to be access-checked
2577          * @return the class that has been access-checked
2578          * @throws IllegalAccessException if the class is not accessible from the lookup class
2579          * and previous lookup class, if present, using the allowed access modes.
2580          * @throws    SecurityException if a security manager is present and it
2581          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2582          * @since 9
2583          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2584          */
2585         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
2586             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2587                 throw new MemberName(targetClass).makeAccessException("access violation", this);
2588             }
2589             checkSecurityManager(targetClass, null);
2590             return targetClass;
2591         }
2592 
2593         /**
2594          * Produces an early-bound method handle for a virtual method.
2595          * It will bypass checks for overriding methods on the receiver,
2596          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2597          * instruction from within the explicitly specified {@code specialCaller}.
2598          * The type of the method handle will be that of the method,
2599          * with a suitably restricted receiver type prepended.
2600          * (The receiver type will be {@code specialCaller} or a subtype.)
2601          * The method and all its argument types must be accessible
2602          * to the lookup object.
2603          * <p>
2604          * Before method resolution,
2605          * if the explicitly specified caller class is not identical with the
2606          * lookup class, or if this lookup object does not have
2607          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2608          * privileges, the access fails.
2609          * <p>
2610          * The returned method handle will have
2611          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2612          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2613          * <p style="font-size:smaller;">
2614          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
2615          * even though the {@code invokespecial} instruction can refer to them
2616          * in special circumstances.  Use {@link #findConstructor findConstructor}
2617          * to access instance initialization methods in a safe manner.)</em>
2618          * <p><b>Example:</b>
2619          * <blockquote><pre>{@code
2620 import static java.lang.invoke.MethodHandles.*;
2621 import static java.lang.invoke.MethodType.*;
2622 ...
2623 static class Listie extends ArrayList {
2624   public String toString() { return "[wee Listie]"; }
2625   static Lookup lookup() { return MethodHandles.lookup(); }
2626 }
2627 ...
2628 // no access to constructor via invokeSpecial:
2629 MethodHandle MH_newListie = Listie.lookup()
2630   .findConstructor(Listie.class, methodType(void.class));
2631 Listie l = (Listie) MH_newListie.invokeExact();
2632 try { assertEquals("impossible", Listie.lookup().findSpecial(
2633         Listie.class, "<init>", methodType(void.class), Listie.class));
2634  } catch (NoSuchMethodException ex) { } // OK
2635 // access to super and self methods via invokeSpecial:
2636 MethodHandle MH_super = Listie.lookup().findSpecial(
2637   ArrayList.class, "toString" , methodType(String.class), Listie.class);
2638 MethodHandle MH_this = Listie.lookup().findSpecial(
2639   Listie.class, "toString" , methodType(String.class), Listie.class);
2640 MethodHandle MH_duper = Listie.lookup().findSpecial(
2641   Object.class, "toString" , methodType(String.class), Listie.class);
2642 assertEquals("[]", (String) MH_super.invokeExact(l));
2643 assertEquals(""+l, (String) MH_this.invokeExact(l));
2644 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
2645 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
2646         String.class, "toString", methodType(String.class), Listie.class));
2647  } catch (IllegalAccessException ex) { } // OK
2648 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
2649 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
2650          * }</pre></blockquote>
2651          *
2652          * @param refc the class or interface from which the method is accessed
2653          * @param name the name of the method (which must not be "&lt;init&gt;")
2654          * @param type the type of the method, with the receiver argument omitted
2655          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
2656          * @return the desired method handle
2657          * @throws NoSuchMethodException if the method does not exist
2658          * @throws IllegalAccessException if access checking fails,
2659          *                                or if the method is {@code static},
2660          *                                or if the method's variable arity modifier bit
2661          *                                is set and {@code asVarargsCollector} fails
2662          * @throws    SecurityException if a security manager is present and it
2663          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2664          * @throws NullPointerException if any argument is null
2665          */
2666         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
2667                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
2668             checkSpecialCaller(specialCaller, refc);
2669             Lookup specialLookup = this.in(specialCaller);
2670             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
2671             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
2672         }
2673 
2674         /**
2675          * Produces a method handle giving read access to a non-static field.
2676          * The type of the method handle will have a return type of the field's
2677          * value type.
2678          * The method handle's single argument will be the instance containing
2679          * the field.
2680          * Access checking is performed immediately on behalf of the lookup class.
2681          * @param refc the class or interface from which the method is accessed
2682          * @param name the field's name
2683          * @param type the field's type
2684          * @return a method handle which can load values from the field
2685          * @throws NoSuchFieldException if the field does not exist
2686          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2687          * @throws    SecurityException if a security manager is present and it
2688          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2689          * @throws NullPointerException if any argument is null
2690          * @see #findVarHandle(Class, String, Class)
2691          */
2692         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2693             MemberName field = resolveOrFail(REF_getField, refc, name, type);
2694             return getDirectField(REF_getField, refc, field);
2695         }
2696 
2697         /**
2698          * Produces a method handle giving write access to a non-static field.
2699          * The type of the method handle will have a void return type.
2700          * The method handle will take two arguments, the instance containing
2701          * the field, and the value to be stored.
2702          * The second argument will be of the field's value type.
2703          * Access checking is performed immediately on behalf of the lookup class.
2704          * @param refc the class or interface from which the method is accessed
2705          * @param name the field's name
2706          * @param type the field's type
2707          * @return a method handle which can store values into the field
2708          * @throws NoSuchFieldException if the field does not exist
2709          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2710          *                                or {@code final}
2711          * @throws    SecurityException if a security manager is present and it
2712          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2713          * @throws NullPointerException if any argument is null
2714          * @see #findVarHandle(Class, String, Class)
2715          */
2716         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2717             MemberName field = resolveOrFail(REF_putField, refc, name, type);
2718             return getDirectField(REF_putField, refc, field);
2719         }
2720 
2721         /**
2722          * Produces a VarHandle giving access to a non-static field {@code name}
2723          * of type {@code type} declared in a class of type {@code recv}.
2724          * The VarHandle's variable type is {@code type} and it has one
2725          * coordinate type, {@code recv}.
2726          * <p>
2727          * Access checking is performed immediately on behalf of the lookup
2728          * class.
2729          * <p>
2730          * Certain access modes of the returned VarHandle are unsupported under
2731          * the following conditions:
2732          * <ul>
2733          * <li>if the field is declared {@code final}, then the write, atomic
2734          *     update, numeric atomic update, and bitwise atomic update access
2735          *     modes are unsupported.
2736          * <li>if the field type is anything other than {@code byte},
2737          *     {@code short}, {@code char}, {@code int}, {@code long},
2738          *     {@code float}, or {@code double} then numeric atomic update
2739          *     access modes are unsupported.
2740          * <li>if the field type is anything other than {@code boolean},
2741          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2742          *     {@code long} then bitwise atomic update access modes are
2743          *     unsupported.
2744          * </ul>
2745          * <p>
2746          * If the field is declared {@code volatile} then the returned VarHandle
2747          * will override access to the field (effectively ignore the
2748          * {@code volatile} declaration) in accordance to its specified
2749          * access modes.
2750          * <p>
2751          * If the field type is {@code float} or {@code double} then numeric
2752          * and atomic update access modes compare values using their bitwise
2753          * representation (see {@link Float#floatToRawIntBits} and
2754          * {@link Double#doubleToRawLongBits}, respectively).
2755          * @apiNote
2756          * Bitwise comparison of {@code float} values or {@code double} values,
2757          * as performed by the numeric and atomic update access modes, differ
2758          * from the primitive {@code ==} operator and the {@link Float#equals}
2759          * and {@link Double#equals} methods, specifically with respect to
2760          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2761          * Care should be taken when performing a compare and set or a compare
2762          * and exchange operation with such values since the operation may
2763          * unexpectedly fail.
2764          * There are many possible NaN values that are considered to be
2765          * {@code NaN} in Java, although no IEEE 754 floating-point operation
2766          * provided by Java can distinguish between them.  Operation failure can
2767          * occur if the expected or witness value is a NaN value and it is
2768          * transformed (perhaps in a platform specific manner) into another NaN
2769          * value, and thus has a different bitwise representation (see
2770          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2771          * details).
2772          * The values {@code -0.0} and {@code +0.0} have different bitwise
2773          * representations but are considered equal when using the primitive
2774          * {@code ==} operator.  Operation failure can occur if, for example, a
2775          * numeric algorithm computes an expected value to be say {@code -0.0}
2776          * and previously computed the witness value to be say {@code +0.0}.
2777          * @param recv the receiver class, of type {@code R}, that declares the
2778          * non-static field
2779          * @param name the field's name
2780          * @param type the field's type, of type {@code T}
2781          * @return a VarHandle giving access to non-static fields.
2782          * @throws NoSuchFieldException if the field does not exist
2783          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2784          * @throws    SecurityException if a security manager is present and it
2785          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2786          * @throws NullPointerException if any argument is null
2787          * @since 9
2788          */
2789         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2790             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
2791             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
2792             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
2793         }
2794 
2795         /**
2796          * Produces a method handle giving read access to a static field.
2797          * The type of the method handle will have a return type of the field's
2798          * value type.
2799          * The method handle will take no arguments.
2800          * Access checking is performed immediately on behalf of the lookup class.
2801          * <p>
2802          * If the returned method handle is invoked, the field's class will
2803          * be initialized, if it has not already been initialized.
2804          * @param refc the class or interface from which the method is accessed
2805          * @param name the field's name
2806          * @param type the field's type
2807          * @return a method handle which can load values from the field
2808          * @throws NoSuchFieldException if the field does not exist
2809          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2810          * @throws    SecurityException if a security manager is present and it
2811          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2812          * @throws NullPointerException if any argument is null
2813          */
2814         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2815             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
2816             return getDirectField(REF_getStatic, refc, field);
2817         }
2818 
2819         /**
2820          * Produces a method handle giving write access to a static field.
2821          * The type of the method handle will have a void return type.
2822          * The method handle will take a single
2823          * argument, of the field's value type, the value to be stored.
2824          * Access checking is performed immediately on behalf of the lookup class.
2825          * <p>
2826          * If the returned method handle is invoked, the field's class will
2827          * be initialized, if it has not already been initialized.
2828          * @param refc the class or interface from which the method is accessed
2829          * @param name the field's name
2830          * @param type the field's type
2831          * @return a method handle which can store values into the field
2832          * @throws NoSuchFieldException if the field does not exist
2833          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2834          *                                or is {@code final}
2835          * @throws    SecurityException if a security manager is present and it
2836          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2837          * @throws NullPointerException if any argument is null
2838          */
2839         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2840             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
2841             return getDirectField(REF_putStatic, refc, field);
2842         }
2843 
2844         /**
2845          * Produces a VarHandle giving access to a static field {@code name} of
2846          * type {@code type} declared in a class of type {@code decl}.
2847          * The VarHandle's variable type is {@code type} and it has no
2848          * coordinate types.
2849          * <p>
2850          * Access checking is performed immediately on behalf of the lookup
2851          * class.
2852          * <p>
2853          * If the returned VarHandle is operated on, the declaring class will be
2854          * initialized, if it has not already been initialized.
2855          * <p>
2856          * Certain access modes of the returned VarHandle are unsupported under
2857          * the following conditions:
2858          * <ul>
2859          * <li>if the field is declared {@code final}, then the write, atomic
2860          *     update, numeric atomic update, and bitwise atomic update access
2861          *     modes are unsupported.
2862          * <li>if the field type is anything other than {@code byte},
2863          *     {@code short}, {@code char}, {@code int}, {@code long},
2864          *     {@code float}, or {@code double}, then numeric atomic update
2865          *     access modes are unsupported.
2866          * <li>if the field type is anything other than {@code boolean},
2867          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2868          *     {@code long} then bitwise atomic update access modes are
2869          *     unsupported.
2870          * </ul>
2871          * <p>
2872          * If the field is declared {@code volatile} then the returned VarHandle
2873          * will override access to the field (effectively ignore the
2874          * {@code volatile} declaration) in accordance to its specified
2875          * access modes.
2876          * <p>
2877          * If the field type is {@code float} or {@code double} then numeric
2878          * and atomic update access modes compare values using their bitwise
2879          * representation (see {@link Float#floatToRawIntBits} and
2880          * {@link Double#doubleToRawLongBits}, respectively).
2881          * @apiNote
2882          * Bitwise comparison of {@code float} values or {@code double} values,
2883          * as performed by the numeric and atomic update access modes, differ
2884          * from the primitive {@code ==} operator and the {@link Float#equals}
2885          * and {@link Double#equals} methods, specifically with respect to
2886          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2887          * Care should be taken when performing a compare and set or a compare
2888          * and exchange operation with such values since the operation may
2889          * unexpectedly fail.
2890          * There are many possible NaN values that are considered to be
2891          * {@code NaN} in Java, although no IEEE 754 floating-point operation
2892          * provided by Java can distinguish between them.  Operation failure can
2893          * occur if the expected or witness value is a NaN value and it is
2894          * transformed (perhaps in a platform specific manner) into another NaN
2895          * value, and thus has a different bitwise representation (see
2896          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2897          * details).
2898          * The values {@code -0.0} and {@code +0.0} have different bitwise
2899          * representations but are considered equal when using the primitive
2900          * {@code ==} operator.  Operation failure can occur if, for example, a
2901          * numeric algorithm computes an expected value to be say {@code -0.0}
2902          * and previously computed the witness value to be say {@code +0.0}.
2903          * @param decl the class that declares the static field
2904          * @param name the field's name
2905          * @param type the field's type, of type {@code T}
2906          * @return a VarHandle giving access to a static field
2907          * @throws NoSuchFieldException if the field does not exist
2908          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2909          * @throws    SecurityException if a security manager is present and it
2910          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2911          * @throws NullPointerException if any argument is null
2912          * @since 9
2913          */
2914         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2915             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
2916             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
2917             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
2918         }
2919 
2920         /**
2921          * Produces an early-bound method handle for a non-static method.
2922          * The receiver must have a supertype {@code defc} in which a method
2923          * of the given name and type is accessible to the lookup class.
2924          * The method and all its argument types must be accessible to the lookup object.
2925          * The type of the method handle will be that of the method,
2926          * without any insertion of an additional receiver parameter.
2927          * The given receiver will be bound into the method handle,
2928          * so that every call to the method handle will invoke the
2929          * requested method on the given receiver.
2930          * <p>
2931          * The returned method handle will have
2932          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2933          * the method's variable arity modifier bit ({@code 0x0080}) is set
2934          * <em>and</em> the trailing array argument is not the only argument.
2935          * (If the trailing array argument is the only argument,
2936          * the given receiver value will be bound to it.)
2937          * <p>
2938          * This is almost equivalent to the following code, with some differences noted below:
2939          * <blockquote><pre>{@code
2940 import static java.lang.invoke.MethodHandles.*;
2941 import static java.lang.invoke.MethodType.*;
2942 ...
2943 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
2944 MethodHandle mh1 = mh0.bindTo(receiver);
2945 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
2946 return mh1;
2947          * }</pre></blockquote>
2948          * where {@code defc} is either {@code receiver.getClass()} or a super
2949          * type of that class, in which the requested method is accessible
2950          * to the lookup class.
2951          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
2952          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
2953          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
2954          * the receiver is restricted by {@code findVirtual} to the lookup class.)
2955          * @param receiver the object from which the method is accessed
2956          * @param name the name of the method
2957          * @param type the type of the method, with the receiver argument omitted
2958          * @return the desired method handle
2959          * @throws NoSuchMethodException if the method does not exist
2960          * @throws IllegalAccessException if access checking fails
2961          *                                or if the method's variable arity modifier bit
2962          *                                is set and {@code asVarargsCollector} fails
2963          * @throws    SecurityException if a security manager is present and it
2964          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2965          * @throws NullPointerException if any argument is null
2966          * @see MethodHandle#bindTo
2967          * @see #findVirtual
2968          */
2969         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2970             Class<? extends Object> refc = receiver.getClass(); // may get NPE
2971             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
2972             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
2973             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
2974                 throw new IllegalAccessException("The restricted defining class " +
2975                                                  mh.type().leadingReferenceParameter().getName() +
2976                                                  " is not assignable from receiver class " +
2977                                                  receiver.getClass().getName());
2978             }
2979             return mh.bindArgumentL(0, receiver).setVarargs(method);
2980         }
2981 
2982         /**
2983          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
2984          * to <i>m</i>, if the lookup class has permission.
2985          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
2986          * If <i>m</i> is virtual, overriding is respected on every call.
2987          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
2988          * The type of the method handle will be that of the method,
2989          * with the receiver type prepended (but only if it is non-static).
2990          * If the method's {@code accessible} flag is not set,
2991          * access checking is performed immediately on behalf of the lookup class.
2992          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
2993          * <p>
2994          * The returned method handle will have
2995          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2996          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2997          * <p>
2998          * If <i>m</i> is static, and
2999          * if the returned method handle is invoked, the method's class will
3000          * be initialized, if it has not already been initialized.
3001          * @param m the reflected method
3002          * @return a method handle which can invoke the reflected method
3003          * @throws IllegalAccessException if access checking fails
3004          *                                or if the method's variable arity modifier bit
3005          *                                is set and {@code asVarargsCollector} fails
3006          * @throws NullPointerException if the argument is null
3007          */
3008         public MethodHandle unreflect(Method m) throws IllegalAccessException {
3009             if (m.getDeclaringClass() == MethodHandle.class) {
3010                 MethodHandle mh = unreflectForMH(m);
3011                 if (mh != null)  return mh;
3012             }
3013             if (m.getDeclaringClass() == VarHandle.class) {
3014                 MethodHandle mh = unreflectForVH(m);
3015                 if (mh != null)  return mh;
3016             }
3017             MemberName method = new MemberName(m);
3018             byte refKind = method.getReferenceKind();
3019             if (refKind == REF_invokeSpecial)
3020                 refKind = REF_invokeVirtual;
3021             assert(method.isMethod());
3022             @SuppressWarnings("deprecation")
3023             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
3024             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3025         }
3026         private MethodHandle unreflectForMH(Method m) {
3027             // these names require special lookups because they throw UnsupportedOperationException
3028             if (MemberName.isMethodHandleInvokeName(m.getName()))
3029                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
3030             return null;
3031         }
3032         private MethodHandle unreflectForVH(Method m) {
3033             // these names require special lookups because they throw UnsupportedOperationException
3034             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
3035                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
3036             return null;
3037         }
3038 
3039         /**
3040          * Produces a method handle for a reflected method.
3041          * It will bypass checks for overriding methods on the receiver,
3042          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
3043          * instruction from within the explicitly specified {@code specialCaller}.
3044          * The type of the method handle will be that of the method,
3045          * with a suitably restricted receiver type prepended.
3046          * (The receiver type will be {@code specialCaller} or a subtype.)
3047          * If the method's {@code accessible} flag is not set,
3048          * access checking is performed immediately on behalf of the lookup class,
3049          * as if {@code invokespecial} instruction were being linked.
3050          * <p>
3051          * Before method resolution,
3052          * if the explicitly specified caller class is not identical with the
3053          * lookup class, or if this lookup object does not have
3054          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3055          * privileges, the access fails.
3056          * <p>
3057          * The returned method handle will have
3058          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3059          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3060          * @param m the reflected method
3061          * @param specialCaller the class nominally calling the method
3062          * @return a method handle which can invoke the reflected method
3063          * @throws IllegalAccessException if access checking fails,
3064          *                                or if the method is {@code static},
3065          *                                or if the method's variable arity modifier bit
3066          *                                is set and {@code asVarargsCollector} fails
3067          * @throws NullPointerException if any argument is null
3068          */
3069         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3070             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3071             Lookup specialLookup = this.in(specialCaller);
3072             MemberName method = new MemberName(m, true);
3073             assert(method.isMethod());
3074             // ignore m.isAccessible:  this is a new kind of access
3075             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3076         }
3077 
3078         /**
3079          * Produces a method handle for a reflected constructor.
3080          * The type of the method handle will be that of the constructor,
3081          * with the return type changed to the declaring class.
3082          * The method handle will perform a {@code newInstance} operation,
3083          * creating a new instance of the constructor's class on the
3084          * arguments passed to the method handle.
3085          * <p>
3086          * If the constructor's {@code accessible} flag is not set,
3087          * access checking is performed immediately on behalf of the lookup class.
3088          * <p>
3089          * The returned method handle will have
3090          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3091          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3092          * <p>
3093          * If the returned method handle is invoked, the constructor's class will
3094          * be initialized, if it has not already been initialized.
3095          * @param c the reflected constructor
3096          * @return a method handle which can invoke the reflected constructor
3097          * @throws IllegalAccessException if access checking fails
3098          *                                or if the method's variable arity modifier bit
3099          *                                is set and {@code asVarargsCollector} fails
3100          * @throws NullPointerException if the argument is null
3101          */
3102         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3103             MemberName ctor = new MemberName(c);
3104             assert(ctor.isConstructor());
3105             @SuppressWarnings("deprecation")
3106             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3107             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
3108         }
3109 
3110         /**
3111          * Produces a method handle giving read access to a reflected field.
3112          * The type of the method handle will have a return type of the field's
3113          * value type.
3114          * If the field is {@code static}, the method handle will take no arguments.
3115          * Otherwise, its single argument will be the instance containing
3116          * the field.
3117          * If the {@code Field} object's {@code accessible} flag is not set,
3118          * access checking is performed immediately on behalf of the lookup class.
3119          * <p>
3120          * If the field is static, and
3121          * if the returned method handle is invoked, the field's class will
3122          * be initialized, if it has not already been initialized.
3123          * @param f the reflected field
3124          * @return a method handle which can load values from the reflected field
3125          * @throws IllegalAccessException if access checking fails
3126          * @throws NullPointerException if the argument is null
3127          */
3128         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3129             return unreflectField(f, false);
3130         }
3131 
3132         /**
3133          * Produces a method handle giving write access to a reflected field.
3134          * The type of the method handle will have a void return type.
3135          * If the field is {@code static}, the method handle will take a single
3136          * argument, of the field's value type, the value to be stored.
3137          * Otherwise, the two arguments will be the instance containing
3138          * the field, and the value to be stored.
3139          * If the {@code Field} object's {@code accessible} flag is not set,
3140          * access checking is performed immediately on behalf of the lookup class.
3141          * <p>
3142          * If the field is {@code final}, write access will not be
3143          * allowed and access checking will fail, except under certain
3144          * narrow circumstances documented for {@link Field#set Field.set}.
3145          * A method handle is returned only if a corresponding call to
3146          * the {@code Field} object's {@code set} method could return
3147          * normally.  In particular, fields which are both {@code static}
3148          * and {@code final} may never be set.
3149          * <p>
3150          * If the field is {@code static}, and
3151          * if the returned method handle is invoked, the field's class will
3152          * be initialized, if it has not already been initialized.
3153          * @param f the reflected field
3154          * @return a method handle which can store values into the reflected field
3155          * @throws IllegalAccessException if access checking fails,
3156          *         or if the field is {@code final} and write access
3157          *         is not enabled on the {@code Field} object
3158          * @throws NullPointerException if the argument is null
3159          */
3160         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3161             return unreflectField(f, true);
3162         }
3163 
3164         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3165             MemberName field = new MemberName(f, isSetter);
3166             if (isSetter && field.isFinal()) {
3167                 if (field.isStatic()) {
3168                     throw field.makeAccessException("static final field has no write access", this);
3169                 } else if (field.getDeclaringClass().isHidden()){
3170                     throw field.makeAccessException("final field in a hidden class has no write access", this);
3171                 }
3172             }
3173             assert(isSetter
3174                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3175                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3176             @SuppressWarnings("deprecation")
3177             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3178             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
3179         }
3180 
3181         /**
3182          * Produces a VarHandle giving access to a reflected field {@code f}
3183          * of type {@code T} declared in a class of type {@code R}.
3184          * The VarHandle's variable type is {@code T}.
3185          * If the field is non-static the VarHandle has one coordinate type,
3186          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3187          * coordinate types.
3188          * <p>
3189          * Access checking is performed immediately on behalf of the lookup
3190          * class, regardless of the value of the field's {@code accessible}
3191          * flag.
3192          * <p>
3193          * If the field is static, and if the returned VarHandle is operated
3194          * on, the field's declaring class will be initialized, if it has not
3195          * already been initialized.
3196          * <p>
3197          * Certain access modes of the returned VarHandle are unsupported under
3198          * the following conditions:
3199          * <ul>
3200          * <li>if the field is declared {@code final}, then the write, atomic
3201          *     update, numeric atomic update, and bitwise atomic update access
3202          *     modes are unsupported.
3203          * <li>if the field type is anything other than {@code byte},
3204          *     {@code short}, {@code char}, {@code int}, {@code long},
3205          *     {@code float}, or {@code double} then numeric atomic update
3206          *     access modes are unsupported.
3207          * <li>if the field type is anything other than {@code boolean},
3208          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3209          *     {@code long} then bitwise atomic update access modes are
3210          *     unsupported.
3211          * </ul>
3212          * <p>
3213          * If the field is declared {@code volatile} then the returned VarHandle
3214          * will override access to the field (effectively ignore the
3215          * {@code volatile} declaration) in accordance to its specified
3216          * access modes.
3217          * <p>
3218          * If the field type is {@code float} or {@code double} then numeric
3219          * and atomic update access modes compare values using their bitwise
3220          * representation (see {@link Float#floatToRawIntBits} and
3221          * {@link Double#doubleToRawLongBits}, respectively).
3222          * @apiNote
3223          * Bitwise comparison of {@code float} values or {@code double} values,
3224          * as performed by the numeric and atomic update access modes, differ
3225          * from the primitive {@code ==} operator and the {@link Float#equals}
3226          * and {@link Double#equals} methods, specifically with respect to
3227          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3228          * Care should be taken when performing a compare and set or a compare
3229          * and exchange operation with such values since the operation may
3230          * unexpectedly fail.
3231          * There are many possible NaN values that are considered to be
3232          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3233          * provided by Java can distinguish between them.  Operation failure can
3234          * occur if the expected or witness value is a NaN value and it is
3235          * transformed (perhaps in a platform specific manner) into another NaN
3236          * value, and thus has a different bitwise representation (see
3237          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3238          * details).
3239          * The values {@code -0.0} and {@code +0.0} have different bitwise
3240          * representations but are considered equal when using the primitive
3241          * {@code ==} operator.  Operation failure can occur if, for example, a
3242          * numeric algorithm computes an expected value to be say {@code -0.0}
3243          * and previously computed the witness value to be say {@code +0.0}.
3244          * @param f the reflected field, with a field of type {@code T}, and
3245          * a declaring class of type {@code R}
3246          * @return a VarHandle giving access to non-static fields or a static
3247          * field
3248          * @throws IllegalAccessException if access checking fails
3249          * @throws NullPointerException if the argument is null
3250          * @since 9
3251          */
3252         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3253             MemberName getField = new MemberName(f, false);
3254             MemberName putField = new MemberName(f, true);
3255             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
3256                                                       f.getDeclaringClass(), getField, putField);
3257         }
3258 
3259         /**
3260          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3261          * created by this lookup object or a similar one.
3262          * Security and access checks are performed to ensure that this lookup object
3263          * is capable of reproducing the target method handle.
3264          * This means that the cracking may fail if target is a direct method handle
3265          * but was created by an unrelated lookup object.
3266          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3267          * and was created by a lookup object for a different class.
3268          * @param target a direct method handle to crack into symbolic reference components
3269          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3270          * @throws    SecurityException if a security manager is present and it
3271          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3272          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3273          * @throws    NullPointerException if the target is {@code null}
3274          * @see MethodHandleInfo
3275          * @since 1.8
3276          */
3277         public MethodHandleInfo revealDirect(MethodHandle target) {
3278             MemberName member = target.internalMemberName();
3279             if (member == null || (!member.isResolved() &&
3280                                    !member.isMethodHandleInvoke() &&
3281                                    !member.isVarHandleMethodInvoke()))
3282                 throw newIllegalArgumentException("not a direct method handle");
3283             Class<?> defc = member.getDeclaringClass();
3284             byte refKind = member.getReferenceKind();
3285             assert(MethodHandleNatives.refKindIsValid(refKind));
3286             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3287                 // Devirtualized method invocation is usually formally virtual.
3288                 // To avoid creating extra MemberName objects for this common case,
3289                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3290                 refKind = REF_invokeVirtual;
3291             if (refKind == REF_invokeVirtual && defc.isInterface())
3292                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3293                 refKind = REF_invokeInterface;
3294             // Check SM permissions and member access before cracking.
3295             try {
3296                 checkAccess(refKind, defc, member);
3297                 checkSecurityManager(defc, member);
3298             } catch (IllegalAccessException ex) {
3299                 throw new IllegalArgumentException(ex);
3300             }
3301             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3302                 Class<?> callerClass = target.internalCallerClass();
3303                 if (!hasFullPrivilegeAccess() || callerClass != lookupClass())
3304                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3305             }
3306             // Produce the handle to the results.
3307             return new InfoFromMemberName(this, member, refKind);
3308         }
3309 
3310         /// Helper methods, all package-private.
3311 
3312         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3313             checkSymbolicClass(refc);  // do this before attempting to resolve
3314             Objects.requireNonNull(name);
3315             Objects.requireNonNull(type);
3316             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
3317                                             NoSuchFieldException.class);
3318         }
3319 
3320         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3321             checkSymbolicClass(refc);  // do this before attempting to resolve
3322             Objects.requireNonNull(name);
3323             Objects.requireNonNull(type);
3324             checkMethodName(refKind, name);  // NPE check on name
3325             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
3326                                             NoSuchMethodException.class);
3327         }
3328 
3329         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3330             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3331             Objects.requireNonNull(member.getName());
3332             Objects.requireNonNull(member.getType());
3333             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
3334                                             ReflectiveOperationException.class);
3335         }
3336 
3337         MemberName resolveOrNull(byte refKind, MemberName member) {
3338             // do this before attempting to resolve
3339             if (!isClassAccessible(member.getDeclaringClass())) {
3340                 return null;
3341             }
3342             Objects.requireNonNull(member.getName());
3343             Objects.requireNonNull(member.getType());
3344             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull());
3345         }
3346 
3347         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3348             if (!isClassAccessible(refc)) {
3349                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3350             }
3351         }
3352 
3353         boolean isClassAccessible(Class<?> refc) {
3354             Objects.requireNonNull(refc);
3355             Class<?> caller = lookupClassOrNull();
3356             return caller == null || VerifyAccess.isClassAccessible(refc, caller, prevLookupClass, allowedModes);
3357         }
3358 
3359         /** Check name for an illegal leading "&lt;" character. */
3360         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3361             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3362                 throw new NoSuchMethodException("illegal method name: "+name);
3363         }
3364 
3365 
3366         /**
3367          * Find my trustable caller class if m is a caller sensitive method.
3368          * If this lookup object has full privilege access, then the caller class is the lookupClass.
3369          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3370          */
3371         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3372             if (MethodHandleNatives.isCallerSensitive(m) && !hasFullPrivilegeAccess()) {
3373                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3374                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3375             }
3376             return this;
3377         }
3378 
3379         /**
3380          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3381          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3382          *
3383          * @deprecated This method was originally designed to test {@code PRIVATE} access
3384          * that implies full privilege access but {@code MODULE} access has since become
3385          * independent of {@code PRIVATE} access.  It is recommended to call
3386          * {@link #hasFullPrivilegeAccess()} instead.
3387          * @since 9
3388          */
3389         @Deprecated(since="14")
3390         public boolean hasPrivateAccess() {
3391             return hasFullPrivilegeAccess();
3392         }
3393 
3394         /**
3395          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3396          * i.e. {@code PRIVATE} and {@code MODULE} access.
3397          * A {@code Lookup} object must have full privilege access in order to
3398          * access all members that are allowed to the {@linkplain #lookupClass() lookup class}.
3399          *
3400          * @return {@code true} if this lookup has full privilege access.
3401          * @since 14
3402          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3403          */
3404         public boolean hasFullPrivilegeAccess() {
3405             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3406         }
3407 
3408         /**
3409          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
3410          * Determines a trustable caller class to compare with refc, the symbolic reference class.
3411          * If this lookup object has full privilege access, then the caller class is the lookupClass.
3412          */
3413         void checkSecurityManager(Class<?> refc, MemberName m) {
3414             if (allowedModes == TRUSTED)  return;
3415 
3416             SecurityManager smgr = System.getSecurityManager();
3417             if (smgr == null)  return;
3418 
3419             // Step 1:
3420             boolean fullPowerLookup = hasFullPrivilegeAccess();
3421             if (!fullPowerLookup ||
3422                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3423                 ReflectUtil.checkPackageAccess(refc);
3424             }
3425 
3426             if (m == null) {  // findClass or accessClass
3427                 // Step 2b:
3428                 if (!fullPowerLookup) {
3429                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
3430                 }
3431                 return;
3432             }
3433 
3434             // Step 2a:
3435             if (m.isPublic()) return;
3436             if (!fullPowerLookup) {
3437                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
3438             }
3439 
3440             // Step 3:
3441             Class<?> defc = m.getDeclaringClass();
3442             if (!fullPowerLookup && defc != refc) {
3443                 ReflectUtil.checkPackageAccess(defc);
3444             }
3445         }
3446 
3447         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3448             boolean wantStatic = (refKind == REF_invokeStatic);
3449             String message;
3450             if (m.isConstructor())
3451                 message = "expected a method, not a constructor";
3452             else if (!m.isMethod())
3453                 message = "expected a method";
3454             else if (wantStatic != m.isStatic())
3455                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3456             else
3457                 { checkAccess(refKind, refc, m); return; }
3458             throw m.makeAccessException(message, this);
3459         }
3460 
3461         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3462             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3463             String message;
3464             if (wantStatic != m.isStatic())
3465                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3466             else
3467                 { checkAccess(refKind, refc, m); return; }
3468             throw m.makeAccessException(message, this);
3469         }
3470 
3471         /** Check public/protected/private bits on the symbolic reference class and its member. */
3472         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3473             assert(m.referenceKindIsConsistentWith(refKind) &&
3474                    MethodHandleNatives.refKindIsValid(refKind) &&
3475                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3476             int allowedModes = this.allowedModes;
3477             if (allowedModes == TRUSTED)  return;
3478             int mods = m.getModifiers();
3479             if (Modifier.isProtected(mods) &&
3480                     refKind == REF_invokeVirtual &&
3481                     m.getDeclaringClass() == Object.class &&
3482                     m.getName().equals("clone") &&
3483                     refc.isArray()) {
3484                 // The JVM does this hack also.
3485                 // (See ClassVerifier::verify_invoke_instructions
3486                 // and LinkResolver::check_method_accessability.)
3487                 // Because the JVM does not allow separate methods on array types,
3488                 // there is no separate method for int[].clone.
3489                 // All arrays simply inherit Object.clone.
3490                 // But for access checking logic, we make Object.clone
3491                 // (normally protected) appear to be public.
3492                 // Later on, when the DirectMethodHandle is created,
3493                 // its leading argument will be restricted to the
3494                 // requested array type.
3495                 // N.B. The return type is not adjusted, because
3496                 // that is *not* the bytecode behavior.
3497                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3498             }
3499             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3500                 // cannot "new" a protected ctor in a different package
3501                 mods ^= Modifier.PROTECTED;
3502             }
3503             if (Modifier.isFinal(mods) &&
3504                     MethodHandleNatives.refKindIsSetter(refKind))
3505                 throw m.makeAccessException("unexpected set of a final field", this);
3506             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3507             if ((requestedModes & allowedModes) != 0) {
3508                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3509                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3510                     return;
3511             } else {
3512                 // Protected members can also be checked as if they were package-private.
3513                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3514                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3515                     return;
3516             }
3517             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3518         }
3519 
3520         String accessFailedMessage(Class<?> refc, MemberName m) {
3521             Class<?> defc = m.getDeclaringClass();
3522             int mods = m.getModifiers();
3523             // check the class first:
3524             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3525                                (defc == refc ||
3526                                 Modifier.isPublic(refc.getModifiers())));
3527             if (!classOK && (allowedModes & PACKAGE) != 0) {
3528                 // ignore previous lookup class to check if default package access
3529                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3530                            (defc == refc ||
3531                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3532             }
3533             if (!classOK)
3534                 return "class is not public";
3535             if (Modifier.isPublic(mods))
3536                 return "access to public member failed";  // (how?, module not readable?)
3537             if (Modifier.isPrivate(mods))
3538                 return "member is private";
3539             if (Modifier.isProtected(mods))
3540                 return "member is protected";
3541             return "member is private to package";
3542         }
3543 
3544         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
3545             int allowedModes = this.allowedModes;
3546             if (allowedModes == TRUSTED)  return;
3547             if ((lookupModes() & PRIVATE) == 0
3548                 || (specialCaller != lookupClass()
3549                        // ensure non-abstract methods in superinterfaces can be special-invoked
3550                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
3551                 throw new MemberName(specialCaller).
3552                     makeAccessException("no private access for invokespecial", this);
3553         }
3554 
3555         private boolean restrictProtectedReceiver(MemberName method) {
3556             // The accessing class only has the right to use a protected member
3557             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
3558             if (!method.isProtected() || method.isStatic()
3559                 || allowedModes == TRUSTED
3560                 || method.getDeclaringClass() == lookupClass()
3561                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
3562                 return false;
3563             return true;
3564         }
3565         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
3566             assert(!method.isStatic());
3567             // receiver type of mh is too wide; narrow to caller
3568             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
3569                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
3570             }
3571             MethodType rawType = mh.type();
3572             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
3573             MethodType narrowType = rawType.changeParameterType(0, caller);
3574             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
3575             assert(mh.viewAsTypeChecks(narrowType, true));
3576             return mh.copyWith(narrowType, mh.form);
3577         }
3578 
3579         /** Check access and get the requested method. */
3580         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3581             final boolean doRestrict    = true;
3582             final boolean checkSecurity = true;
3583             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3584         }
3585         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
3586         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3587             final boolean doRestrict    = false;
3588             final boolean checkSecurity = true;
3589             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, callerLookup);
3590         }
3591         /** Check access and get the requested method, eliding security manager checks. */
3592         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3593             final boolean doRestrict    = true;
3594             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3595             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3596         }
3597         /** Common code for all methods; do not call directly except from immediately above. */
3598         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
3599                                                    boolean checkSecurity,
3600                                                    boolean doRestrict,
3601                                                    Lookup boundCaller) throws IllegalAccessException {
3602             checkMethod(refKind, refc, method);
3603             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3604             if (checkSecurity)
3605                 checkSecurityManager(refc, method);
3606             assert(!method.isMethodHandleInvoke());
3607 
3608             if (refKind == REF_invokeSpecial &&
3609                 refc != lookupClass() &&
3610                 !refc.isInterface() &&
3611                 refc != lookupClass().getSuperclass() &&
3612                 refc.isAssignableFrom(lookupClass())) {
3613                 assert(!method.getName().equals("<init>"));  // not this code path
3614 
3615                 // Per JVMS 6.5, desc. of invokespecial instruction:
3616                 // If the method is in a superclass of the LC,
3617                 // and if our original search was above LC.super,
3618                 // repeat the search (symbolic lookup) from LC.super
3619                 // and continue with the direct superclass of that class,
3620                 // and so forth, until a match is found or no further superclasses exist.
3621                 // FIXME: MemberName.resolve should handle this instead.
3622                 Class<?> refcAsSuper = lookupClass();
3623                 MemberName m2;
3624                 do {
3625                     refcAsSuper = refcAsSuper.getSuperclass();
3626                     m2 = new MemberName(refcAsSuper,
3627                                         method.getName(),
3628                                         method.getMethodType(),
3629                                         REF_invokeSpecial);
3630                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
3631                 } while (m2 == null &&         // no method is found yet
3632                          refc != refcAsSuper); // search up to refc
3633                 if (m2 == null)  throw new InternalError(method.toString());
3634                 method = m2;
3635                 refc = refcAsSuper;
3636                 // redo basic checks
3637                 checkMethod(refKind, refc, method);
3638             }
3639             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
3640             MethodHandle mh = dmh;
3641             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
3642             if ((doRestrict && refKind == REF_invokeSpecial) ||
3643                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
3644                 mh = restrictReceiver(method, dmh, lookupClass());
3645             }
3646             mh = maybeBindCaller(method, mh, boundCaller);
3647             mh = mh.setVarargs(method);
3648             return mh;
3649         }
3650         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
3651                                              throws IllegalAccessException {
3652             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
3653                 return mh;
3654 
3655             // boundCaller must have full privilege access.
3656             // It should have been checked by findBoundCallerLookup. Safe to check this again.
3657             if (!boundCaller.hasFullPrivilegeAccess())
3658                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3659 
3660             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
3661             // Note: caller will apply varargs after this step happens.
3662             return cbmh;
3663         }
3664 
3665         /** Check access and get the requested field. */
3666         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3667             final boolean checkSecurity = true;
3668             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
3669         }
3670         /** Check access and get the requested field, eliding security manager checks. */
3671         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3672             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3673             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
3674         }
3675         /** Common code for all fields; do not call directly except from immediately above. */
3676         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
3677                                                   boolean checkSecurity) throws IllegalAccessException {
3678             checkField(refKind, refc, field);
3679             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3680             if (checkSecurity)
3681                 checkSecurityManager(refc, field);
3682             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
3683             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
3684                                     restrictProtectedReceiver(field));
3685             if (doRestrict)
3686                 return restrictReceiver(field, dmh, lookupClass());
3687             return dmh;
3688         }
3689         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
3690                                             Class<?> refc, MemberName getField, MemberName putField)
3691                 throws IllegalAccessException {
3692             final boolean checkSecurity = true;
3693             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
3694         }
3695         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
3696                                                              Class<?> refc, MemberName getField, MemberName putField)
3697                 throws IllegalAccessException {
3698             final boolean checkSecurity = false;
3699             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
3700         }
3701         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
3702                                                   Class<?> refc, MemberName getField, MemberName putField,
3703                                                   boolean checkSecurity) throws IllegalAccessException {
3704             assert getField.isStatic() == putField.isStatic();
3705             assert getField.isGetter() && putField.isSetter();
3706             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
3707             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
3708 
3709             checkField(getRefKind, refc, getField);
3710             if (checkSecurity)
3711                 checkSecurityManager(refc, getField);
3712 
3713             if (!putField.isFinal()) {
3714                 // A VarHandle does not support updates to final fields, any
3715                 // such VarHandle to a final field will be read-only and
3716                 // therefore the following write-based accessibility checks are
3717                 // only required for non-final fields
3718                 checkField(putRefKind, refc, putField);
3719                 if (checkSecurity)
3720                     checkSecurityManager(refc, putField);
3721             }
3722 
3723             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
3724                                   restrictProtectedReceiver(getField));
3725             if (doRestrict) {
3726                 assert !getField.isStatic();
3727                 // receiver type of VarHandle is too wide; narrow to caller
3728                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
3729                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
3730                 }
3731                 refc = lookupClass();
3732             }
3733             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(),
3734                                              this.allowedModes == TRUSTED && !getField.getDeclaringClass().isHidden());
3735         }
3736         /** Check access and get the requested constructor. */
3737         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3738             final boolean checkSecurity = true;
3739             return getDirectConstructorCommon(refc, ctor, checkSecurity);
3740         }
3741         /** Check access and get the requested constructor, eliding security manager checks. */
3742         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3743             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3744             return getDirectConstructorCommon(refc, ctor, checkSecurity);
3745         }
3746         /** Common code for all constructors; do not call directly except from immediately above. */
3747         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
3748                                                   boolean checkSecurity) throws IllegalAccessException {
3749             assert(ctor.isConstructor());
3750             checkAccess(REF_newInvokeSpecial, refc, ctor);
3751             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3752             if (checkSecurity)
3753                 checkSecurityManager(refc, ctor);
3754             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3755             return DirectMethodHandle.make(ctor).setVarargs(ctor);
3756         }
3757 
3758         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
3759          */
3760         /*non-public*/
3761         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
3762                 throws ReflectiveOperationException {
3763             if (!(type instanceof Class || type instanceof MethodType))
3764                 throw new InternalError("unresolved MemberName");
3765             MemberName member = new MemberName(refKind, defc, name, type);
3766             MethodHandle mh = LOOKASIDE_TABLE.get(member);
3767             if (mh != null) {
3768                 checkSymbolicClass(defc);
3769                 return mh;
3770             }
3771             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
3772                 // Treat MethodHandle.invoke and invokeExact specially.
3773                 mh = findVirtualForMH(member.getName(), member.getMethodType());
3774                 if (mh != null) {
3775                     return mh;
3776                 }
3777             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
3778                 // Treat signature-polymorphic methods on VarHandle specially.
3779                 mh = findVirtualForVH(member.getName(), member.getMethodType());
3780                 if (mh != null) {
3781                     return mh;
3782                 }
3783             }
3784             MemberName resolved = resolveOrFail(refKind, member);
3785             mh = getDirectMethodForConstant(refKind, defc, resolved);
3786             if (mh instanceof DirectMethodHandle
3787                     && canBeCached(refKind, defc, resolved)) {
3788                 MemberName key = mh.internalMemberName();
3789                 if (key != null) {
3790                     key = key.asNormalOriginal();
3791                 }
3792                 if (member.equals(key)) {  // better safe than sorry
3793                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
3794                 }
3795             }
3796             return mh;
3797         }
3798         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
3799             if (refKind == REF_invokeSpecial) {
3800                 return false;
3801             }
3802             if (!Modifier.isPublic(defc.getModifiers()) ||
3803                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
3804                     !member.isPublic() ||
3805                     member.isCallerSensitive()) {
3806                 return false;
3807             }
3808             ClassLoader loader = defc.getClassLoader();
3809             if (loader != null) {
3810                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
3811                 boolean found = false;
3812                 while (sysl != null) {
3813                     if (loader == sysl) { found = true; break; }
3814                     sysl = sysl.getParent();
3815                 }
3816                 if (!found) {
3817                     return false;
3818                 }
3819             }
3820             try {
3821                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
3822                     new MemberName(refKind, defc, member.getName(), member.getType()));
3823                 if (resolved2 == null) {
3824                     return false;
3825                 }
3826                 checkSecurityManager(defc, resolved2);
3827             } catch (SecurityException ex) {
3828                 return false;
3829             }
3830             return true;
3831         }
3832         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
3833                 throws ReflectiveOperationException {
3834             if (MethodHandleNatives.refKindIsField(refKind)) {
3835                 return getDirectFieldNoSecurityManager(refKind, defc, member);
3836             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
3837                 return getDirectMethodNoSecurityManager(refKind, defc, member, findBoundCallerLookup(member));
3838             } else if (refKind == REF_newInvokeSpecial) {
3839                 return getDirectConstructorNoSecurityManager(defc, member);
3840             }
3841             // oops
3842             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
3843         }
3844 
3845         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
3846     }
3847 
3848     /**
3849      * Produces a method handle constructing arrays of a desired type,
3850      * as if by the {@code anewarray} bytecode.
3851      * The return type of the method handle will be the array type.
3852      * The type of its sole argument will be {@code int}, which specifies the size of the array.
3853      *
3854      * <p> If the returned method handle is invoked with a negative
3855      * array size, a {@code NegativeArraySizeException} will be thrown.
3856      *
3857      * @param arrayClass an array type
3858      * @return a method handle which can create arrays of the given type
3859      * @throws NullPointerException if the argument is {@code null}
3860      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
3861      * @see java.lang.reflect.Array#newInstance(Class, int)
3862      * @jvms 6.5 {@code anewarray} Instruction
3863      * @since 9
3864      */
3865     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
3866         if (!arrayClass.isArray()) {
3867             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
3868         }
3869         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
3870                 bindTo(arrayClass.getComponentType());
3871         return ani.asType(ani.type().changeReturnType(arrayClass));
3872     }
3873 
3874     /**
3875      * Produces a method handle returning the length of an array,
3876      * as if by the {@code arraylength} bytecode.
3877      * The type of the method handle will have {@code int} as return type,
3878      * and its sole argument will be the array type.
3879      *
3880      * <p> If the returned method handle is invoked with a {@code null}
3881      * array reference, a {@code NullPointerException} will be thrown.
3882      *
3883      * @param arrayClass an array type
3884      * @return a method handle which can retrieve the length of an array of the given array type
3885      * @throws NullPointerException if the argument is {@code null}
3886      * @throws IllegalArgumentException if arrayClass is not an array type
3887      * @jvms 6.5 {@code arraylength} Instruction
3888      * @since 9
3889      */
3890     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
3891         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
3892     }
3893 
3894     /**
3895      * Produces a method handle giving read access to elements of an array,
3896      * as if by the {@code aaload} bytecode.
3897      * The type of the method handle will have a return type of the array's
3898      * element type.  Its first argument will be the array type,
3899      * and the second will be {@code int}.
3900      *
3901      * <p> When the returned method handle is invoked,
3902      * the array reference and array index are checked.
3903      * A {@code NullPointerException} will be thrown if the array reference
3904      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3905      * thrown if the index is negative or if it is greater than or equal to
3906      * the length of the array.
3907      *
3908      * @param arrayClass an array type
3909      * @return a method handle which can load values from the given array type
3910      * @throws NullPointerException if the argument is null
3911      * @throws  IllegalArgumentException if arrayClass is not an array type
3912      * @jvms 6.5 {@code aaload} Instruction
3913      */
3914     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
3915         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
3916     }
3917 
3918     /**
3919      * Produces a method handle giving write access to elements of an array,
3920      * as if by the {@code astore} bytecode.
3921      * The type of the method handle will have a void return type.
3922      * Its last argument will be the array's element type.
3923      * The first and second arguments will be the array type and int.
3924      *
3925      * <p> When the returned method handle is invoked,
3926      * the array reference and array index are checked.
3927      * A {@code NullPointerException} will be thrown if the array reference
3928      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3929      * thrown if the index is negative or if it is greater than or equal to
3930      * the length of the array.
3931      *
3932      * @param arrayClass the class of an array
3933      * @return a method handle which can store values into the array type
3934      * @throws NullPointerException if the argument is null
3935      * @throws IllegalArgumentException if arrayClass is not an array type
3936      * @jvms 6.5 {@code aastore} Instruction
3937      */
3938     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
3939         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
3940     }
3941 
3942     /**
3943      * Produces a VarHandle giving access to elements of an array of type
3944      * {@code arrayClass}.  The VarHandle's variable type is the component type
3945      * of {@code arrayClass} and the list of coordinate types is
3946      * {@code (arrayClass, int)}, where the {@code int} coordinate type
3947      * corresponds to an argument that is an index into an array.
3948      * <p>
3949      * Certain access modes of the returned VarHandle are unsupported under
3950      * the following conditions:
3951      * <ul>
3952      * <li>if the component type is anything other than {@code byte},
3953      *     {@code short}, {@code char}, {@code int}, {@code long},
3954      *     {@code float}, or {@code double} then numeric atomic update access
3955      *     modes are unsupported.
3956      * <li>if the field type is anything other than {@code boolean},
3957      *     {@code byte}, {@code short}, {@code char}, {@code int} or
3958      *     {@code long} then bitwise atomic update access modes are
3959      *     unsupported.
3960      * </ul>
3961      * <p>
3962      * If the component type is {@code float} or {@code double} then numeric
3963      * and atomic update access modes compare values using their bitwise
3964      * representation (see {@link Float#floatToRawIntBits} and
3965      * {@link Double#doubleToRawLongBits}, respectively).
3966      *
3967      * <p> When the returned {@code VarHandle} is invoked,
3968      * the array reference and array index are checked.
3969      * A {@code NullPointerException} will be thrown if the array reference
3970      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3971      * thrown if the index is negative or if it is greater than or equal to
3972      * the length of the array.
3973      *
3974      * @apiNote
3975      * Bitwise comparison of {@code float} values or {@code double} values,
3976      * as performed by the numeric and atomic update access modes, differ
3977      * from the primitive {@code ==} operator and the {@link Float#equals}
3978      * and {@link Double#equals} methods, specifically with respect to
3979      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3980      * Care should be taken when performing a compare and set or a compare
3981      * and exchange operation with such values since the operation may
3982      * unexpectedly fail.
3983      * There are many possible NaN values that are considered to be
3984      * {@code NaN} in Java, although no IEEE 754 floating-point operation
3985      * provided by Java can distinguish between them.  Operation failure can
3986      * occur if the expected or witness value is a NaN value and it is
3987      * transformed (perhaps in a platform specific manner) into another NaN
3988      * value, and thus has a different bitwise representation (see
3989      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3990      * details).
3991      * The values {@code -0.0} and {@code +0.0} have different bitwise
3992      * representations but are considered equal when using the primitive
3993      * {@code ==} operator.  Operation failure can occur if, for example, a
3994      * numeric algorithm computes an expected value to be say {@code -0.0}
3995      * and previously computed the witness value to be say {@code +0.0}.
3996      * @param arrayClass the class of an array, of type {@code T[]}
3997      * @return a VarHandle giving access to elements of an array
3998      * @throws NullPointerException if the arrayClass is null
3999      * @throws IllegalArgumentException if arrayClass is not an array type
4000      * @since 9
4001      */
4002     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
4003         return VarHandles.makeArrayElementHandle(arrayClass);
4004     }
4005 
4006     /**
4007      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4008      * viewed as if it were a different primitive array type, such as
4009      * {@code int[]} or {@code long[]}.
4010      * The VarHandle's variable type is the component type of
4011      * {@code viewArrayClass} and the list of coordinate types is
4012      * {@code (byte[], int)}, where the {@code int} coordinate type
4013      * corresponds to an argument that is an index into a {@code byte[]} array.
4014      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4015      * array, composing bytes to or from a value of the component type of
4016      * {@code viewArrayClass} according to the given endianness.
4017      * <p>
4018      * The supported component types (variables types) are {@code short},
4019      * {@code char}, {@code int}, {@code long}, {@code float} and
4020      * {@code double}.
4021      * <p>
4022      * Access of bytes at a given index will result in an
4023      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4024      * or greater than the {@code byte[]} array length minus the size (in bytes)
4025      * of {@code T}.
4026      * <p>
4027      * Access of bytes at an index may be aligned or misaligned for {@code T},
4028      * with respect to the underlying memory address, {@code A} say, associated
4029      * with the array and index.
4030      * If access is misaligned then access for anything other than the
4031      * {@code get} and {@code set} access modes will result in an
4032      * {@code IllegalStateException}.  In such cases atomic access is only
4033      * guaranteed with respect to the largest power of two that divides the GCD
4034      * of {@code A} and the size (in bytes) of {@code T}.
4035      * If access is aligned then following access modes are supported and are
4036      * guaranteed to support atomic access:
4037      * <ul>
4038      * <li>read write access modes for all {@code T}, with the exception of
4039      *     access modes {@code get} and {@code set} for {@code long} and
4040      *     {@code double} on 32-bit platforms.
4041      * <li>atomic update access modes for {@code int}, {@code long},
4042      *     {@code float} or {@code double}.
4043      *     (Future major platform releases of the JDK may support additional
4044      *     types for certain currently unsupported access modes.)
4045      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4046      *     (Future major platform releases of the JDK may support additional
4047      *     numeric types for certain currently unsupported access modes.)
4048      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4049      *     (Future major platform releases of the JDK may support additional
4050      *     numeric types for certain currently unsupported access modes.)
4051      * </ul>
4052      * <p>
4053      * Misaligned access, and therefore atomicity guarantees, may be determined
4054      * for {@code byte[]} arrays without operating on a specific array.  Given
4055      * an {@code index}, {@code T} and it's corresponding boxed type,
4056      * {@code T_BOX}, misalignment may be determined as follows:
4057      * <pre>{@code
4058      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4059      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
4060      *     alignmentOffset(0, sizeOfT);
4061      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
4062      * boolean isMisaligned = misalignedAtIndex != 0;
4063      * }</pre>
4064      * <p>
4065      * If the variable type is {@code float} or {@code double} then atomic
4066      * update access modes compare values using their bitwise representation
4067      * (see {@link Float#floatToRawIntBits} and
4068      * {@link Double#doubleToRawLongBits}, respectively).
4069      * @param viewArrayClass the view array class, with a component type of
4070      * type {@code T}
4071      * @param byteOrder the endianness of the view array elements, as
4072      * stored in the underlying {@code byte} array
4073      * @return a VarHandle giving access to elements of a {@code byte[]} array
4074      * viewed as if elements corresponding to the components type of the view
4075      * array class
4076      * @throws NullPointerException if viewArrayClass or byteOrder is null
4077      * @throws IllegalArgumentException if viewArrayClass is not an array type
4078      * @throws UnsupportedOperationException if the component type of
4079      * viewArrayClass is not supported as a variable type
4080      * @since 9
4081      */
4082     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4083                                      ByteOrder byteOrder) throws IllegalArgumentException {
4084         Objects.requireNonNull(byteOrder);
4085         return VarHandles.byteArrayViewHandle(viewArrayClass,
4086                                               byteOrder == ByteOrder.BIG_ENDIAN);
4087     }
4088 
4089     /**
4090      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4091      * viewed as if it were an array of elements of a different primitive
4092      * component type to that of {@code byte}, such as {@code int[]} or
4093      * {@code long[]}.
4094      * The VarHandle's variable type is the component type of
4095      * {@code viewArrayClass} and the list of coordinate types is
4096      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4097      * corresponds to an argument that is an index into a {@code byte[]} array.
4098      * The returned VarHandle accesses bytes at an index in a
4099      * {@code ByteBuffer}, composing bytes to or from a value of the component
4100      * type of {@code viewArrayClass} according to the given endianness.
4101      * <p>
4102      * The supported component types (variables types) are {@code short},
4103      * {@code char}, {@code int}, {@code long}, {@code float} and
4104      * {@code double}.
4105      * <p>
4106      * Access will result in a {@code ReadOnlyBufferException} for anything
4107      * other than the read access modes if the {@code ByteBuffer} is read-only.
4108      * <p>
4109      * Access of bytes at a given index will result in an
4110      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4111      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4112      * {@code T}.
4113      * <p>
4114      * Access of bytes at an index may be aligned or misaligned for {@code T},
4115      * with respect to the underlying memory address, {@code A} say, associated
4116      * with the {@code ByteBuffer} and index.
4117      * If access is misaligned then access for anything other than the
4118      * {@code get} and {@code set} access modes will result in an
4119      * {@code IllegalStateException}.  In such cases atomic access is only
4120      * guaranteed with respect to the largest power of two that divides the GCD
4121      * of {@code A} and the size (in bytes) of {@code T}.
4122      * If access is aligned then following access modes are supported and are
4123      * guaranteed to support atomic access:
4124      * <ul>
4125      * <li>read write access modes for all {@code T}, with the exception of
4126      *     access modes {@code get} and {@code set} for {@code long} and
4127      *     {@code double} on 32-bit platforms.
4128      * <li>atomic update access modes for {@code int}, {@code long},
4129      *     {@code float} or {@code double}.
4130      *     (Future major platform releases of the JDK may support additional
4131      *     types for certain currently unsupported access modes.)
4132      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4133      *     (Future major platform releases of the JDK may support additional
4134      *     numeric types for certain currently unsupported access modes.)
4135      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4136      *     (Future major platform releases of the JDK may support additional
4137      *     numeric types for certain currently unsupported access modes.)
4138      * </ul>
4139      * <p>
4140      * Misaligned access, and therefore atomicity guarantees, may be determined
4141      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4142      * {@code index}, {@code T} and it's corresponding boxed type,
4143      * {@code T_BOX}, as follows:
4144      * <pre>{@code
4145      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4146      * ByteBuffer bb = ...
4147      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4148      * boolean isMisaligned = misalignedAtIndex != 0;
4149      * }</pre>
4150      * <p>
4151      * If the variable type is {@code float} or {@code double} then atomic
4152      * update access modes compare values using their bitwise representation
4153      * (see {@link Float#floatToRawIntBits} and
4154      * {@link Double#doubleToRawLongBits}, respectively).
4155      * @param viewArrayClass the view array class, with a component type of
4156      * type {@code T}
4157      * @param byteOrder the endianness of the view array elements, as
4158      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4159      * endianness of a {@code ByteBuffer})
4160      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4161      * viewed as if elements corresponding to the components type of the view
4162      * array class
4163      * @throws NullPointerException if viewArrayClass or byteOrder is null
4164      * @throws IllegalArgumentException if viewArrayClass is not an array type
4165      * @throws UnsupportedOperationException if the component type of
4166      * viewArrayClass is not supported as a variable type
4167      * @since 9
4168      */
4169     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4170                                       ByteOrder byteOrder) throws IllegalArgumentException {
4171         Objects.requireNonNull(byteOrder);
4172         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4173                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4174     }
4175 
4176 
4177     /// method handle invocation (reflective style)
4178 
4179     /**
4180      * Produces a method handle which will invoke any method handle of the
4181      * given {@code type}, with a given number of trailing arguments replaced by
4182      * a single trailing {@code Object[]} array.
4183      * The resulting invoker will be a method handle with the following
4184      * arguments:
4185      * <ul>
4186      * <li>a single {@code MethodHandle} target
4187      * <li>zero or more leading values (counted by {@code leadingArgCount})
4188      * <li>an {@code Object[]} array containing trailing arguments
4189      * </ul>
4190      * <p>
4191      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4192      * the indicated {@code type}.
4193      * That is, if the target is exactly of the given {@code type}, it will behave
4194      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4195      * is used to convert the target to the required {@code type}.
4196      * <p>
4197      * The type of the returned invoker will not be the given {@code type}, but rather
4198      * will have all parameters except the first {@code leadingArgCount}
4199      * replaced by a single array of type {@code Object[]}, which will be
4200      * the final parameter.
4201      * <p>
4202      * Before invoking its target, the invoker will spread the final array, apply
4203      * reference casts as necessary, and unbox and widen primitive arguments.
4204      * If, when the invoker is called, the supplied array argument does
4205      * not have the correct number of elements, the invoker will throw
4206      * an {@link IllegalArgumentException} instead of invoking the target.
4207      * <p>
4208      * This method is equivalent to the following code (though it may be more efficient):
4209      * <blockquote><pre>{@code
4210 MethodHandle invoker = MethodHandles.invoker(type);
4211 int spreadArgCount = type.parameterCount() - leadingArgCount;
4212 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4213 return invoker;
4214      * }</pre></blockquote>
4215      * This method throws no reflective or security exceptions.
4216      * @param type the desired target type
4217      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4218      * @return a method handle suitable for invoking any method handle of the given type
4219      * @throws NullPointerException if {@code type} is null
4220      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4221      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4222      *                  or if the resulting method handle's type would have
4223      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4224      */
4225     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4226         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4227             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4228         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4229         return type.invokers().spreadInvoker(leadingArgCount);
4230     }
4231 
4232     /**
4233      * Produces a special <em>invoker method handle</em> which can be used to
4234      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4235      * The resulting invoker will have a type which is
4236      * exactly equal to the desired type, except that it will accept
4237      * an additional leading argument of type {@code MethodHandle}.
4238      * <p>
4239      * This method is equivalent to the following code (though it may be more efficient):
4240      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4241      *
4242      * <p style="font-size:smaller;">
4243      * <em>Discussion:</em>
4244      * Invoker method handles can be useful when working with variable method handles
4245      * of unknown types.
4246      * For example, to emulate an {@code invokeExact} call to a variable method
4247      * handle {@code M}, extract its type {@code T},
4248      * look up the invoker method {@code X} for {@code T},
4249      * and call the invoker method, as {@code X.invoke(T, A...)}.
4250      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4251      * is unknown.)
4252      * If spreading, collecting, or other argument transformations are required,
4253      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4254      * method handle values, as long as they are compatible with the type of {@code X}.
4255      * <p style="font-size:smaller;">
4256      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4257      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4258      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4259      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4260      * <p>
4261      * This method throws no reflective or security exceptions.
4262      * @param type the desired target type
4263      * @return a method handle suitable for invoking any method handle of the given type
4264      * @throws IllegalArgumentException if the resulting method handle's type would have
4265      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4266      */
4267     public static MethodHandle exactInvoker(MethodType type) {
4268         return type.invokers().exactInvoker();
4269     }
4270 
4271     /**
4272      * Produces a special <em>invoker method handle</em> which can be used to
4273      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4274      * The resulting invoker will have a type which is
4275      * exactly equal to the desired type, except that it will accept
4276      * an additional leading argument of type {@code MethodHandle}.
4277      * <p>
4278      * Before invoking its target, if the target differs from the expected type,
4279      * the invoker will apply reference casts as
4280      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4281      * Similarly, the return value will be converted as necessary.
4282      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4283      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4284      * <p>
4285      * This method is equivalent to the following code (though it may be more efficient):
4286      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4287      * <p style="font-size:smaller;">
4288      * <em>Discussion:</em>
4289      * A {@linkplain MethodType#genericMethodType general method type} is one which
4290      * mentions only {@code Object} arguments and return values.
4291      * An invoker for such a type is capable of calling any method handle
4292      * of the same arity as the general type.
4293      * <p style="font-size:smaller;">
4294      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4295      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4296      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4297      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4298      * <p>
4299      * This method throws no reflective or security exceptions.
4300      * @param type the desired target type
4301      * @return a method handle suitable for invoking any method handle convertible to the given type
4302      * @throws IllegalArgumentException if the resulting method handle's type would have
4303      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4304      */
4305     public static MethodHandle invoker(MethodType type) {
4306         return type.invokers().genericInvoker();
4307     }
4308 
4309     /**
4310      * Produces a special <em>invoker method handle</em> which can be used to
4311      * invoke a signature-polymorphic access mode method on any VarHandle whose
4312      * associated access mode type is compatible with the given type.
4313      * The resulting invoker will have a type which is exactly equal to the
4314      * desired given type, except that it will accept an additional leading
4315      * argument of type {@code VarHandle}.
4316      *
4317      * @param accessMode the VarHandle access mode
4318      * @param type the desired target type
4319      * @return a method handle suitable for invoking an access mode method of
4320      *         any VarHandle whose access mode type is of the given type.
4321      * @since 9
4322      */
4323     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4324         return type.invokers().varHandleMethodExactInvoker(accessMode);
4325     }
4326 
4327     /**
4328      * Produces a special <em>invoker method handle</em> which can be used to
4329      * invoke a signature-polymorphic access mode method on any VarHandle whose
4330      * associated access mode type is compatible with the given type.
4331      * The resulting invoker will have a type which is exactly equal to the
4332      * desired given type, except that it will accept an additional leading
4333      * argument of type {@code VarHandle}.
4334      * <p>
4335      * Before invoking its target, if the access mode type differs from the
4336      * desired given type, the invoker will apply reference casts as necessary
4337      * and box, unbox, or widen primitive values, as if by
4338      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4339      * converted as necessary.
4340      * <p>
4341      * This method is equivalent to the following code (though it may be more
4342      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4343      *
4344      * @param accessMode the VarHandle access mode
4345      * @param type the desired target type
4346      * @return a method handle suitable for invoking an access mode method of
4347      *         any VarHandle whose access mode type is convertible to the given
4348      *         type.
4349      * @since 9
4350      */
4351     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4352         return type.invokers().varHandleMethodInvoker(accessMode);
4353     }
4354 
4355     /*non-public*/
4356     static MethodHandle basicInvoker(MethodType type) {
4357         return type.invokers().basicInvoker();
4358     }
4359 
4360      /// method handle modification (creation from other method handles)
4361 
4362     /**
4363      * Produces a method handle which adapts the type of the
4364      * given method handle to a new type by pairwise argument and return type conversion.
4365      * The original type and new type must have the same number of arguments.
4366      * The resulting method handle is guaranteed to report a type
4367      * which is equal to the desired new type.
4368      * <p>
4369      * If the original type and new type are equal, returns target.
4370      * <p>
4371      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4372      * and some additional conversions are also applied if those conversions fail.
4373      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4374      * if possible, before or instead of any conversions done by {@code asType}:
4375      * <ul>
4376      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4377      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4378      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4379      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4380      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4381      *     (This treatment follows the usage of the bytecode verifier.)
4382      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4383      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
4384      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4385      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4386      *     then a Java casting conversion (JLS 5.5) is applied.
4387      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4388      *     widening and/or narrowing.)
4389      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4390      *     conversion will be applied at runtime, possibly followed
4391      *     by a Java casting conversion (JLS 5.5) on the primitive value,
4392      *     possibly followed by a conversion from byte to boolean by testing
4393      *     the low-order bit.
4394      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4395      *     and if the reference is null at runtime, a zero value is introduced.
4396      * </ul>
4397      * @param target the method handle to invoke after arguments are retyped
4398      * @param newType the expected type of the new method handle
4399      * @return a method handle which delegates to the target after performing
4400      *           any necessary argument conversions, and arranges for any
4401      *           necessary return value conversions
4402      * @throws NullPointerException if either argument is null
4403      * @throws WrongMethodTypeException if the conversion cannot be made
4404      * @see MethodHandle#asType
4405      */
4406     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4407         explicitCastArgumentsChecks(target, newType);
4408         // use the asTypeCache when possible:
4409         MethodType oldType = target.type();
4410         if (oldType == newType)  return target;
4411         if (oldType.explicitCastEquivalentToAsType(newType)) {
4412             return target.asFixedArity().asType(newType);
4413         }
4414         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4415     }
4416 
4417     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4418         if (target.type().parameterCount() != newType.parameterCount()) {
4419             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4420         }
4421     }
4422 
4423     /**
4424      * Produces a method handle which adapts the calling sequence of the
4425      * given method handle to a new type, by reordering the arguments.
4426      * The resulting method handle is guaranteed to report a type
4427      * which is equal to the desired new type.
4428      * <p>
4429      * The given array controls the reordering.
4430      * Call {@code #I} the number of incoming parameters (the value
4431      * {@code newType.parameterCount()}, and call {@code #O} the number
4432      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4433      * Then the length of the reordering array must be {@code #O},
4434      * and each element must be a non-negative number less than {@code #I}.
4435      * For every {@code N} less than {@code #O}, the {@code N}-th
4436      * outgoing argument will be taken from the {@code I}-th incoming
4437      * argument, where {@code I} is {@code reorder[N]}.
4438      * <p>
4439      * No argument or return value conversions are applied.
4440      * The type of each incoming argument, as determined by {@code newType},
4441      * must be identical to the type of the corresponding outgoing parameter
4442      * or parameters in the target method handle.
4443      * The return type of {@code newType} must be identical to the return
4444      * type of the original target.
4445      * <p>
4446      * The reordering array need not specify an actual permutation.
4447      * An incoming argument will be duplicated if its index appears
4448      * more than once in the array, and an incoming argument will be dropped
4449      * if its index does not appear in the array.
4450      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4451      * incoming arguments which are not mentioned in the reordering array
4452      * may be of any type, as determined only by {@code newType}.
4453      * <blockquote><pre>{@code
4454 import static java.lang.invoke.MethodHandles.*;
4455 import static java.lang.invoke.MethodType.*;
4456 ...
4457 MethodType intfn1 = methodType(int.class, int.class);
4458 MethodType intfn2 = methodType(int.class, int.class, int.class);
4459 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4460 assert(sub.type().equals(intfn2));
4461 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4462 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4463 assert((int)rsub.invokeExact(1, 100) == 99);
4464 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4465 assert(add.type().equals(intfn2));
4466 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4467 assert(twice.type().equals(intfn1));
4468 assert((int)twice.invokeExact(21) == 42);
4469      * }</pre></blockquote>
4470      * <p>
4471      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4472      * variable-arity method handle}, even if the original target method handle was.
4473      * @param target the method handle to invoke after arguments are reordered
4474      * @param newType the expected type of the new method handle
4475      * @param reorder an index array which controls the reordering
4476      * @return a method handle which delegates to the target after it
4477      *           drops unused arguments and moves and/or duplicates the other arguments
4478      * @throws NullPointerException if any argument is null
4479      * @throws IllegalArgumentException if the index array length is not equal to
4480      *                  the arity of the target, or if any index array element
4481      *                  not a valid index for a parameter of {@code newType},
4482      *                  or if two corresponding parameter types in
4483      *                  {@code target.type()} and {@code newType} are not identical,
4484      */
4485     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4486         reorder = reorder.clone();  // get a private copy
4487         MethodType oldType = target.type();
4488         permuteArgumentChecks(reorder, newType, oldType);
4489         // first detect dropped arguments and handle them separately
4490         int[] originalReorder = reorder;
4491         BoundMethodHandle result = target.rebind();
4492         LambdaForm form = result.form;
4493         int newArity = newType.parameterCount();
4494         // Normalize the reordering into a real permutation,
4495         // by removing duplicates and adding dropped elements.
4496         // This somewhat improves lambda form caching, as well
4497         // as simplifying the transform by breaking it up into steps.
4498         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4499             if (ddIdx > 0) {
4500                 // We found a duplicated entry at reorder[ddIdx].
4501                 // Example:  (x,y,z)->asList(x,y,z)
4502                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4503                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4504                 // The starred element corresponds to the argument
4505                 // deleted by the dupArgumentForm transform.
4506                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4507                 boolean killFirst = false;
4508                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4509                     // Set killFirst if the dup is larger than an intervening position.
4510                     // This will remove at least one inversion from the permutation.
4511                     if (dupVal > val) killFirst = true;
4512                 }
4513                 if (!killFirst) {
4514                     srcPos = dstPos;
4515                     dstPos = ddIdx;
4516                 }
4517                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4518                 assert (reorder[srcPos] == reorder[dstPos]);
4519                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4520                 // contract the reordering by removing the element at dstPos
4521                 int tailPos = dstPos + 1;
4522                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4523                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4524             } else {
4525                 int dropVal = ~ddIdx, insPos = 0;
4526                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4527                     // Find first element of reorder larger than dropVal.
4528                     // This is where we will insert the dropVal.
4529                     insPos += 1;
4530                 }
4531                 Class<?> ptype = newType.parameterType(dropVal);
4532                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4533                 oldType = oldType.insertParameterTypes(insPos, ptype);
4534                 // expand the reordering by inserting an element at insPos
4535                 int tailPos = insPos + 1;
4536                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4537                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4538                 reorder[insPos] = dropVal;
4539             }
4540             assert (permuteArgumentChecks(reorder, newType, oldType));
4541         }
4542         assert (reorder.length == newArity);  // a perfect permutation
4543         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4544         form = form.editor().permuteArgumentsForm(1, reorder);
4545         if (newType == result.type() && form == result.internalForm())
4546             return result;
4547         return result.copyWith(newType, form);
4548     }
4549 
4550     /**
4551      * Return an indication of any duplicate or omission in reorder.
4552      * If the reorder contains a duplicate entry, return the index of the second occurrence.
4553      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
4554      * Otherwise, return zero.
4555      * If an element not in [0..newArity-1] is encountered, return reorder.length.
4556      */
4557     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
4558         final int BIT_LIMIT = 63;  // max number of bits in bit mask
4559         if (newArity < BIT_LIMIT) {
4560             long mask = 0;
4561             for (int i = 0; i < reorder.length; i++) {
4562                 int arg = reorder[i];
4563                 if (arg >= newArity) {
4564                     return reorder.length;
4565                 }
4566                 long bit = 1L << arg;
4567                 if ((mask & bit) != 0) {
4568                     return i;  // >0 indicates a dup
4569                 }
4570                 mask |= bit;
4571             }
4572             if (mask == (1L << newArity) - 1) {
4573                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
4574                 return 0;
4575             }
4576             // find first zero
4577             long zeroBit = Long.lowestOneBit(~mask);
4578             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
4579             assert(zeroPos <= newArity);
4580             if (zeroPos == newArity) {
4581                 return 0;
4582             }
4583             return ~zeroPos;
4584         } else {
4585             // same algorithm, different bit set
4586             BitSet mask = new BitSet(newArity);
4587             for (int i = 0; i < reorder.length; i++) {
4588                 int arg = reorder[i];
4589                 if (arg >= newArity) {
4590                     return reorder.length;
4591                 }
4592                 if (mask.get(arg)) {
4593                     return i;  // >0 indicates a dup
4594                 }
4595                 mask.set(arg);
4596             }
4597             int zeroPos = mask.nextClearBit(0);
4598             assert(zeroPos <= newArity);
4599             if (zeroPos == newArity) {
4600                 return 0;
4601             }
4602             return ~zeroPos;
4603         }
4604     }
4605 
4606     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
4607         if (newType.returnType() != oldType.returnType())
4608             throw newIllegalArgumentException("return types do not match",
4609                     oldType, newType);
4610         if (reorder.length == oldType.parameterCount()) {
4611             int limit = newType.parameterCount();
4612             boolean bad = false;
4613             for (int j = 0; j < reorder.length; j++) {
4614                 int i = reorder[j];
4615                 if (i < 0 || i >= limit) {
4616                     bad = true; break;
4617                 }
4618                 Class<?> src = newType.parameterType(i);
4619                 Class<?> dst = oldType.parameterType(j);
4620                 if (src != dst)
4621                     throw newIllegalArgumentException("parameter types do not match after reorder",
4622                             oldType, newType);
4623             }
4624             if (!bad)  return true;
4625         }
4626         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
4627     }
4628 
4629     /**
4630      * Produces a method handle of the requested return type which returns the given
4631      * constant value every time it is invoked.
4632      * <p>
4633      * Before the method handle is returned, the passed-in value is converted to the requested type.
4634      * If the requested type is primitive, widening primitive conversions are attempted,
4635      * else reference conversions are attempted.
4636      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
4637      * @param type the return type of the desired method handle
4638      * @param value the value to return
4639      * @return a method handle of the given return type and no arguments, which always returns the given value
4640      * @throws NullPointerException if the {@code type} argument is null
4641      * @throws ClassCastException if the value cannot be converted to the required return type
4642      * @throws IllegalArgumentException if the given type is {@code void.class}
4643      */
4644     public static MethodHandle constant(Class<?> type, Object value) {
4645         if (type.isPrimitive()) {
4646             if (type == void.class)
4647                 throw newIllegalArgumentException("void type");
4648             Wrapper w = Wrapper.forPrimitiveType(type);
4649             value = w.convert(value, type);
4650             if (w.zero().equals(value))
4651                 return zero(w, type);
4652             return insertArguments(identity(type), 0, value);
4653         } else {
4654             if (value == null)
4655                 return zero(Wrapper.OBJECT, type);
4656             return identity(type).bindTo(value);
4657         }
4658     }
4659 
4660     /**
4661      * Produces a method handle which returns its sole argument when invoked.
4662      * @param type the type of the sole parameter and return value of the desired method handle
4663      * @return a unary method handle which accepts and returns the given type
4664      * @throws NullPointerException if the argument is null
4665      * @throws IllegalArgumentException if the given type is {@code void.class}
4666      */
4667     public static MethodHandle identity(Class<?> type) {
4668         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
4669         int pos = btw.ordinal();
4670         MethodHandle ident = IDENTITY_MHS[pos];
4671         if (ident == null) {
4672             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
4673         }
4674         if (ident.type().returnType() == type)
4675             return ident;
4676         // something like identity(Foo.class); do not bother to intern these
4677         assert (btw == Wrapper.OBJECT);
4678         return makeIdentity(type);
4679     }
4680 
4681     /**
4682      * Produces a constant method handle of the requested return type which
4683      * returns the default value for that type every time it is invoked.
4684      * The resulting constant method handle will have no side effects.
4685      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
4686      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
4687      * since {@code explicitCastArguments} converts {@code null} to default values.
4688      * @param type the expected return type of the desired method handle
4689      * @return a constant method handle that takes no arguments
4690      *         and returns the default value of the given type (or void, if the type is void)
4691      * @throws NullPointerException if the argument is null
4692      * @see MethodHandles#constant
4693      * @see MethodHandles#empty
4694      * @see MethodHandles#explicitCastArguments
4695      * @since 9
4696      */
4697     public static MethodHandle zero(Class<?> type) {
4698         Objects.requireNonNull(type);
4699         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
4700     }
4701 
4702     private static MethodHandle identityOrVoid(Class<?> type) {
4703         return type == void.class ? zero(type) : identity(type);
4704     }
4705 
4706     /**
4707      * Produces a method handle of the requested type which ignores any arguments, does nothing,
4708      * and returns a suitable default depending on the return type.
4709      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
4710      * <p>The returned method handle is equivalent to
4711      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
4712      *
4713      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
4714      * {@code guardWithTest(pred, target, empty(target.type())}.
4715      * @param type the type of the desired method handle
4716      * @return a constant method handle of the given type, which returns a default value of the given return type
4717      * @throws NullPointerException if the argument is null
4718      * @see MethodHandles#zero
4719      * @see MethodHandles#constant
4720      * @since 9
4721      */
4722     public static  MethodHandle empty(MethodType type) {
4723         Objects.requireNonNull(type);
4724         return dropArguments(zero(type.returnType()), 0, type.parameterList());
4725     }
4726 
4727     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
4728     private static MethodHandle makeIdentity(Class<?> ptype) {
4729         MethodType mtype = methodType(ptype, ptype);
4730         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
4731         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
4732     }
4733 
4734     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
4735         int pos = btw.ordinal();
4736         MethodHandle zero = ZERO_MHS[pos];
4737         if (zero == null) {
4738             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
4739         }
4740         if (zero.type().returnType() == rtype)
4741             return zero;
4742         assert(btw == Wrapper.OBJECT);
4743         return makeZero(rtype);
4744     }
4745     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
4746     private static MethodHandle makeZero(Class<?> rtype) {
4747         MethodType mtype = methodType(rtype);
4748         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
4749         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
4750     }
4751 
4752     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
4753         // Simulate a CAS, to avoid racy duplication of results.
4754         MethodHandle prev = cache[pos];
4755         if (prev != null) return prev;
4756         return cache[pos] = value;
4757     }
4758 
4759     /**
4760      * Provides a target method handle with one or more <em>bound arguments</em>
4761      * in advance of the method handle's invocation.
4762      * The formal parameters to the target corresponding to the bound
4763      * arguments are called <em>bound parameters</em>.
4764      * Returns a new method handle which saves away the bound arguments.
4765      * When it is invoked, it receives arguments for any non-bound parameters,
4766      * binds the saved arguments to their corresponding parameters,
4767      * and calls the original target.
4768      * <p>
4769      * The type of the new method handle will drop the types for the bound
4770      * parameters from the original target type, since the new method handle
4771      * will no longer require those arguments to be supplied by its callers.
4772      * <p>
4773      * Each given argument object must match the corresponding bound parameter type.
4774      * If a bound parameter type is a primitive, the argument object
4775      * must be a wrapper, and will be unboxed to produce the primitive value.
4776      * <p>
4777      * The {@code pos} argument selects which parameters are to be bound.
4778      * It may range between zero and <i>N-L</i> (inclusively),
4779      * where <i>N</i> is the arity of the target method handle
4780      * and <i>L</i> is the length of the values array.
4781      * <p>
4782      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4783      * variable-arity method handle}, even if the original target method handle was.
4784      * @param target the method handle to invoke after the argument is inserted
4785      * @param pos where to insert the argument (zero for the first)
4786      * @param values the series of arguments to insert
4787      * @return a method handle which inserts an additional argument,
4788      *         before calling the original method handle
4789      * @throws NullPointerException if the target or the {@code values} array is null
4790      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
4791      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
4792      *         is the length of the values array.
4793      * @throws ClassCastException if an argument does not match the corresponding bound parameter
4794      *         type.
4795      * @see MethodHandle#bindTo
4796      */
4797     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
4798         int insCount = values.length;
4799         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
4800         if (insCount == 0)  return target;
4801         BoundMethodHandle result = target.rebind();
4802         for (int i = 0; i < insCount; i++) {
4803             Object value = values[i];
4804             Class<?> ptype = ptypes[pos+i];
4805             if (ptype.isPrimitive()) {
4806                 result = insertArgumentPrimitive(result, pos, ptype, value);
4807             } else {
4808                 value = ptype.cast(value);  // throw CCE if needed
4809                 result = result.bindArgumentL(pos, value);
4810             }
4811         }
4812         return result;
4813     }
4814 
4815     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
4816                                                              Class<?> ptype, Object value) {
4817         Wrapper w = Wrapper.forPrimitiveType(ptype);
4818         // perform unboxing and/or primitive conversion
4819         value = w.convert(value, ptype);
4820         switch (w) {
4821         case INT:     return result.bindArgumentI(pos, (int)value);
4822         case LONG:    return result.bindArgumentJ(pos, (long)value);
4823         case FLOAT:   return result.bindArgumentF(pos, (float)value);
4824         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
4825         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
4826         }
4827     }
4828 
4829     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
4830         MethodType oldType = target.type();
4831         int outargs = oldType.parameterCount();
4832         int inargs  = outargs - insCount;
4833         if (inargs < 0)
4834             throw newIllegalArgumentException("too many values to insert");
4835         if (pos < 0 || pos > inargs)
4836             throw newIllegalArgumentException("no argument type to append");
4837         return oldType.ptypes();
4838     }
4839 
4840     /**
4841      * Produces a method handle which will discard some dummy arguments
4842      * before calling some other specified <i>target</i> method handle.
4843      * The type of the new method handle will be the same as the target's type,
4844      * except it will also include the dummy argument types,
4845      * at some given position.
4846      * <p>
4847      * The {@code pos} argument may range between zero and <i>N</i>,
4848      * where <i>N</i> is the arity of the target.
4849      * If {@code pos} is zero, the dummy arguments will precede
4850      * the target's real arguments; if {@code pos} is <i>N</i>
4851      * they will come after.
4852      * <p>
4853      * <b>Example:</b>
4854      * <blockquote><pre>{@code
4855 import static java.lang.invoke.MethodHandles.*;
4856 import static java.lang.invoke.MethodType.*;
4857 ...
4858 MethodHandle cat = lookup().findVirtual(String.class,
4859   "concat", methodType(String.class, String.class));
4860 assertEquals("xy", (String) cat.invokeExact("x", "y"));
4861 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
4862 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
4863 assertEquals(bigType, d0.type());
4864 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
4865      * }</pre></blockquote>
4866      * <p>
4867      * This method is also equivalent to the following code:
4868      * <blockquote><pre>
4869      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
4870      * </pre></blockquote>
4871      * @param target the method handle to invoke after the arguments are dropped
4872      * @param pos position of first argument to drop (zero for the leftmost)
4873      * @param valueTypes the type(s) of the argument(s) to drop
4874      * @return a method handle which drops arguments of the given types,
4875      *         before calling the original method handle
4876      * @throws NullPointerException if the target is null,
4877      *                              or if the {@code valueTypes} list or any of its elements is null
4878      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
4879      *                  or if {@code pos} is negative or greater than the arity of the target,
4880      *                  or if the new method handle's type would have too many parameters
4881      */
4882     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
4883         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
4884     }
4885 
4886     private static List<Class<?>> copyTypes(Object[] array) {
4887         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
4888     }
4889 
4890     private static MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
4891         MethodType oldType = target.type();  // get NPE
4892         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
4893         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
4894         if (dropped == 0)  return target;
4895         BoundMethodHandle result = target.rebind();
4896         LambdaForm lform = result.form;
4897         int insertFormArg = 1 + pos;
4898         for (Class<?> ptype : valueTypes) {
4899             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
4900         }
4901         result = result.copyWith(newType, lform);
4902         return result;
4903     }
4904 
4905     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
4906         int dropped = valueTypes.size();
4907         MethodType.checkSlotCount(dropped);
4908         int outargs = oldType.parameterCount();
4909         int inargs  = outargs + dropped;
4910         if (pos < 0 || pos > outargs)
4911             throw newIllegalArgumentException("no argument type to remove"
4912                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
4913                     );
4914         return dropped;
4915     }
4916 
4917     /**
4918      * Produces a method handle which will discard some dummy arguments
4919      * before calling some other specified <i>target</i> method handle.
4920      * The type of the new method handle will be the same as the target's type,
4921      * except it will also include the dummy argument types,
4922      * at some given position.
4923      * <p>
4924      * The {@code pos} argument may range between zero and <i>N</i>,
4925      * where <i>N</i> is the arity of the target.
4926      * If {@code pos} is zero, the dummy arguments will precede
4927      * the target's real arguments; if {@code pos} is <i>N</i>
4928      * they will come after.
4929      * @apiNote
4930      * <blockquote><pre>{@code
4931 import static java.lang.invoke.MethodHandles.*;
4932 import static java.lang.invoke.MethodType.*;
4933 ...
4934 MethodHandle cat = lookup().findVirtual(String.class,
4935   "concat", methodType(String.class, String.class));
4936 assertEquals("xy", (String) cat.invokeExact("x", "y"));
4937 MethodHandle d0 = dropArguments(cat, 0, String.class);
4938 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
4939 MethodHandle d1 = dropArguments(cat, 1, String.class);
4940 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
4941 MethodHandle d2 = dropArguments(cat, 2, String.class);
4942 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
4943 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
4944 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
4945      * }</pre></blockquote>
4946      * <p>
4947      * This method is also equivalent to the following code:
4948      * <blockquote><pre>
4949      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
4950      * </pre></blockquote>
4951      * @param target the method handle to invoke after the arguments are dropped
4952      * @param pos position of first argument to drop (zero for the leftmost)
4953      * @param valueTypes the type(s) of the argument(s) to drop
4954      * @return a method handle which drops arguments of the given types,
4955      *         before calling the original method handle
4956      * @throws NullPointerException if the target is null,
4957      *                              or if the {@code valueTypes} array or any of its elements is null
4958      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
4959      *                  or if {@code pos} is negative or greater than the arity of the target,
4960      *                  or if the new method handle's type would have
4961      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
4962      */
4963     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
4964         return dropArguments0(target, pos, copyTypes(valueTypes));
4965     }
4966 
4967     // private version which allows caller some freedom with error handling
4968     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
4969                                       boolean nullOnFailure) {
4970         newTypes = copyTypes(newTypes.toArray());
4971         List<Class<?>> oldTypes = target.type().parameterList();
4972         int match = oldTypes.size();
4973         if (skip != 0) {
4974             if (skip < 0 || skip > match) {
4975                 throw newIllegalArgumentException("illegal skip", skip, target);
4976             }
4977             oldTypes = oldTypes.subList(skip, match);
4978             match -= skip;
4979         }
4980         List<Class<?>> addTypes = newTypes;
4981         int add = addTypes.size();
4982         if (pos != 0) {
4983             if (pos < 0 || pos > add) {
4984                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
4985             }
4986             addTypes = addTypes.subList(pos, add);
4987             add -= pos;
4988             assert(addTypes.size() == add);
4989         }
4990         // Do not add types which already match the existing arguments.
4991         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
4992             if (nullOnFailure) {
4993                 return null;
4994             }
4995             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
4996         }
4997         addTypes = addTypes.subList(match, add);
4998         add -= match;
4999         assert(addTypes.size() == add);
5000         // newTypes:     (   P*[pos], M*[match], A*[add] )
5001         // target: ( S*[skip],        M*[match]  )
5002         MethodHandle adapter = target;
5003         if (add > 0) {
5004             adapter = dropArguments0(adapter, skip+ match, addTypes);
5005         }
5006         // adapter: (S*[skip],        M*[match], A*[add] )
5007         if (pos > 0) {
5008             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
5009         }
5010         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5011         return adapter;
5012     }
5013 
5014     /**
5015      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5016      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5017      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5018      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5019      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5020      * {@link #dropArguments(MethodHandle, int, Class[])}.
5021      * <p>
5022      * The resulting handle will have the same return type as the target handle.
5023      * <p>
5024      * In more formal terms, assume these two type lists:<ul>
5025      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5026      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5027      * {@code newTypes}.
5028      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5029      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5030      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5031      * sub-list.
5032      * </ul>
5033      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5034      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5035      * {@link #dropArguments(MethodHandle, int, Class[])}.
5036      *
5037      * @apiNote
5038      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5039      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5040      * <blockquote><pre>{@code
5041 import static java.lang.invoke.MethodHandles.*;
5042 import static java.lang.invoke.MethodType.*;
5043 ...
5044 ...
5045 MethodHandle h0 = constant(boolean.class, true);
5046 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5047 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5048 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5049 if (h1.type().parameterCount() < h2.type().parameterCount())
5050     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5051 else
5052     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5053 MethodHandle h3 = guardWithTest(h0, h1, h2);
5054 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5055      * }</pre></blockquote>
5056      * @param target the method handle to adapt
5057      * @param skip number of targets parameters to disregard (they will be unchanged)
5058      * @param newTypes the list of types to match {@code target}'s parameter type list to
5059      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5060      * @return a possibly adapted method handle
5061      * @throws NullPointerException if either argument is null
5062      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5063      *         or if {@code skip} is negative or greater than the arity of the target,
5064      *         or if {@code pos} is negative or greater than the newTypes list size,
5065      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5066      *         {@code pos}.
5067      * @since 9
5068      */
5069     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5070         Objects.requireNonNull(target);
5071         Objects.requireNonNull(newTypes);
5072         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
5073     }
5074 
5075     /**
5076      * Adapts a target method handle by pre-processing
5077      * one or more of its arguments, each with its own unary filter function,
5078      * and then calling the target with each pre-processed argument
5079      * replaced by the result of its corresponding filter function.
5080      * <p>
5081      * The pre-processing is performed by one or more method handles,
5082      * specified in the elements of the {@code filters} array.
5083      * The first element of the filter array corresponds to the {@code pos}
5084      * argument of the target, and so on in sequence.
5085      * The filter functions are invoked in left to right order.
5086      * <p>
5087      * Null arguments in the array are treated as identity functions,
5088      * and the corresponding arguments left unchanged.
5089      * (If there are no non-null elements in the array, the original target is returned.)
5090      * Each filter is applied to the corresponding argument of the adapter.
5091      * <p>
5092      * If a filter {@code F} applies to the {@code N}th argument of
5093      * the target, then {@code F} must be a method handle which
5094      * takes exactly one argument.  The type of {@code F}'s sole argument
5095      * replaces the corresponding argument type of the target
5096      * in the resulting adapted method handle.
5097      * The return type of {@code F} must be identical to the corresponding
5098      * parameter type of the target.
5099      * <p>
5100      * It is an error if there are elements of {@code filters}
5101      * (null or not)
5102      * which do not correspond to argument positions in the target.
5103      * <p><b>Example:</b>
5104      * <blockquote><pre>{@code
5105 import static java.lang.invoke.MethodHandles.*;
5106 import static java.lang.invoke.MethodType.*;
5107 ...
5108 MethodHandle cat = lookup().findVirtual(String.class,
5109   "concat", methodType(String.class, String.class));
5110 MethodHandle upcase = lookup().findVirtual(String.class,
5111   "toUpperCase", methodType(String.class));
5112 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5113 MethodHandle f0 = filterArguments(cat, 0, upcase);
5114 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5115 MethodHandle f1 = filterArguments(cat, 1, upcase);
5116 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5117 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5118 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5119      * }</pre></blockquote>
5120      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5121      * denotes the return type of both the {@code target} and resulting adapter.
5122      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5123      * of the parameters and arguments that precede and follow the filter position
5124      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5125      * values of the filtered parameters and arguments; they also represent the
5126      * return types of the {@code filter[i]} handles. The latter accept arguments
5127      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5128      * the resulting adapter.
5129      * <blockquote><pre>{@code
5130      * T target(P... p, A[i]... a[i], B... b);
5131      * A[i] filter[i](V[i]);
5132      * T adapter(P... p, V[i]... v[i], B... b) {
5133      *   return target(p..., filter[i](v[i])..., b...);
5134      * }
5135      * }</pre></blockquote>
5136      * <p>
5137      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5138      * variable-arity method handle}, even if the original target method handle was.
5139      *
5140      * @param target the method handle to invoke after arguments are filtered
5141      * @param pos the position of the first argument to filter
5142      * @param filters method handles to call initially on filtered arguments
5143      * @return method handle which incorporates the specified argument filtering logic
5144      * @throws NullPointerException if the target is null
5145      *                              or if the {@code filters} array is null
5146      * @throws IllegalArgumentException if a non-null element of {@code filters}
5147      *          does not match a corresponding argument type of target as described above,
5148      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5149      *          or if the resulting method handle's type would have
5150      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5151      */
5152     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5153         // In method types arguments start at index 0, while the LF
5154         // editor have the MH receiver at position 0 - adjust appropriately.
5155         final int MH_RECEIVER_OFFSET = 1;
5156         filterArgumentsCheckArity(target, pos, filters);
5157         MethodHandle adapter = target;
5158 
5159         // keep track of currently matched filters, as to optimize repeated filters
5160         int index = 0;
5161         int[] positions = new int[filters.length];
5162         MethodHandle filter = null;
5163 
5164         // process filters in reverse order so that the invocation of
5165         // the resulting adapter will invoke the filters in left-to-right order
5166         for (int i = filters.length - 1; i >= 0; --i) {
5167             MethodHandle newFilter = filters[i];
5168             if (newFilter == null) continue;  // ignore null elements of filters
5169 
5170             // flush changes on update
5171             if (filter != newFilter) {
5172                 if (filter != null) {
5173                     if (index > 1) {
5174                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5175                     } else {
5176                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5177                     }
5178                 }
5179                 filter = newFilter;
5180                 index = 0;
5181             }
5182 
5183             filterArgumentChecks(target, pos + i, newFilter);
5184             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5185         }
5186         if (index > 1) {
5187             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5188         } else if (index == 1) {
5189             adapter = filterArgument(adapter, positions[0] - 1, filter);
5190         }
5191         return adapter;
5192     }
5193 
5194     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5195         MethodType targetType = adapter.type();
5196         MethodType filterType = filter.type();
5197         BoundMethodHandle result = adapter.rebind();
5198         Class<?> newParamType = filterType.parameterType(0);
5199 
5200         Class<?>[] ptypes = targetType.ptypes().clone();
5201         for (int pos : positions) {
5202             ptypes[pos - 1] = newParamType;
5203         }
5204         MethodType newType = MethodType.makeImpl(targetType.rtype(), ptypes, true);
5205 
5206         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5207         return result.copyWithExtendL(newType, lform, filter);
5208     }
5209 
5210     /*non-public*/
5211     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5212         filterArgumentChecks(target, pos, filter);
5213         MethodType targetType = target.type();
5214         MethodType filterType = filter.type();
5215         BoundMethodHandle result = target.rebind();
5216         Class<?> newParamType = filterType.parameterType(0);
5217         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5218         MethodType newType = targetType.changeParameterType(pos, newParamType);
5219         result = result.copyWithExtendL(newType, lform, filter);
5220         return result;
5221     }
5222 
5223     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5224         MethodType targetType = target.type();
5225         int maxPos = targetType.parameterCount();
5226         if (pos + filters.length > maxPos)
5227             throw newIllegalArgumentException("too many filters");
5228     }
5229 
5230     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5231         MethodType targetType = target.type();
5232         MethodType filterType = filter.type();
5233         if (filterType.parameterCount() != 1
5234             || filterType.returnType() != targetType.parameterType(pos))
5235             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5236     }
5237 
5238     /**
5239      * Adapts a target method handle by pre-processing
5240      * a sub-sequence of its arguments with a filter (another method handle).
5241      * The pre-processed arguments are replaced by the result (if any) of the
5242      * filter function.
5243      * The target is then called on the modified (usually shortened) argument list.
5244      * <p>
5245      * If the filter returns a value, the target must accept that value as
5246      * its argument in position {@code pos}, preceded and/or followed by
5247      * any arguments not passed to the filter.
5248      * If the filter returns void, the target must accept all arguments
5249      * not passed to the filter.
5250      * No arguments are reordered, and a result returned from the filter
5251      * replaces (in order) the whole subsequence of arguments originally
5252      * passed to the adapter.
5253      * <p>
5254      * The argument types (if any) of the filter
5255      * replace zero or one argument types of the target, at position {@code pos},
5256      * in the resulting adapted method handle.
5257      * The return type of the filter (if any) must be identical to the
5258      * argument type of the target at position {@code pos}, and that target argument
5259      * is supplied by the return value of the filter.
5260      * <p>
5261      * In all cases, {@code pos} must be greater than or equal to zero, and
5262      * {@code pos} must also be less than or equal to the target's arity.
5263      * <p><b>Example:</b>
5264      * <blockquote><pre>{@code
5265 import static java.lang.invoke.MethodHandles.*;
5266 import static java.lang.invoke.MethodType.*;
5267 ...
5268 MethodHandle deepToString = publicLookup()
5269   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5270 
5271 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5272 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5273 
5274 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5275 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5276 
5277 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5278 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5279 assertEquals("[top, [up, down], strange]",
5280              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5281 
5282 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5283 assertEquals("[top, [up, down], [strange]]",
5284              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5285 
5286 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5287 assertEquals("[top, [[up, down, strange], charm], bottom]",
5288              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5289      * }</pre></blockquote>
5290      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5291      * represents the return type of the {@code target} and resulting adapter.
5292      * {@code V}/{@code v} stand for the return type and value of the
5293      * {@code filter}, which are also found in the signature and arguments of
5294      * the {@code target}, respectively, unless {@code V} is {@code void}.
5295      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5296      * and values preceding and following the collection position, {@code pos},
5297      * in the {@code target}'s signature. They also turn up in the resulting
5298      * adapter's signature and arguments, where they surround
5299      * {@code B}/{@code b}, which represent the parameter types and arguments
5300      * to the {@code filter} (if any).
5301      * <blockquote><pre>{@code
5302      * T target(A...,V,C...);
5303      * V filter(B...);
5304      * T adapter(A... a,B... b,C... c) {
5305      *   V v = filter(b...);
5306      *   return target(a...,v,c...);
5307      * }
5308      * // and if the filter has no arguments:
5309      * T target2(A...,V,C...);
5310      * V filter2();
5311      * T adapter2(A... a,C... c) {
5312      *   V v = filter2();
5313      *   return target2(a...,v,c...);
5314      * }
5315      * // and if the filter has a void return:
5316      * T target3(A...,C...);
5317      * void filter3(B...);
5318      * T adapter3(A... a,B... b,C... c) {
5319      *   filter3(b...);
5320      *   return target3(a...,c...);
5321      * }
5322      * }</pre></blockquote>
5323      * <p>
5324      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5325      * one which first "folds" the affected arguments, and then drops them, in separate
5326      * steps as follows:
5327      * <blockquote><pre>{@code
5328      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5329      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5330      * }</pre></blockquote>
5331      * If the target method handle consumes no arguments besides than the result
5332      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5333      * is equivalent to {@code filterReturnValue(coll, mh)}.
5334      * If the filter method handle {@code coll} consumes one argument and produces
5335      * a non-void result, then {@code collectArguments(mh, N, coll)}
5336      * is equivalent to {@code filterArguments(mh, N, coll)}.
5337      * Other equivalences are possible but would require argument permutation.
5338      * <p>
5339      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5340      * variable-arity method handle}, even if the original target method handle was.
5341      *
5342      * @param target the method handle to invoke after filtering the subsequence of arguments
5343      * @param pos the position of the first adapter argument to pass to the filter,
5344      *            and/or the target argument which receives the result of the filter
5345      * @param filter method handle to call on the subsequence of arguments
5346      * @return method handle which incorporates the specified argument subsequence filtering logic
5347      * @throws NullPointerException if either argument is null
5348      * @throws IllegalArgumentException if the return type of {@code filter}
5349      *          is non-void and is not the same as the {@code pos} argument of the target,
5350      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5351      *          or if the resulting method handle's type would have
5352      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5353      * @see MethodHandles#foldArguments
5354      * @see MethodHandles#filterArguments
5355      * @see MethodHandles#filterReturnValue
5356      */
5357     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5358         MethodType newType = collectArgumentsChecks(target, pos, filter);
5359         MethodType collectorType = filter.type();
5360         BoundMethodHandle result = target.rebind();
5361         LambdaForm lform;
5362         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
5363             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
5364             if (lform != null) {
5365                 return result.copyWith(newType, lform);
5366             }
5367         }
5368         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5369         return result.copyWithExtendL(newType, lform, filter);
5370     }
5371 
5372     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5373         MethodType targetType = target.type();
5374         MethodType filterType = filter.type();
5375         Class<?> rtype = filterType.returnType();
5376         List<Class<?>> filterArgs = filterType.parameterList();
5377         if (rtype == void.class) {
5378             return targetType.insertParameterTypes(pos, filterArgs);
5379         }
5380         if (rtype != targetType.parameterType(pos)) {
5381             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5382         }
5383         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
5384     }
5385 
5386     /**
5387      * Adapts a target method handle by post-processing
5388      * its return value (if any) with a filter (another method handle).
5389      * The result of the filter is returned from the adapter.
5390      * <p>
5391      * If the target returns a value, the filter must accept that value as
5392      * its only argument.
5393      * If the target returns void, the filter must accept no arguments.
5394      * <p>
5395      * The return type of the filter
5396      * replaces the return type of the target
5397      * in the resulting adapted method handle.
5398      * The argument type of the filter (if any) must be identical to the
5399      * return type of the target.
5400      * <p><b>Example:</b>
5401      * <blockquote><pre>{@code
5402 import static java.lang.invoke.MethodHandles.*;
5403 import static java.lang.invoke.MethodType.*;
5404 ...
5405 MethodHandle cat = lookup().findVirtual(String.class,
5406   "concat", methodType(String.class, String.class));
5407 MethodHandle length = lookup().findVirtual(String.class,
5408   "length", methodType(int.class));
5409 System.out.println((String) cat.invokeExact("x", "y")); // xy
5410 MethodHandle f0 = filterReturnValue(cat, length);
5411 System.out.println((int) f0.invokeExact("x", "y")); // 2
5412      * }</pre></blockquote>
5413      * <p>Here is pseudocode for the resulting adapter. In the code,
5414      * {@code T}/{@code t} represent the result type and value of the
5415      * {@code target}; {@code V}, the result type of the {@code filter}; and
5416      * {@code A}/{@code a}, the types and values of the parameters and arguments
5417      * of the {@code target} as well as the resulting adapter.
5418      * <blockquote><pre>{@code
5419      * T target(A...);
5420      * V filter(T);
5421      * V adapter(A... a) {
5422      *   T t = target(a...);
5423      *   return filter(t);
5424      * }
5425      * // and if the target has a void return:
5426      * void target2(A...);
5427      * V filter2();
5428      * V adapter2(A... a) {
5429      *   target2(a...);
5430      *   return filter2();
5431      * }
5432      * // and if the filter has a void return:
5433      * T target3(A...);
5434      * void filter3(V);
5435      * void adapter3(A... a) {
5436      *   T t = target3(a...);
5437      *   filter3(t);
5438      * }
5439      * }</pre></blockquote>
5440      * <p>
5441      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5442      * variable-arity method handle}, even if the original target method handle was.
5443      * @param target the method handle to invoke before filtering the return value
5444      * @param filter method handle to call on the return value
5445      * @return method handle which incorporates the specified return value filtering logic
5446      * @throws NullPointerException if either argument is null
5447      * @throws IllegalArgumentException if the argument list of {@code filter}
5448      *          does not match the return type of target as described above
5449      */
5450     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5451         MethodType targetType = target.type();
5452         MethodType filterType = filter.type();
5453         filterReturnValueChecks(targetType, filterType);
5454         BoundMethodHandle result = target.rebind();
5455         BasicType rtype = BasicType.basicType(filterType.returnType());
5456         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5457         MethodType newType = targetType.changeReturnType(filterType.returnType());
5458         result = result.copyWithExtendL(newType, lform, filter);
5459         return result;
5460     }
5461 
5462     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5463         Class<?> rtype = targetType.returnType();
5464         int filterValues = filterType.parameterCount();
5465         if (filterValues == 0
5466                 ? (rtype != void.class)
5467                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5468             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5469     }
5470 
5471     /**
5472      * Adapts a target method handle by pre-processing
5473      * some of its arguments, and then calling the target with
5474      * the result of the pre-processing, inserted into the original
5475      * sequence of arguments.
5476      * <p>
5477      * The pre-processing is performed by {@code combiner}, a second method handle.
5478      * Of the arguments passed to the adapter, the first {@code N} arguments
5479      * are copied to the combiner, which is then called.
5480      * (Here, {@code N} is defined as the parameter count of the combiner.)
5481      * After this, control passes to the target, with any result
5482      * from the combiner inserted before the original {@code N} incoming
5483      * arguments.
5484      * <p>
5485      * If the combiner returns a value, the first parameter type of the target
5486      * must be identical with the return type of the combiner, and the next
5487      * {@code N} parameter types of the target must exactly match the parameters
5488      * of the combiner.
5489      * <p>
5490      * If the combiner has a void return, no result will be inserted,
5491      * and the first {@code N} parameter types of the target
5492      * must exactly match the parameters of the combiner.
5493      * <p>
5494      * The resulting adapter is the same type as the target, except that the
5495      * first parameter type is dropped,
5496      * if it corresponds to the result of the combiner.
5497      * <p>
5498      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
5499      * that either the combiner or the target does not wish to receive.
5500      * If some of the incoming arguments are destined only for the combiner,
5501      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
5502      * arguments will not need to be live on the stack on entry to the
5503      * target.)
5504      * <p><b>Example:</b>
5505      * <blockquote><pre>{@code
5506 import static java.lang.invoke.MethodHandles.*;
5507 import static java.lang.invoke.MethodType.*;
5508 ...
5509 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5510   "println", methodType(void.class, String.class))
5511     .bindTo(System.out);
5512 MethodHandle cat = lookup().findVirtual(String.class,
5513   "concat", methodType(String.class, String.class));
5514 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5515 MethodHandle catTrace = foldArguments(cat, trace);
5516 // also prints "boo":
5517 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5518      * }</pre></blockquote>
5519      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5520      * represents the result type of the {@code target} and resulting adapter.
5521      * {@code V}/{@code v} represent the type and value of the parameter and argument
5522      * of {@code target} that precedes the folding position; {@code V} also is
5523      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5524      * types and values of the {@code N} parameters and arguments at the folding
5525      * position. {@code B}/{@code b} represent the types and values of the
5526      * {@code target} parameters and arguments that follow the folded parameters
5527      * and arguments.
5528      * <blockquote><pre>{@code
5529      * // there are N arguments in A...
5530      * T target(V, A[N]..., B...);
5531      * V combiner(A...);
5532      * T adapter(A... a, B... b) {
5533      *   V v = combiner(a...);
5534      *   return target(v, a..., b...);
5535      * }
5536      * // and if the combiner has a void return:
5537      * T target2(A[N]..., B...);
5538      * void combiner2(A...);
5539      * T adapter2(A... a, B... b) {
5540      *   combiner2(a...);
5541      *   return target2(a..., b...);
5542      * }
5543      * }</pre></blockquote>
5544      * <p>
5545      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5546      * variable-arity method handle}, even if the original target method handle was.
5547      * @param target the method handle to invoke after arguments are combined
5548      * @param combiner method handle to call initially on the incoming arguments
5549      * @return method handle which incorporates the specified argument folding logic
5550      * @throws NullPointerException if either argument is null
5551      * @throws IllegalArgumentException if {@code combiner}'s return type
5552      *          is non-void and not the same as the first argument type of
5553      *          the target, or if the initial {@code N} argument types
5554      *          of the target
5555      *          (skipping one matching the {@code combiner}'s return type)
5556      *          are not identical with the argument types of {@code combiner}
5557      */
5558     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
5559         return foldArguments(target, 0, combiner);
5560     }
5561 
5562     /**
5563      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
5564      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
5565      * before the folded arguments.
5566      * <p>
5567      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
5568      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
5569      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
5570      * 0.
5571      *
5572      * @apiNote Example:
5573      * <blockquote><pre>{@code
5574     import static java.lang.invoke.MethodHandles.*;
5575     import static java.lang.invoke.MethodType.*;
5576     ...
5577     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5578     "println", methodType(void.class, String.class))
5579     .bindTo(System.out);
5580     MethodHandle cat = lookup().findVirtual(String.class,
5581     "concat", methodType(String.class, String.class));
5582     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5583     MethodHandle catTrace = foldArguments(cat, 1, trace);
5584     // also prints "jum":
5585     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5586      * }</pre></blockquote>
5587      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5588      * represents the result type of the {@code target} and resulting adapter.
5589      * {@code V}/{@code v} represent the type and value of the parameter and argument
5590      * of {@code target} that precedes the folding position; {@code V} also is
5591      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5592      * types and values of the {@code N} parameters and arguments at the folding
5593      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
5594      * and values of the {@code target} parameters and arguments that precede and
5595      * follow the folded parameters and arguments starting at {@code pos},
5596      * respectively.
5597      * <blockquote><pre>{@code
5598      * // there are N arguments in A...
5599      * T target(Z..., V, A[N]..., B...);
5600      * V combiner(A...);
5601      * T adapter(Z... z, A... a, B... b) {
5602      *   V v = combiner(a...);
5603      *   return target(z..., v, a..., b...);
5604      * }
5605      * // and if the combiner has a void return:
5606      * T target2(Z..., A[N]..., B...);
5607      * void combiner2(A...);
5608      * T adapter2(Z... z, A... a, B... b) {
5609      *   combiner2(a...);
5610      *   return target2(z..., a..., b...);
5611      * }
5612      * }</pre></blockquote>
5613      * <p>
5614      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5615      * variable-arity method handle}, even if the original target method handle was.
5616      *
5617      * @param target the method handle to invoke after arguments are combined
5618      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
5619      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5620      * @param combiner method handle to call initially on the incoming arguments
5621      * @return method handle which incorporates the specified argument folding logic
5622      * @throws NullPointerException if either argument is null
5623      * @throws IllegalArgumentException if either of the following two conditions holds:
5624      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5625      *              {@code pos} of the target signature;
5626      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
5627      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
5628      *
5629      * @see #foldArguments(MethodHandle, MethodHandle)
5630      * @since 9
5631      */
5632     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
5633         MethodType targetType = target.type();
5634         MethodType combinerType = combiner.type();
5635         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
5636         BoundMethodHandle result = target.rebind();
5637         boolean dropResult = rtype == void.class;
5638         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
5639         MethodType newType = targetType;
5640         if (!dropResult) {
5641             newType = newType.dropParameterTypes(pos, pos + 1);
5642         }
5643         result = result.copyWithExtendL(newType, lform, combiner);
5644         return result;
5645     }
5646 
5647     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
5648         int foldArgs   = combinerType.parameterCount();
5649         Class<?> rtype = combinerType.returnType();
5650         int foldVals = rtype == void.class ? 0 : 1;
5651         int afterInsertPos = foldPos + foldVals;
5652         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
5653         if (ok) {
5654             for (int i = 0; i < foldArgs; i++) {
5655                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
5656                     ok = false;
5657                     break;
5658                 }
5659             }
5660         }
5661         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
5662             ok = false;
5663         if (!ok)
5664             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5665         return rtype;
5666     }
5667 
5668     /**
5669      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
5670      * of the pre-processing replacing the argument at the given position.
5671      *
5672      * @param target the method handle to invoke after arguments are combined
5673      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5674      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5675      * @param combiner method handle to call initially on the incoming arguments
5676      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5677      * @return method handle which incorporates the specified argument folding logic
5678      * @throws NullPointerException if either argument is null
5679      * @throws IllegalArgumentException if either of the following two conditions holds:
5680      *          (1) {@code combiner}'s return type is not the same as the argument type at position
5681      *              {@code pos} of the target signature;
5682      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
5683      *              not identical with the argument types of {@code combiner}.
5684      */
5685     /*non-public*/
5686     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5687         return argumentsWithCombiner(true, target, position, combiner, argPositions);
5688     }
5689 
5690     /**
5691      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
5692      * the pre-processing inserted into the original sequence of arguments at the given position.
5693      *
5694      * @param target the method handle to invoke after arguments are combined
5695      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5696      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5697      * @param combiner method handle to call initially on the incoming arguments
5698      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5699      * @return method handle which incorporates the specified argument folding logic
5700      * @throws NullPointerException if either argument is null
5701      * @throws IllegalArgumentException if either of the following two conditions holds:
5702      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5703      *              {@code pos} of the target signature;
5704      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
5705      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
5706      *              with the argument types of {@code combiner}.
5707      */
5708     /*non-public*/
5709     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5710         return argumentsWithCombiner(false, target, position, combiner, argPositions);
5711     }
5712 
5713     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5714         MethodType targetType = target.type();
5715         MethodType combinerType = combiner.type();
5716         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
5717         BoundMethodHandle result = target.rebind();
5718 
5719         MethodType newType = targetType;
5720         LambdaForm lform;
5721         if (filter) {
5722             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
5723         } else {
5724             boolean dropResult = rtype == void.class;
5725             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
5726             if (!dropResult) {
5727                 newType = newType.dropParameterTypes(position, position + 1);
5728             }
5729         }
5730         result = result.copyWithExtendL(newType, lform, combiner);
5731         return result;
5732     }
5733 
5734     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
5735         int combinerArgs = combinerType.parameterCount();
5736         if (argPos.length != combinerArgs) {
5737             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
5738         }
5739         Class<?> rtype = combinerType.returnType();
5740 
5741         for (int i = 0; i < combinerArgs; i++) {
5742             int arg = argPos[i];
5743             if (arg < 0 || arg > targetType.parameterCount()) {
5744                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
5745             }
5746             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
5747                 throw newIllegalArgumentException("target argument type at position " + arg
5748                         + " must match combiner argument type at index " + i + ": " + targetType
5749                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
5750             }
5751         }
5752         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
5753             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5754         }
5755         return rtype;
5756     }
5757 
5758     /**
5759      * Makes a method handle which adapts a target method handle,
5760      * by guarding it with a test, a boolean-valued method handle.
5761      * If the guard fails, a fallback handle is called instead.
5762      * All three method handles must have the same corresponding
5763      * argument and return types, except that the return type
5764      * of the test must be boolean, and the test is allowed
5765      * to have fewer arguments than the other two method handles.
5766      * <p>
5767      * Here is pseudocode for the resulting adapter. In the code, {@code T}
5768      * represents the uniform result type of the three involved handles;
5769      * {@code A}/{@code a}, the types and values of the {@code target}
5770      * parameters and arguments that are consumed by the {@code test}; and
5771      * {@code B}/{@code b}, those types and values of the {@code target}
5772      * parameters and arguments that are not consumed by the {@code test}.
5773      * <blockquote><pre>{@code
5774      * boolean test(A...);
5775      * T target(A...,B...);
5776      * T fallback(A...,B...);
5777      * T adapter(A... a,B... b) {
5778      *   if (test(a...))
5779      *     return target(a..., b...);
5780      *   else
5781      *     return fallback(a..., b...);
5782      * }
5783      * }</pre></blockquote>
5784      * Note that the test arguments ({@code a...} in the pseudocode) cannot
5785      * be modified by execution of the test, and so are passed unchanged
5786      * from the caller to the target or fallback as appropriate.
5787      * @param test method handle used for test, must return boolean
5788      * @param target method handle to call if test passes
5789      * @param fallback method handle to call if test fails
5790      * @return method handle which incorporates the specified if/then/else logic
5791      * @throws NullPointerException if any argument is null
5792      * @throws IllegalArgumentException if {@code test} does not return boolean,
5793      *          or if all three method types do not match (with the return
5794      *          type of {@code test} changed to match that of the target).
5795      */
5796     public static MethodHandle guardWithTest(MethodHandle test,
5797                                MethodHandle target,
5798                                MethodHandle fallback) {
5799         MethodType gtype = test.type();
5800         MethodType ttype = target.type();
5801         MethodType ftype = fallback.type();
5802         if (!ttype.equals(ftype))
5803             throw misMatchedTypes("target and fallback types", ttype, ftype);
5804         if (gtype.returnType() != boolean.class)
5805             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
5806         List<Class<?>> targs = ttype.parameterList();
5807         test = dropArgumentsToMatch(test, 0, targs, 0, true);
5808         if (test == null) {
5809             throw misMatchedTypes("target and test types", ttype, gtype);
5810         }
5811         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
5812     }
5813 
5814     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
5815         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
5816     }
5817 
5818     /**
5819      * Makes a method handle which adapts a target method handle,
5820      * by running it inside an exception handler.
5821      * If the target returns normally, the adapter returns that value.
5822      * If an exception matching the specified type is thrown, the fallback
5823      * handle is called instead on the exception, plus the original arguments.
5824      * <p>
5825      * The target and handler must have the same corresponding
5826      * argument and return types, except that handler may omit trailing arguments
5827      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
5828      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
5829      * <p>
5830      * Here is pseudocode for the resulting adapter. In the code, {@code T}
5831      * represents the return type of the {@code target} and {@code handler},
5832      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
5833      * the types and values of arguments to the resulting handle consumed by
5834      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
5835      * resulting handle discarded by {@code handler}.
5836      * <blockquote><pre>{@code
5837      * T target(A..., B...);
5838      * T handler(ExType, A...);
5839      * T adapter(A... a, B... b) {
5840      *   try {
5841      *     return target(a..., b...);
5842      *   } catch (ExType ex) {
5843      *     return handler(ex, a...);
5844      *   }
5845      * }
5846      * }</pre></blockquote>
5847      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
5848      * be modified by execution of the target, and so are passed unchanged
5849      * from the caller to the handler, if the handler is invoked.
5850      * <p>
5851      * The target and handler must return the same type, even if the handler
5852      * always throws.  (This might happen, for instance, because the handler
5853      * is simulating a {@code finally} clause).
5854      * To create such a throwing handler, compose the handler creation logic
5855      * with {@link #throwException throwException},
5856      * in order to create a method handle of the correct return type.
5857      * @param target method handle to call
5858      * @param exType the type of exception which the handler will catch
5859      * @param handler method handle to call if a matching exception is thrown
5860      * @return method handle which incorporates the specified try/catch logic
5861      * @throws NullPointerException if any argument is null
5862      * @throws IllegalArgumentException if {@code handler} does not accept
5863      *          the given exception type, or if the method handle types do
5864      *          not match in their return types and their
5865      *          corresponding parameters
5866      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
5867      */
5868     public static MethodHandle catchException(MethodHandle target,
5869                                 Class<? extends Throwable> exType,
5870                                 MethodHandle handler) {
5871         MethodType ttype = target.type();
5872         MethodType htype = handler.type();
5873         if (!Throwable.class.isAssignableFrom(exType))
5874             throw new ClassCastException(exType.getName());
5875         if (htype.parameterCount() < 1 ||
5876             !htype.parameterType(0).isAssignableFrom(exType))
5877             throw newIllegalArgumentException("handler does not accept exception type "+exType);
5878         if (htype.returnType() != ttype.returnType())
5879             throw misMatchedTypes("target and handler return types", ttype, htype);
5880         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
5881         if (handler == null) {
5882             throw misMatchedTypes("target and handler types", ttype, htype);
5883         }
5884         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
5885     }
5886 
5887     /**
5888      * Produces a method handle which will throw exceptions of the given {@code exType}.
5889      * The method handle will accept a single argument of {@code exType},
5890      * and immediately throw it as an exception.
5891      * The method type will nominally specify a return of {@code returnType}.
5892      * The return type may be anything convenient:  It doesn't matter to the
5893      * method handle's behavior, since it will never return normally.
5894      * @param returnType the return type of the desired method handle
5895      * @param exType the parameter type of the desired method handle
5896      * @return method handle which can throw the given exceptions
5897      * @throws NullPointerException if either argument is null
5898      */
5899     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
5900         if (!Throwable.class.isAssignableFrom(exType))
5901             throw new ClassCastException(exType.getName());
5902         return MethodHandleImpl.throwException(methodType(returnType, exType));
5903     }
5904 
5905     /**
5906      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
5907      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
5908      * delivers the loop's result, which is the return value of the resulting handle.
5909      * <p>
5910      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
5911      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
5912      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
5913      * terms of method handles, each clause will specify up to four independent actions:<ul>
5914      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
5915      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
5916      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
5917      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
5918      * </ul>
5919      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
5920      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
5921      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
5922      * <p>
5923      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
5924      * this case. See below for a detailed description.
5925      * <p>
5926      * <em>Parameters optional everywhere:</em>
5927      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
5928      * As an exception, the init functions cannot take any {@code v} parameters,
5929      * because those values are not yet computed when the init functions are executed.
5930      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
5931      * In fact, any clause function may take no arguments at all.
5932      * <p>
5933      * <em>Loop parameters:</em>
5934      * A clause function may take all the iteration variable values it is entitled to, in which case
5935      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
5936      * with their types and values notated as {@code (A...)} and {@code (a...)}.
5937      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
5938      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
5939      * init function is automatically a loop parameter {@code a}.)
5940      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
5941      * These loop parameters act as loop-invariant values visible across the whole loop.
5942      * <p>
5943      * <em>Parameters visible everywhere:</em>
5944      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
5945      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
5946      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
5947      * Most clause functions will not need all of this information, but they will be formally connected to it
5948      * as if by {@link #dropArguments}.
5949      * <a id="astar"></a>
5950      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
5951      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
5952      * In that notation, the general form of an init function parameter list
5953      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
5954      * <p>
5955      * <em>Checking clause structure:</em>
5956      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
5957      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
5958      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
5959      * met by the inputs to the loop combinator.
5960      * <p>
5961      * <em>Effectively identical sequences:</em>
5962      * <a id="effid"></a>
5963      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
5964      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
5965      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
5966      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
5967      * that longest list.
5968      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
5969      * and the same is true if more sequences of the form {@code (V... A*)} are added.
5970      * <p>
5971      * <em>Step 0: Determine clause structure.</em><ol type="a">
5972      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
5973      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
5974      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
5975      * four. Padding takes place by appending elements to the array.
5976      * <li>Clauses with all {@code null}s are disregarded.
5977      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
5978      * </ol>
5979      * <p>
5980      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
5981      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
5982      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
5983      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
5984      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
5985      * iteration variable type.
5986      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
5987      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
5988      * </ol>
5989      * <p>
5990      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
5991      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
5992      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
5993      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
5994      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
5995      * (These types will be checked in step 2, along with all the clause function types.)
5996      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
5997      * <li>All of the collected parameter lists must be effectively identical.
5998      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
5999      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
6000      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
6001      * the "internal parameter list".
6002      * </ul>
6003      * <p>
6004      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6005      * <li>Examine fini function return types, disregarding omitted fini functions.
6006      * <li>If there are no fini functions, the loop return type is {@code void}.
6007      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6008      * type.
6009      * </ol>
6010      * <p>
6011      * <em>Step 1D: Check other types.</em><ol type="a">
6012      * <li>There must be at least one non-omitted pred function.
6013      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6014      * </ol>
6015      * <p>
6016      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6017      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6018      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6019      * (Note that their parameter lists are already effectively identical to this list.)
6020      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6021      * effectively identical to the internal parameter list {@code (V... A...)}.
6022      * </ol>
6023      * <p>
6024      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6025      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6026      * type.
6027      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6028      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6029      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6030      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6031      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6032      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6033      * loop return type.
6034      * </ol>
6035      * <p>
6036      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6037      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6038      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6039      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6040      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6041      * pad out the end of the list.
6042      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6043      * </ol>
6044      * <p>
6045      * <em>Final observations.</em><ol type="a">
6046      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6047      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6048      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6049      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6050      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6051      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6052      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6053      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6054      * </ol>
6055      * <p>
6056      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6057      * <ul>
6058      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6059      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6060      * (Only one {@code Pn} has to be non-{@code null}.)
6061      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6062      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6063      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6064      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6065      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6066      * the resulting loop handle's parameter types {@code (A...)}.
6067      * </ul>
6068      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6069      * which is natural if most of the loop computation happens in the steps.  For some loops,
6070      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6071      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6072      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6073      * where the init functions will need the extra parameters.  For such reasons, the rules for
6074      * determining these parameters are as symmetric as possible, across all clause parts.
6075      * In general, the loop parameters function as common invariant values across the whole
6076      * loop, while the iteration variables function as common variant values, or (if there is
6077      * no step function) as internal loop invariant temporaries.
6078      * <p>
6079      * <em>Loop execution.</em><ol type="a">
6080      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6081      * every clause function. These locals are loop invariant.
6082      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6083      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6084      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6085      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6086      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6087      * (in argument order).
6088      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6089      * returns {@code false}.
6090      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6091      * sequence {@code (v...)} of loop variables.
6092      * The updated value is immediately visible to all subsequent function calls.
6093      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6094      * (of type {@code R}) is returned from the loop as a whole.
6095      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6096      * except by throwing an exception.
6097      * </ol>
6098      * <p>
6099      * <em>Usage tips.</em>
6100      * <ul>
6101      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6102      * sometimes a step function only needs to observe the current value of its own variable.
6103      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6104      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6105      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6106      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6107      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6108      * <li>If some of the clause functions are virtual methods on an instance, the instance
6109      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6110      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6111      * will be the first iteration variable value, and it will be easy to use virtual
6112      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6113      * </ul>
6114      * <p>
6115      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6116      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6117      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6118      * <blockquote><pre>{@code
6119      * V... init...(A...);
6120      * boolean pred...(V..., A...);
6121      * V... step...(V..., A...);
6122      * R fini...(V..., A...);
6123      * R loop(A... a) {
6124      *   V... v... = init...(a...);
6125      *   for (;;) {
6126      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6127      *       v = s(v..., a...);
6128      *       if (!p(v..., a...)) {
6129      *         return f(v..., a...);
6130      *       }
6131      *     }
6132      *   }
6133      * }
6134      * }</pre></blockquote>
6135      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6136      * to their full length, even though individual clause functions may neglect to take them all.
6137      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6138      *
6139      * @apiNote Example:
6140      * <blockquote><pre>{@code
6141      * // iterative implementation of the factorial function as a loop handle
6142      * static int one(int k) { return 1; }
6143      * static int inc(int i, int acc, int k) { return i + 1; }
6144      * static int mult(int i, int acc, int k) { return i * acc; }
6145      * static boolean pred(int i, int acc, int k) { return i < k; }
6146      * static int fin(int i, int acc, int k) { return acc; }
6147      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6148      * // null initializer for counter, should initialize to 0
6149      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6150      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6151      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6152      * assertEquals(120, loop.invoke(5));
6153      * }</pre></blockquote>
6154      * The same example, dropping arguments and using combinators:
6155      * <blockquote><pre>{@code
6156      * // simplified implementation of the factorial function as a loop handle
6157      * static int inc(int i) { return i + 1; } // drop acc, k
6158      * static int mult(int i, int acc) { return i * acc; } //drop k
6159      * static boolean cmp(int i, int k) { return i < k; }
6160      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6161      * // null initializer for counter, should initialize to 0
6162      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6163      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6164      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6165      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6166      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6167      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6168      * assertEquals(720, loop.invoke(6));
6169      * }</pre></blockquote>
6170      * A similar example, using a helper object to hold a loop parameter:
6171      * <blockquote><pre>{@code
6172      * // instance-based implementation of the factorial function as a loop handle
6173      * static class FacLoop {
6174      *   final int k;
6175      *   FacLoop(int k) { this.k = k; }
6176      *   int inc(int i) { return i + 1; }
6177      *   int mult(int i, int acc) { return i * acc; }
6178      *   boolean pred(int i) { return i < k; }
6179      *   int fin(int i, int acc) { return acc; }
6180      * }
6181      * // assume MH_FacLoop is a handle to the constructor
6182      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6183      * // null initializer for counter, should initialize to 0
6184      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6185      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6186      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6187      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6188      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6189      * assertEquals(5040, loop.invoke(7));
6190      * }</pre></blockquote>
6191      *
6192      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6193      *
6194      * @return a method handle embodying the looping behavior as defined by the arguments.
6195      *
6196      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6197      *
6198      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6199      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6200      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6201      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6202      * @since 9
6203      */
6204     public static MethodHandle loop(MethodHandle[]... clauses) {
6205         // Step 0: determine clause structure.
6206         loopChecks0(clauses);
6207 
6208         List<MethodHandle> init = new ArrayList<>();
6209         List<MethodHandle> step = new ArrayList<>();
6210         List<MethodHandle> pred = new ArrayList<>();
6211         List<MethodHandle> fini = new ArrayList<>();
6212 
6213         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6214             init.add(clause[0]); // all clauses have at least length 1
6215             step.add(clause.length <= 1 ? null : clause[1]);
6216             pred.add(clause.length <= 2 ? null : clause[2]);
6217             fini.add(clause.length <= 3 ? null : clause[3]);
6218         });
6219 
6220         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6221         final int nclauses = init.size();
6222 
6223         // Step 1A: determine iteration variables (V...).
6224         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6225         for (int i = 0; i < nclauses; ++i) {
6226             MethodHandle in = init.get(i);
6227             MethodHandle st = step.get(i);
6228             if (in == null && st == null) {
6229                 iterationVariableTypes.add(void.class);
6230             } else if (in != null && st != null) {
6231                 loopChecks1a(i, in, st);
6232                 iterationVariableTypes.add(in.type().returnType());
6233             } else {
6234                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6235             }
6236         }
6237         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
6238                 collect(Collectors.toList());
6239 
6240         // Step 1B: determine loop parameters (A...).
6241         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6242         loopChecks1b(init, commonSuffix);
6243 
6244         // Step 1C: determine loop return type.
6245         // Step 1D: check other types.
6246         // local variable required here; see JDK-8223553
6247         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6248                 .map(MethodType::returnType);
6249         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6250         loopChecks1cd(pred, fini, loopReturnType);
6251 
6252         // Step 2: determine parameter lists.
6253         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6254         commonParameterSequence.addAll(commonSuffix);
6255         loopChecks2(step, pred, fini, commonParameterSequence);
6256 
6257         // Step 3: fill in omitted functions.
6258         for (int i = 0; i < nclauses; ++i) {
6259             Class<?> t = iterationVariableTypes.get(i);
6260             if (init.get(i) == null) {
6261                 init.set(i, empty(methodType(t, commonSuffix)));
6262             }
6263             if (step.get(i) == null) {
6264                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6265             }
6266             if (pred.get(i) == null) {
6267                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
6268             }
6269             if (fini.get(i) == null) {
6270                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6271             }
6272         }
6273 
6274         // Step 4: fill in missing parameter types.
6275         // Also convert all handles to fixed-arity handles.
6276         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6277         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6278         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6279         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6280 
6281         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6282                 allMatch(pl -> pl.equals(commonSuffix));
6283         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6284                 allMatch(pl -> pl.equals(commonParameterSequence));
6285 
6286         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6287     }
6288 
6289     private static void loopChecks0(MethodHandle[][] clauses) {
6290         if (clauses == null || clauses.length == 0) {
6291             throw newIllegalArgumentException("null or no clauses passed");
6292         }
6293         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6294             throw newIllegalArgumentException("null clauses are not allowed");
6295         }
6296         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6297             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6298         }
6299     }
6300 
6301     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6302         if (in.type().returnType() != st.type().returnType()) {
6303             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6304                     st.type().returnType());
6305         }
6306     }
6307 
6308     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6309         final List<Class<?>> empty = List.of();
6310         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
6311                 // take only those that can contribute to a common suffix because they are longer than the prefix
6312                         map(MethodHandle::type).
6313                         filter(t -> t.parameterCount() > skipSize).
6314                         map(MethodType::parameterList).
6315                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6316         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
6317     }
6318 
6319     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
6320         final List<Class<?>> empty = List.of();
6321         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6322     }
6323 
6324     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6325         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6326         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6327         return longestParameterList(Arrays.asList(longest1, longest2));
6328     }
6329 
6330     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6331         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6332                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6333             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6334                     " (common suffix: " + commonSuffix + ")");
6335         }
6336     }
6337 
6338     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6339         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6340                 anyMatch(t -> t != loopReturnType)) {
6341             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6342                     loopReturnType + ")");
6343         }
6344 
6345         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
6346             throw newIllegalArgumentException("no predicate found", pred);
6347         }
6348         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6349                 anyMatch(t -> t != boolean.class)) {
6350             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6351         }
6352     }
6353 
6354     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6355         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6356                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6357             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6358                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6359         }
6360     }
6361 
6362     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6363         return hs.stream().map(h -> {
6364             int pc = h.type().parameterCount();
6365             int tpsize = targetParams.size();
6366             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
6367         }).collect(Collectors.toList());
6368     }
6369 
6370     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6371         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
6372     }
6373 
6374     /**
6375      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6376      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6377      * <p>
6378      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6379      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6380      * evaluates to {@code true}).
6381      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6382      * <p>
6383      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6384      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6385      * and updated with the value returned from its invocation. The result of loop execution will be
6386      * the final value of the additional loop-local variable (if present).
6387      * <p>
6388      * The following rules hold for these argument handles:<ul>
6389      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6390      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6391      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6392      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6393      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6394      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6395      * It will constrain the parameter lists of the other loop parts.
6396      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6397      * list {@code (A...)} is called the <em>external parameter list</em>.
6398      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6399      * additional state variable of the loop.
6400      * The body must both accept and return a value of this type {@code V}.
6401      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6402      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6403      * <a href="MethodHandles.html#effid">effectively identical</a>
6404      * to the external parameter list {@code (A...)}.
6405      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6406      * {@linkplain #empty default value}.
6407      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6408      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6409      * effectively identical to the internal parameter list.
6410      * </ul>
6411      * <p>
6412      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6413      * <li>The loop handle's result type is the result type {@code V} of the body.
6414      * <li>The loop handle's parameter types are the types {@code (A...)},
6415      * from the external parameter list.
6416      * </ul>
6417      * <p>
6418      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6419      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6420      * passed to the loop.
6421      * <blockquote><pre>{@code
6422      * V init(A...);
6423      * boolean pred(V, A...);
6424      * V body(V, A...);
6425      * V whileLoop(A... a...) {
6426      *   V v = init(a...);
6427      *   while (pred(v, a...)) {
6428      *     v = body(v, a...);
6429      *   }
6430      *   return v;
6431      * }
6432      * }</pre></blockquote>
6433      *
6434      * @apiNote Example:
6435      * <blockquote><pre>{@code
6436      * // implement the zip function for lists as a loop handle
6437      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6438      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6439      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6440      *   zip.add(a.next());
6441      *   zip.add(b.next());
6442      *   return zip;
6443      * }
6444      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6445      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6446      * List<String> a = Arrays.asList("a", "b", "c", "d");
6447      * List<String> b = Arrays.asList("e", "f", "g", "h");
6448      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6449      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6450      * }</pre></blockquote>
6451      *
6452      *
6453      * @apiNote The implementation of this method can be expressed as follows:
6454      * <blockquote><pre>{@code
6455      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6456      *     MethodHandle fini = (body.type().returnType() == void.class
6457      *                         ? null : identity(body.type().returnType()));
6458      *     MethodHandle[]
6459      *         checkExit = { null, null, pred, fini },
6460      *         varBody   = { init, body };
6461      *     return loop(checkExit, varBody);
6462      * }
6463      * }</pre></blockquote>
6464      *
6465      * @param init optional initializer, providing the initial value of the loop variable.
6466      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6467      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6468      *             above for other constraints.
6469      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6470      *             See above for other constraints.
6471      *
6472      * @return a method handle implementing the {@code while} loop as described by the arguments.
6473      * @throws IllegalArgumentException if the rules for the arguments are violated.
6474      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6475      *
6476      * @see #loop(MethodHandle[][])
6477      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6478      * @since 9
6479      */
6480     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6481         whileLoopChecks(init, pred, body);
6482         MethodHandle fini = identityOrVoid(body.type().returnType());
6483         MethodHandle[] checkExit = { null, null, pred, fini };
6484         MethodHandle[] varBody = { init, body };
6485         return loop(checkExit, varBody);
6486     }
6487 
6488     /**
6489      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6490      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6491      * <p>
6492      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6493      * method will, in each iteration, first execute its body and then evaluate the predicate.
6494      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6495      * <p>
6496      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6497      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6498      * and updated with the value returned from its invocation. The result of loop execution will be
6499      * the final value of the additional loop-local variable (if present).
6500      * <p>
6501      * The following rules hold for these argument handles:<ul>
6502      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6503      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6504      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6505      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6506      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6507      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6508      * It will constrain the parameter lists of the other loop parts.
6509      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6510      * list {@code (A...)} is called the <em>external parameter list</em>.
6511      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6512      * additional state variable of the loop.
6513      * The body must both accept and return a value of this type {@code V}.
6514      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6515      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6516      * <a href="MethodHandles.html#effid">effectively identical</a>
6517      * to the external parameter list {@code (A...)}.
6518      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6519      * {@linkplain #empty default value}.
6520      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6521      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6522      * effectively identical to the internal parameter list.
6523      * </ul>
6524      * <p>
6525      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6526      * <li>The loop handle's result type is the result type {@code V} of the body.
6527      * <li>The loop handle's parameter types are the types {@code (A...)},
6528      * from the external parameter list.
6529      * </ul>
6530      * <p>
6531      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6532      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6533      * passed to the loop.
6534      * <blockquote><pre>{@code
6535      * V init(A...);
6536      * boolean pred(V, A...);
6537      * V body(V, A...);
6538      * V doWhileLoop(A... a...) {
6539      *   V v = init(a...);
6540      *   do {
6541      *     v = body(v, a...);
6542      *   } while (pred(v, a...));
6543      *   return v;
6544      * }
6545      * }</pre></blockquote>
6546      *
6547      * @apiNote Example:
6548      * <blockquote><pre>{@code
6549      * // int i = 0; while (i < limit) { ++i; } return i; => limit
6550      * static int zero(int limit) { return 0; }
6551      * static int step(int i, int limit) { return i + 1; }
6552      * static boolean pred(int i, int limit) { return i < limit; }
6553      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
6554      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
6555      * assertEquals(23, loop.invoke(23));
6556      * }</pre></blockquote>
6557      *
6558      *
6559      * @apiNote The implementation of this method can be expressed as follows:
6560      * <blockquote><pre>{@code
6561      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6562      *     MethodHandle fini = (body.type().returnType() == void.class
6563      *                         ? null : identity(body.type().returnType()));
6564      *     MethodHandle[] clause = { init, body, pred, fini };
6565      *     return loop(clause);
6566      * }
6567      * }</pre></blockquote>
6568      *
6569      * @param init optional initializer, providing the initial value of the loop variable.
6570      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6571      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6572      *             See above for other constraints.
6573      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6574      *             above for other constraints.
6575      *
6576      * @return a method handle implementing the {@code while} loop as described by the arguments.
6577      * @throws IllegalArgumentException if the rules for the arguments are violated.
6578      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6579      *
6580      * @see #loop(MethodHandle[][])
6581      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
6582      * @since 9
6583      */
6584     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6585         whileLoopChecks(init, pred, body);
6586         MethodHandle fini = identityOrVoid(body.type().returnType());
6587         MethodHandle[] clause = {init, body, pred, fini };
6588         return loop(clause);
6589     }
6590 
6591     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
6592         Objects.requireNonNull(pred);
6593         Objects.requireNonNull(body);
6594         MethodType bodyType = body.type();
6595         Class<?> returnType = bodyType.returnType();
6596         List<Class<?>> innerList = bodyType.parameterList();
6597         List<Class<?>> outerList = innerList;
6598         if (returnType == void.class) {
6599             // OK
6600         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
6601             // leading V argument missing => error
6602             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6603             throw misMatchedTypes("body function", bodyType, expected);
6604         } else {
6605             outerList = innerList.subList(1, innerList.size());
6606         }
6607         MethodType predType = pred.type();
6608         if (predType.returnType() != boolean.class ||
6609                 !predType.effectivelyIdenticalParameters(0, innerList)) {
6610             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
6611         }
6612         if (init != null) {
6613             MethodType initType = init.type();
6614             if (initType.returnType() != returnType ||
6615                     !initType.effectivelyIdenticalParameters(0, outerList)) {
6616                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6617             }
6618         }
6619     }
6620 
6621     /**
6622      * Constructs a loop that runs a given number of iterations.
6623      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6624      * <p>
6625      * The number of iterations is determined by the {@code iterations} handle evaluation result.
6626      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
6627      * It will be initialized to 0 and incremented by 1 in each iteration.
6628      * <p>
6629      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6630      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6631      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6632      * <p>
6633      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6634      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6635      * iteration variable.
6636      * The result of the loop handle execution will be the final {@code V} value of that variable
6637      * (or {@code void} if there is no {@code V} variable).
6638      * <p>
6639      * The following rules hold for the argument handles:<ul>
6640      * <li>The {@code iterations} handle must not be {@code null}, and must return
6641      * the type {@code int}, referred to here as {@code I} in parameter type lists.
6642      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6643      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6644      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6645      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6646      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6647      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6648      * of types called the <em>internal parameter list</em>.
6649      * It will constrain the parameter lists of the other loop parts.
6650      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6651      * with no additional {@code A} types, then the internal parameter list is extended by
6652      * the argument types {@code A...} of the {@code iterations} handle.
6653      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6654      * list {@code (A...)} is called the <em>external parameter list</em>.
6655      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6656      * additional state variable of the loop.
6657      * The body must both accept a leading parameter and return a value of this type {@code V}.
6658      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6659      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6660      * <a href="MethodHandles.html#effid">effectively identical</a>
6661      * to the external parameter list {@code (A...)}.
6662      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6663      * {@linkplain #empty default value}.
6664      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
6665      * effectively identical to the external parameter list {@code (A...)}.
6666      * </ul>
6667      * <p>
6668      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6669      * <li>The loop handle's result type is the result type {@code V} of the body.
6670      * <li>The loop handle's parameter types are the types {@code (A...)},
6671      * from the external parameter list.
6672      * </ul>
6673      * <p>
6674      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6675      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6676      * arguments passed to the loop.
6677      * <blockquote><pre>{@code
6678      * int iterations(A...);
6679      * V init(A...);
6680      * V body(V, int, A...);
6681      * V countedLoop(A... a...) {
6682      *   int end = iterations(a...);
6683      *   V v = init(a...);
6684      *   for (int i = 0; i < end; ++i) {
6685      *     v = body(v, i, a...);
6686      *   }
6687      *   return v;
6688      * }
6689      * }</pre></blockquote>
6690      *
6691      * @apiNote Example with a fully conformant body method:
6692      * <blockquote><pre>{@code
6693      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6694      * // => a variation on a well known theme
6695      * static String step(String v, int counter, String init) { return "na " + v; }
6696      * // assume MH_step is a handle to the method above
6697      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
6698      * MethodHandle start = MethodHandles.identity(String.class);
6699      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
6700      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
6701      * }</pre></blockquote>
6702      *
6703      * @apiNote Example with the simplest possible body method type,
6704      * and passing the number of iterations to the loop invocation:
6705      * <blockquote><pre>{@code
6706      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6707      * // => a variation on a well known theme
6708      * static String step(String v, int counter ) { return "na " + v; }
6709      * // assume MH_step is a handle to the method above
6710      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
6711      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
6712      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
6713      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
6714      * }</pre></blockquote>
6715      *
6716      * @apiNote Example that treats the number of iterations, string to append to, and string to append
6717      * as loop parameters:
6718      * <blockquote><pre>{@code
6719      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6720      * // => a variation on a well known theme
6721      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
6722      * // assume MH_step is a handle to the method above
6723      * MethodHandle count = MethodHandles.identity(int.class);
6724      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
6725      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
6726      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
6727      * }</pre></blockquote>
6728      *
6729      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
6730      * to enforce a loop type:
6731      * <blockquote><pre>{@code
6732      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6733      * // => a variation on a well known theme
6734      * static String step(String v, int counter, String pre) { return pre + " " + v; }
6735      * // assume MH_step is a handle to the method above
6736      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
6737      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
6738      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
6739      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
6740      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
6741      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
6742      * }</pre></blockquote>
6743      *
6744      * @apiNote The implementation of this method can be expressed as follows:
6745      * <blockquote><pre>{@code
6746      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6747      *     return countedLoop(empty(iterations.type()), iterations, init, body);
6748      * }
6749      * }</pre></blockquote>
6750      *
6751      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
6752      *                   result type must be {@code int}. See above for other constraints.
6753      * @param init optional initializer, providing the initial value of the loop variable.
6754      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6755      * @param body body of the loop, which may not be {@code null}.
6756      *             It controls the loop parameters and result type in the standard case (see above for details).
6757      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6758      *             and may accept any number of additional types.
6759      *             See above for other constraints.
6760      *
6761      * @return a method handle representing the loop.
6762      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
6763      * @throws IllegalArgumentException if any argument violates the rules formulated above.
6764      *
6765      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
6766      * @since 9
6767      */
6768     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6769         return countedLoop(empty(iterations.type()), iterations, init, body);
6770     }
6771 
6772     /**
6773      * Constructs a loop that counts over a range of numbers.
6774      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6775      * <p>
6776      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
6777      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
6778      * values of the loop counter.
6779      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
6780      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
6781      * <p>
6782      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6783      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6784      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6785      * <p>
6786      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6787      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6788      * iteration variable.
6789      * The result of the loop handle execution will be the final {@code V} value of that variable
6790      * (or {@code void} if there is no {@code V} variable).
6791      * <p>
6792      * The following rules hold for the argument handles:<ul>
6793      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
6794      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
6795      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6796      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6797      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6798      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6799      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6800      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6801      * of types called the <em>internal parameter list</em>.
6802      * It will constrain the parameter lists of the other loop parts.
6803      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6804      * with no additional {@code A} types, then the internal parameter list is extended by
6805      * the argument types {@code A...} of the {@code end} handle.
6806      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6807      * list {@code (A...)} is called the <em>external parameter list</em>.
6808      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6809      * additional state variable of the loop.
6810      * The body must both accept a leading parameter and return a value of this type {@code V}.
6811      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6812      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6813      * <a href="MethodHandles.html#effid">effectively identical</a>
6814      * to the external parameter list {@code (A...)}.
6815      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6816      * {@linkplain #empty default value}.
6817      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
6818      * effectively identical to the external parameter list {@code (A...)}.
6819      * <li>Likewise, the parameter list of {@code end} must be effectively identical
6820      * to the external parameter list.
6821      * </ul>
6822      * <p>
6823      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6824      * <li>The loop handle's result type is the result type {@code V} of the body.
6825      * <li>The loop handle's parameter types are the types {@code (A...)},
6826      * from the external parameter list.
6827      * </ul>
6828      * <p>
6829      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6830      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6831      * arguments passed to the loop.
6832      * <blockquote><pre>{@code
6833      * int start(A...);
6834      * int end(A...);
6835      * V init(A...);
6836      * V body(V, int, A...);
6837      * V countedLoop(A... a...) {
6838      *   int e = end(a...);
6839      *   int s = start(a...);
6840      *   V v = init(a...);
6841      *   for (int i = s; i < e; ++i) {
6842      *     v = body(v, i, a...);
6843      *   }
6844      *   return v;
6845      * }
6846      * }</pre></blockquote>
6847      *
6848      * @apiNote The implementation of this method can be expressed as follows:
6849      * <blockquote><pre>{@code
6850      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6851      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
6852      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
6853      *     // the following semantics:
6854      *     // MH_increment: (int limit, int counter) -> counter + 1
6855      *     // MH_predicate: (int limit, int counter) -> counter < limit
6856      *     Class<?> counterType = start.type().returnType();  // int
6857      *     Class<?> returnType = body.type().returnType();
6858      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
6859      *     if (returnType != void.class) {  // ignore the V variable
6860      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
6861      *         pred = dropArguments(pred, 1, returnType);  // ditto
6862      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
6863      *     }
6864      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
6865      *     MethodHandle[]
6866      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
6867      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
6868      *         indexVar   = { start, incr };           // i = start(); i = i + 1
6869      *     return loop(loopLimit, bodyClause, indexVar);
6870      * }
6871      * }</pre></blockquote>
6872      *
6873      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
6874      *              See above for other constraints.
6875      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
6876      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
6877      * @param init optional initializer, providing the initial value of the loop variable.
6878      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6879      * @param body body of the loop, which may not be {@code null}.
6880      *             It controls the loop parameters and result type in the standard case (see above for details).
6881      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6882      *             and may accept any number of additional types.
6883      *             See above for other constraints.
6884      *
6885      * @return a method handle representing the loop.
6886      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
6887      * @throws IllegalArgumentException if any argument violates the rules formulated above.
6888      *
6889      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
6890      * @since 9
6891      */
6892     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6893         countedLoopChecks(start, end, init, body);
6894         Class<?> counterType = start.type().returnType();  // int, but who's counting?
6895         Class<?> limitType   = end.type().returnType();    // yes, int again
6896         Class<?> returnType  = body.type().returnType();
6897         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
6898         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
6899         MethodHandle retv = null;
6900         if (returnType != void.class) {
6901             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
6902             pred = dropArguments(pred, 1, returnType);  // ditto
6903             retv = dropArguments(identity(returnType), 0, counterType);
6904         }
6905         body = dropArguments(body, 0, counterType);  // ignore the limit variable
6906         MethodHandle[]
6907             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
6908             bodyClause = { init, body },            // v = init(); v = body(v, i)
6909             indexVar   = { start, incr };           // i = start(); i = i + 1
6910         return loop(loopLimit, bodyClause, indexVar);
6911     }
6912 
6913     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6914         Objects.requireNonNull(start);
6915         Objects.requireNonNull(end);
6916         Objects.requireNonNull(body);
6917         Class<?> counterType = start.type().returnType();
6918         if (counterType != int.class) {
6919             MethodType expected = start.type().changeReturnType(int.class);
6920             throw misMatchedTypes("start function", start.type(), expected);
6921         } else if (end.type().returnType() != counterType) {
6922             MethodType expected = end.type().changeReturnType(counterType);
6923             throw misMatchedTypes("end function", end.type(), expected);
6924         }
6925         MethodType bodyType = body.type();
6926         Class<?> returnType = bodyType.returnType();
6927         List<Class<?>> innerList = bodyType.parameterList();
6928         // strip leading V value if present
6929         int vsize = (returnType == void.class ? 0 : 1);
6930         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
6931             // argument list has no "V" => error
6932             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6933             throw misMatchedTypes("body function", bodyType, expected);
6934         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
6935             // missing I type => error
6936             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
6937             throw misMatchedTypes("body function", bodyType, expected);
6938         }
6939         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
6940         if (outerList.isEmpty()) {
6941             // special case; take lists from end handle
6942             outerList = end.type().parameterList();
6943             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
6944         }
6945         MethodType expected = methodType(counterType, outerList);
6946         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
6947             throw misMatchedTypes("start parameter types", start.type(), expected);
6948         }
6949         if (end.type() != start.type() &&
6950             !end.type().effectivelyIdenticalParameters(0, outerList)) {
6951             throw misMatchedTypes("end parameter types", end.type(), expected);
6952         }
6953         if (init != null) {
6954             MethodType initType = init.type();
6955             if (initType.returnType() != returnType ||
6956                 !initType.effectivelyIdenticalParameters(0, outerList)) {
6957                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6958             }
6959         }
6960     }
6961 
6962     /**
6963      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
6964      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6965      * <p>
6966      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
6967      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
6968      * <p>
6969      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6970      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6971      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6972      * <p>
6973      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6974      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6975      * iteration variable.
6976      * The result of the loop handle execution will be the final {@code V} value of that variable
6977      * (or {@code void} if there is no {@code V} variable).
6978      * <p>
6979      * The following rules hold for the argument handles:<ul>
6980      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6981      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
6982      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6983      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
6984      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
6985      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
6986      * of types called the <em>internal parameter list</em>.
6987      * It will constrain the parameter lists of the other loop parts.
6988      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
6989      * with no additional {@code A} types, then the internal parameter list is extended by
6990      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
6991      * single type {@code Iterable} is added and constitutes the {@code A...} list.
6992      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
6993      * list {@code (A...)} is called the <em>external parameter list</em>.
6994      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6995      * additional state variable of the loop.
6996      * The body must both accept a leading parameter and return a value of this type {@code V}.
6997      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6998      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6999      * <a href="MethodHandles.html#effid">effectively identical</a>
7000      * to the external parameter list {@code (A...)}.
7001      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7002      * {@linkplain #empty default value}.
7003      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7004      * type {@code java.util.Iterator} or a subtype thereof.
7005      * The iterator it produces when the loop is executed will be assumed
7006      * to yield values which can be converted to type {@code T}.
7007      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7008      * effectively identical to the external parameter list {@code (A...)}.
7009      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7010      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7011      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7012      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7013      * the {@link MethodHandle#asType asType} conversion method.
7014      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7015      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7016      * </ul>
7017      * <p>
7018      * The type {@code T} may be either a primitive or reference.
7019      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7020      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7021      * as if by the {@link MethodHandle#asType asType} conversion method.
7022      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7023      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7024      * <p>
7025      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7026      * <li>The loop handle's result type is the result type {@code V} of the body.
7027      * <li>The loop handle's parameter types are the types {@code (A...)},
7028      * from the external parameter list.
7029      * </ul>
7030      * <p>
7031      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7032      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7033      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7034      * <blockquote><pre>{@code
7035      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7036      * V init(A...);
7037      * V body(V,T,A...);
7038      * V iteratedLoop(A... a...) {
7039      *   Iterator<T> it = iterator(a...);
7040      *   V v = init(a...);
7041      *   while (it.hasNext()) {
7042      *     T t = it.next();
7043      *     v = body(v, t, a...);
7044      *   }
7045      *   return v;
7046      * }
7047      * }</pre></blockquote>
7048      *
7049      * @apiNote Example:
7050      * <blockquote><pre>{@code
7051      * // get an iterator from a list
7052      * static List<String> reverseStep(List<String> r, String e) {
7053      *   r.add(0, e);
7054      *   return r;
7055      * }
7056      * static List<String> newArrayList() { return new ArrayList<>(); }
7057      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7058      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7059      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7060      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7061      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7062      * }</pre></blockquote>
7063      *
7064      * @apiNote The implementation of this method can be expressed approximately as follows:
7065      * <blockquote><pre>{@code
7066      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7067      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7068      *     Class<?> returnType = body.type().returnType();
7069      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7070      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7071      *     MethodHandle retv = null, step = body, startIter = iterator;
7072      *     if (returnType != void.class) {
7073      *         // the simple thing first:  in (I V A...), drop the I to get V
7074      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7075      *         // body type signature (V T A...), internal loop types (I V A...)
7076      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7077      *     }
7078      *     if (startIter == null)  startIter = MH_getIter;
7079      *     MethodHandle[]
7080      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7081      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7082      *     return loop(iterVar, bodyClause);
7083      * }
7084      * }</pre></blockquote>
7085      *
7086      * @param iterator an optional handle to return the iterator to start the loop.
7087      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7088      *                 See above for other constraints.
7089      * @param init optional initializer, providing the initial value of the loop variable.
7090      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7091      * @param body body of the loop, which may not be {@code null}.
7092      *             It controls the loop parameters and result type in the standard case (see above for details).
7093      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7094      *             and may accept any number of additional types.
7095      *             See above for other constraints.
7096      *
7097      * @return a method handle embodying the iteration loop functionality.
7098      * @throws NullPointerException if the {@code body} handle is {@code null}.
7099      * @throws IllegalArgumentException if any argument violates the above requirements.
7100      *
7101      * @since 9
7102      */
7103     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7104         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7105         Class<?> returnType = body.type().returnType();
7106         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7107         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7108         MethodHandle startIter;
7109         MethodHandle nextVal;
7110         {
7111             MethodType iteratorType;
7112             if (iterator == null) {
7113                 // derive argument type from body, if available, else use Iterable
7114                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7115                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7116             } else {
7117                 // force return type to the internal iterator class
7118                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7119                 startIter = iterator;
7120             }
7121             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7122             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7123 
7124             // perform the asType transforms under an exception transformer, as per spec.:
7125             try {
7126                 startIter = startIter.asType(iteratorType);
7127                 nextVal = nextRaw.asType(nextValType);
7128             } catch (WrongMethodTypeException ex) {
7129                 throw new IllegalArgumentException(ex);
7130             }
7131         }
7132 
7133         MethodHandle retv = null, step = body;
7134         if (returnType != void.class) {
7135             // the simple thing first:  in (I V A...), drop the I to get V
7136             retv = dropArguments(identity(returnType), 0, Iterator.class);
7137             // body type signature (V T A...), internal loop types (I V A...)
7138             step = swapArguments(body, 0, 1);  // swap V <-> T
7139         }
7140 
7141         MethodHandle[]
7142             iterVar    = { startIter, null, hasNext, retv },
7143             bodyClause = { init, filterArgument(step, 0, nextVal) };
7144         return loop(iterVar, bodyClause);
7145     }
7146 
7147     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7148         Objects.requireNonNull(body);
7149         MethodType bodyType = body.type();
7150         Class<?> returnType = bodyType.returnType();
7151         List<Class<?>> internalParamList = bodyType.parameterList();
7152         // strip leading V value if present
7153         int vsize = (returnType == void.class ? 0 : 1);
7154         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
7155             // argument list has no "V" => error
7156             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7157             throw misMatchedTypes("body function", bodyType, expected);
7158         } else if (internalParamList.size() <= vsize) {
7159             // missing T type => error
7160             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7161             throw misMatchedTypes("body function", bodyType, expected);
7162         }
7163         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7164         Class<?> iterableType = null;
7165         if (iterator != null) {
7166             // special case; if the body handle only declares V and T then
7167             // the external parameter list is obtained from iterator handle
7168             if (externalParamList.isEmpty()) {
7169                 externalParamList = iterator.type().parameterList();
7170             }
7171             MethodType itype = iterator.type();
7172             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7173                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7174             }
7175             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7176                 MethodType expected = methodType(itype.returnType(), externalParamList);
7177                 throw misMatchedTypes("iterator parameters", itype, expected);
7178             }
7179         } else {
7180             if (externalParamList.isEmpty()) {
7181                 // special case; if the iterator handle is null and the body handle
7182                 // only declares V and T then the external parameter list consists
7183                 // of Iterable
7184                 externalParamList = Arrays.asList(Iterable.class);
7185                 iterableType = Iterable.class;
7186             } else {
7187                 // special case; if the iterator handle is null and the external
7188                 // parameter list is not empty then the first parameter must be
7189                 // assignable to Iterable
7190                 iterableType = externalParamList.get(0);
7191                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7192                     throw newIllegalArgumentException(
7193                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7194                 }
7195             }
7196         }
7197         if (init != null) {
7198             MethodType initType = init.type();
7199             if (initType.returnType() != returnType ||
7200                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7201                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7202             }
7203         }
7204         return iterableType;  // help the caller a bit
7205     }
7206 
7207     /*non-public*/
7208     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7209         // there should be a better way to uncross my wires
7210         int arity = mh.type().parameterCount();
7211         int[] order = new int[arity];
7212         for (int k = 0; k < arity; k++)  order[k] = k;
7213         order[i] = j; order[j] = i;
7214         Class<?>[] types = mh.type().parameterArray();
7215         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7216         MethodType swapType = methodType(mh.type().returnType(), types);
7217         return permuteArguments(mh, swapType, order);
7218     }
7219 
7220     /**
7221      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7222      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7223      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7224      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7225      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7226      * {@code try-finally} handle.
7227      * <p>
7228      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7229      * The first is the exception thrown during the
7230      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7231      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7232      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7233      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7234      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7235      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7236      * <p>
7237      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7238      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7239      * two extra leading parameters:<ul>
7240      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7241      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7242      * the result from the execution of the {@code target} handle.
7243      * This parameter is not present if the {@code target} returns {@code void}.
7244      * </ul>
7245      * <p>
7246      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7247      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7248      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7249      * the cleanup.
7250      * <blockquote><pre>{@code
7251      * V target(A..., B...);
7252      * V cleanup(Throwable, V, A...);
7253      * V adapter(A... a, B... b) {
7254      *   V result = (zero value for V);
7255      *   Throwable throwable = null;
7256      *   try {
7257      *     result = target(a..., b...);
7258      *   } catch (Throwable t) {
7259      *     throwable = t;
7260      *     throw t;
7261      *   } finally {
7262      *     result = cleanup(throwable, result, a...);
7263      *   }
7264      *   return result;
7265      * }
7266      * }</pre></blockquote>
7267      * <p>
7268      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7269      * be modified by execution of the target, and so are passed unchanged
7270      * from the caller to the cleanup, if it is invoked.
7271      * <p>
7272      * The target and cleanup must return the same type, even if the cleanup
7273      * always throws.
7274      * To create such a throwing cleanup, compose the cleanup logic
7275      * with {@link #throwException throwException},
7276      * in order to create a method handle of the correct return type.
7277      * <p>
7278      * Note that {@code tryFinally} never converts exceptions into normal returns.
7279      * In rare cases where exceptions must be converted in that way, first wrap
7280      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7281      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7282      * <p>
7283      * It is recommended that the first parameter type of {@code cleanup} be
7284      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7285      * {@code cleanup} will always be invoked with whatever exception that
7286      * {@code target} throws.  Declaring a narrower type may result in a
7287      * {@code ClassCastException} being thrown by the {@code try-finally}
7288      * handle if the type of the exception thrown by {@code target} is not
7289      * assignable to the first parameter type of {@code cleanup}.  Note that
7290      * various exception types of {@code VirtualMachineError},
7291      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7292      * thrown by almost any kind of Java code, and a finally clause that
7293      * catches (say) only {@code IOException} would mask any of the others
7294      * behind a {@code ClassCastException}.
7295      *
7296      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7297      * @param cleanup the handle that is invoked in the finally block.
7298      *
7299      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7300      * @throws NullPointerException if any argument is null
7301      * @throws IllegalArgumentException if {@code cleanup} does not accept
7302      *          the required leading arguments, or if the method handle types do
7303      *          not match in their return types and their
7304      *          corresponding trailing parameters
7305      *
7306      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7307      * @since 9
7308      */
7309     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7310         List<Class<?>> targetParamTypes = target.type().parameterList();
7311         Class<?> rtype = target.type().returnType();
7312 
7313         tryFinallyChecks(target, cleanup);
7314 
7315         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7316         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7317         // target parameter list.
7318         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
7319 
7320         // Ensure that the intrinsic type checks the instance thrown by the
7321         // target against the first parameter of cleanup
7322         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7323 
7324         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7325         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7326     }
7327 
7328     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7329         Class<?> rtype = target.type().returnType();
7330         if (rtype != cleanup.type().returnType()) {
7331             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7332         }
7333         MethodType cleanupType = cleanup.type();
7334         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7335             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7336         }
7337         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7338             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7339         }
7340         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7341         // target parameter list.
7342         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7343         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7344             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7345                     cleanup.type(), target.type());
7346         }
7347     }
7348 
7349 }
--- EOF ---