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 {@jmvs 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 and links 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 IllegalArgumentException the bytes are for a class in a different package
1663          * to the lookup class
1664          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1665          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
1666          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
1667          * @throws SecurityException if a security manager is present and it
1668          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1669          * @throws NullPointerException if {@code bytes} is {@code null}
1670          * @since 9
1671          * @spec JPMS
1672          * @see Lookup#privateLookupIn
1673          * @see Lookup#dropLookupMode
1674          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1675          */
1676         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1677             ensureDefineClassPermission();
1678             if ((lookupModes() & PACKAGE) == 0)
1679                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1680             return makeClassDefiner(bytes.clone()).defineClass(false);
1681         }
1682 
1683         private void ensureDefineClassPermission() {
1684             if (allowedModes == TRUSTED)  return;
1685 
1686             if (!hasFullPrivilegeAccess()) {
1687                 SecurityManager sm = System.getSecurityManager();
1688                 if (sm != null)
1689                     sm.checkPermission(new RuntimePermission("defineClass"));
1690             }
1691         }
1692 
1693         /**
1694          * Creates a <em>hidden</em> class or interface from {@code bytes},
1695          * returning a {@code Lookup} on the newly created class or interface.
1696          *
1697          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1698          * which either defines {@code C} directly or delegates to another class loader.
1699          * A class loader defines {@code C} directly by invoking
1700          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1701          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1702          * to derive {@code C} from a purported representation in {@code class} file format.
1703          * In situations where use of a class loader is undesirable, a class or interface
1704          * {@code C} can be created by this method instead. This method is capable of
1705          * defining {@code C}, and thereby creating it, without invoking
1706          * {@code ClassLoader::defineClass}.
1707          * Instead, this method defines {@code C} as if by arranging for
1708          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1709          * from a purported representation in {@code class} file format
1710          * using the following rules:
1711          *
1712          * <ol>
1713          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1714          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1715          * This level of access is needed to create {@code C} in the module
1716          * of the lookup class of this {@code Lookup}.</li>
1717          *
1718          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1719          * structure of a supported major and minor version. The major and minor version
1720          * may differ from the {@code class} file version of the lookup class of this
1721          * {@code Lookup}.</li>
1722          *
1723          * <li> The value of {@code this_class} must be a valid index in the
1724          * {@code constant_pool} table, and the entry at that index must be a valid
1725          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1726          * encoded in internal form that is specified by this structure. {@code N} must
1727          * denote a class or interface in the same package as the lookup class.</li>
1728          *
1729          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1730          * where {@code <suffix>} is an unqualified name that is guaranteed to be unique
1731          * during this execution of the JVM.
1732          *
1733          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1734          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1735          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1736          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1737          * refers to the new {@code CONSTANT_Utf8_info} structure.
1738          *
1739          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1740          *
1741          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1742          * purported representation {@code newBytes} as if by the rules of JVMS 5.3.5,
1743          * with the following adjustments:
1744          * <ul>
1745          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1746          * that includes a single {@code "."} character, even though this is not a valid
1747          * binary class or interface name in internal form.</li>
1748          *
1749          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1750          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1751          *
1752          * <li> {@code C} is considered to have the same runtime package and
1753          * {@linkplain java.security.ProtectionDomain protection domain}
1754          * as the lookup class of this {@code Lookup}.
1755          *
1756          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1757          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1758          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C},
1759          * {@link Class#getName()} returns the string {@code GN + "/" + <suffix>}, even though
1760          * this is not a valid binary class or interface name.</li>
1761          * </ul>
1762          * </li>
1763          * </ol>
1764          *
1765          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
1766          * Linkage occurs as specified in JVMS 5.4.3, with the following adjustments:
1767          * <ul>
1768          * <li> During verification, whenever it is necessary to load the class named
1769          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
1770          * made of any class loader.</li>
1771          *
1772          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
1773          * by {@code this_class}, the symbolic reference is considered to be resolved to
1774          * {@code C} and resolution always succeeds immediately.</li>
1775          * </ul>
1776          *
1777          * <p> If the {@code initialize} parameter is {@code true},
1778          * then {@code C} is initialized by the Java Virtual Machine.
1779          *
1780          * <p> The newly created class or interface {@code C} is <em>hidden</em>, in the sense that
1781          * no other class or interface can refer to {@code C} via a constant pool entry.
1782          * That is, a hidden class or interface cannot be named as a supertype, a field type,
1783          * a method parameter type, or a method return type by any other class.
1784          * This is because a hidden class or interface does not have a binary name, so
1785          * there is no internal form available to record in any class's constant pool.
1786          * (Given the {@code Lookup} object returned this method, its lookup class
1787          * is a {@code Class} object for which {@link Class#getName()} returns a string
1788          * that is not a binary name.)
1789          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
1790          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
1791          * is not {@linkplain java.lang.instrument.Instrumentation#isModifiableClass(Class)
1792          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
1793          * JVM Tool Interface</a>.
1794          *
1795          * <p> A class or interface created by
1796          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1797          * a class loader} has a strong relationship with that class loader.
1798          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
1799          * that {@linkplain Class#getClassLoader() defined it}.
1800          * This means that a class created by a class loader may be unloaded if and
1801          * only if its defining loader is not reachable and thus may be reclaimed
1802          * by a garbage collector (JLS 12.7).
1803          *
1804          * By default, however, a hidden class or interface may be unloaded even if
1805          * the class loader that is marked as its defining loader is
1806          * <a href="../ref/package.html#reachability">reachable</a>.
1807          * This behavior is useful when a hidden class or interface serves multiple
1808          * classes defined by arbitrary class loaders.  In other cases, a hidden
1809          * class or interface may be linked to a single class (or a small number of classes)
1810          * with the same defining loader as the hidden class or interface.
1811          * In such cases, where the hidden class or interface must be coterminous
1812          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
1813          * option may be passed in {@code options}.
1814          * This arranges for a hidden class to have the same strong relationship
1815          * with the class loader marked as its defining loader,
1816          * as a normal class or interface has with its own defining loader.
1817          *
1818          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
1819          * may still prevent a hidden class or interface from being
1820          * unloaded by ensuring that the {@code Class} object is reachable.
1821          *
1822          * <p> The unloading characteristics are set for each hidden class when it is
1823          * defined, and cannot be changed later.  An advantage of allowing hidden classes
1824          * to be unloaded independently of the class loader marked as their defining loader
1825          * is that a very large number of hidden classes may be created by an application.
1826          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
1827          * just as if normal classes were created by class loaders.
1828          *
1829          * <p> Classes and interfaces in a nest are allowed to have mutual access to
1830          * their private members.  The nest relationship is determined by
1831          * the {@code NestHost} attribute (JVMS 4.7.28) and
1832          * the {@code NestMembers} attribute (JVMS 4.7.29) in a {@code class} file.
1833          * By default, a hidden class belongs to a nest consisting only of itself
1834          * because a hidden class has no binary name.
1835          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
1836          * to create a hidden class or interface {@code C} as a member of a nest.
1837          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
1838          * in the {@code ClassFile} structure from which {@code C} was derived.
1839          * Instead, the following rules determine the nest host of {@code C}:
1840          * <ul>
1841          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
1842          *     been determined, then let {@code H} be the nest host of the lookup class.
1843          *     Otherwise, the nest host of the lookup class is determined using the
1844          *     algorithm in JVMS 5.4.4, yielding {@code H}.</li>
1845          * <li>The nest host of {@code C} is determined to be {@code H},
1846          *     the nest host of the lookup class.</li>
1847          * </ul>
1848          *
1849          * <p> A hidden class or interface may be serializable, but this requires a custom
1850          * serialization mechanism in order to ensure that instances are properly serialized
1851          * and deserialized. The default serialization mechanism supports only classes and
1852          * interfaces that are discoverable by their class name.
1853          *
1854          * @param bytes the bytes that make up the class data,
1855          * in the format of a valid {@code class} file as defined by
1856          * <cite>The Java Virtual Machine Specification</cite>.
1857          * @param initialize if {@code true} the class will be initialized.
1858          * @param options {@linkplain ClassOption class options}
1859          * @return the {@code Lookup} object on the hidden class
1860          *
1861          * @throws IllegalAccessException if this {@code Lookup} does not have
1862          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1863          * @throws SecurityException if a security manager is present and it
1864          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1865          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1866          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1867          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
1868          * {@bytes} denotes a class in a different package than the lookup class
1869          * @throws IncompatibleClassChangeError if the class or interface named as
1870          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1871          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1872          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
1873          * {@code C} is {@code C} itself
1874          * @throws VerifyError if the newly created class cannot be verified
1875          * @throws LinkageError if the newly created class cannot be linked for any other reason
1876          * @throws NullPointerException if any parameter is {@code null}
1877          *
1878          * @since 15
1879          * @see Class#isHiddenClass()
1880          * @jvms 4.2.1 Binary Class and Interface Names
1881          * @jvms 4.2.2 Unqualified Names
1882          * @jvms 4.7.28 The {@code NestHost} Attribute
1883          * @jvms 4.7.29 The {@code NestMembers} Attribute
1884          * @jvms 5.4.3.1 Class and Interface Resolution
1885          * @jvms 5.4.4 Access Control
1886          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
1887          * @jvms 5.4 Linking
1888          * @jvms 5.5 Initialization
1889          * @jls 12.7 Unloading of Classes and Interfaces
1890          */
1891         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
1892                 throws IllegalAccessException
1893         {
1894             Objects.requireNonNull(bytes);
1895             Objects.requireNonNull(options);
1896 
1897             ensureDefineClassPermission();
1898             if (!hasFullPrivilegeAccess()) {
1899                 throw new IllegalAccessException(this + " does not have full privilege access");
1900             }
1901 
1902             Set<ClassOption> opts = options.length > 0 ? Set.of(options) : Set.of();
1903             return makeHiddenClassDefiner(bytes.clone(), opts, false).defineClassAsLookup(initialize);
1904         }
1905 
1906         /**
1907          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
1908          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
1909          * returning a {@code Lookup} on the newly created class or interface.
1910          *
1911          * <p> This method is equivalent to calling
1912          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, true, options)}
1913          * as if the hidden class has a private static final unnamed field whose value
1914          * is initialized to {@code classData} right before the class initializer is
1915          * executed.  The newly created class is linked and initialized by the Java
1916          * Virtual Machine.
1917          *
1918          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
1919          * method can be used to retrieve the {@code classData}.
1920          *
1921          * @param bytes     the class bytes
1922          * @param classData pre-initialized class data
1923          * @param options   {@linkplain ClassOption class options}
1924          * @return the {@code Lookup} object on the hidden class
1925          *
1926          * @throws IllegalAccessException if this {@code Lookup} does not have
1927          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1928          * @throws SecurityException if a security manager is present and it
1929          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1930          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1931          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1932          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
1933          * {@bytes} denotes a class in a different package than the lookup class
1934          * @throws IncompatibleClassChangeError if the class or interface named as
1935          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1936          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1937          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
1938          * {@code C} is {@code C} itself
1939          * @throws VerifyError if the newly created class cannot be verified
1940          * @throws LinkageError if the newly created class cannot be linked for any other reason
1941          * @throws NullPointerException if any parameter is {@code null}
1942          *
1943          * @since 15
1944          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1945          * @see Class#isHiddenClass()
1946          */
1947         /* package-private */ Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, ClassOption... options)
1948                 throws IllegalAccessException
1949         {
1950             Objects.requireNonNull(bytes);
1951             Objects.requireNonNull(classData);
1952             Objects.requireNonNull(options);
1953 
1954             ensureDefineClassPermission();
1955             if (!hasFullPrivilegeAccess()) {
1956                 throw new IllegalAccessException(this + " does not have full privilege access");
1957             }
1958 
1959             Set<ClassOption> opts = options.length > 0 ? Set.of(options) : Set.of();
1960             return makeHiddenClassDefiner(bytes.clone(), opts, false)
1961                        .defineClassAsLookup(true, classData);
1962         }
1963 
1964         private ClassDefiner makeClassDefiner(byte[] bytes) {
1965             return new ClassDefiner(this, bytes, STRONG_LOADER_LINK);
1966         }
1967 
1968         /**
1969          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
1970          * from the given bytes.
1971          *
1972          * @param bytes   class bytes
1973          * @return ClassDefiner that defines a hidden class of the given bytes.
1974          */
1975         ClassDefiner makeHiddenClassDefiner(byte[] bytes) {
1976             return makeHiddenClassDefiner(bytes, Set.of(), false);
1977         }
1978 
1979         /**
1980          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
1981          * from the given bytes.
1982          *
1983          * @param name    fully-qualified name that specifies the prefix of the hidden class
1984          * @param bytes   class bytes
1985          * @return ClassDefiner that defines a hidden class of the given bytes.
1986          */
1987         ClassDefiner makeHiddenClassDefiner(String name, byte[] bytes) {
1988             return makeHiddenClassDefiner(name, bytes, Set.of(), false);
1989         }
1990 
1991         /**
1992          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
1993          * from the given bytes and options.  This method will read the class file
1994          * and obtain the class name.
1995          *
1996          * @param bytes   class bytes
1997          * @param options class options
1998          * @param accessVmAnnotations true to give the hidden class access to VM annotations
1999          * @return ClassDefiner that defines a hidden class of the given bytes and options
2000          */
2001         ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2002                                             Set<ClassOption> options,
2003                                             boolean accessVmAnnotations) {
2004             int flags = HIDDEN_CLASS | ClassOption.optionsToFlag(options);
2005             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2006                 // jdk.internal.vm.annotations are permitted for classes
2007                 // defined to boot loader and platform loader
2008                 flags |= ACCESS_VM_ANNOTATIONS;
2009             }
2010 
2011             return new ClassDefiner(this, bytes, flags);
2012         }
2013 
2014         /**
2015          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2016          * from the given bytes and options.
2017          *
2018          * @param name the name of the class and the name in the class bytes is ignored.
2019          * @param bytes class bytes
2020          * @param options class options
2021          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2022          */
2023         ClassDefiner makeHiddenClassDefiner(String name,
2024                                             byte[] bytes,
2025                                             Set<ClassOption> options,
2026                                             boolean accessVmAnnotations) {
2027             int flags = HIDDEN_CLASS | ClassOption.optionsToFlag(options);
2028             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2029                 // jdk.internal.vm.annotations are permitted for classes
2030                 // defined to boot loader and platform loader
2031                 flags |= ACCESS_VM_ANNOTATIONS;
2032             }
2033 
2034             return new ClassDefiner(this, name, bytes, flags);
2035         }
2036 
2037         static class ClassDefiner {
2038             private final Lookup lookup;
2039             private final String name;
2040             private final byte[] bytes;
2041             private final int classFlags;
2042 
2043             // caller should make a defensive copy of the arguments if needed
2044             // before calling this constructor
2045             private ClassDefiner(Lookup lookup, byte[] bytes, int flags) {
2046                 // defining an ordinary class which must be a strongly referenced by its defining loader
2047                 assert ((flags & HIDDEN_CLASS) != 0 || (flags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2048                 this.lookup = lookup;
2049                 this.bytes = bytes;
2050                 this.classFlags = flags;
2051                 this.name = className(bytes);
2052 
2053                 int index = name.lastIndexOf('.');
2054                 String pn = (index == -1) ? "" : name.substring(0, index);
2055                 if (!pn.equals(lookup.lookupClass().getPackageName())) {
2056                     throw newIllegalArgumentException(name + " not in same package as lookup class: " +
2057                             lookup.lookupClass().getName());
2058                 }
2059             }
2060 
2061             // skip package name check
2062             private ClassDefiner(Lookup lookup, String name, byte[] bytes, int flags) {
2063                 assert ((flags & HIDDEN_CLASS) != 0 || (flags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2064                 this.lookup = lookup;
2065                 this.bytes = bytes;
2066                 this.classFlags = flags;
2067                 this.name = name;
2068             }
2069 
2070             String className() {
2071                 return name;
2072             }
2073 
2074             Class<?> defineClass(boolean initialize) {
2075                 return defineClass(initialize, null);
2076             }
2077 
2078             Lookup defineClassAsLookup(boolean initialize) {
2079                 Class<?> c = defineClass(initialize, null);
2080                 return new Lookup(c, null, FULL_POWER_MODES);
2081             }
2082 
2083             /**
2084              * Defines the class of the given bytes and the given classData.
2085              * If {@code initialize} parameter is true, then the class will be initialized.
2086              *
2087              * @param initialize true if the class to be initialized
2088              * @param classData classData or null
2089              * @return the class
2090              *
2091              * @throws LinkageError linkage error
2092              */
2093             Class<?> defineClass(boolean initialize, Object classData) {
2094                 Class<?> lookupClass = lookup.lookupClass();
2095                 ClassLoader loader = lookupClass.getClassLoader();
2096                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2097                 Class<?> c = JLA.defineClass(loader, lookupClass, name, bytes, pd, initialize, classFlags, classData);
2098                 assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2099                 return c;
2100             }
2101 
2102             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2103                 // initialize must be true if classData is non-null
2104                 assert classData == null || initialize == true;
2105                 Class<?> c = defineClass(initialize, classData);
2106                 return new Lookup(c, null, FULL_POWER_MODES);
2107             }
2108 
2109             private boolean isNestmate() {
2110                 return (classFlags & NESTMATE_CLASS) != 0;
2111             }
2112 
2113             private static String className(byte[] bytes) {
2114                 try {
2115                     ClassReader reader = new ClassReader(bytes);
2116                     if ((reader.getAccess() & Opcodes.ACC_MODULE) != 0) {
2117                         throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2118                     }
2119                     String name = reader.getClassName();
2120                     return name.replace('/', '.');
2121                 } catch (IllegalArgumentException e) {
2122                     throw e;
2123                 } catch (RuntimeException e) {
2124                     // ASM exceptions are poorly specified
2125                     ClassFormatError cfe = new ClassFormatError();
2126                     cfe.initCause(e);
2127                     throw cfe;
2128                 }
2129             }
2130         }
2131 
2132         private ProtectionDomain lookupClassProtectionDomain() {
2133             ProtectionDomain pd = cachedProtectionDomain;
2134             if (pd == null) {
2135                 cachedProtectionDomain = pd = JLA.protectionDomain(lookupClass);
2136             }
2137             return pd;
2138         }
2139 
2140         // cached protection domain
2141         private volatile ProtectionDomain cachedProtectionDomain;
2142 
2143         // Make sure outer class is initialized first.
2144         static { IMPL_NAMES.getClass(); }
2145 
2146         /** Package-private version of lookup which is trusted. */
2147         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2148 
2149         /** Version of lookup which is trusted minimally.
2150          *  It can only be used to create method handles to publicly accessible
2151          *  members in packages that are exported unconditionally.
2152          */
2153         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2154 
2155         static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
2156 
2157         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2158             String name = lookupClass.getName();
2159             if (name.startsWith("java.lang.invoke."))
2160                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2161         }
2162 
2163         /**
2164          * Displays the name of the class from which lookups are to be made.
2165          * followed with "/" and the name of the {@linkplain #previousLookupClass()
2166          * previous lookup class} if present.
2167          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2168          * If there are restrictions on the access permitted to this lookup,
2169          * this is indicated by adding a suffix to the class name, consisting
2170          * of a slash and a keyword.  The keyword represents the strongest
2171          * allowed access, and is chosen as follows:
2172          * <ul>
2173          * <li>If no access is allowed, the suffix is "/noaccess".
2174          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2175          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2176          * <li>If only public and module access are allowed, the suffix is "/module".
2177          * <li>If public and package access are allowed, the suffix is "/package".
2178          * <li>If public, package, and private access are allowed, the suffix is "/private".
2179          * </ul>
2180          * If none of the above cases apply, it is the case that full access
2181          * (public, module, package, private, and protected) is allowed.
2182          * In this case, no suffix is added.
2183          * This is true only of an object obtained originally from
2184          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2185          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2186          * always have restricted access, and will display a suffix.
2187          * <p>
2188          * (It may seem strange that protected access should be
2189          * stronger than private access.  Viewed independently from
2190          * package access, protected access is the first to be lost,
2191          * because it requires a direct subclass relationship between
2192          * caller and callee.)
2193          * @see #in
2194          *
2195          * @revised 9
2196          * @spec JPMS
2197          */
2198         @Override
2199         public String toString() {
2200             String cname = lookupClass.getName();
2201             if (prevLookupClass != null)
2202                 cname += "/" + prevLookupClass.getName();
2203             switch (allowedModes) {
2204             case 0:  // no privileges
2205                 return cname + "/noaccess";
2206             case UNCONDITIONAL:
2207                 return cname + "/publicLookup";
2208             case PUBLIC:
2209                 return cname + "/public";
2210             case PUBLIC|MODULE:
2211                 return cname + "/module";
2212             case PUBLIC|PACKAGE:
2213             case PUBLIC|MODULE|PACKAGE:
2214                 return cname + "/package";
2215             case FULL_POWER_MODES & (~PROTECTED):
2216             case FULL_POWER_MODES & ~(PROTECTED|MODULE):
2217                 return cname + "/private";
2218             case FULL_POWER_MODES:
2219             case FULL_POWER_MODES & (~MODULE):
2220                 return cname;
2221             case TRUSTED:
2222                 return "/trusted";  // internal only; not exported
2223             default:  // Should not happen, but it's a bitfield...
2224                 cname = cname + "/" + Integer.toHexString(allowedModes);
2225                 assert(false) : cname;
2226                 return cname;
2227             }
2228         }
2229 
2230         /**
2231          * Produces a method handle for a static method.
2232          * The type of the method handle will be that of the method.
2233          * (Since static methods do not take receivers, there is no
2234          * additional receiver argument inserted into the method handle type,
2235          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2236          * The method and all its argument types must be accessible to the lookup object.
2237          * <p>
2238          * The returned method handle will have
2239          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2240          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2241          * <p>
2242          * If the returned method handle is invoked, the method's class will
2243          * be initialized, if it has not already been initialized.
2244          * <p><b>Example:</b>
2245          * <blockquote><pre>{@code
2246 import static java.lang.invoke.MethodHandles.*;
2247 import static java.lang.invoke.MethodType.*;
2248 ...
2249 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2250   "asList", methodType(List.class, Object[].class));
2251 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2252          * }</pre></blockquote>
2253          * @param refc the class from which the method is accessed
2254          * @param name the name of the method
2255          * @param type the type of the method
2256          * @return the desired method handle
2257          * @throws NoSuchMethodException if the method does not exist
2258          * @throws IllegalAccessException if access checking fails,
2259          *                                or if the method is not {@code static},
2260          *                                or if the method's variable arity modifier bit
2261          *                                is set and {@code asVarargsCollector} fails
2262          * @throws    SecurityException if a security manager is present and it
2263          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2264          * @throws NullPointerException if any argument is null
2265          */
2266         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2267             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2268             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2269         }
2270 
2271         /**
2272          * Produces a method handle for a virtual method.
2273          * The type of the method handle will be that of the method,
2274          * with the receiver type (usually {@code refc}) prepended.
2275          * The method and all its argument types must be accessible to the lookup object.
2276          * <p>
2277          * When called, the handle will treat the first argument as a receiver
2278          * and, for non-private methods, dispatch on the receiver's type to determine which method
2279          * implementation to enter.
2280          * For private methods the named method in {@code refc} will be invoked on the receiver.
2281          * (The dispatching action is identical with that performed by an
2282          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2283          * <p>
2284          * The first argument will be of type {@code refc} if the lookup
2285          * class has full privileges to access the member.  Otherwise
2286          * the member must be {@code protected} and the first argument
2287          * will be restricted in type to the lookup class.
2288          * <p>
2289          * The returned method handle will have
2290          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2291          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2292          * <p>
2293          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2294          * instructions and method handles produced by {@code findVirtual},
2295          * if the class is {@code MethodHandle} and the name string is
2296          * {@code invokeExact} or {@code invoke}, the resulting
2297          * method handle is equivalent to one produced by
2298          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2299          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2300          * with the same {@code type} argument.
2301          * <p>
2302          * If the class is {@code VarHandle} and the name string corresponds to
2303          * the name of a signature-polymorphic access mode method, the resulting
2304          * method handle is equivalent to one produced by
2305          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2306          * the access mode corresponding to the name string and with the same
2307          * {@code type} arguments.
2308          * <p>
2309          * <b>Example:</b>
2310          * <blockquote><pre>{@code
2311 import static java.lang.invoke.MethodHandles.*;
2312 import static java.lang.invoke.MethodType.*;
2313 ...
2314 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2315   "concat", methodType(String.class, String.class));
2316 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2317   "hashCode", methodType(int.class));
2318 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2319   "hashCode", methodType(int.class));
2320 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2321 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2322 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2323 // interface method:
2324 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2325   "subSequence", methodType(CharSequence.class, int.class, int.class));
2326 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2327 // constructor "internal method" must be accessed differently:
2328 MethodType MT_newString = methodType(void.class); //()V for new String()
2329 try { assertEquals("impossible", lookup()
2330         .findVirtual(String.class, "<init>", MT_newString));
2331  } catch (NoSuchMethodException ex) { } // OK
2332 MethodHandle MH_newString = publicLookup()
2333   .findConstructor(String.class, MT_newString);
2334 assertEquals("", (String) MH_newString.invokeExact());
2335          * }</pre></blockquote>
2336          *
2337          * @param refc the class or interface from which the method is accessed
2338          * @param name the name of the method
2339          * @param type the type of the method, with the receiver argument omitted
2340          * @return the desired method handle
2341          * @throws NoSuchMethodException if the method does not exist
2342          * @throws IllegalAccessException if access checking fails,
2343          *                                or if the method is {@code static},
2344          *                                or if the method's variable arity modifier bit
2345          *                                is set and {@code asVarargsCollector} fails
2346          * @throws    SecurityException if a security manager is present and it
2347          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2348          * @throws NullPointerException if any argument is null
2349          */
2350         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2351             if (refc == MethodHandle.class) {
2352                 MethodHandle mh = findVirtualForMH(name, type);
2353                 if (mh != null)  return mh;
2354             } else if (refc == VarHandle.class) {
2355                 MethodHandle mh = findVirtualForVH(name, type);
2356                 if (mh != null)  return mh;
2357             }
2358             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2359             MemberName method = resolveOrFail(refKind, refc, name, type);
2360             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2361         }
2362         private MethodHandle findVirtualForMH(String name, MethodType type) {
2363             // these names require special lookups because of the implicit MethodType argument
2364             if ("invoke".equals(name))
2365                 return invoker(type);
2366             if ("invokeExact".equals(name))
2367                 return exactInvoker(type);
2368             assert(!MemberName.isMethodHandleInvokeName(name));
2369             return null;
2370         }
2371         private MethodHandle findVirtualForVH(String name, MethodType type) {
2372             try {
2373                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2374             } catch (IllegalArgumentException e) {
2375                 return null;
2376             }
2377         }
2378 
2379         /**
2380          * Produces a method handle which creates an object and initializes it, using
2381          * the constructor of the specified type.
2382          * The parameter types of the method handle will be those of the constructor,
2383          * while the return type will be a reference to the constructor's class.
2384          * The constructor and all its argument types must be accessible to the lookup object.
2385          * <p>
2386          * The requested type must have a return type of {@code void}.
2387          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2388          * <p>
2389          * The returned method handle will have
2390          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2391          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2392          * <p>
2393          * If the returned method handle is invoked, the constructor's class will
2394          * be initialized, if it has not already been initialized.
2395          * <p><b>Example:</b>
2396          * <blockquote><pre>{@code
2397 import static java.lang.invoke.MethodHandles.*;
2398 import static java.lang.invoke.MethodType.*;
2399 ...
2400 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2401   ArrayList.class, methodType(void.class, Collection.class));
2402 Collection orig = Arrays.asList("x", "y");
2403 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2404 assert(orig != copy);
2405 assertEquals(orig, copy);
2406 // a variable-arity constructor:
2407 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2408   ProcessBuilder.class, methodType(void.class, String[].class));
2409 ProcessBuilder pb = (ProcessBuilder)
2410   MH_newProcessBuilder.invoke("x", "y", "z");
2411 assertEquals("[x, y, z]", pb.command().toString());
2412          * }</pre></blockquote>
2413          * @param refc the class or interface from which the method is accessed
2414          * @param type the type of the method, with the receiver argument omitted, and a void return type
2415          * @return the desired method handle
2416          * @throws NoSuchMethodException if the constructor does not exist
2417          * @throws IllegalAccessException if access checking fails
2418          *                                or if the method's variable arity modifier bit
2419          *                                is set and {@code asVarargsCollector} fails
2420          * @throws    SecurityException if a security manager is present and it
2421          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2422          * @throws NullPointerException if any argument is null
2423          */
2424         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2425             if (refc.isArray()) {
2426                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2427             }
2428             String name = "<init>";
2429             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2430             return getDirectConstructor(refc, ctor);
2431         }
2432 
2433         /**
2434          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2435          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2436          * Such a resolution, as specified in JVMS 5.4.3.1 section, attempts to locate and load the class,
2437          * and then determines whether the class is accessible to this lookup object.
2438          * <p>
2439          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2440          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2441          *
2442          * @param targetName the fully qualified name of the class to be looked up.
2443          * @return the requested class.
2444          * @throws SecurityException if a security manager is present and it
2445          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2446          * @throws LinkageError if the linkage fails
2447          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2448          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2449          * modes.
2450          * @since 9
2451          * @jvms 5.4.3.1 Class and Interface Resolution
2452          */
2453         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2454             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2455             return accessClass(targetClass);
2456         }
2457 
2458         /**
2459          * Determines if a class can be accessed from the lookup context defined by
2460          * this {@code Lookup} object. The static initializer of the class is not run.
2461          * <p>
2462          * If the {@code targetClass} is in the same module as the lookup class,
2463          * the lookup class is {@code LC} in module {@code M1} and
2464          * the previous lookup class is in module {@code M0} or
2465          * {@code null} if not present,
2466          * {@code targetClass} is accessible if and only if one of the following is true:
2467          * <ul>
2468          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2469          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2470          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2471          *     in the same runtime package of {@code LC}.</li>
2472          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2473          *     a public type in {@code M1}.</li>
2474          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2475          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2476          *     if the previous lookup class is present; otherwise, {@code targetClass}
2477          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2478          * </ul>
2479          *
2480          * <p>
2481          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2482          * can access public types in all modules when the type is in a package
2483          * that is exported unconditionally.
2484          * <p>
2485          * Otherwise, the target class is in a different module from {@code lookupClass},
2486          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2487          * is inaccessible.
2488          * <p>
2489          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2490          * {@code M1} is the module containing {@code lookupClass} and
2491          * {@code M2} is the module containing {@code targetClass},
2492          * then {@code targetClass} is accessible if and only if
2493          * <ul>
2494          * <li>{@code M1} reads {@code M2}, and
2495          * <li>{@code targetClass} is public and in a package exported by
2496          *     {@code M2} at least to {@code M1}.
2497          * </ul>
2498          * <p>
2499          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2500          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2501          * containing the previous lookup class, then {@code targetClass} is accessible
2502          * if and only if one of the following is true:
2503          * <ul>
2504          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2505          *     {@linkplain Module#reads reads} {@code M0} and the type is
2506          *     in a package that is exported to at least {@code M1}.
2507          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2508          *     {@linkplain Module#reads reads} {@code M1} and the type is
2509          *     in a package that is exported to at least {@code M0}.
2510          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2511          *     and {@code M1} reads {@code M2} and the type is in a package
2512          *     that is exported to at least both {@code M0} and {@code M2}.
2513          * </ul>
2514          * <p>
2515          * Otherwise, {@code targetClass} is not accessible.
2516          *
2517          * @param targetClass the class to be access-checked
2518          * @return the class that has been access-checked
2519          * @throws IllegalAccessException if the class is not accessible from the lookup class
2520          * and previous lookup class, if present, using the allowed access modes.
2521          * @throws    SecurityException if a security manager is present and it
2522          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2523          * @since 9
2524          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2525          */
2526         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
2527             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2528                 throw new MemberName(targetClass).makeAccessException("access violation", this);
2529             }
2530             checkSecurityManager(targetClass, null);
2531             return targetClass;
2532         }
2533 
2534         /**
2535          * Produces an early-bound method handle for a virtual method.
2536          * It will bypass checks for overriding methods on the receiver,
2537          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2538          * instruction from within the explicitly specified {@code specialCaller}.
2539          * The type of the method handle will be that of the method,
2540          * with a suitably restricted receiver type prepended.
2541          * (The receiver type will be {@code specialCaller} or a subtype.)
2542          * The method and all its argument types must be accessible
2543          * to the lookup object.
2544          * <p>
2545          * Before method resolution,
2546          * if the explicitly specified caller class is not identical with the
2547          * lookup class, or if this lookup object does not have
2548          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2549          * privileges, the access fails.
2550          * <p>
2551          * The returned method handle will have
2552          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2553          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2554          * <p style="font-size:smaller;">
2555          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
2556          * even though the {@code invokespecial} instruction can refer to them
2557          * in special circumstances.  Use {@link #findConstructor findConstructor}
2558          * to access instance initialization methods in a safe manner.)</em>
2559          * <p><b>Example:</b>
2560          * <blockquote><pre>{@code
2561 import static java.lang.invoke.MethodHandles.*;
2562 import static java.lang.invoke.MethodType.*;
2563 ...
2564 static class Listie extends ArrayList {
2565   public String toString() { return "[wee Listie]"; }
2566   static Lookup lookup() { return MethodHandles.lookup(); }
2567 }
2568 ...
2569 // no access to constructor via invokeSpecial:
2570 MethodHandle MH_newListie = Listie.lookup()
2571   .findConstructor(Listie.class, methodType(void.class));
2572 Listie l = (Listie) MH_newListie.invokeExact();
2573 try { assertEquals("impossible", Listie.lookup().findSpecial(
2574         Listie.class, "<init>", methodType(void.class), Listie.class));
2575  } catch (NoSuchMethodException ex) { } // OK
2576 // access to super and self methods via invokeSpecial:
2577 MethodHandle MH_super = Listie.lookup().findSpecial(
2578   ArrayList.class, "toString" , methodType(String.class), Listie.class);
2579 MethodHandle MH_this = Listie.lookup().findSpecial(
2580   Listie.class, "toString" , methodType(String.class), Listie.class);
2581 MethodHandle MH_duper = Listie.lookup().findSpecial(
2582   Object.class, "toString" , methodType(String.class), Listie.class);
2583 assertEquals("[]", (String) MH_super.invokeExact(l));
2584 assertEquals(""+l, (String) MH_this.invokeExact(l));
2585 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
2586 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
2587         String.class, "toString", methodType(String.class), Listie.class));
2588  } catch (IllegalAccessException ex) { } // OK
2589 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
2590 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
2591          * }</pre></blockquote>
2592          *
2593          * @param refc the class or interface from which the method is accessed
2594          * @param name the name of the method (which must not be "&lt;init&gt;")
2595          * @param type the type of the method, with the receiver argument omitted
2596          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
2597          * @return the desired method handle
2598          * @throws NoSuchMethodException if the method does not exist
2599          * @throws IllegalAccessException if access checking fails,
2600          *                                or if the method is {@code static},
2601          *                                or if the method's variable arity modifier bit
2602          *                                is set and {@code asVarargsCollector} fails
2603          * @throws    SecurityException if a security manager is present and it
2604          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2605          * @throws NullPointerException if any argument is null
2606          */
2607         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
2608                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
2609             checkSpecialCaller(specialCaller, refc);
2610             Lookup specialLookup = this.in(specialCaller);
2611             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
2612             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
2613         }
2614 
2615         /**
2616          * Produces a method handle giving read access to a non-static field.
2617          * The type of the method handle will have a return type of the field's
2618          * value type.
2619          * The method handle's single argument will be the instance containing
2620          * the field.
2621          * Access checking is performed immediately on behalf of the lookup class.
2622          * @param refc the class or interface from which the method is accessed
2623          * @param name the field's name
2624          * @param type the field's type
2625          * @return a method handle which can load values from the field
2626          * @throws NoSuchFieldException if the field does not exist
2627          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2628          * @throws    SecurityException if a security manager is present and it
2629          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2630          * @throws NullPointerException if any argument is null
2631          * @see #findVarHandle(Class, String, Class)
2632          */
2633         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2634             MemberName field = resolveOrFail(REF_getField, refc, name, type);
2635             return getDirectField(REF_getField, refc, field);
2636         }
2637 
2638         /**
2639          * Produces a method handle giving write access to a non-static field.
2640          * The type of the method handle will have a void return type.
2641          * The method handle will take two arguments, the instance containing
2642          * the field, and the value to be stored.
2643          * The second argument will be of the field's value type.
2644          * Access checking is performed immediately on behalf of the lookup class.
2645          * @param refc the class or interface from which the method is accessed
2646          * @param name the field's name
2647          * @param type the field's type
2648          * @return a method handle which can store values into the field
2649          * @throws NoSuchFieldException if the field does not exist
2650          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2651          *                                or {@code final}
2652          * @throws    SecurityException if a security manager is present and it
2653          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2654          * @throws NullPointerException if any argument is null
2655          * @see #findVarHandle(Class, String, Class)
2656          */
2657         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2658             MemberName field = resolveOrFail(REF_putField, refc, name, type);
2659             return getDirectField(REF_putField, refc, field);
2660         }
2661 
2662         /**
2663          * Produces a VarHandle giving access to a non-static field {@code name}
2664          * of type {@code type} declared in a class of type {@code recv}.
2665          * The VarHandle's variable type is {@code type} and it has one
2666          * coordinate type, {@code recv}.
2667          * <p>
2668          * Access checking is performed immediately on behalf of the lookup
2669          * class.
2670          * <p>
2671          * Certain access modes of the returned VarHandle are unsupported under
2672          * the following conditions:
2673          * <ul>
2674          * <li>if the field is declared {@code final}, then the write, atomic
2675          *     update, numeric atomic update, and bitwise atomic update access
2676          *     modes are unsupported.
2677          * <li>if the field type is anything other than {@code byte},
2678          *     {@code short}, {@code char}, {@code int}, {@code long},
2679          *     {@code float}, or {@code double} then numeric atomic update
2680          *     access modes are unsupported.
2681          * <li>if the field type is anything other than {@code boolean},
2682          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2683          *     {@code long} then bitwise atomic update access modes are
2684          *     unsupported.
2685          * </ul>
2686          * <p>
2687          * If the field is declared {@code volatile} then the returned VarHandle
2688          * will override access to the field (effectively ignore the
2689          * {@code volatile} declaration) in accordance to its specified
2690          * access modes.
2691          * <p>
2692          * If the field type is {@code float} or {@code double} then numeric
2693          * and atomic update access modes compare values using their bitwise
2694          * representation (see {@link Float#floatToRawIntBits} and
2695          * {@link Double#doubleToRawLongBits}, respectively).
2696          * @apiNote
2697          * Bitwise comparison of {@code float} values or {@code double} values,
2698          * as performed by the numeric and atomic update access modes, differ
2699          * from the primitive {@code ==} operator and the {@link Float#equals}
2700          * and {@link Double#equals} methods, specifically with respect to
2701          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2702          * Care should be taken when performing a compare and set or a compare
2703          * and exchange operation with such values since the operation may
2704          * unexpectedly fail.
2705          * There are many possible NaN values that are considered to be
2706          * {@code NaN} in Java, although no IEEE 754 floating-point operation
2707          * provided by Java can distinguish between them.  Operation failure can
2708          * occur if the expected or witness value is a NaN value and it is
2709          * transformed (perhaps in a platform specific manner) into another NaN
2710          * value, and thus has a different bitwise representation (see
2711          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2712          * details).
2713          * The values {@code -0.0} and {@code +0.0} have different bitwise
2714          * representations but are considered equal when using the primitive
2715          * {@code ==} operator.  Operation failure can occur if, for example, a
2716          * numeric algorithm computes an expected value to be say {@code -0.0}
2717          * and previously computed the witness value to be say {@code +0.0}.
2718          * @param recv the receiver class, of type {@code R}, that declares the
2719          * non-static field
2720          * @param name the field's name
2721          * @param type the field's type, of type {@code T}
2722          * @return a VarHandle giving access to non-static fields.
2723          * @throws NoSuchFieldException if the field does not exist
2724          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2725          * @throws    SecurityException if a security manager is present and it
2726          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2727          * @throws NullPointerException if any argument is null
2728          * @since 9
2729          */
2730         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2731             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
2732             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
2733             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
2734         }
2735 
2736         /**
2737          * Produces a method handle giving read access to a static field.
2738          * The type of the method handle will have a return type of the field's
2739          * value type.
2740          * The method handle will take no arguments.
2741          * Access checking is performed immediately on behalf of the lookup class.
2742          * <p>
2743          * If the returned method handle is invoked, the field's class will
2744          * be initialized, if it has not already been initialized.
2745          * @param refc the class or interface from which the method is accessed
2746          * @param name the field's name
2747          * @param type the field's type
2748          * @return a method handle which can load values from the field
2749          * @throws NoSuchFieldException if the field does not exist
2750          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2751          * @throws    SecurityException if a security manager is present and it
2752          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2753          * @throws NullPointerException if any argument is null
2754          */
2755         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2756             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
2757             return getDirectField(REF_getStatic, refc, field);
2758         }
2759 
2760         /**
2761          * Produces a method handle giving write access to a static field.
2762          * The type of the method handle will have a void return type.
2763          * The method handle will take a single
2764          * argument, of the field's value type, the value to be stored.
2765          * Access checking is performed immediately on behalf of the lookup class.
2766          * <p>
2767          * If the returned method handle is invoked, the field's class will
2768          * be initialized, if it has not already been initialized.
2769          * @param refc the class or interface from which the method is accessed
2770          * @param name the field's name
2771          * @param type the field's type
2772          * @return a method handle which can store values into the field
2773          * @throws NoSuchFieldException if the field does not exist
2774          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2775          *                                or is {@code final}
2776          * @throws    SecurityException if a security manager is present and it
2777          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2778          * @throws NullPointerException if any argument is null
2779          */
2780         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2781             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
2782             return getDirectField(REF_putStatic, refc, field);
2783         }
2784 
2785         /**
2786          * Produces a VarHandle giving access to a static field {@code name} of
2787          * type {@code type} declared in a class of type {@code decl}.
2788          * The VarHandle's variable type is {@code type} and it has no
2789          * coordinate types.
2790          * <p>
2791          * Access checking is performed immediately on behalf of the lookup
2792          * class.
2793          * <p>
2794          * If the returned VarHandle is operated on, the declaring class will be
2795          * initialized, if it has not already been initialized.
2796          * <p>
2797          * Certain access modes of the returned VarHandle are unsupported under
2798          * the following conditions:
2799          * <ul>
2800          * <li>if the field is declared {@code final}, then the write, atomic
2801          *     update, numeric atomic update, and bitwise atomic update access
2802          *     modes are unsupported.
2803          * <li>if the field type is anything other than {@code byte},
2804          *     {@code short}, {@code char}, {@code int}, {@code long},
2805          *     {@code float}, or {@code double}, then numeric atomic update
2806          *     access modes are unsupported.
2807          * <li>if the field type is anything other than {@code boolean},
2808          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2809          *     {@code long} then bitwise atomic update access modes are
2810          *     unsupported.
2811          * </ul>
2812          * <p>
2813          * If the field is declared {@code volatile} then the returned VarHandle
2814          * will override access to the field (effectively ignore the
2815          * {@code volatile} declaration) in accordance to its specified
2816          * access modes.
2817          * <p>
2818          * If the field type is {@code float} or {@code double} then numeric
2819          * and atomic update access modes compare values using their bitwise
2820          * representation (see {@link Float#floatToRawIntBits} and
2821          * {@link Double#doubleToRawLongBits}, respectively).
2822          * @apiNote
2823          * Bitwise comparison of {@code float} values or {@code double} values,
2824          * as performed by the numeric and atomic update access modes, differ
2825          * from the primitive {@code ==} operator and the {@link Float#equals}
2826          * and {@link Double#equals} methods, specifically with respect to
2827          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2828          * Care should be taken when performing a compare and set or a compare
2829          * and exchange operation with such values since the operation may
2830          * unexpectedly fail.
2831          * There are many possible NaN values that are considered to be
2832          * {@code NaN} in Java, although no IEEE 754 floating-point operation
2833          * provided by Java can distinguish between them.  Operation failure can
2834          * occur if the expected or witness value is a NaN value and it is
2835          * transformed (perhaps in a platform specific manner) into another NaN
2836          * value, and thus has a different bitwise representation (see
2837          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2838          * details).
2839          * The values {@code -0.0} and {@code +0.0} have different bitwise
2840          * representations but are considered equal when using the primitive
2841          * {@code ==} operator.  Operation failure can occur if, for example, a
2842          * numeric algorithm computes an expected value to be say {@code -0.0}
2843          * and previously computed the witness value to be say {@code +0.0}.
2844          * @param decl the class that declares the static field
2845          * @param name the field's name
2846          * @param type the field's type, of type {@code T}
2847          * @return a VarHandle giving access to a static field
2848          * @throws NoSuchFieldException if the field does not exist
2849          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
2850          * @throws    SecurityException if a security manager is present and it
2851          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2852          * @throws NullPointerException if any argument is null
2853          * @since 9
2854          */
2855         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2856             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
2857             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
2858             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
2859         }
2860 
2861         /**
2862          * Produces an early-bound method handle for a non-static method.
2863          * The receiver must have a supertype {@code defc} in which a method
2864          * of the given name and type is accessible to the lookup class.
2865          * The method and all its argument types must be accessible to the lookup object.
2866          * The type of the method handle will be that of the method,
2867          * without any insertion of an additional receiver parameter.
2868          * The given receiver will be bound into the method handle,
2869          * so that every call to the method handle will invoke the
2870          * requested method on the given receiver.
2871          * <p>
2872          * The returned method handle will have
2873          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2874          * the method's variable arity modifier bit ({@code 0x0080}) is set
2875          * <em>and</em> the trailing array argument is not the only argument.
2876          * (If the trailing array argument is the only argument,
2877          * the given receiver value will be bound to it.)
2878          * <p>
2879          * This is almost equivalent to the following code, with some differences noted below:
2880          * <blockquote><pre>{@code
2881 import static java.lang.invoke.MethodHandles.*;
2882 import static java.lang.invoke.MethodType.*;
2883 ...
2884 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
2885 MethodHandle mh1 = mh0.bindTo(receiver);
2886 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
2887 return mh1;
2888          * }</pre></blockquote>
2889          * where {@code defc} is either {@code receiver.getClass()} or a super
2890          * type of that class, in which the requested method is accessible
2891          * to the lookup class.
2892          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
2893          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
2894          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
2895          * the receiver is restricted by {@code findVirtual} to the lookup class.)
2896          * @param receiver the object from which the method is accessed
2897          * @param name the name of the method
2898          * @param type the type of the method, with the receiver argument omitted
2899          * @return the desired method handle
2900          * @throws NoSuchMethodException if the method does not exist
2901          * @throws IllegalAccessException if access checking fails
2902          *                                or if the method's variable arity modifier bit
2903          *                                is set and {@code asVarargsCollector} fails
2904          * @throws    SecurityException if a security manager is present and it
2905          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2906          * @throws NullPointerException if any argument is null
2907          * @see MethodHandle#bindTo
2908          * @see #findVirtual
2909          */
2910         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2911             Class<? extends Object> refc = receiver.getClass(); // may get NPE
2912             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
2913             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
2914             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
2915                 throw new IllegalAccessException("The restricted defining class " +
2916                                                  mh.type().leadingReferenceParameter().getName() +
2917                                                  " is not assignable from receiver class " +
2918                                                  receiver.getClass().getName());
2919             }
2920             return mh.bindArgumentL(0, receiver).setVarargs(method);
2921         }
2922 
2923         /**
2924          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
2925          * to <i>m</i>, if the lookup class has permission.
2926          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
2927          * If <i>m</i> is virtual, overriding is respected on every call.
2928          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
2929          * The type of the method handle will be that of the method,
2930          * with the receiver type prepended (but only if it is non-static).
2931          * If the method's {@code accessible} flag is not set,
2932          * access checking is performed immediately on behalf of the lookup class.
2933          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
2934          * <p>
2935          * The returned method handle will have
2936          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2937          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2938          * <p>
2939          * If <i>m</i> is static, and
2940          * if the returned method handle is invoked, the method's class will
2941          * be initialized, if it has not already been initialized.
2942          * @param m the reflected method
2943          * @return a method handle which can invoke the reflected method
2944          * @throws IllegalAccessException if access checking fails
2945          *                                or if the method's variable arity modifier bit
2946          *                                is set and {@code asVarargsCollector} fails
2947          * @throws NullPointerException if the argument is null
2948          */
2949         public MethodHandle unreflect(Method m) throws IllegalAccessException {
2950             if (m.getDeclaringClass() == MethodHandle.class) {
2951                 MethodHandle mh = unreflectForMH(m);
2952                 if (mh != null)  return mh;
2953             }
2954             if (m.getDeclaringClass() == VarHandle.class) {
2955                 MethodHandle mh = unreflectForVH(m);
2956                 if (mh != null)  return mh;
2957             }
2958             MemberName method = new MemberName(m);
2959             byte refKind = method.getReferenceKind();
2960             if (refKind == REF_invokeSpecial)
2961                 refKind = REF_invokeVirtual;
2962             assert(method.isMethod());
2963             @SuppressWarnings("deprecation")
2964             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
2965             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
2966         }
2967         private MethodHandle unreflectForMH(Method m) {
2968             // these names require special lookups because they throw UnsupportedOperationException
2969             if (MemberName.isMethodHandleInvokeName(m.getName()))
2970                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
2971             return null;
2972         }
2973         private MethodHandle unreflectForVH(Method m) {
2974             // these names require special lookups because they throw UnsupportedOperationException
2975             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
2976                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
2977             return null;
2978         }
2979 
2980         /**
2981          * Produces a method handle for a reflected method.
2982          * It will bypass checks for overriding methods on the receiver,
2983          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2984          * instruction from within the explicitly specified {@code specialCaller}.
2985          * The type of the method handle will be that of the method,
2986          * with a suitably restricted receiver type prepended.
2987          * (The receiver type will be {@code specialCaller} or a subtype.)
2988          * If the method's {@code accessible} flag is not set,
2989          * access checking is performed immediately on behalf of the lookup class,
2990          * as if {@code invokespecial} instruction were being linked.
2991          * <p>
2992          * Before method resolution,
2993          * if the explicitly specified caller class is not identical with the
2994          * lookup class, or if this lookup object does not have
2995          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2996          * privileges, the access fails.
2997          * <p>
2998          * The returned method handle will have
2999          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3000          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3001          * @param m the reflected method
3002          * @param specialCaller the class nominally calling the method
3003          * @return a method handle which can invoke the reflected method
3004          * @throws IllegalAccessException if access checking fails,
3005          *                                or if the method is {@code static},
3006          *                                or if the method's variable arity modifier bit
3007          *                                is set and {@code asVarargsCollector} fails
3008          * @throws NullPointerException if any argument is null
3009          */
3010         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3011             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3012             Lookup specialLookup = this.in(specialCaller);
3013             MemberName method = new MemberName(m, true);
3014             assert(method.isMethod());
3015             // ignore m.isAccessible:  this is a new kind of access
3016             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3017         }
3018 
3019         /**
3020          * Produces a method handle for a reflected constructor.
3021          * The type of the method handle will be that of the constructor,
3022          * with the return type changed to the declaring class.
3023          * The method handle will perform a {@code newInstance} operation,
3024          * creating a new instance of the constructor's class on the
3025          * arguments passed to the method handle.
3026          * <p>
3027          * If the constructor's {@code accessible} flag is not set,
3028          * access checking is performed immediately on behalf of the lookup class.
3029          * <p>
3030          * The returned method handle will have
3031          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3032          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3033          * <p>
3034          * If the returned method handle is invoked, the constructor's class will
3035          * be initialized, if it has not already been initialized.
3036          * @param c the reflected constructor
3037          * @return a method handle which can invoke the reflected constructor
3038          * @throws IllegalAccessException if access checking fails
3039          *                                or if the method's variable arity modifier bit
3040          *                                is set and {@code asVarargsCollector} fails
3041          * @throws NullPointerException if the argument is null
3042          */
3043         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3044             MemberName ctor = new MemberName(c);
3045             assert(ctor.isConstructor());
3046             @SuppressWarnings("deprecation")
3047             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3048             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
3049         }
3050 
3051         /**
3052          * Produces a method handle giving read access to a reflected field.
3053          * The type of the method handle will have a return type of the field's
3054          * value type.
3055          * If the field is {@code static}, the method handle will take no arguments.
3056          * Otherwise, its single argument will be the instance containing
3057          * the field.
3058          * If the {@code Field} object's {@code accessible} flag is not set,
3059          * access checking is performed immediately on behalf of the lookup class.
3060          * <p>
3061          * If the field is static, and
3062          * if the returned method handle is invoked, the field's class will
3063          * be initialized, if it has not already been initialized.
3064          * @param f the reflected field
3065          * @return a method handle which can load values from the reflected field
3066          * @throws IllegalAccessException if access checking fails
3067          * @throws NullPointerException if the argument is null
3068          */
3069         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3070             return unreflectField(f, false);
3071         }
3072 
3073         /**
3074          * Produces a method handle giving write access to a reflected field.
3075          * The type of the method handle will have a void return type.
3076          * If the field is {@code static}, the method handle will take a single
3077          * argument, of the field's value type, the value to be stored.
3078          * Otherwise, the two arguments will be the instance containing
3079          * the field, and the value to be stored.
3080          * If the {@code Field} object's {@code accessible} flag is not set,
3081          * access checking is performed immediately on behalf of the lookup class.
3082          * <p>
3083          * If the field is {@code final}, write access will not be
3084          * allowed and access checking will fail, except under certain
3085          * narrow circumstances documented for {@link Field#set Field.set}.
3086          * A method handle is returned only if a corresponding call to
3087          * the {@code Field} object's {@code set} method could return
3088          * normally.  In particular, fields which are both {@code static}
3089          * and {@code final} may never be set.
3090          * <p>
3091          * If the field is {@code static}, and
3092          * if the returned method handle is invoked, the field's class will
3093          * be initialized, if it has not already been initialized.
3094          * @param f the reflected field
3095          * @return a method handle which can store values into the reflected field
3096          * @throws IllegalAccessException if access checking fails,
3097          *         or if the field is {@code final} and write access
3098          *         is not enabled on the {@code Field} object
3099          * @throws NullPointerException if the argument is null
3100          */
3101         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3102             return unreflectField(f, true);
3103         }
3104 
3105         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3106             MemberName field = new MemberName(f, isSetter);
3107             if (isSetter && field.isFinal()) {
3108                 if (field.isStatic()) {
3109                     throw field.makeAccessException("static final field has no write access", this);
3110                 } else if (field.getDeclaringClass().isHiddenClass()){
3111                     throw field.makeAccessException("final field in a hidden class has no write access", this);
3112                 }
3113             }
3114             assert(isSetter
3115                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3116                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3117             @SuppressWarnings("deprecation")
3118             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3119             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
3120         }
3121 
3122         /**
3123          * Produces a VarHandle giving access to a reflected field {@code f}
3124          * of type {@code T} declared in a class of type {@code R}.
3125          * The VarHandle's variable type is {@code T}.
3126          * If the field is non-static the VarHandle has one coordinate type,
3127          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3128          * coordinate types.
3129          * <p>
3130          * Access checking is performed immediately on behalf of the lookup
3131          * class, regardless of the value of the field's {@code accessible}
3132          * flag.
3133          * <p>
3134          * If the field is static, and if the returned VarHandle is operated
3135          * on, the field's declaring class will be initialized, if it has not
3136          * already been initialized.
3137          * <p>
3138          * Certain access modes of the returned VarHandle are unsupported under
3139          * the following conditions:
3140          * <ul>
3141          * <li>if the field is declared {@code final}, then the write, atomic
3142          *     update, numeric atomic update, and bitwise atomic update access
3143          *     modes are unsupported.
3144          * <li>if the field type is anything other than {@code byte},
3145          *     {@code short}, {@code char}, {@code int}, {@code long},
3146          *     {@code float}, or {@code double} then numeric atomic update
3147          *     access modes are unsupported.
3148          * <li>if the field type is anything other than {@code boolean},
3149          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3150          *     {@code long} then bitwise atomic update access modes are
3151          *     unsupported.
3152          * </ul>
3153          * <p>
3154          * If the field is declared {@code volatile} then the returned VarHandle
3155          * will override access to the field (effectively ignore the
3156          * {@code volatile} declaration) in accordance to its specified
3157          * access modes.
3158          * <p>
3159          * If the field type is {@code float} or {@code double} then numeric
3160          * and atomic update access modes compare values using their bitwise
3161          * representation (see {@link Float#floatToRawIntBits} and
3162          * {@link Double#doubleToRawLongBits}, respectively).
3163          * @apiNote
3164          * Bitwise comparison of {@code float} values or {@code double} values,
3165          * as performed by the numeric and atomic update access modes, differ
3166          * from the primitive {@code ==} operator and the {@link Float#equals}
3167          * and {@link Double#equals} methods, specifically with respect to
3168          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3169          * Care should be taken when performing a compare and set or a compare
3170          * and exchange operation with such values since the operation may
3171          * unexpectedly fail.
3172          * There are many possible NaN values that are considered to be
3173          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3174          * provided by Java can distinguish between them.  Operation failure can
3175          * occur if the expected or witness value is a NaN value and it is
3176          * transformed (perhaps in a platform specific manner) into another NaN
3177          * value, and thus has a different bitwise representation (see
3178          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3179          * details).
3180          * The values {@code -0.0} and {@code +0.0} have different bitwise
3181          * representations but are considered equal when using the primitive
3182          * {@code ==} operator.  Operation failure can occur if, for example, a
3183          * numeric algorithm computes an expected value to be say {@code -0.0}
3184          * and previously computed the witness value to be say {@code +0.0}.
3185          * @param f the reflected field, with a field of type {@code T}, and
3186          * a declaring class of type {@code R}
3187          * @return a VarHandle giving access to non-static fields or a static
3188          * field
3189          * @throws IllegalAccessException if access checking fails
3190          * @throws NullPointerException if the argument is null
3191          * @since 9
3192          */
3193         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3194             MemberName getField = new MemberName(f, false);
3195             MemberName putField = new MemberName(f, true);
3196             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
3197                                                       f.getDeclaringClass(), getField, putField);
3198         }
3199 
3200         /**
3201          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3202          * created by this lookup object or a similar one.
3203          * Security and access checks are performed to ensure that this lookup object
3204          * is capable of reproducing the target method handle.
3205          * This means that the cracking may fail if target is a direct method handle
3206          * but was created by an unrelated lookup object.
3207          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3208          * and was created by a lookup object for a different class.
3209          * @param target a direct method handle to crack into symbolic reference components
3210          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3211          * @throws    SecurityException if a security manager is present and it
3212          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3213          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3214          * @throws    NullPointerException if the target is {@code null}
3215          * @see MethodHandleInfo
3216          * @since 1.8
3217          */
3218         public MethodHandleInfo revealDirect(MethodHandle target) {
3219             MemberName member = target.internalMemberName();
3220             if (member == null || (!member.isResolved() &&
3221                                    !member.isMethodHandleInvoke() &&
3222                                    !member.isVarHandleMethodInvoke()))
3223                 throw newIllegalArgumentException("not a direct method handle");
3224             Class<?> defc = member.getDeclaringClass();
3225             byte refKind = member.getReferenceKind();
3226             assert(MethodHandleNatives.refKindIsValid(refKind));
3227             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3228                 // Devirtualized method invocation is usually formally virtual.
3229                 // To avoid creating extra MemberName objects for this common case,
3230                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3231                 refKind = REF_invokeVirtual;
3232             if (refKind == REF_invokeVirtual && defc.isInterface())
3233                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3234                 refKind = REF_invokeInterface;
3235             // Check SM permissions and member access before cracking.
3236             try {
3237                 checkAccess(refKind, defc, member);
3238                 checkSecurityManager(defc, member);
3239             } catch (IllegalAccessException ex) {
3240                 throw new IllegalArgumentException(ex);
3241             }
3242             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3243                 Class<?> callerClass = target.internalCallerClass();
3244                 if (!hasFullPrivilegeAccess() || callerClass != lookupClass())
3245                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3246             }
3247             // Produce the handle to the results.
3248             return new InfoFromMemberName(this, member, refKind);
3249         }
3250 
3251         /// Helper methods, all package-private.
3252 
3253         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3254             checkSymbolicClass(refc);  // do this before attempting to resolve
3255             Objects.requireNonNull(name);
3256             Objects.requireNonNull(type);
3257             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
3258                                             NoSuchFieldException.class);
3259         }
3260 
3261         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3262             checkSymbolicClass(refc);  // do this before attempting to resolve
3263             Objects.requireNonNull(name);
3264             Objects.requireNonNull(type);
3265             checkMethodName(refKind, name);  // NPE check on name
3266             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
3267                                             NoSuchMethodException.class);
3268         }
3269 
3270         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3271             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3272             Objects.requireNonNull(member.getName());
3273             Objects.requireNonNull(member.getType());
3274             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
3275                                             ReflectiveOperationException.class);
3276         }
3277 
3278         MemberName resolveOrNull(byte refKind, MemberName member) {
3279             // do this before attempting to resolve
3280             if (!isClassAccessible(member.getDeclaringClass())) {
3281                 return null;
3282             }
3283             Objects.requireNonNull(member.getName());
3284             Objects.requireNonNull(member.getType());
3285             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull());
3286         }
3287 
3288         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3289             if (!isClassAccessible(refc)) {
3290                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3291             }
3292         }
3293 
3294         boolean isClassAccessible(Class<?> refc) {
3295             Objects.requireNonNull(refc);
3296             Class<?> caller = lookupClassOrNull();
3297             return caller == null || VerifyAccess.isClassAccessible(refc, caller, prevLookupClass, allowedModes);
3298         }
3299 
3300         /** Check name for an illegal leading "&lt;" character. */
3301         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3302             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3303                 throw new NoSuchMethodException("illegal method name: "+name);
3304         }
3305 
3306 
3307         /**
3308          * Find my trustable caller class if m is a caller sensitive method.
3309          * If this lookup object has full privilege access, then the caller class is the lookupClass.
3310          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3311          */
3312         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3313             if (MethodHandleNatives.isCallerSensitive(m) && !hasFullPrivilegeAccess()) {
3314                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3315                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3316             }
3317             return this;
3318         }
3319 
3320         /**
3321          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3322          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3323          *
3324          * @deprecated This method was originally designed to test {@code PRIVATE} access
3325          * that implies full privilege access but {@code MODULE} access has since become
3326          * independent of {@code PRIVATE} access.  It is recommended to call
3327          * {@link #hasFullPrivilegeAccess()} instead.
3328          * @since 9
3329          */
3330         @Deprecated(since="14")
3331         public boolean hasPrivateAccess() {
3332             return hasFullPrivilegeAccess();
3333         }
3334 
3335         /**
3336          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3337          * i.e. {@code PRIVATE} and {@code MODULE} access.
3338          * A {@code Lookup} object must have full privilege access in order to
3339          * access all members that are allowed to the {@linkplain #lookupClass() lookup class}.
3340          *
3341          * @return {@code true} if this lookup has full privilege access.
3342          * @since 14
3343          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3344          */
3345         public boolean hasFullPrivilegeAccess() {
3346             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3347         }
3348 
3349         /**
3350          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
3351          * Determines a trustable caller class to compare with refc, the symbolic reference class.
3352          * If this lookup object has full privilege access, then the caller class is the lookupClass.
3353          */
3354         void checkSecurityManager(Class<?> refc, MemberName m) {
3355             if (allowedModes == TRUSTED)  return;
3356 
3357             SecurityManager smgr = System.getSecurityManager();
3358             if (smgr == null)  return;
3359 
3360             // Step 1:
3361             boolean fullPowerLookup = hasFullPrivilegeAccess();
3362             if (!fullPowerLookup ||
3363                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3364                 ReflectUtil.checkPackageAccess(refc);
3365             }
3366 
3367             if (m == null) {  // findClass or accessClass
3368                 // Step 2b:
3369                 if (!fullPowerLookup) {
3370                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
3371                 }
3372                 return;
3373             }
3374 
3375             // Step 2a:
3376             if (m.isPublic()) return;
3377             if (!fullPowerLookup) {
3378                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
3379             }
3380 
3381             // Step 3:
3382             Class<?> defc = m.getDeclaringClass();
3383             if (!fullPowerLookup && defc != refc) {
3384                 ReflectUtil.checkPackageAccess(defc);
3385             }
3386         }
3387 
3388         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3389             boolean wantStatic = (refKind == REF_invokeStatic);
3390             String message;
3391             if (m.isConstructor())
3392                 message = "expected a method, not a constructor";
3393             else if (!m.isMethod())
3394                 message = "expected a method";
3395             else if (wantStatic != m.isStatic())
3396                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3397             else
3398                 { checkAccess(refKind, refc, m); return; }
3399             throw m.makeAccessException(message, this);
3400         }
3401 
3402         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3403             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3404             String message;
3405             if (wantStatic != m.isStatic())
3406                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3407             else
3408                 { checkAccess(refKind, refc, m); return; }
3409             throw m.makeAccessException(message, this);
3410         }
3411 
3412         /** Check public/protected/private bits on the symbolic reference class and its member. */
3413         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3414             assert(m.referenceKindIsConsistentWith(refKind) &&
3415                    MethodHandleNatives.refKindIsValid(refKind) &&
3416                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3417             int allowedModes = this.allowedModes;
3418             if (allowedModes == TRUSTED)  return;
3419             int mods = m.getModifiers();
3420             if (Modifier.isProtected(mods) &&
3421                     refKind == REF_invokeVirtual &&
3422                     m.getDeclaringClass() == Object.class &&
3423                     m.getName().equals("clone") &&
3424                     refc.isArray()) {
3425                 // The JVM does this hack also.
3426                 // (See ClassVerifier::verify_invoke_instructions
3427                 // and LinkResolver::check_method_accessability.)
3428                 // Because the JVM does not allow separate methods on array types,
3429                 // there is no separate method for int[].clone.
3430                 // All arrays simply inherit Object.clone.
3431                 // But for access checking logic, we make Object.clone
3432                 // (normally protected) appear to be public.
3433                 // Later on, when the DirectMethodHandle is created,
3434                 // its leading argument will be restricted to the
3435                 // requested array type.
3436                 // N.B. The return type is not adjusted, because
3437                 // that is *not* the bytecode behavior.
3438                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3439             }
3440             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3441                 // cannot "new" a protected ctor in a different package
3442                 mods ^= Modifier.PROTECTED;
3443             }
3444             if (Modifier.isFinal(mods) &&
3445                     MethodHandleNatives.refKindIsSetter(refKind))
3446                 throw m.makeAccessException("unexpected set of a final field", this);
3447             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3448             if ((requestedModes & allowedModes) != 0) {
3449                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3450                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3451                     return;
3452             } else {
3453                 // Protected members can also be checked as if they were package-private.
3454                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3455                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3456                     return;
3457             }
3458             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3459         }
3460 
3461         String accessFailedMessage(Class<?> refc, MemberName m) {
3462             Class<?> defc = m.getDeclaringClass();
3463             int mods = m.getModifiers();
3464             // check the class first:
3465             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3466                                (defc == refc ||
3467                                 Modifier.isPublic(refc.getModifiers())));
3468             if (!classOK && (allowedModes & PACKAGE) != 0) {
3469                 // ignore previous lookup class to check if default package access
3470                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3471                            (defc == refc ||
3472                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3473             }
3474             if (!classOK)
3475                 return "class is not public";
3476             if (Modifier.isPublic(mods))
3477                 return "access to public member failed";  // (how?, module not readable?)
3478             if (Modifier.isPrivate(mods))
3479                 return "member is private";
3480             if (Modifier.isProtected(mods))
3481                 return "member is protected";
3482             return "member is private to package";
3483         }
3484 
3485         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
3486             int allowedModes = this.allowedModes;
3487             if (allowedModes == TRUSTED)  return;
3488             if ((lookupModes() & PRIVATE) == 0
3489                 || (specialCaller != lookupClass()
3490                        // ensure non-abstract methods in superinterfaces can be special-invoked
3491                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
3492                 throw new MemberName(specialCaller).
3493                     makeAccessException("no private access for invokespecial", this);
3494         }
3495 
3496         private boolean restrictProtectedReceiver(MemberName method) {
3497             // The accessing class only has the right to use a protected member
3498             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
3499             if (!method.isProtected() || method.isStatic()
3500                 || allowedModes == TRUSTED
3501                 || method.getDeclaringClass() == lookupClass()
3502                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
3503                 return false;
3504             return true;
3505         }
3506         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
3507             assert(!method.isStatic());
3508             // receiver type of mh is too wide; narrow to caller
3509             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
3510                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
3511             }
3512             MethodType rawType = mh.type();
3513             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
3514             MethodType narrowType = rawType.changeParameterType(0, caller);
3515             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
3516             assert(mh.viewAsTypeChecks(narrowType, true));
3517             return mh.copyWith(narrowType, mh.form);
3518         }
3519 
3520         /** Check access and get the requested method. */
3521         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3522             final boolean doRestrict    = true;
3523             final boolean checkSecurity = true;
3524             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3525         }
3526         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
3527         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3528             final boolean doRestrict    = false;
3529             final boolean checkSecurity = true;
3530             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, callerLookup);
3531         }
3532         /** Check access and get the requested method, eliding security manager checks. */
3533         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3534             final boolean doRestrict    = true;
3535             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3536             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3537         }
3538         /** Common code for all methods; do not call directly except from immediately above. */
3539         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
3540                                                    boolean checkSecurity,
3541                                                    boolean doRestrict,
3542                                                    Lookup boundCaller) throws IllegalAccessException {
3543             checkMethod(refKind, refc, method);
3544             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3545             if (checkSecurity)
3546                 checkSecurityManager(refc, method);
3547             assert(!method.isMethodHandleInvoke());
3548 
3549             if (refKind == REF_invokeSpecial &&
3550                 refc != lookupClass() &&
3551                 !refc.isInterface() &&
3552                 refc != lookupClass().getSuperclass() &&
3553                 refc.isAssignableFrom(lookupClass())) {
3554                 assert(!method.getName().equals("<init>"));  // not this code path
3555 
3556                 // Per JVMS 6.5, desc. of invokespecial instruction:
3557                 // If the method is in a superclass of the LC,
3558                 // and if our original search was above LC.super,
3559                 // repeat the search (symbolic lookup) from LC.super
3560                 // and continue with the direct superclass of that class,
3561                 // and so forth, until a match is found or no further superclasses exist.
3562                 // FIXME: MemberName.resolve should handle this instead.
3563                 Class<?> refcAsSuper = lookupClass();
3564                 MemberName m2;
3565                 do {
3566                     refcAsSuper = refcAsSuper.getSuperclass();
3567                     m2 = new MemberName(refcAsSuper,
3568                                         method.getName(),
3569                                         method.getMethodType(),
3570                                         REF_invokeSpecial);
3571                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
3572                 } while (m2 == null &&         // no method is found yet
3573                          refc != refcAsSuper); // search up to refc
3574                 if (m2 == null)  throw new InternalError(method.toString());
3575                 method = m2;
3576                 refc = refcAsSuper;
3577                 // redo basic checks
3578                 checkMethod(refKind, refc, method);
3579             }
3580             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
3581             MethodHandle mh = dmh;
3582             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
3583             if ((doRestrict && refKind == REF_invokeSpecial) ||
3584                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
3585                 mh = restrictReceiver(method, dmh, lookupClass());
3586             }
3587             mh = maybeBindCaller(method, mh, boundCaller);
3588             mh = mh.setVarargs(method);
3589             return mh;
3590         }
3591         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
3592                                              throws IllegalAccessException {
3593             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
3594                 return mh;
3595 
3596             // boundCaller must have full privilege access.
3597             // It should have been checked by findBoundCallerLookup. Safe to check this again.
3598             if (!boundCaller.hasFullPrivilegeAccess())
3599                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3600 
3601             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
3602             // Note: caller will apply varargs after this step happens.
3603             return cbmh;
3604         }
3605 
3606         /** Check access and get the requested field. */
3607         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3608             final boolean checkSecurity = true;
3609             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
3610         }
3611         /** Check access and get the requested field, eliding security manager checks. */
3612         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3613             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3614             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
3615         }
3616         /** Common code for all fields; do not call directly except from immediately above. */
3617         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
3618                                                   boolean checkSecurity) throws IllegalAccessException {
3619             checkField(refKind, refc, field);
3620             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3621             if (checkSecurity)
3622                 checkSecurityManager(refc, field);
3623             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
3624             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
3625                                     restrictProtectedReceiver(field));
3626             if (doRestrict)
3627                 return restrictReceiver(field, dmh, lookupClass());
3628             return dmh;
3629         }
3630         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
3631                                             Class<?> refc, MemberName getField, MemberName putField)
3632                 throws IllegalAccessException {
3633             final boolean checkSecurity = true;
3634             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
3635         }
3636         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
3637                                                              Class<?> refc, MemberName getField, MemberName putField)
3638                 throws IllegalAccessException {
3639             final boolean checkSecurity = false;
3640             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
3641         }
3642         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
3643                                                   Class<?> refc, MemberName getField, MemberName putField,
3644                                                   boolean checkSecurity) throws IllegalAccessException {
3645             assert getField.isStatic() == putField.isStatic();
3646             assert getField.isGetter() && putField.isSetter();
3647             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
3648             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
3649 
3650             checkField(getRefKind, refc, getField);
3651             if (checkSecurity)
3652                 checkSecurityManager(refc, getField);
3653 
3654             if (!putField.isFinal()) {
3655                 // A VarHandle does not support updates to final fields, any
3656                 // such VarHandle to a final field will be read-only and
3657                 // therefore the following write-based accessibility checks are
3658                 // only required for non-final fields
3659                 checkField(putRefKind, refc, putField);
3660                 if (checkSecurity)
3661                     checkSecurityManager(refc, putField);
3662             }
3663 
3664             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
3665                                   restrictProtectedReceiver(getField));
3666             if (doRestrict) {
3667                 assert !getField.isStatic();
3668                 // receiver type of VarHandle is too wide; narrow to caller
3669                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
3670                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
3671                 }
3672                 refc = lookupClass();
3673             }
3674             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(),
3675                                              this.allowedModes == TRUSTED && !getField.getDeclaringClass().isHiddenClass());
3676         }
3677         /** Check access and get the requested constructor. */
3678         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3679             final boolean checkSecurity = true;
3680             return getDirectConstructorCommon(refc, ctor, checkSecurity);
3681         }
3682         /** Check access and get the requested constructor, eliding security manager checks. */
3683         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3684             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3685             return getDirectConstructorCommon(refc, ctor, checkSecurity);
3686         }
3687         /** Common code for all constructors; do not call directly except from immediately above. */
3688         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
3689                                                   boolean checkSecurity) throws IllegalAccessException {
3690             assert(ctor.isConstructor());
3691             checkAccess(REF_newInvokeSpecial, refc, ctor);
3692             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3693             if (checkSecurity)
3694                 checkSecurityManager(refc, ctor);
3695             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3696             return DirectMethodHandle.make(ctor).setVarargs(ctor);
3697         }
3698 
3699         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
3700          */
3701         /*non-public*/
3702         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
3703                 throws ReflectiveOperationException {
3704             if (!(type instanceof Class || type instanceof MethodType))
3705                 throw new InternalError("unresolved MemberName");
3706             MemberName member = new MemberName(refKind, defc, name, type);
3707             MethodHandle mh = LOOKASIDE_TABLE.get(member);
3708             if (mh != null) {
3709                 checkSymbolicClass(defc);
3710                 return mh;
3711             }
3712             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
3713                 // Treat MethodHandle.invoke and invokeExact specially.
3714                 mh = findVirtualForMH(member.getName(), member.getMethodType());
3715                 if (mh != null) {
3716                     return mh;
3717                 }
3718             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
3719                 // Treat signature-polymorphic methods on VarHandle specially.
3720                 mh = findVirtualForVH(member.getName(), member.getMethodType());
3721                 if (mh != null) {
3722                     return mh;
3723                 }
3724             }
3725             MemberName resolved = resolveOrFail(refKind, member);
3726             mh = getDirectMethodForConstant(refKind, defc, resolved);
3727             if (mh instanceof DirectMethodHandle
3728                     && canBeCached(refKind, defc, resolved)) {
3729                 MemberName key = mh.internalMemberName();
3730                 if (key != null) {
3731                     key = key.asNormalOriginal();
3732                 }
3733                 if (member.equals(key)) {  // better safe than sorry
3734                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
3735                 }
3736             }
3737             return mh;
3738         }
3739         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
3740             if (refKind == REF_invokeSpecial) {
3741                 return false;
3742             }
3743             if (!Modifier.isPublic(defc.getModifiers()) ||
3744                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
3745                     !member.isPublic() ||
3746                     member.isCallerSensitive()) {
3747                 return false;
3748             }
3749             ClassLoader loader = defc.getClassLoader();
3750             if (loader != null) {
3751                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
3752                 boolean found = false;
3753                 while (sysl != null) {
3754                     if (loader == sysl) { found = true; break; }
3755                     sysl = sysl.getParent();
3756                 }
3757                 if (!found) {
3758                     return false;
3759                 }
3760             }
3761             try {
3762                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
3763                     new MemberName(refKind, defc, member.getName(), member.getType()));
3764                 if (resolved2 == null) {
3765                     return false;
3766                 }
3767                 checkSecurityManager(defc, resolved2);
3768             } catch (SecurityException ex) {
3769                 return false;
3770             }
3771             return true;
3772         }
3773         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
3774                 throws ReflectiveOperationException {
3775             if (MethodHandleNatives.refKindIsField(refKind)) {
3776                 return getDirectFieldNoSecurityManager(refKind, defc, member);
3777             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
3778                 return getDirectMethodNoSecurityManager(refKind, defc, member, findBoundCallerLookup(member));
3779             } else if (refKind == REF_newInvokeSpecial) {
3780                 return getDirectConstructorNoSecurityManager(defc, member);
3781             }
3782             // oops
3783             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
3784         }
3785 
3786         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
3787 
3788         /**
3789          * The set of class options that specify whether a hidden class created by
3790          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
3791          * Lookup::defineHiddenClass} method is dynamically added as a new member
3792          * to the nest of a lookup class and/or whether a hidden class has
3793          * a strong relationship with the class loader marked as its defining loader.
3794          *
3795          * @since 15
3796          */
3797         public enum ClassOption {
3798             /**
3799              * This class option specifies the hidden class be added to
3800              * {@linkplain Class#getNestHost nest} of a lookup class as
3801              * a nestmate.
3802              *
3803              * <p> A hidden nestmate class has access to the private members of all
3804              * classes and interfaces in the same nest.
3805              *
3806              * @see Class#getNestHost()
3807              */
3808             NESTMATE(NESTMATE_CLASS),
3809 
3810             /**
3811              *
3812              * This class option specifies the hidden class to have a <em>strong</em>
3813              * relationship with the class loader marked as its defining loader,
3814              * as a normal class or interface has with its own defining loader.
3815              * This means that the hidden class may be unloaded if and only if
3816              * its defining loader is not reachable and thus may be reclaimed
3817              * by a garbage collector (JLS 12.7).
3818              *
3819              * <p> By default, a hidden class or interface may be unloaded
3820              * even if the class loader that is marked as its defining loader is
3821              * <a href="../ref/package.html#reachability">reachable</a>.
3822 
3823              *
3824              * @jls 12.7 Unloading of Classes and Interfaces
3825              */
3826             STRONG(STRONG_LOADER_LINK);
3827 
3828             /* the flag value is used by VM at define class time */
3829             private final int flag;
3830             ClassOption(int flag) {
3831                 this.flag = flag;
3832             }
3833 
3834             static int optionsToFlag(Set<ClassOption> options) {
3835                 int flags = 0;
3836                 for (ClassOption cp : options) {
3837                     flags |= cp.flag;
3838                 }
3839                 return flags;
3840             }
3841         }
3842     }
3843 
3844     /**
3845      * Produces a method handle constructing arrays of a desired type,
3846      * as if by the {@code anewarray} bytecode.
3847      * The return type of the method handle will be the array type.
3848      * The type of its sole argument will be {@code int}, which specifies the size of the array.
3849      *
3850      * <p> If the returned method handle is invoked with a negative
3851      * array size, a {@code NegativeArraySizeException} will be thrown.
3852      *
3853      * @param arrayClass an array type
3854      * @return a method handle which can create arrays of the given type
3855      * @throws NullPointerException if the argument is {@code null}
3856      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
3857      * @see java.lang.reflect.Array#newInstance(Class, int)
3858      * @jvms 6.5 {@code anewarray} Instruction
3859      * @since 9
3860      */
3861     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
3862         if (!arrayClass.isArray()) {
3863             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
3864         }
3865         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
3866                 bindTo(arrayClass.getComponentType());
3867         return ani.asType(ani.type().changeReturnType(arrayClass));
3868     }
3869 
3870     /**
3871      * Produces a method handle returning the length of an array,
3872      * as if by the {@code arraylength} bytecode.
3873      * The type of the method handle will have {@code int} as return type,
3874      * and its sole argument will be the array type.
3875      *
3876      * <p> If the returned method handle is invoked with a {@code null}
3877      * array reference, a {@code NullPointerException} will be thrown.
3878      *
3879      * @param arrayClass an array type
3880      * @return a method handle which can retrieve the length of an array of the given array type
3881      * @throws NullPointerException if the argument is {@code null}
3882      * @throws IllegalArgumentException if arrayClass is not an array type
3883      * @jvms 6.5 {@code arraylength} Instruction
3884      * @since 9
3885      */
3886     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
3887         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
3888     }
3889 
3890     /**
3891      * Produces a method handle giving read access to elements of an array,
3892      * as if by the {@code aaload} bytecode.
3893      * The type of the method handle will have a return type of the array's
3894      * element type.  Its first argument will be the array type,
3895      * and the second will be {@code int}.
3896      *
3897      * <p> When the returned method handle is invoked,
3898      * the array reference and array index are checked.
3899      * A {@code NullPointerException} will be thrown if the array reference
3900      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3901      * thrown if the index is negative or if it is greater than or equal to
3902      * the length of the array.
3903      *
3904      * @param arrayClass an array type
3905      * @return a method handle which can load values from the given array type
3906      * @throws NullPointerException if the argument is null
3907      * @throws  IllegalArgumentException if arrayClass is not an array type
3908      * @jvms 6.5 {@code aaload} Instruction
3909      */
3910     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
3911         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
3912     }
3913 
3914     /**
3915      * Produces a method handle giving write access to elements of an array,
3916      * as if by the {@code astore} bytecode.
3917      * The type of the method handle will have a void return type.
3918      * Its last argument will be the array's element type.
3919      * The first and second arguments will be the array type and int.
3920      *
3921      * <p> When the returned method handle is invoked,
3922      * the array reference and array index are checked.
3923      * A {@code NullPointerException} will be thrown if the array reference
3924      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3925      * thrown if the index is negative or if it is greater than or equal to
3926      * the length of the array.
3927      *
3928      * @param arrayClass the class of an array
3929      * @return a method handle which can store values into the array type
3930      * @throws NullPointerException if the argument is null
3931      * @throws IllegalArgumentException if arrayClass is not an array type
3932      * @jvms 6.5 {@code aastore} Instruction
3933      */
3934     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
3935         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
3936     }
3937 
3938     /**
3939      * Produces a VarHandle giving access to elements of an array of type
3940      * {@code arrayClass}.  The VarHandle's variable type is the component type
3941      * of {@code arrayClass} and the list of coordinate types is
3942      * {@code (arrayClass, int)}, where the {@code int} coordinate type
3943      * corresponds to an argument that is an index into an array.
3944      * <p>
3945      * Certain access modes of the returned VarHandle are unsupported under
3946      * the following conditions:
3947      * <ul>
3948      * <li>if the component type is anything other than {@code byte},
3949      *     {@code short}, {@code char}, {@code int}, {@code long},
3950      *     {@code float}, or {@code double} then numeric atomic update access
3951      *     modes are unsupported.
3952      * <li>if the field type is anything other than {@code boolean},
3953      *     {@code byte}, {@code short}, {@code char}, {@code int} or
3954      *     {@code long} then bitwise atomic update access modes are
3955      *     unsupported.
3956      * </ul>
3957      * <p>
3958      * If the component type is {@code float} or {@code double} then numeric
3959      * and atomic update access modes compare values using their bitwise
3960      * representation (see {@link Float#floatToRawIntBits} and
3961      * {@link Double#doubleToRawLongBits}, respectively).
3962      *
3963      * <p> When the returned {@code VarHandle} is invoked,
3964      * the array reference and array index are checked.
3965      * A {@code NullPointerException} will be thrown if the array reference
3966      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3967      * thrown if the index is negative or if it is greater than or equal to
3968      * the length of the array.
3969      *
3970      * @apiNote
3971      * Bitwise comparison of {@code float} values or {@code double} values,
3972      * as performed by the numeric and atomic update access modes, differ
3973      * from the primitive {@code ==} operator and the {@link Float#equals}
3974      * and {@link Double#equals} methods, specifically with respect to
3975      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3976      * Care should be taken when performing a compare and set or a compare
3977      * and exchange operation with such values since the operation may
3978      * unexpectedly fail.
3979      * There are many possible NaN values that are considered to be
3980      * {@code NaN} in Java, although no IEEE 754 floating-point operation
3981      * provided by Java can distinguish between them.  Operation failure can
3982      * occur if the expected or witness value is a NaN value and it is
3983      * transformed (perhaps in a platform specific manner) into another NaN
3984      * value, and thus has a different bitwise representation (see
3985      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3986      * details).
3987      * The values {@code -0.0} and {@code +0.0} have different bitwise
3988      * representations but are considered equal when using the primitive
3989      * {@code ==} operator.  Operation failure can occur if, for example, a
3990      * numeric algorithm computes an expected value to be say {@code -0.0}
3991      * and previously computed the witness value to be say {@code +0.0}.
3992      * @param arrayClass the class of an array, of type {@code T[]}
3993      * @return a VarHandle giving access to elements of an array
3994      * @throws NullPointerException if the arrayClass is null
3995      * @throws IllegalArgumentException if arrayClass is not an array type
3996      * @since 9
3997      */
3998     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
3999         return VarHandles.makeArrayElementHandle(arrayClass);
4000     }
4001 
4002     /**
4003      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4004      * viewed as if it were a different primitive array type, such as
4005      * {@code int[]} or {@code long[]}.
4006      * The VarHandle's variable type is the component type of
4007      * {@code viewArrayClass} and the list of coordinate types is
4008      * {@code (byte[], int)}, where the {@code int} coordinate type
4009      * corresponds to an argument that is an index into a {@code byte[]} array.
4010      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4011      * array, composing bytes to or from a value of the component type of
4012      * {@code viewArrayClass} according to the given endianness.
4013      * <p>
4014      * The supported component types (variables types) are {@code short},
4015      * {@code char}, {@code int}, {@code long}, {@code float} and
4016      * {@code double}.
4017      * <p>
4018      * Access of bytes at a given index will result in an
4019      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4020      * or greater than the {@code byte[]} array length minus the size (in bytes)
4021      * of {@code T}.
4022      * <p>
4023      * Access of bytes at an index may be aligned or misaligned for {@code T},
4024      * with respect to the underlying memory address, {@code A} say, associated
4025      * with the array and index.
4026      * If access is misaligned then access for anything other than the
4027      * {@code get} and {@code set} access modes will result in an
4028      * {@code IllegalStateException}.  In such cases atomic access is only
4029      * guaranteed with respect to the largest power of two that divides the GCD
4030      * of {@code A} and the size (in bytes) of {@code T}.
4031      * If access is aligned then following access modes are supported and are
4032      * guaranteed to support atomic access:
4033      * <ul>
4034      * <li>read write access modes for all {@code T}, with the exception of
4035      *     access modes {@code get} and {@code set} for {@code long} and
4036      *     {@code double} on 32-bit platforms.
4037      * <li>atomic update access modes for {@code int}, {@code long},
4038      *     {@code float} or {@code double}.
4039      *     (Future major platform releases of the JDK may support additional
4040      *     types for certain currently unsupported access modes.)
4041      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4042      *     (Future major platform releases of the JDK may support additional
4043      *     numeric types for certain currently unsupported access modes.)
4044      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4045      *     (Future major platform releases of the JDK may support additional
4046      *     numeric types for certain currently unsupported access modes.)
4047      * </ul>
4048      * <p>
4049      * Misaligned access, and therefore atomicity guarantees, may be determined
4050      * for {@code byte[]} arrays without operating on a specific array.  Given
4051      * an {@code index}, {@code T} and it's corresponding boxed type,
4052      * {@code T_BOX}, misalignment may be determined as follows:
4053      * <pre>{@code
4054      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4055      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
4056      *     alignmentOffset(0, sizeOfT);
4057      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
4058      * boolean isMisaligned = misalignedAtIndex != 0;
4059      * }</pre>
4060      * <p>
4061      * If the variable type is {@code float} or {@code double} then atomic
4062      * update access modes compare values using their bitwise representation
4063      * (see {@link Float#floatToRawIntBits} and
4064      * {@link Double#doubleToRawLongBits}, respectively).
4065      * @param viewArrayClass the view array class, with a component type of
4066      * type {@code T}
4067      * @param byteOrder the endianness of the view array elements, as
4068      * stored in the underlying {@code byte} array
4069      * @return a VarHandle giving access to elements of a {@code byte[]} array
4070      * viewed as if elements corresponding to the components type of the view
4071      * array class
4072      * @throws NullPointerException if viewArrayClass or byteOrder is null
4073      * @throws IllegalArgumentException if viewArrayClass is not an array type
4074      * @throws UnsupportedOperationException if the component type of
4075      * viewArrayClass is not supported as a variable type
4076      * @since 9
4077      */
4078     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4079                                      ByteOrder byteOrder) throws IllegalArgumentException {
4080         Objects.requireNonNull(byteOrder);
4081         return VarHandles.byteArrayViewHandle(viewArrayClass,
4082                                               byteOrder == ByteOrder.BIG_ENDIAN);
4083     }
4084 
4085     /**
4086      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4087      * viewed as if it were an array of elements of a different primitive
4088      * component type to that of {@code byte}, such as {@code int[]} or
4089      * {@code long[]}.
4090      * The VarHandle's variable type is the component type of
4091      * {@code viewArrayClass} and the list of coordinate types is
4092      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4093      * corresponds to an argument that is an index into a {@code byte[]} array.
4094      * The returned VarHandle accesses bytes at an index in a
4095      * {@code ByteBuffer}, composing bytes to or from a value of the component
4096      * type of {@code viewArrayClass} according to the given endianness.
4097      * <p>
4098      * The supported component types (variables types) are {@code short},
4099      * {@code char}, {@code int}, {@code long}, {@code float} and
4100      * {@code double}.
4101      * <p>
4102      * Access will result in a {@code ReadOnlyBufferException} for anything
4103      * other than the read access modes if the {@code ByteBuffer} is read-only.
4104      * <p>
4105      * Access of bytes at a given index will result in an
4106      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4107      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4108      * {@code T}.
4109      * <p>
4110      * Access of bytes at an index may be aligned or misaligned for {@code T},
4111      * with respect to the underlying memory address, {@code A} say, associated
4112      * with the {@code ByteBuffer} and index.
4113      * If access is misaligned then access for anything other than the
4114      * {@code get} and {@code set} access modes will result in an
4115      * {@code IllegalStateException}.  In such cases atomic access is only
4116      * guaranteed with respect to the largest power of two that divides the GCD
4117      * of {@code A} and the size (in bytes) of {@code T}.
4118      * If access is aligned then following access modes are supported and are
4119      * guaranteed to support atomic access:
4120      * <ul>
4121      * <li>read write access modes for all {@code T}, with the exception of
4122      *     access modes {@code get} and {@code set} for {@code long} and
4123      *     {@code double} on 32-bit platforms.
4124      * <li>atomic update access modes for {@code int}, {@code long},
4125      *     {@code float} or {@code double}.
4126      *     (Future major platform releases of the JDK may support additional
4127      *     types for certain currently unsupported access modes.)
4128      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4129      *     (Future major platform releases of the JDK may support additional
4130      *     numeric types for certain currently unsupported access modes.)
4131      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4132      *     (Future major platform releases of the JDK may support additional
4133      *     numeric types for certain currently unsupported access modes.)
4134      * </ul>
4135      * <p>
4136      * Misaligned access, and therefore atomicity guarantees, may be determined
4137      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4138      * {@code index}, {@code T} and it's corresponding boxed type,
4139      * {@code T_BOX}, as follows:
4140      * <pre>{@code
4141      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4142      * ByteBuffer bb = ...
4143      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4144      * boolean isMisaligned = misalignedAtIndex != 0;
4145      * }</pre>
4146      * <p>
4147      * If the variable type is {@code float} or {@code double} then atomic
4148      * update access modes compare values using their bitwise representation
4149      * (see {@link Float#floatToRawIntBits} and
4150      * {@link Double#doubleToRawLongBits}, respectively).
4151      * @param viewArrayClass the view array class, with a component type of
4152      * type {@code T}
4153      * @param byteOrder the endianness of the view array elements, as
4154      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4155      * endianness of a {@code ByteBuffer})
4156      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4157      * viewed as if elements corresponding to the components type of the view
4158      * array class
4159      * @throws NullPointerException if viewArrayClass or byteOrder is null
4160      * @throws IllegalArgumentException if viewArrayClass is not an array type
4161      * @throws UnsupportedOperationException if the component type of
4162      * viewArrayClass is not supported as a variable type
4163      * @since 9
4164      */
4165     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4166                                       ByteOrder byteOrder) throws IllegalArgumentException {
4167         Objects.requireNonNull(byteOrder);
4168         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4169                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4170     }
4171 
4172 
4173     /// method handle invocation (reflective style)
4174 
4175     /**
4176      * Produces a method handle which will invoke any method handle of the
4177      * given {@code type}, with a given number of trailing arguments replaced by
4178      * a single trailing {@code Object[]} array.
4179      * The resulting invoker will be a method handle with the following
4180      * arguments:
4181      * <ul>
4182      * <li>a single {@code MethodHandle} target
4183      * <li>zero or more leading values (counted by {@code leadingArgCount})
4184      * <li>an {@code Object[]} array containing trailing arguments
4185      * </ul>
4186      * <p>
4187      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4188      * the indicated {@code type}.
4189      * That is, if the target is exactly of the given {@code type}, it will behave
4190      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4191      * is used to convert the target to the required {@code type}.
4192      * <p>
4193      * The type of the returned invoker will not be the given {@code type}, but rather
4194      * will have all parameters except the first {@code leadingArgCount}
4195      * replaced by a single array of type {@code Object[]}, which will be
4196      * the final parameter.
4197      * <p>
4198      * Before invoking its target, the invoker will spread the final array, apply
4199      * reference casts as necessary, and unbox and widen primitive arguments.
4200      * If, when the invoker is called, the supplied array argument does
4201      * not have the correct number of elements, the invoker will throw
4202      * an {@link IllegalArgumentException} instead of invoking the target.
4203      * <p>
4204      * This method is equivalent to the following code (though it may be more efficient):
4205      * <blockquote><pre>{@code
4206 MethodHandle invoker = MethodHandles.invoker(type);
4207 int spreadArgCount = type.parameterCount() - leadingArgCount;
4208 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4209 return invoker;
4210      * }</pre></blockquote>
4211      * This method throws no reflective or security exceptions.
4212      * @param type the desired target type
4213      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4214      * @return a method handle suitable for invoking any method handle of the given type
4215      * @throws NullPointerException if {@code type} is null
4216      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4217      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4218      *                  or if the resulting method handle's type would have
4219      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4220      */
4221     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4222         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4223             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4224         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4225         return type.invokers().spreadInvoker(leadingArgCount);
4226     }
4227 
4228     /**
4229      * Produces a special <em>invoker method handle</em> which can be used to
4230      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4231      * The resulting invoker will have a type which is
4232      * exactly equal to the desired type, except that it will accept
4233      * an additional leading argument of type {@code MethodHandle}.
4234      * <p>
4235      * This method is equivalent to the following code (though it may be more efficient):
4236      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4237      *
4238      * <p style="font-size:smaller;">
4239      * <em>Discussion:</em>
4240      * Invoker method handles can be useful when working with variable method handles
4241      * of unknown types.
4242      * For example, to emulate an {@code invokeExact} call to a variable method
4243      * handle {@code M}, extract its type {@code T},
4244      * look up the invoker method {@code X} for {@code T},
4245      * and call the invoker method, as {@code X.invoke(T, A...)}.
4246      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4247      * is unknown.)
4248      * If spreading, collecting, or other argument transformations are required,
4249      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4250      * method handle values, as long as they are compatible with the type of {@code X}.
4251      * <p style="font-size:smaller;">
4252      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4253      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4254      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4255      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4256      * <p>
4257      * This method throws no reflective or security exceptions.
4258      * @param type the desired target type
4259      * @return a method handle suitable for invoking any method handle of the given type
4260      * @throws IllegalArgumentException if the resulting method handle's type would have
4261      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4262      */
4263     public static MethodHandle exactInvoker(MethodType type) {
4264         return type.invokers().exactInvoker();
4265     }
4266 
4267     /**
4268      * Produces a special <em>invoker method handle</em> which can be used to
4269      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4270      * The resulting invoker will have a type which is
4271      * exactly equal to the desired type, except that it will accept
4272      * an additional leading argument of type {@code MethodHandle}.
4273      * <p>
4274      * Before invoking its target, if the target differs from the expected type,
4275      * the invoker will apply reference casts as
4276      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4277      * Similarly, the return value will be converted as necessary.
4278      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4279      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4280      * <p>
4281      * This method is equivalent to the following code (though it may be more efficient):
4282      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4283      * <p style="font-size:smaller;">
4284      * <em>Discussion:</em>
4285      * A {@linkplain MethodType#genericMethodType general method type} is one which
4286      * mentions only {@code Object} arguments and return values.
4287      * An invoker for such a type is capable of calling any method handle
4288      * of the same arity as the general type.
4289      * <p style="font-size:smaller;">
4290      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4291      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4292      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4293      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4294      * <p>
4295      * This method throws no reflective or security exceptions.
4296      * @param type the desired target type
4297      * @return a method handle suitable for invoking any method handle convertible to the given type
4298      * @throws IllegalArgumentException if the resulting method handle's type would have
4299      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4300      */
4301     public static MethodHandle invoker(MethodType type) {
4302         return type.invokers().genericInvoker();
4303     }
4304 
4305     /**
4306      * Produces a special <em>invoker method handle</em> which can be used to
4307      * invoke a signature-polymorphic access mode method on any VarHandle whose
4308      * associated access mode type is compatible with the given type.
4309      * The resulting invoker will have a type which is exactly equal to the
4310      * desired given type, except that it will accept an additional leading
4311      * argument of type {@code VarHandle}.
4312      *
4313      * @param accessMode the VarHandle access mode
4314      * @param type the desired target type
4315      * @return a method handle suitable for invoking an access mode method of
4316      *         any VarHandle whose access mode type is of the given type.
4317      * @since 9
4318      */
4319     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4320         return type.invokers().varHandleMethodExactInvoker(accessMode);
4321     }
4322 
4323     /**
4324      * Produces a special <em>invoker method handle</em> which can be used to
4325      * invoke a signature-polymorphic access mode method on any VarHandle whose
4326      * associated access mode type is compatible with the given type.
4327      * The resulting invoker will have a type which is exactly equal to the
4328      * desired given type, except that it will accept an additional leading
4329      * argument of type {@code VarHandle}.
4330      * <p>
4331      * Before invoking its target, if the access mode type differs from the
4332      * desired given type, the invoker will apply reference casts as necessary
4333      * and box, unbox, or widen primitive values, as if by
4334      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4335      * converted as necessary.
4336      * <p>
4337      * This method is equivalent to the following code (though it may be more
4338      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4339      *
4340      * @param accessMode the VarHandle access mode
4341      * @param type the desired target type
4342      * @return a method handle suitable for invoking an access mode method of
4343      *         any VarHandle whose access mode type is convertible to the given
4344      *         type.
4345      * @since 9
4346      */
4347     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4348         return type.invokers().varHandleMethodInvoker(accessMode);
4349     }
4350 
4351     /*non-public*/
4352     static MethodHandle basicInvoker(MethodType type) {
4353         return type.invokers().basicInvoker();
4354     }
4355 
4356      /// method handle modification (creation from other method handles)
4357 
4358     /**
4359      * Produces a method handle which adapts the type of the
4360      * given method handle to a new type by pairwise argument and return type conversion.
4361      * The original type and new type must have the same number of arguments.
4362      * The resulting method handle is guaranteed to report a type
4363      * which is equal to the desired new type.
4364      * <p>
4365      * If the original type and new type are equal, returns target.
4366      * <p>
4367      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4368      * and some additional conversions are also applied if those conversions fail.
4369      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4370      * if possible, before or instead of any conversions done by {@code asType}:
4371      * <ul>
4372      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4373      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4374      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4375      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4376      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4377      *     (This treatment follows the usage of the bytecode verifier.)
4378      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4379      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
4380      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4381      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4382      *     then a Java casting conversion (JLS 5.5) is applied.
4383      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4384      *     widening and/or narrowing.)
4385      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4386      *     conversion will be applied at runtime, possibly followed
4387      *     by a Java casting conversion (JLS 5.5) on the primitive value,
4388      *     possibly followed by a conversion from byte to boolean by testing
4389      *     the low-order bit.
4390      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4391      *     and if the reference is null at runtime, a zero value is introduced.
4392      * </ul>
4393      * @param target the method handle to invoke after arguments are retyped
4394      * @param newType the expected type of the new method handle
4395      * @return a method handle which delegates to the target after performing
4396      *           any necessary argument conversions, and arranges for any
4397      *           necessary return value conversions
4398      * @throws NullPointerException if either argument is null
4399      * @throws WrongMethodTypeException if the conversion cannot be made
4400      * @see MethodHandle#asType
4401      */
4402     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4403         explicitCastArgumentsChecks(target, newType);
4404         // use the asTypeCache when possible:
4405         MethodType oldType = target.type();
4406         if (oldType == newType)  return target;
4407         if (oldType.explicitCastEquivalentToAsType(newType)) {
4408             return target.asFixedArity().asType(newType);
4409         }
4410         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4411     }
4412 
4413     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4414         if (target.type().parameterCount() != newType.parameterCount()) {
4415             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4416         }
4417     }
4418 
4419     /**
4420      * Produces a method handle which adapts the calling sequence of the
4421      * given method handle to a new type, by reordering the arguments.
4422      * The resulting method handle is guaranteed to report a type
4423      * which is equal to the desired new type.
4424      * <p>
4425      * The given array controls the reordering.
4426      * Call {@code #I} the number of incoming parameters (the value
4427      * {@code newType.parameterCount()}, and call {@code #O} the number
4428      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4429      * Then the length of the reordering array must be {@code #O},
4430      * and each element must be a non-negative number less than {@code #I}.
4431      * For every {@code N} less than {@code #O}, the {@code N}-th
4432      * outgoing argument will be taken from the {@code I}-th incoming
4433      * argument, where {@code I} is {@code reorder[N]}.
4434      * <p>
4435      * No argument or return value conversions are applied.
4436      * The type of each incoming argument, as determined by {@code newType},
4437      * must be identical to the type of the corresponding outgoing parameter
4438      * or parameters in the target method handle.
4439      * The return type of {@code newType} must be identical to the return
4440      * type of the original target.
4441      * <p>
4442      * The reordering array need not specify an actual permutation.
4443      * An incoming argument will be duplicated if its index appears
4444      * more than once in the array, and an incoming argument will be dropped
4445      * if its index does not appear in the array.
4446      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4447      * incoming arguments which are not mentioned in the reordering array
4448      * may be of any type, as determined only by {@code newType}.
4449      * <blockquote><pre>{@code
4450 import static java.lang.invoke.MethodHandles.*;
4451 import static java.lang.invoke.MethodType.*;
4452 ...
4453 MethodType intfn1 = methodType(int.class, int.class);
4454 MethodType intfn2 = methodType(int.class, int.class, int.class);
4455 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4456 assert(sub.type().equals(intfn2));
4457 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4458 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4459 assert((int)rsub.invokeExact(1, 100) == 99);
4460 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4461 assert(add.type().equals(intfn2));
4462 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4463 assert(twice.type().equals(intfn1));
4464 assert((int)twice.invokeExact(21) == 42);
4465      * }</pre></blockquote>
4466      * <p>
4467      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4468      * variable-arity method handle}, even if the original target method handle was.
4469      * @param target the method handle to invoke after arguments are reordered
4470      * @param newType the expected type of the new method handle
4471      * @param reorder an index array which controls the reordering
4472      * @return a method handle which delegates to the target after it
4473      *           drops unused arguments and moves and/or duplicates the other arguments
4474      * @throws NullPointerException if any argument is null
4475      * @throws IllegalArgumentException if the index array length is not equal to
4476      *                  the arity of the target, or if any index array element
4477      *                  not a valid index for a parameter of {@code newType},
4478      *                  or if two corresponding parameter types in
4479      *                  {@code target.type()} and {@code newType} are not identical,
4480      */
4481     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4482         reorder = reorder.clone();  // get a private copy
4483         MethodType oldType = target.type();
4484         permuteArgumentChecks(reorder, newType, oldType);
4485         // first detect dropped arguments and handle them separately
4486         int[] originalReorder = reorder;
4487         BoundMethodHandle result = target.rebind();
4488         LambdaForm form = result.form;
4489         int newArity = newType.parameterCount();
4490         // Normalize the reordering into a real permutation,
4491         // by removing duplicates and adding dropped elements.
4492         // This somewhat improves lambda form caching, as well
4493         // as simplifying the transform by breaking it up into steps.
4494         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4495             if (ddIdx > 0) {
4496                 // We found a duplicated entry at reorder[ddIdx].
4497                 // Example:  (x,y,z)->asList(x,y,z)
4498                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4499                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4500                 // The starred element corresponds to the argument
4501                 // deleted by the dupArgumentForm transform.
4502                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4503                 boolean killFirst = false;
4504                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4505                     // Set killFirst if the dup is larger than an intervening position.
4506                     // This will remove at least one inversion from the permutation.
4507                     if (dupVal > val) killFirst = true;
4508                 }
4509                 if (!killFirst) {
4510                     srcPos = dstPos;
4511                     dstPos = ddIdx;
4512                 }
4513                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4514                 assert (reorder[srcPos] == reorder[dstPos]);
4515                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4516                 // contract the reordering by removing the element at dstPos
4517                 int tailPos = dstPos + 1;
4518                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4519                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4520             } else {
4521                 int dropVal = ~ddIdx, insPos = 0;
4522                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4523                     // Find first element of reorder larger than dropVal.
4524                     // This is where we will insert the dropVal.
4525                     insPos += 1;
4526                 }
4527                 Class<?> ptype = newType.parameterType(dropVal);
4528                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4529                 oldType = oldType.insertParameterTypes(insPos, ptype);
4530                 // expand the reordering by inserting an element at insPos
4531                 int tailPos = insPos + 1;
4532                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4533                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4534                 reorder[insPos] = dropVal;
4535             }
4536             assert (permuteArgumentChecks(reorder, newType, oldType));
4537         }
4538         assert (reorder.length == newArity);  // a perfect permutation
4539         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4540         form = form.editor().permuteArgumentsForm(1, reorder);
4541         if (newType == result.type() && form == result.internalForm())
4542             return result;
4543         return result.copyWith(newType, form);
4544     }
4545 
4546     /**
4547      * Return an indication of any duplicate or omission in reorder.
4548      * If the reorder contains a duplicate entry, return the index of the second occurrence.
4549      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
4550      * Otherwise, return zero.
4551      * If an element not in [0..newArity-1] is encountered, return reorder.length.
4552      */
4553     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
4554         final int BIT_LIMIT = 63;  // max number of bits in bit mask
4555         if (newArity < BIT_LIMIT) {
4556             long mask = 0;
4557             for (int i = 0; i < reorder.length; i++) {
4558                 int arg = reorder[i];
4559                 if (arg >= newArity) {
4560                     return reorder.length;
4561                 }
4562                 long bit = 1L << arg;
4563                 if ((mask & bit) != 0) {
4564                     return i;  // >0 indicates a dup
4565                 }
4566                 mask |= bit;
4567             }
4568             if (mask == (1L << newArity) - 1) {
4569                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
4570                 return 0;
4571             }
4572             // find first zero
4573             long zeroBit = Long.lowestOneBit(~mask);
4574             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
4575             assert(zeroPos <= newArity);
4576             if (zeroPos == newArity) {
4577                 return 0;
4578             }
4579             return ~zeroPos;
4580         } else {
4581             // same algorithm, different bit set
4582             BitSet mask = new BitSet(newArity);
4583             for (int i = 0; i < reorder.length; i++) {
4584                 int arg = reorder[i];
4585                 if (arg >= newArity) {
4586                     return reorder.length;
4587                 }
4588                 if (mask.get(arg)) {
4589                     return i;  // >0 indicates a dup
4590                 }
4591                 mask.set(arg);
4592             }
4593             int zeroPos = mask.nextClearBit(0);
4594             assert(zeroPos <= newArity);
4595             if (zeroPos == newArity) {
4596                 return 0;
4597             }
4598             return ~zeroPos;
4599         }
4600     }
4601 
4602     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
4603         if (newType.returnType() != oldType.returnType())
4604             throw newIllegalArgumentException("return types do not match",
4605                     oldType, newType);
4606         if (reorder.length == oldType.parameterCount()) {
4607             int limit = newType.parameterCount();
4608             boolean bad = false;
4609             for (int j = 0; j < reorder.length; j++) {
4610                 int i = reorder[j];
4611                 if (i < 0 || i >= limit) {
4612                     bad = true; break;
4613                 }
4614                 Class<?> src = newType.parameterType(i);
4615                 Class<?> dst = oldType.parameterType(j);
4616                 if (src != dst)
4617                     throw newIllegalArgumentException("parameter types do not match after reorder",
4618                             oldType, newType);
4619             }
4620             if (!bad)  return true;
4621         }
4622         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
4623     }
4624 
4625     /**
4626      * Produces a method handle of the requested return type which returns the given
4627      * constant value every time it is invoked.
4628      * <p>
4629      * Before the method handle is returned, the passed-in value is converted to the requested type.
4630      * If the requested type is primitive, widening primitive conversions are attempted,
4631      * else reference conversions are attempted.
4632      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
4633      * @param type the return type of the desired method handle
4634      * @param value the value to return
4635      * @return a method handle of the given return type and no arguments, which always returns the given value
4636      * @throws NullPointerException if the {@code type} argument is null
4637      * @throws ClassCastException if the value cannot be converted to the required return type
4638      * @throws IllegalArgumentException if the given type is {@code void.class}
4639      */
4640     public static MethodHandle constant(Class<?> type, Object value) {
4641         if (type.isPrimitive()) {
4642             if (type == void.class)
4643                 throw newIllegalArgumentException("void type");
4644             Wrapper w = Wrapper.forPrimitiveType(type);
4645             value = w.convert(value, type);
4646             if (w.zero().equals(value))
4647                 return zero(w, type);
4648             return insertArguments(identity(type), 0, value);
4649         } else {
4650             if (value == null)
4651                 return zero(Wrapper.OBJECT, type);
4652             return identity(type).bindTo(value);
4653         }
4654     }
4655 
4656     /**
4657      * Produces a method handle which returns its sole argument when invoked.
4658      * @param type the type of the sole parameter and return value of the desired method handle
4659      * @return a unary method handle which accepts and returns the given type
4660      * @throws NullPointerException if the argument is null
4661      * @throws IllegalArgumentException if the given type is {@code void.class}
4662      */
4663     public static MethodHandle identity(Class<?> type) {
4664         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
4665         int pos = btw.ordinal();
4666         MethodHandle ident = IDENTITY_MHS[pos];
4667         if (ident == null) {
4668             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
4669         }
4670         if (ident.type().returnType() == type)
4671             return ident;
4672         // something like identity(Foo.class); do not bother to intern these
4673         assert (btw == Wrapper.OBJECT);
4674         return makeIdentity(type);
4675     }
4676 
4677     /**
4678      * Produces a constant method handle of the requested return type which
4679      * returns the default value for that type every time it is invoked.
4680      * The resulting constant method handle will have no side effects.
4681      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
4682      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
4683      * since {@code explicitCastArguments} converts {@code null} to default values.
4684      * @param type the expected return type of the desired method handle
4685      * @return a constant method handle that takes no arguments
4686      *         and returns the default value of the given type (or void, if the type is void)
4687      * @throws NullPointerException if the argument is null
4688      * @see MethodHandles#constant
4689      * @see MethodHandles#empty
4690      * @see MethodHandles#explicitCastArguments
4691      * @since 9
4692      */
4693     public static MethodHandle zero(Class<?> type) {
4694         Objects.requireNonNull(type);
4695         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
4696     }
4697 
4698     private static MethodHandle identityOrVoid(Class<?> type) {
4699         return type == void.class ? zero(type) : identity(type);
4700     }
4701 
4702     /**
4703      * Produces a method handle of the requested type which ignores any arguments, does nothing,
4704      * and returns a suitable default depending on the return type.
4705      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
4706      * <p>The returned method handle is equivalent to
4707      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
4708      *
4709      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
4710      * {@code guardWithTest(pred, target, empty(target.type())}.
4711      * @param type the type of the desired method handle
4712      * @return a constant method handle of the given type, which returns a default value of the given return type
4713      * @throws NullPointerException if the argument is null
4714      * @see MethodHandles#zero
4715      * @see MethodHandles#constant
4716      * @since 9
4717      */
4718     public static  MethodHandle empty(MethodType type) {
4719         Objects.requireNonNull(type);
4720         return dropArguments(zero(type.returnType()), 0, type.parameterList());
4721     }
4722 
4723     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
4724     private static MethodHandle makeIdentity(Class<?> ptype) {
4725         MethodType mtype = methodType(ptype, ptype);
4726         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
4727         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
4728     }
4729 
4730     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
4731         int pos = btw.ordinal();
4732         MethodHandle zero = ZERO_MHS[pos];
4733         if (zero == null) {
4734             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
4735         }
4736         if (zero.type().returnType() == rtype)
4737             return zero;
4738         assert(btw == Wrapper.OBJECT);
4739         return makeZero(rtype);
4740     }
4741     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
4742     private static MethodHandle makeZero(Class<?> rtype) {
4743         MethodType mtype = methodType(rtype);
4744         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
4745         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
4746     }
4747 
4748     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
4749         // Simulate a CAS, to avoid racy duplication of results.
4750         MethodHandle prev = cache[pos];
4751         if (prev != null) return prev;
4752         return cache[pos] = value;
4753     }
4754 
4755     /**
4756      * Provides a target method handle with one or more <em>bound arguments</em>
4757      * in advance of the method handle's invocation.
4758      * The formal parameters to the target corresponding to the bound
4759      * arguments are called <em>bound parameters</em>.
4760      * Returns a new method handle which saves away the bound arguments.
4761      * When it is invoked, it receives arguments for any non-bound parameters,
4762      * binds the saved arguments to their corresponding parameters,
4763      * and calls the original target.
4764      * <p>
4765      * The type of the new method handle will drop the types for the bound
4766      * parameters from the original target type, since the new method handle
4767      * will no longer require those arguments to be supplied by its callers.
4768      * <p>
4769      * Each given argument object must match the corresponding bound parameter type.
4770      * If a bound parameter type is a primitive, the argument object
4771      * must be a wrapper, and will be unboxed to produce the primitive value.
4772      * <p>
4773      * The {@code pos} argument selects which parameters are to be bound.
4774      * It may range between zero and <i>N-L</i> (inclusively),
4775      * where <i>N</i> is the arity of the target method handle
4776      * and <i>L</i> is the length of the values array.
4777      * <p>
4778      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4779      * variable-arity method handle}, even if the original target method handle was.
4780      * @param target the method handle to invoke after the argument is inserted
4781      * @param pos where to insert the argument (zero for the first)
4782      * @param values the series of arguments to insert
4783      * @return a method handle which inserts an additional argument,
4784      *         before calling the original method handle
4785      * @throws NullPointerException if the target or the {@code values} array is null
4786      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
4787      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
4788      *         is the length of the values array.
4789      * @throws ClassCastException if an argument does not match the corresponding bound parameter
4790      *         type.
4791      * @see MethodHandle#bindTo
4792      */
4793     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
4794         int insCount = values.length;
4795         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
4796         if (insCount == 0)  return target;
4797         BoundMethodHandle result = target.rebind();
4798         for (int i = 0; i < insCount; i++) {
4799             Object value = values[i];
4800             Class<?> ptype = ptypes[pos+i];
4801             if (ptype.isPrimitive()) {
4802                 result = insertArgumentPrimitive(result, pos, ptype, value);
4803             } else {
4804                 value = ptype.cast(value);  // throw CCE if needed
4805                 result = result.bindArgumentL(pos, value);
4806             }
4807         }
4808         return result;
4809     }
4810 
4811     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
4812                                                              Class<?> ptype, Object value) {
4813         Wrapper w = Wrapper.forPrimitiveType(ptype);
4814         // perform unboxing and/or primitive conversion
4815         value = w.convert(value, ptype);
4816         switch (w) {
4817         case INT:     return result.bindArgumentI(pos, (int)value);
4818         case LONG:    return result.bindArgumentJ(pos, (long)value);
4819         case FLOAT:   return result.bindArgumentF(pos, (float)value);
4820         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
4821         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
4822         }
4823     }
4824 
4825     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
4826         MethodType oldType = target.type();
4827         int outargs = oldType.parameterCount();
4828         int inargs  = outargs - insCount;
4829         if (inargs < 0)
4830             throw newIllegalArgumentException("too many values to insert");
4831         if (pos < 0 || pos > inargs)
4832             throw newIllegalArgumentException("no argument type to append");
4833         return oldType.ptypes();
4834     }
4835 
4836     /**
4837      * Produces a method handle which will discard some dummy arguments
4838      * before calling some other specified <i>target</i> method handle.
4839      * The type of the new method handle will be the same as the target's type,
4840      * except it will also include the dummy argument types,
4841      * at some given position.
4842      * <p>
4843      * The {@code pos} argument may range between zero and <i>N</i>,
4844      * where <i>N</i> is the arity of the target.
4845      * If {@code pos} is zero, the dummy arguments will precede
4846      * the target's real arguments; if {@code pos} is <i>N</i>
4847      * they will come after.
4848      * <p>
4849      * <b>Example:</b>
4850      * <blockquote><pre>{@code
4851 import static java.lang.invoke.MethodHandles.*;
4852 import static java.lang.invoke.MethodType.*;
4853 ...
4854 MethodHandle cat = lookup().findVirtual(String.class,
4855   "concat", methodType(String.class, String.class));
4856 assertEquals("xy", (String) cat.invokeExact("x", "y"));
4857 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
4858 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
4859 assertEquals(bigType, d0.type());
4860 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
4861      * }</pre></blockquote>
4862      * <p>
4863      * This method is also equivalent to the following code:
4864      * <blockquote><pre>
4865      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
4866      * </pre></blockquote>
4867      * @param target the method handle to invoke after the arguments are dropped
4868      * @param pos position of first argument to drop (zero for the leftmost)
4869      * @param valueTypes the type(s) of the argument(s) to drop
4870      * @return a method handle which drops arguments of the given types,
4871      *         before calling the original method handle
4872      * @throws NullPointerException if the target is null,
4873      *                              or if the {@code valueTypes} list or any of its elements is null
4874      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
4875      *                  or if {@code pos} is negative or greater than the arity of the target,
4876      *                  or if the new method handle's type would have too many parameters
4877      */
4878     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
4879         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
4880     }
4881 
4882     private static List<Class<?>> copyTypes(Object[] array) {
4883         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
4884     }
4885 
4886     private static MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
4887         MethodType oldType = target.type();  // get NPE
4888         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
4889         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
4890         if (dropped == 0)  return target;
4891         BoundMethodHandle result = target.rebind();
4892         LambdaForm lform = result.form;
4893         int insertFormArg = 1 + pos;
4894         for (Class<?> ptype : valueTypes) {
4895             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
4896         }
4897         result = result.copyWith(newType, lform);
4898         return result;
4899     }
4900 
4901     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
4902         int dropped = valueTypes.size();
4903         MethodType.checkSlotCount(dropped);
4904         int outargs = oldType.parameterCount();
4905         int inargs  = outargs + dropped;
4906         if (pos < 0 || pos > outargs)
4907             throw newIllegalArgumentException("no argument type to remove"
4908                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
4909                     );
4910         return dropped;
4911     }
4912 
4913     /**
4914      * Produces a method handle which will discard some dummy arguments
4915      * before calling some other specified <i>target</i> method handle.
4916      * The type of the new method handle will be the same as the target's type,
4917      * except it will also include the dummy argument types,
4918      * at some given position.
4919      * <p>
4920      * The {@code pos} argument may range between zero and <i>N</i>,
4921      * where <i>N</i> is the arity of the target.
4922      * If {@code pos} is zero, the dummy arguments will precede
4923      * the target's real arguments; if {@code pos} is <i>N</i>
4924      * they will come after.
4925      * @apiNote
4926      * <blockquote><pre>{@code
4927 import static java.lang.invoke.MethodHandles.*;
4928 import static java.lang.invoke.MethodType.*;
4929 ...
4930 MethodHandle cat = lookup().findVirtual(String.class,
4931   "concat", methodType(String.class, String.class));
4932 assertEquals("xy", (String) cat.invokeExact("x", "y"));
4933 MethodHandle d0 = dropArguments(cat, 0, String.class);
4934 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
4935 MethodHandle d1 = dropArguments(cat, 1, String.class);
4936 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
4937 MethodHandle d2 = dropArguments(cat, 2, String.class);
4938 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
4939 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
4940 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
4941      * }</pre></blockquote>
4942      * <p>
4943      * This method is also equivalent to the following code:
4944      * <blockquote><pre>
4945      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
4946      * </pre></blockquote>
4947      * @param target the method handle to invoke after the arguments are dropped
4948      * @param pos position of first argument to drop (zero for the leftmost)
4949      * @param valueTypes the type(s) of the argument(s) to drop
4950      * @return a method handle which drops arguments of the given types,
4951      *         before calling the original method handle
4952      * @throws NullPointerException if the target is null,
4953      *                              or if the {@code valueTypes} array or any of its elements is null
4954      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
4955      *                  or if {@code pos} is negative or greater than the arity of the target,
4956      *                  or if the new method handle's type would have
4957      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
4958      */
4959     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
4960         return dropArguments0(target, pos, copyTypes(valueTypes));
4961     }
4962 
4963     // private version which allows caller some freedom with error handling
4964     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
4965                                       boolean nullOnFailure) {
4966         newTypes = copyTypes(newTypes.toArray());
4967         List<Class<?>> oldTypes = target.type().parameterList();
4968         int match = oldTypes.size();
4969         if (skip != 0) {
4970             if (skip < 0 || skip > match) {
4971                 throw newIllegalArgumentException("illegal skip", skip, target);
4972             }
4973             oldTypes = oldTypes.subList(skip, match);
4974             match -= skip;
4975         }
4976         List<Class<?>> addTypes = newTypes;
4977         int add = addTypes.size();
4978         if (pos != 0) {
4979             if (pos < 0 || pos > add) {
4980                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
4981             }
4982             addTypes = addTypes.subList(pos, add);
4983             add -= pos;
4984             assert(addTypes.size() == add);
4985         }
4986         // Do not add types which already match the existing arguments.
4987         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
4988             if (nullOnFailure) {
4989                 return null;
4990             }
4991             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
4992         }
4993         addTypes = addTypes.subList(match, add);
4994         add -= match;
4995         assert(addTypes.size() == add);
4996         // newTypes:     (   P*[pos], M*[match], A*[add] )
4997         // target: ( S*[skip],        M*[match]  )
4998         MethodHandle adapter = target;
4999         if (add > 0) {
5000             adapter = dropArguments0(adapter, skip+ match, addTypes);
5001         }
5002         // adapter: (S*[skip],        M*[match], A*[add] )
5003         if (pos > 0) {
5004             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
5005         }
5006         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5007         return adapter;
5008     }
5009 
5010     /**
5011      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5012      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5013      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5014      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5015      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5016      * {@link #dropArguments(MethodHandle, int, Class[])}.
5017      * <p>
5018      * The resulting handle will have the same return type as the target handle.
5019      * <p>
5020      * In more formal terms, assume these two type lists:<ul>
5021      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5022      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5023      * {@code newTypes}.
5024      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5025      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5026      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5027      * sub-list.
5028      * </ul>
5029      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5030      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5031      * {@link #dropArguments(MethodHandle, int, Class[])}.
5032      *
5033      * @apiNote
5034      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5035      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5036      * <blockquote><pre>{@code
5037 import static java.lang.invoke.MethodHandles.*;
5038 import static java.lang.invoke.MethodType.*;
5039 ...
5040 ...
5041 MethodHandle h0 = constant(boolean.class, true);
5042 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5043 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5044 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5045 if (h1.type().parameterCount() < h2.type().parameterCount())
5046     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5047 else
5048     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5049 MethodHandle h3 = guardWithTest(h0, h1, h2);
5050 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5051      * }</pre></blockquote>
5052      * @param target the method handle to adapt
5053      * @param skip number of targets parameters to disregard (they will be unchanged)
5054      * @param newTypes the list of types to match {@code target}'s parameter type list to
5055      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5056      * @return a possibly adapted method handle
5057      * @throws NullPointerException if either argument is null
5058      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5059      *         or if {@code skip} is negative or greater than the arity of the target,
5060      *         or if {@code pos} is negative or greater than the newTypes list size,
5061      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5062      *         {@code pos}.
5063      * @since 9
5064      */
5065     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5066         Objects.requireNonNull(target);
5067         Objects.requireNonNull(newTypes);
5068         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
5069     }
5070 
5071     /**
5072      * Adapts a target method handle by pre-processing
5073      * one or more of its arguments, each with its own unary filter function,
5074      * and then calling the target with each pre-processed argument
5075      * replaced by the result of its corresponding filter function.
5076      * <p>
5077      * The pre-processing is performed by one or more method handles,
5078      * specified in the elements of the {@code filters} array.
5079      * The first element of the filter array corresponds to the {@code pos}
5080      * argument of the target, and so on in sequence.
5081      * The filter functions are invoked in left to right order.
5082      * <p>
5083      * Null arguments in the array are treated as identity functions,
5084      * and the corresponding arguments left unchanged.
5085      * (If there are no non-null elements in the array, the original target is returned.)
5086      * Each filter is applied to the corresponding argument of the adapter.
5087      * <p>
5088      * If a filter {@code F} applies to the {@code N}th argument of
5089      * the target, then {@code F} must be a method handle which
5090      * takes exactly one argument.  The type of {@code F}'s sole argument
5091      * replaces the corresponding argument type of the target
5092      * in the resulting adapted method handle.
5093      * The return type of {@code F} must be identical to the corresponding
5094      * parameter type of the target.
5095      * <p>
5096      * It is an error if there are elements of {@code filters}
5097      * (null or not)
5098      * which do not correspond to argument positions in the target.
5099      * <p><b>Example:</b>
5100      * <blockquote><pre>{@code
5101 import static java.lang.invoke.MethodHandles.*;
5102 import static java.lang.invoke.MethodType.*;
5103 ...
5104 MethodHandle cat = lookup().findVirtual(String.class,
5105   "concat", methodType(String.class, String.class));
5106 MethodHandle upcase = lookup().findVirtual(String.class,
5107   "toUpperCase", methodType(String.class));
5108 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5109 MethodHandle f0 = filterArguments(cat, 0, upcase);
5110 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5111 MethodHandle f1 = filterArguments(cat, 1, upcase);
5112 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5113 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5114 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5115      * }</pre></blockquote>
5116      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5117      * denotes the return type of both the {@code target} and resulting adapter.
5118      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5119      * of the parameters and arguments that precede and follow the filter position
5120      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5121      * values of the filtered parameters and arguments; they also represent the
5122      * return types of the {@code filter[i]} handles. The latter accept arguments
5123      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5124      * the resulting adapter.
5125      * <blockquote><pre>{@code
5126      * T target(P... p, A[i]... a[i], B... b);
5127      * A[i] filter[i](V[i]);
5128      * T adapter(P... p, V[i]... v[i], B... b) {
5129      *   return target(p..., filter[i](v[i])..., b...);
5130      * }
5131      * }</pre></blockquote>
5132      * <p>
5133      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5134      * variable-arity method handle}, even if the original target method handle was.
5135      *
5136      * @param target the method handle to invoke after arguments are filtered
5137      * @param pos the position of the first argument to filter
5138      * @param filters method handles to call initially on filtered arguments
5139      * @return method handle which incorporates the specified argument filtering logic
5140      * @throws NullPointerException if the target is null
5141      *                              or if the {@code filters} array is null
5142      * @throws IllegalArgumentException if a non-null element of {@code filters}
5143      *          does not match a corresponding argument type of target as described above,
5144      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5145      *          or if the resulting method handle's type would have
5146      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5147      */
5148     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5149         // In method types arguments start at index 0, while the LF
5150         // editor have the MH receiver at position 0 - adjust appropriately.
5151         final int MH_RECEIVER_OFFSET = 1;
5152         filterArgumentsCheckArity(target, pos, filters);
5153         MethodHandle adapter = target;
5154 
5155         // keep track of currently matched filters, as to optimize repeated filters
5156         int index = 0;
5157         int[] positions = new int[filters.length];
5158         MethodHandle filter = null;
5159 
5160         // process filters in reverse order so that the invocation of
5161         // the resulting adapter will invoke the filters in left-to-right order
5162         for (int i = filters.length - 1; i >= 0; --i) {
5163             MethodHandle newFilter = filters[i];
5164             if (newFilter == null) continue;  // ignore null elements of filters
5165 
5166             // flush changes on update
5167             if (filter != newFilter) {
5168                 if (filter != null) {
5169                     if (index > 1) {
5170                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5171                     } else {
5172                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5173                     }
5174                 }
5175                 filter = newFilter;
5176                 index = 0;
5177             }
5178 
5179             filterArgumentChecks(target, pos + i, newFilter);
5180             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5181         }
5182         if (index > 1) {
5183             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5184         } else if (index == 1) {
5185             adapter = filterArgument(adapter, positions[0] - 1, filter);
5186         }
5187         return adapter;
5188     }
5189 
5190     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5191         MethodType targetType = adapter.type();
5192         MethodType filterType = filter.type();
5193         BoundMethodHandle result = adapter.rebind();
5194         Class<?> newParamType = filterType.parameterType(0);
5195 
5196         Class<?>[] ptypes = targetType.ptypes().clone();
5197         for (int pos : positions) {
5198             ptypes[pos - 1] = newParamType;
5199         }
5200         MethodType newType = MethodType.makeImpl(targetType.rtype(), ptypes, true);
5201 
5202         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5203         return result.copyWithExtendL(newType, lform, filter);
5204     }
5205 
5206     /*non-public*/
5207     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5208         filterArgumentChecks(target, pos, filter);
5209         MethodType targetType = target.type();
5210         MethodType filterType = filter.type();
5211         BoundMethodHandle result = target.rebind();
5212         Class<?> newParamType = filterType.parameterType(0);
5213         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5214         MethodType newType = targetType.changeParameterType(pos, newParamType);
5215         result = result.copyWithExtendL(newType, lform, filter);
5216         return result;
5217     }
5218 
5219     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5220         MethodType targetType = target.type();
5221         int maxPos = targetType.parameterCount();
5222         if (pos + filters.length > maxPos)
5223             throw newIllegalArgumentException("too many filters");
5224     }
5225 
5226     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5227         MethodType targetType = target.type();
5228         MethodType filterType = filter.type();
5229         if (filterType.parameterCount() != 1
5230             || filterType.returnType() != targetType.parameterType(pos))
5231             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5232     }
5233 
5234     /**
5235      * Adapts a target method handle by pre-processing
5236      * a sub-sequence of its arguments with a filter (another method handle).
5237      * The pre-processed arguments are replaced by the result (if any) of the
5238      * filter function.
5239      * The target is then called on the modified (usually shortened) argument list.
5240      * <p>
5241      * If the filter returns a value, the target must accept that value as
5242      * its argument in position {@code pos}, preceded and/or followed by
5243      * any arguments not passed to the filter.
5244      * If the filter returns void, the target must accept all arguments
5245      * not passed to the filter.
5246      * No arguments are reordered, and a result returned from the filter
5247      * replaces (in order) the whole subsequence of arguments originally
5248      * passed to the adapter.
5249      * <p>
5250      * The argument types (if any) of the filter
5251      * replace zero or one argument types of the target, at position {@code pos},
5252      * in the resulting adapted method handle.
5253      * The return type of the filter (if any) must be identical to the
5254      * argument type of the target at position {@code pos}, and that target argument
5255      * is supplied by the return value of the filter.
5256      * <p>
5257      * In all cases, {@code pos} must be greater than or equal to zero, and
5258      * {@code pos} must also be less than or equal to the target's arity.
5259      * <p><b>Example:</b>
5260      * <blockquote><pre>{@code
5261 import static java.lang.invoke.MethodHandles.*;
5262 import static java.lang.invoke.MethodType.*;
5263 ...
5264 MethodHandle deepToString = publicLookup()
5265   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5266 
5267 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5268 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5269 
5270 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5271 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5272 
5273 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5274 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5275 assertEquals("[top, [up, down], strange]",
5276              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5277 
5278 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5279 assertEquals("[top, [up, down], [strange]]",
5280              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5281 
5282 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5283 assertEquals("[top, [[up, down, strange], charm], bottom]",
5284              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5285      * }</pre></blockquote>
5286      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5287      * represents the return type of the {@code target} and resulting adapter.
5288      * {@code V}/{@code v} stand for the return type and value of the
5289      * {@code filter}, which are also found in the signature and arguments of
5290      * the {@code target}, respectively, unless {@code V} is {@code void}.
5291      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5292      * and values preceding and following the collection position, {@code pos},
5293      * in the {@code target}'s signature. They also turn up in the resulting
5294      * adapter's signature and arguments, where they surround
5295      * {@code B}/{@code b}, which represent the parameter types and arguments
5296      * to the {@code filter} (if any).
5297      * <blockquote><pre>{@code
5298      * T target(A...,V,C...);
5299      * V filter(B...);
5300      * T adapter(A... a,B... b,C... c) {
5301      *   V v = filter(b...);
5302      *   return target(a...,v,c...);
5303      * }
5304      * // and if the filter has no arguments:
5305      * T target2(A...,V,C...);
5306      * V filter2();
5307      * T adapter2(A... a,C... c) {
5308      *   V v = filter2();
5309      *   return target2(a...,v,c...);
5310      * }
5311      * // and if the filter has a void return:
5312      * T target3(A...,C...);
5313      * void filter3(B...);
5314      * T adapter3(A... a,B... b,C... c) {
5315      *   filter3(b...);
5316      *   return target3(a...,c...);
5317      * }
5318      * }</pre></blockquote>
5319      * <p>
5320      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5321      * one which first "folds" the affected arguments, and then drops them, in separate
5322      * steps as follows:
5323      * <blockquote><pre>{@code
5324      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5325      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5326      * }</pre></blockquote>
5327      * If the target method handle consumes no arguments besides than the result
5328      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5329      * is equivalent to {@code filterReturnValue(coll, mh)}.
5330      * If the filter method handle {@code coll} consumes one argument and produces
5331      * a non-void result, then {@code collectArguments(mh, N, coll)}
5332      * is equivalent to {@code filterArguments(mh, N, coll)}.
5333      * Other equivalences are possible but would require argument permutation.
5334      * <p>
5335      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5336      * variable-arity method handle}, even if the original target method handle was.
5337      *
5338      * @param target the method handle to invoke after filtering the subsequence of arguments
5339      * @param pos the position of the first adapter argument to pass to the filter,
5340      *            and/or the target argument which receives the result of the filter
5341      * @param filter method handle to call on the subsequence of arguments
5342      * @return method handle which incorporates the specified argument subsequence filtering logic
5343      * @throws NullPointerException if either argument is null
5344      * @throws IllegalArgumentException if the return type of {@code filter}
5345      *          is non-void and is not the same as the {@code pos} argument of the target,
5346      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5347      *          or if the resulting method handle's type would have
5348      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5349      * @see MethodHandles#foldArguments
5350      * @see MethodHandles#filterArguments
5351      * @see MethodHandles#filterReturnValue
5352      */
5353     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5354         MethodType newType = collectArgumentsChecks(target, pos, filter);
5355         MethodType collectorType = filter.type();
5356         BoundMethodHandle result = target.rebind();
5357         LambdaForm lform;
5358         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
5359             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
5360             if (lform != null) {
5361                 return result.copyWith(newType, lform);
5362             }
5363         }
5364         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5365         return result.copyWithExtendL(newType, lform, filter);
5366     }
5367 
5368     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5369         MethodType targetType = target.type();
5370         MethodType filterType = filter.type();
5371         Class<?> rtype = filterType.returnType();
5372         List<Class<?>> filterArgs = filterType.parameterList();
5373         if (rtype == void.class) {
5374             return targetType.insertParameterTypes(pos, filterArgs);
5375         }
5376         if (rtype != targetType.parameterType(pos)) {
5377             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5378         }
5379         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
5380     }
5381 
5382     /**
5383      * Adapts a target method handle by post-processing
5384      * its return value (if any) with a filter (another method handle).
5385      * The result of the filter is returned from the adapter.
5386      * <p>
5387      * If the target returns a value, the filter must accept that value as
5388      * its only argument.
5389      * If the target returns void, the filter must accept no arguments.
5390      * <p>
5391      * The return type of the filter
5392      * replaces the return type of the target
5393      * in the resulting adapted method handle.
5394      * The argument type of the filter (if any) must be identical to the
5395      * return type of the target.
5396      * <p><b>Example:</b>
5397      * <blockquote><pre>{@code
5398 import static java.lang.invoke.MethodHandles.*;
5399 import static java.lang.invoke.MethodType.*;
5400 ...
5401 MethodHandle cat = lookup().findVirtual(String.class,
5402   "concat", methodType(String.class, String.class));
5403 MethodHandle length = lookup().findVirtual(String.class,
5404   "length", methodType(int.class));
5405 System.out.println((String) cat.invokeExact("x", "y")); // xy
5406 MethodHandle f0 = filterReturnValue(cat, length);
5407 System.out.println((int) f0.invokeExact("x", "y")); // 2
5408      * }</pre></blockquote>
5409      * <p>Here is pseudocode for the resulting adapter. In the code,
5410      * {@code T}/{@code t} represent the result type and value of the
5411      * {@code target}; {@code V}, the result type of the {@code filter}; and
5412      * {@code A}/{@code a}, the types and values of the parameters and arguments
5413      * of the {@code target} as well as the resulting adapter.
5414      * <blockquote><pre>{@code
5415      * T target(A...);
5416      * V filter(T);
5417      * V adapter(A... a) {
5418      *   T t = target(a...);
5419      *   return filter(t);
5420      * }
5421      * // and if the target has a void return:
5422      * void target2(A...);
5423      * V filter2();
5424      * V adapter2(A... a) {
5425      *   target2(a...);
5426      *   return filter2();
5427      * }
5428      * // and if the filter has a void return:
5429      * T target3(A...);
5430      * void filter3(V);
5431      * void adapter3(A... a) {
5432      *   T t = target3(a...);
5433      *   filter3(t);
5434      * }
5435      * }</pre></blockquote>
5436      * <p>
5437      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5438      * variable-arity method handle}, even if the original target method handle was.
5439      * @param target the method handle to invoke before filtering the return value
5440      * @param filter method handle to call on the return value
5441      * @return method handle which incorporates the specified return value filtering logic
5442      * @throws NullPointerException if either argument is null
5443      * @throws IllegalArgumentException if the argument list of {@code filter}
5444      *          does not match the return type of target as described above
5445      */
5446     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5447         MethodType targetType = target.type();
5448         MethodType filterType = filter.type();
5449         filterReturnValueChecks(targetType, filterType);
5450         BoundMethodHandle result = target.rebind();
5451         BasicType rtype = BasicType.basicType(filterType.returnType());
5452         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5453         MethodType newType = targetType.changeReturnType(filterType.returnType());
5454         result = result.copyWithExtendL(newType, lform, filter);
5455         return result;
5456     }
5457 
5458     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5459         Class<?> rtype = targetType.returnType();
5460         int filterValues = filterType.parameterCount();
5461         if (filterValues == 0
5462                 ? (rtype != void.class)
5463                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5464             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5465     }
5466 
5467     /**
5468      * Adapts a target method handle by pre-processing
5469      * some of its arguments, and then calling the target with
5470      * the result of the pre-processing, inserted into the original
5471      * sequence of arguments.
5472      * <p>
5473      * The pre-processing is performed by {@code combiner}, a second method handle.
5474      * Of the arguments passed to the adapter, the first {@code N} arguments
5475      * are copied to the combiner, which is then called.
5476      * (Here, {@code N} is defined as the parameter count of the combiner.)
5477      * After this, control passes to the target, with any result
5478      * from the combiner inserted before the original {@code N} incoming
5479      * arguments.
5480      * <p>
5481      * If the combiner returns a value, the first parameter type of the target
5482      * must be identical with the return type of the combiner, and the next
5483      * {@code N} parameter types of the target must exactly match the parameters
5484      * of the combiner.
5485      * <p>
5486      * If the combiner has a void return, no result will be inserted,
5487      * and the first {@code N} parameter types of the target
5488      * must exactly match the parameters of the combiner.
5489      * <p>
5490      * The resulting adapter is the same type as the target, except that the
5491      * first parameter type is dropped,
5492      * if it corresponds to the result of the combiner.
5493      * <p>
5494      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
5495      * that either the combiner or the target does not wish to receive.
5496      * If some of the incoming arguments are destined only for the combiner,
5497      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
5498      * arguments will not need to be live on the stack on entry to the
5499      * target.)
5500      * <p><b>Example:</b>
5501      * <blockquote><pre>{@code
5502 import static java.lang.invoke.MethodHandles.*;
5503 import static java.lang.invoke.MethodType.*;
5504 ...
5505 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5506   "println", methodType(void.class, String.class))
5507     .bindTo(System.out);
5508 MethodHandle cat = lookup().findVirtual(String.class,
5509   "concat", methodType(String.class, String.class));
5510 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5511 MethodHandle catTrace = foldArguments(cat, trace);
5512 // also prints "boo":
5513 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5514      * }</pre></blockquote>
5515      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5516      * represents the result type of the {@code target} and resulting adapter.
5517      * {@code V}/{@code v} represent the type and value of the parameter and argument
5518      * of {@code target} that precedes the folding position; {@code V} also is
5519      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5520      * types and values of the {@code N} parameters and arguments at the folding
5521      * position. {@code B}/{@code b} represent the types and values of the
5522      * {@code target} parameters and arguments that follow the folded parameters
5523      * and arguments.
5524      * <blockquote><pre>{@code
5525      * // there are N arguments in A...
5526      * T target(V, A[N]..., B...);
5527      * V combiner(A...);
5528      * T adapter(A... a, B... b) {
5529      *   V v = combiner(a...);
5530      *   return target(v, a..., b...);
5531      * }
5532      * // and if the combiner has a void return:
5533      * T target2(A[N]..., B...);
5534      * void combiner2(A...);
5535      * T adapter2(A... a, B... b) {
5536      *   combiner2(a...);
5537      *   return target2(a..., b...);
5538      * }
5539      * }</pre></blockquote>
5540      * <p>
5541      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5542      * variable-arity method handle}, even if the original target method handle was.
5543      * @param target the method handle to invoke after arguments are combined
5544      * @param combiner method handle to call initially on the incoming arguments
5545      * @return method handle which incorporates the specified argument folding logic
5546      * @throws NullPointerException if either argument is null
5547      * @throws IllegalArgumentException if {@code combiner}'s return type
5548      *          is non-void and not the same as the first argument type of
5549      *          the target, or if the initial {@code N} argument types
5550      *          of the target
5551      *          (skipping one matching the {@code combiner}'s return type)
5552      *          are not identical with the argument types of {@code combiner}
5553      */
5554     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
5555         return foldArguments(target, 0, combiner);
5556     }
5557 
5558     /**
5559      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
5560      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
5561      * before the folded arguments.
5562      * <p>
5563      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
5564      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
5565      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
5566      * 0.
5567      *
5568      * @apiNote Example:
5569      * <blockquote><pre>{@code
5570     import static java.lang.invoke.MethodHandles.*;
5571     import static java.lang.invoke.MethodType.*;
5572     ...
5573     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5574     "println", methodType(void.class, String.class))
5575     .bindTo(System.out);
5576     MethodHandle cat = lookup().findVirtual(String.class,
5577     "concat", methodType(String.class, String.class));
5578     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5579     MethodHandle catTrace = foldArguments(cat, 1, trace);
5580     // also prints "jum":
5581     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5582      * }</pre></blockquote>
5583      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5584      * represents the result type of the {@code target} and resulting adapter.
5585      * {@code V}/{@code v} represent the type and value of the parameter and argument
5586      * of {@code target} that precedes the folding position; {@code V} also is
5587      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5588      * types and values of the {@code N} parameters and arguments at the folding
5589      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
5590      * and values of the {@code target} parameters and arguments that precede and
5591      * follow the folded parameters and arguments starting at {@code pos},
5592      * respectively.
5593      * <blockquote><pre>{@code
5594      * // there are N arguments in A...
5595      * T target(Z..., V, A[N]..., B...);
5596      * V combiner(A...);
5597      * T adapter(Z... z, A... a, B... b) {
5598      *   V v = combiner(a...);
5599      *   return target(z..., v, a..., b...);
5600      * }
5601      * // and if the combiner has a void return:
5602      * T target2(Z..., A[N]..., B...);
5603      * void combiner2(A...);
5604      * T adapter2(Z... z, A... a, B... b) {
5605      *   combiner2(a...);
5606      *   return target2(z..., a..., b...);
5607      * }
5608      * }</pre></blockquote>
5609      * <p>
5610      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5611      * variable-arity method handle}, even if the original target method handle was.
5612      *
5613      * @param target the method handle to invoke after arguments are combined
5614      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
5615      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5616      * @param combiner method handle to call initially on the incoming arguments
5617      * @return method handle which incorporates the specified argument folding logic
5618      * @throws NullPointerException if either argument is null
5619      * @throws IllegalArgumentException if either of the following two conditions holds:
5620      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5621      *              {@code pos} of the target signature;
5622      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
5623      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
5624      *
5625      * @see #foldArguments(MethodHandle, MethodHandle)
5626      * @since 9
5627      */
5628     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
5629         MethodType targetType = target.type();
5630         MethodType combinerType = combiner.type();
5631         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
5632         BoundMethodHandle result = target.rebind();
5633         boolean dropResult = rtype == void.class;
5634         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
5635         MethodType newType = targetType;
5636         if (!dropResult) {
5637             newType = newType.dropParameterTypes(pos, pos + 1);
5638         }
5639         result = result.copyWithExtendL(newType, lform, combiner);
5640         return result;
5641     }
5642 
5643     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
5644         int foldArgs   = combinerType.parameterCount();
5645         Class<?> rtype = combinerType.returnType();
5646         int foldVals = rtype == void.class ? 0 : 1;
5647         int afterInsertPos = foldPos + foldVals;
5648         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
5649         if (ok) {
5650             for (int i = 0; i < foldArgs; i++) {
5651                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
5652                     ok = false;
5653                     break;
5654                 }
5655             }
5656         }
5657         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
5658             ok = false;
5659         if (!ok)
5660             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5661         return rtype;
5662     }
5663 
5664     /**
5665      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
5666      * of the pre-processing replacing the argument at the given position.
5667      *
5668      * @param target the method handle to invoke after arguments are combined
5669      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5670      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5671      * @param combiner method handle to call initially on the incoming arguments
5672      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5673      * @return method handle which incorporates the specified argument folding logic
5674      * @throws NullPointerException if either argument is null
5675      * @throws IllegalArgumentException if either of the following two conditions holds:
5676      *          (1) {@code combiner}'s return type is not the same as the argument type at position
5677      *              {@code pos} of the target signature;
5678      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
5679      *              not identical with the argument types of {@code combiner}.
5680      */
5681     /*non-public*/
5682     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5683         return argumentsWithCombiner(true, target, position, combiner, argPositions);
5684     }
5685 
5686     /**
5687      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
5688      * the pre-processing inserted into the original sequence of arguments at the given position.
5689      *
5690      * @param target the method handle to invoke after arguments are combined
5691      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5692      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5693      * @param combiner method handle to call initially on the incoming arguments
5694      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5695      * @return method handle which incorporates the specified argument folding logic
5696      * @throws NullPointerException if either argument is null
5697      * @throws IllegalArgumentException if either of the following two conditions holds:
5698      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5699      *              {@code pos} of the target signature;
5700      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
5701      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
5702      *              with the argument types of {@code combiner}.
5703      */
5704     /*non-public*/
5705     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5706         return argumentsWithCombiner(false, target, position, combiner, argPositions);
5707     }
5708 
5709     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5710         MethodType targetType = target.type();
5711         MethodType combinerType = combiner.type();
5712         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
5713         BoundMethodHandle result = target.rebind();
5714 
5715         MethodType newType = targetType;
5716         LambdaForm lform;
5717         if (filter) {
5718             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
5719         } else {
5720             boolean dropResult = rtype == void.class;
5721             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
5722             if (!dropResult) {
5723                 newType = newType.dropParameterTypes(position, position + 1);
5724             }
5725         }
5726         result = result.copyWithExtendL(newType, lform, combiner);
5727         return result;
5728     }
5729 
5730     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
5731         int combinerArgs = combinerType.parameterCount();
5732         if (argPos.length != combinerArgs) {
5733             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
5734         }
5735         Class<?> rtype = combinerType.returnType();
5736 
5737         for (int i = 0; i < combinerArgs; i++) {
5738             int arg = argPos[i];
5739             if (arg < 0 || arg > targetType.parameterCount()) {
5740                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
5741             }
5742             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
5743                 throw newIllegalArgumentException("target argument type at position " + arg
5744                         + " must match combiner argument type at index " + i + ": " + targetType
5745                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
5746             }
5747         }
5748         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
5749             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5750         }
5751         return rtype;
5752     }
5753 
5754     /**
5755      * Makes a method handle which adapts a target method handle,
5756      * by guarding it with a test, a boolean-valued method handle.
5757      * If the guard fails, a fallback handle is called instead.
5758      * All three method handles must have the same corresponding
5759      * argument and return types, except that the return type
5760      * of the test must be boolean, and the test is allowed
5761      * to have fewer arguments than the other two method handles.
5762      * <p>
5763      * Here is pseudocode for the resulting adapter. In the code, {@code T}
5764      * represents the uniform result type of the three involved handles;
5765      * {@code A}/{@code a}, the types and values of the {@code target}
5766      * parameters and arguments that are consumed by the {@code test}; and
5767      * {@code B}/{@code b}, those types and values of the {@code target}
5768      * parameters and arguments that are not consumed by the {@code test}.
5769      * <blockquote><pre>{@code
5770      * boolean test(A...);
5771      * T target(A...,B...);
5772      * T fallback(A...,B...);
5773      * T adapter(A... a,B... b) {
5774      *   if (test(a...))
5775      *     return target(a..., b...);
5776      *   else
5777      *     return fallback(a..., b...);
5778      * }
5779      * }</pre></blockquote>
5780      * Note that the test arguments ({@code a...} in the pseudocode) cannot
5781      * be modified by execution of the test, and so are passed unchanged
5782      * from the caller to the target or fallback as appropriate.
5783      * @param test method handle used for test, must return boolean
5784      * @param target method handle to call if test passes
5785      * @param fallback method handle to call if test fails
5786      * @return method handle which incorporates the specified if/then/else logic
5787      * @throws NullPointerException if any argument is null
5788      * @throws IllegalArgumentException if {@code test} does not return boolean,
5789      *          or if all three method types do not match (with the return
5790      *          type of {@code test} changed to match that of the target).
5791      */
5792     public static MethodHandle guardWithTest(MethodHandle test,
5793                                MethodHandle target,
5794                                MethodHandle fallback) {
5795         MethodType gtype = test.type();
5796         MethodType ttype = target.type();
5797         MethodType ftype = fallback.type();
5798         if (!ttype.equals(ftype))
5799             throw misMatchedTypes("target and fallback types", ttype, ftype);
5800         if (gtype.returnType() != boolean.class)
5801             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
5802         List<Class<?>> targs = ttype.parameterList();
5803         test = dropArgumentsToMatch(test, 0, targs, 0, true);
5804         if (test == null) {
5805             throw misMatchedTypes("target and test types", ttype, gtype);
5806         }
5807         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
5808     }
5809 
5810     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
5811         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
5812     }
5813 
5814     /**
5815      * Makes a method handle which adapts a target method handle,
5816      * by running it inside an exception handler.
5817      * If the target returns normally, the adapter returns that value.
5818      * If an exception matching the specified type is thrown, the fallback
5819      * handle is called instead on the exception, plus the original arguments.
5820      * <p>
5821      * The target and handler must have the same corresponding
5822      * argument and return types, except that handler may omit trailing arguments
5823      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
5824      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
5825      * <p>
5826      * Here is pseudocode for the resulting adapter. In the code, {@code T}
5827      * represents the return type of the {@code target} and {@code handler},
5828      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
5829      * the types and values of arguments to the resulting handle consumed by
5830      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
5831      * resulting handle discarded by {@code handler}.
5832      * <blockquote><pre>{@code
5833      * T target(A..., B...);
5834      * T handler(ExType, A...);
5835      * T adapter(A... a, B... b) {
5836      *   try {
5837      *     return target(a..., b...);
5838      *   } catch (ExType ex) {
5839      *     return handler(ex, a...);
5840      *   }
5841      * }
5842      * }</pre></blockquote>
5843      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
5844      * be modified by execution of the target, and so are passed unchanged
5845      * from the caller to the handler, if the handler is invoked.
5846      * <p>
5847      * The target and handler must return the same type, even if the handler
5848      * always throws.  (This might happen, for instance, because the handler
5849      * is simulating a {@code finally} clause).
5850      * To create such a throwing handler, compose the handler creation logic
5851      * with {@link #throwException throwException},
5852      * in order to create a method handle of the correct return type.
5853      * @param target method handle to call
5854      * @param exType the type of exception which the handler will catch
5855      * @param handler method handle to call if a matching exception is thrown
5856      * @return method handle which incorporates the specified try/catch logic
5857      * @throws NullPointerException if any argument is null
5858      * @throws IllegalArgumentException if {@code handler} does not accept
5859      *          the given exception type, or if the method handle types do
5860      *          not match in their return types and their
5861      *          corresponding parameters
5862      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
5863      */
5864     public static MethodHandle catchException(MethodHandle target,
5865                                 Class<? extends Throwable> exType,
5866                                 MethodHandle handler) {
5867         MethodType ttype = target.type();
5868         MethodType htype = handler.type();
5869         if (!Throwable.class.isAssignableFrom(exType))
5870             throw new ClassCastException(exType.getName());
5871         if (htype.parameterCount() < 1 ||
5872             !htype.parameterType(0).isAssignableFrom(exType))
5873             throw newIllegalArgumentException("handler does not accept exception type "+exType);
5874         if (htype.returnType() != ttype.returnType())
5875             throw misMatchedTypes("target and handler return types", ttype, htype);
5876         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
5877         if (handler == null) {
5878             throw misMatchedTypes("target and handler types", ttype, htype);
5879         }
5880         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
5881     }
5882 
5883     /**
5884      * Produces a method handle which will throw exceptions of the given {@code exType}.
5885      * The method handle will accept a single argument of {@code exType},
5886      * and immediately throw it as an exception.
5887      * The method type will nominally specify a return of {@code returnType}.
5888      * The return type may be anything convenient:  It doesn't matter to the
5889      * method handle's behavior, since it will never return normally.
5890      * @param returnType the return type of the desired method handle
5891      * @param exType the parameter type of the desired method handle
5892      * @return method handle which can throw the given exceptions
5893      * @throws NullPointerException if either argument is null
5894      */
5895     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
5896         if (!Throwable.class.isAssignableFrom(exType))
5897             throw new ClassCastException(exType.getName());
5898         return MethodHandleImpl.throwException(methodType(returnType, exType));
5899     }
5900 
5901     /**
5902      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
5903      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
5904      * delivers the loop's result, which is the return value of the resulting handle.
5905      * <p>
5906      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
5907      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
5908      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
5909      * terms of method handles, each clause will specify up to four independent actions:<ul>
5910      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
5911      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
5912      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
5913      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
5914      * </ul>
5915      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
5916      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
5917      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
5918      * <p>
5919      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
5920      * this case. See below for a detailed description.
5921      * <p>
5922      * <em>Parameters optional everywhere:</em>
5923      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
5924      * As an exception, the init functions cannot take any {@code v} parameters,
5925      * because those values are not yet computed when the init functions are executed.
5926      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
5927      * In fact, any clause function may take no arguments at all.
5928      * <p>
5929      * <em>Loop parameters:</em>
5930      * A clause function may take all the iteration variable values it is entitled to, in which case
5931      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
5932      * with their types and values notated as {@code (A...)} and {@code (a...)}.
5933      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
5934      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
5935      * init function is automatically a loop parameter {@code a}.)
5936      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
5937      * These loop parameters act as loop-invariant values visible across the whole loop.
5938      * <p>
5939      * <em>Parameters visible everywhere:</em>
5940      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
5941      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
5942      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
5943      * Most clause functions will not need all of this information, but they will be formally connected to it
5944      * as if by {@link #dropArguments}.
5945      * <a id="astar"></a>
5946      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
5947      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
5948      * In that notation, the general form of an init function parameter list
5949      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
5950      * <p>
5951      * <em>Checking clause structure:</em>
5952      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
5953      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
5954      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
5955      * met by the inputs to the loop combinator.
5956      * <p>
5957      * <em>Effectively identical sequences:</em>
5958      * <a id="effid"></a>
5959      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
5960      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
5961      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
5962      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
5963      * that longest list.
5964      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
5965      * and the same is true if more sequences of the form {@code (V... A*)} are added.
5966      * <p>
5967      * <em>Step 0: Determine clause structure.</em><ol type="a">
5968      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
5969      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
5970      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
5971      * four. Padding takes place by appending elements to the array.
5972      * <li>Clauses with all {@code null}s are disregarded.
5973      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
5974      * </ol>
5975      * <p>
5976      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
5977      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
5978      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
5979      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
5980      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
5981      * iteration variable type.
5982      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
5983      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
5984      * </ol>
5985      * <p>
5986      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
5987      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
5988      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
5989      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
5990      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
5991      * (These types will be checked in step 2, along with all the clause function types.)
5992      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
5993      * <li>All of the collected parameter lists must be effectively identical.
5994      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
5995      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
5996      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
5997      * the "internal parameter list".
5998      * </ul>
5999      * <p>
6000      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6001      * <li>Examine fini function return types, disregarding omitted fini functions.
6002      * <li>If there are no fini functions, the loop return type is {@code void}.
6003      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6004      * type.
6005      * </ol>
6006      * <p>
6007      * <em>Step 1D: Check other types.</em><ol type="a">
6008      * <li>There must be at least one non-omitted pred function.
6009      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6010      * </ol>
6011      * <p>
6012      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6013      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6014      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6015      * (Note that their parameter lists are already effectively identical to this list.)
6016      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6017      * effectively identical to the internal parameter list {@code (V... A...)}.
6018      * </ol>
6019      * <p>
6020      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6021      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6022      * type.
6023      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6024      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6025      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6026      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6027      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6028      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6029      * loop return type.
6030      * </ol>
6031      * <p>
6032      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6033      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6034      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6035      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6036      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6037      * pad out the end of the list.
6038      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6039      * </ol>
6040      * <p>
6041      * <em>Final observations.</em><ol type="a">
6042      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6043      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6044      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6045      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6046      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6047      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6048      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6049      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6050      * </ol>
6051      * <p>
6052      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6053      * <ul>
6054      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6055      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6056      * (Only one {@code Pn} has to be non-{@code null}.)
6057      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6058      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6059      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6060      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6061      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6062      * the resulting loop handle's parameter types {@code (A...)}.
6063      * </ul>
6064      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6065      * which is natural if most of the loop computation happens in the steps.  For some loops,
6066      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6067      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6068      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6069      * where the init functions will need the extra parameters.  For such reasons, the rules for
6070      * determining these parameters are as symmetric as possible, across all clause parts.
6071      * In general, the loop parameters function as common invariant values across the whole
6072      * loop, while the iteration variables function as common variant values, or (if there is
6073      * no step function) as internal loop invariant temporaries.
6074      * <p>
6075      * <em>Loop execution.</em><ol type="a">
6076      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6077      * every clause function. These locals are loop invariant.
6078      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6079      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6080      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6081      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6082      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6083      * (in argument order).
6084      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6085      * returns {@code false}.
6086      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6087      * sequence {@code (v...)} of loop variables.
6088      * The updated value is immediately visible to all subsequent function calls.
6089      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6090      * (of type {@code R}) is returned from the loop as a whole.
6091      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6092      * except by throwing an exception.
6093      * </ol>
6094      * <p>
6095      * <em>Usage tips.</em>
6096      * <ul>
6097      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6098      * sometimes a step function only needs to observe the current value of its own variable.
6099      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6100      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6101      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6102      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6103      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6104      * <li>If some of the clause functions are virtual methods on an instance, the instance
6105      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6106      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6107      * will be the first iteration variable value, and it will be easy to use virtual
6108      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6109      * </ul>
6110      * <p>
6111      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6112      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6113      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6114      * <blockquote><pre>{@code
6115      * V... init...(A...);
6116      * boolean pred...(V..., A...);
6117      * V... step...(V..., A...);
6118      * R fini...(V..., A...);
6119      * R loop(A... a) {
6120      *   V... v... = init...(a...);
6121      *   for (;;) {
6122      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6123      *       v = s(v..., a...);
6124      *       if (!p(v..., a...)) {
6125      *         return f(v..., a...);
6126      *       }
6127      *     }
6128      *   }
6129      * }
6130      * }</pre></blockquote>
6131      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6132      * to their full length, even though individual clause functions may neglect to take them all.
6133      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6134      *
6135      * @apiNote Example:
6136      * <blockquote><pre>{@code
6137      * // iterative implementation of the factorial function as a loop handle
6138      * static int one(int k) { return 1; }
6139      * static int inc(int i, int acc, int k) { return i + 1; }
6140      * static int mult(int i, int acc, int k) { return i * acc; }
6141      * static boolean pred(int i, int acc, int k) { return i < k; }
6142      * static int fin(int i, int acc, int k) { return acc; }
6143      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6144      * // null initializer for counter, should initialize to 0
6145      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6146      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6147      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6148      * assertEquals(120, loop.invoke(5));
6149      * }</pre></blockquote>
6150      * The same example, dropping arguments and using combinators:
6151      * <blockquote><pre>{@code
6152      * // simplified implementation of the factorial function as a loop handle
6153      * static int inc(int i) { return i + 1; } // drop acc, k
6154      * static int mult(int i, int acc) { return i * acc; } //drop k
6155      * static boolean cmp(int i, int k) { return i < k; }
6156      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6157      * // null initializer for counter, should initialize to 0
6158      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6159      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6160      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6161      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6162      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6163      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6164      * assertEquals(720, loop.invoke(6));
6165      * }</pre></blockquote>
6166      * A similar example, using a helper object to hold a loop parameter:
6167      * <blockquote><pre>{@code
6168      * // instance-based implementation of the factorial function as a loop handle
6169      * static class FacLoop {
6170      *   final int k;
6171      *   FacLoop(int k) { this.k = k; }
6172      *   int inc(int i) { return i + 1; }
6173      *   int mult(int i, int acc) { return i * acc; }
6174      *   boolean pred(int i) { return i < k; }
6175      *   int fin(int i, int acc) { return acc; }
6176      * }
6177      * // assume MH_FacLoop is a handle to the constructor
6178      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6179      * // null initializer for counter, should initialize to 0
6180      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6181      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6182      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6183      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6184      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6185      * assertEquals(5040, loop.invoke(7));
6186      * }</pre></blockquote>
6187      *
6188      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6189      *
6190      * @return a method handle embodying the looping behavior as defined by the arguments.
6191      *
6192      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6193      *
6194      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6195      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6196      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6197      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6198      * @since 9
6199      */
6200     public static MethodHandle loop(MethodHandle[]... clauses) {
6201         // Step 0: determine clause structure.
6202         loopChecks0(clauses);
6203 
6204         List<MethodHandle> init = new ArrayList<>();
6205         List<MethodHandle> step = new ArrayList<>();
6206         List<MethodHandle> pred = new ArrayList<>();
6207         List<MethodHandle> fini = new ArrayList<>();
6208 
6209         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6210             init.add(clause[0]); // all clauses have at least length 1
6211             step.add(clause.length <= 1 ? null : clause[1]);
6212             pred.add(clause.length <= 2 ? null : clause[2]);
6213             fini.add(clause.length <= 3 ? null : clause[3]);
6214         });
6215 
6216         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6217         final int nclauses = init.size();
6218 
6219         // Step 1A: determine iteration variables (V...).
6220         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6221         for (int i = 0; i < nclauses; ++i) {
6222             MethodHandle in = init.get(i);
6223             MethodHandle st = step.get(i);
6224             if (in == null && st == null) {
6225                 iterationVariableTypes.add(void.class);
6226             } else if (in != null && st != null) {
6227                 loopChecks1a(i, in, st);
6228                 iterationVariableTypes.add(in.type().returnType());
6229             } else {
6230                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6231             }
6232         }
6233         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
6234                 collect(Collectors.toList());
6235 
6236         // Step 1B: determine loop parameters (A...).
6237         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6238         loopChecks1b(init, commonSuffix);
6239 
6240         // Step 1C: determine loop return type.
6241         // Step 1D: check other types.
6242         // local variable required here; see JDK-8223553
6243         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6244                 .map(MethodType::returnType);
6245         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6246         loopChecks1cd(pred, fini, loopReturnType);
6247 
6248         // Step 2: determine parameter lists.
6249         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6250         commonParameterSequence.addAll(commonSuffix);
6251         loopChecks2(step, pred, fini, commonParameterSequence);
6252 
6253         // Step 3: fill in omitted functions.
6254         for (int i = 0; i < nclauses; ++i) {
6255             Class<?> t = iterationVariableTypes.get(i);
6256             if (init.get(i) == null) {
6257                 init.set(i, empty(methodType(t, commonSuffix)));
6258             }
6259             if (step.get(i) == null) {
6260                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6261             }
6262             if (pred.get(i) == null) {
6263                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
6264             }
6265             if (fini.get(i) == null) {
6266                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6267             }
6268         }
6269 
6270         // Step 4: fill in missing parameter types.
6271         // Also convert all handles to fixed-arity handles.
6272         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6273         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6274         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6275         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6276 
6277         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6278                 allMatch(pl -> pl.equals(commonSuffix));
6279         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6280                 allMatch(pl -> pl.equals(commonParameterSequence));
6281 
6282         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6283     }
6284 
6285     private static void loopChecks0(MethodHandle[][] clauses) {
6286         if (clauses == null || clauses.length == 0) {
6287             throw newIllegalArgumentException("null or no clauses passed");
6288         }
6289         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6290             throw newIllegalArgumentException("null clauses are not allowed");
6291         }
6292         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6293             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6294         }
6295     }
6296 
6297     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6298         if (in.type().returnType() != st.type().returnType()) {
6299             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6300                     st.type().returnType());
6301         }
6302     }
6303 
6304     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6305         final List<Class<?>> empty = List.of();
6306         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
6307                 // take only those that can contribute to a common suffix because they are longer than the prefix
6308                         map(MethodHandle::type).
6309                         filter(t -> t.parameterCount() > skipSize).
6310                         map(MethodType::parameterList).
6311                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6312         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
6313     }
6314 
6315     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
6316         final List<Class<?>> empty = List.of();
6317         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6318     }
6319 
6320     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6321         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6322         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6323         return longestParameterList(Arrays.asList(longest1, longest2));
6324     }
6325 
6326     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6327         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6328                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6329             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6330                     " (common suffix: " + commonSuffix + ")");
6331         }
6332     }
6333 
6334     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6335         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6336                 anyMatch(t -> t != loopReturnType)) {
6337             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6338                     loopReturnType + ")");
6339         }
6340 
6341         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
6342             throw newIllegalArgumentException("no predicate found", pred);
6343         }
6344         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6345                 anyMatch(t -> t != boolean.class)) {
6346             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6347         }
6348     }
6349 
6350     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6351         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6352                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6353             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6354                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6355         }
6356     }
6357 
6358     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6359         return hs.stream().map(h -> {
6360             int pc = h.type().parameterCount();
6361             int tpsize = targetParams.size();
6362             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
6363         }).collect(Collectors.toList());
6364     }
6365 
6366     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6367         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
6368     }
6369 
6370     /**
6371      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6372      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6373      * <p>
6374      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6375      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6376      * evaluates to {@code true}).
6377      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6378      * <p>
6379      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6380      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6381      * and updated with the value returned from its invocation. The result of loop execution will be
6382      * the final value of the additional loop-local variable (if present).
6383      * <p>
6384      * The following rules hold for these argument handles:<ul>
6385      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6386      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6387      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6388      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6389      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6390      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6391      * It will constrain the parameter lists of the other loop parts.
6392      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6393      * list {@code (A...)} is called the <em>external parameter list</em>.
6394      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6395      * additional state variable of the loop.
6396      * The body must both accept and return a value of this type {@code V}.
6397      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6398      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6399      * <a href="MethodHandles.html#effid">effectively identical</a>
6400      * to the external parameter list {@code (A...)}.
6401      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6402      * {@linkplain #empty default value}.
6403      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6404      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6405      * effectively identical to the internal parameter list.
6406      * </ul>
6407      * <p>
6408      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6409      * <li>The loop handle's result type is the result type {@code V} of the body.
6410      * <li>The loop handle's parameter types are the types {@code (A...)},
6411      * from the external parameter list.
6412      * </ul>
6413      * <p>
6414      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6415      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6416      * passed to the loop.
6417      * <blockquote><pre>{@code
6418      * V init(A...);
6419      * boolean pred(V, A...);
6420      * V body(V, A...);
6421      * V whileLoop(A... a...) {
6422      *   V v = init(a...);
6423      *   while (pred(v, a...)) {
6424      *     v = body(v, a...);
6425      *   }
6426      *   return v;
6427      * }
6428      * }</pre></blockquote>
6429      *
6430      * @apiNote Example:
6431      * <blockquote><pre>{@code
6432      * // implement the zip function for lists as a loop handle
6433      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6434      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6435      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6436      *   zip.add(a.next());
6437      *   zip.add(b.next());
6438      *   return zip;
6439      * }
6440      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6441      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6442      * List<String> a = Arrays.asList("a", "b", "c", "d");
6443      * List<String> b = Arrays.asList("e", "f", "g", "h");
6444      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6445      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6446      * }</pre></blockquote>
6447      *
6448      *
6449      * @apiNote The implementation of this method can be expressed as follows:
6450      * <blockquote><pre>{@code
6451      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6452      *     MethodHandle fini = (body.type().returnType() == void.class
6453      *                         ? null : identity(body.type().returnType()));
6454      *     MethodHandle[]
6455      *         checkExit = { null, null, pred, fini },
6456      *         varBody   = { init, body };
6457      *     return loop(checkExit, varBody);
6458      * }
6459      * }</pre></blockquote>
6460      *
6461      * @param init optional initializer, providing the initial value of the loop variable.
6462      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6463      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6464      *             above for other constraints.
6465      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6466      *             See above for other constraints.
6467      *
6468      * @return a method handle implementing the {@code while} loop as described by the arguments.
6469      * @throws IllegalArgumentException if the rules for the arguments are violated.
6470      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6471      *
6472      * @see #loop(MethodHandle[][])
6473      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6474      * @since 9
6475      */
6476     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6477         whileLoopChecks(init, pred, body);
6478         MethodHandle fini = identityOrVoid(body.type().returnType());
6479         MethodHandle[] checkExit = { null, null, pred, fini };
6480         MethodHandle[] varBody = { init, body };
6481         return loop(checkExit, varBody);
6482     }
6483 
6484     /**
6485      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6486      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6487      * <p>
6488      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6489      * method will, in each iteration, first execute its body and then evaluate the predicate.
6490      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6491      * <p>
6492      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6493      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6494      * and updated with the value returned from its invocation. The result of loop execution will be
6495      * the final value of the additional loop-local variable (if present).
6496      * <p>
6497      * The following rules hold for these argument handles:<ul>
6498      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6499      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6500      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6501      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6502      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6503      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6504      * It will constrain the parameter lists of the other loop parts.
6505      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6506      * list {@code (A...)} is called the <em>external parameter list</em>.
6507      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6508      * additional state variable of the loop.
6509      * The body must both accept and return a value of this type {@code V}.
6510      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6511      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6512      * <a href="MethodHandles.html#effid">effectively identical</a>
6513      * to the external parameter list {@code (A...)}.
6514      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6515      * {@linkplain #empty default value}.
6516      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6517      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6518      * effectively identical to the internal parameter list.
6519      * </ul>
6520      * <p>
6521      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6522      * <li>The loop handle's result type is the result type {@code V} of the body.
6523      * <li>The loop handle's parameter types are the types {@code (A...)},
6524      * from the external parameter list.
6525      * </ul>
6526      * <p>
6527      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6528      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6529      * passed to the loop.
6530      * <blockquote><pre>{@code
6531      * V init(A...);
6532      * boolean pred(V, A...);
6533      * V body(V, A...);
6534      * V doWhileLoop(A... a...) {
6535      *   V v = init(a...);
6536      *   do {
6537      *     v = body(v, a...);
6538      *   } while (pred(v, a...));
6539      *   return v;
6540      * }
6541      * }</pre></blockquote>
6542      *
6543      * @apiNote Example:
6544      * <blockquote><pre>{@code
6545      * // int i = 0; while (i < limit) { ++i; } return i; => limit
6546      * static int zero(int limit) { return 0; }
6547      * static int step(int i, int limit) { return i + 1; }
6548      * static boolean pred(int i, int limit) { return i < limit; }
6549      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
6550      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
6551      * assertEquals(23, loop.invoke(23));
6552      * }</pre></blockquote>
6553      *
6554      *
6555      * @apiNote The implementation of this method can be expressed as follows:
6556      * <blockquote><pre>{@code
6557      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6558      *     MethodHandle fini = (body.type().returnType() == void.class
6559      *                         ? null : identity(body.type().returnType()));
6560      *     MethodHandle[] clause = { init, body, pred, fini };
6561      *     return loop(clause);
6562      * }
6563      * }</pre></blockquote>
6564      *
6565      * @param init optional initializer, providing the initial value of the loop variable.
6566      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6567      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6568      *             See above for other constraints.
6569      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6570      *             above for other constraints.
6571      *
6572      * @return a method handle implementing the {@code while} loop as described by the arguments.
6573      * @throws IllegalArgumentException if the rules for the arguments are violated.
6574      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6575      *
6576      * @see #loop(MethodHandle[][])
6577      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
6578      * @since 9
6579      */
6580     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6581         whileLoopChecks(init, pred, body);
6582         MethodHandle fini = identityOrVoid(body.type().returnType());
6583         MethodHandle[] clause = {init, body, pred, fini };
6584         return loop(clause);
6585     }
6586 
6587     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
6588         Objects.requireNonNull(pred);
6589         Objects.requireNonNull(body);
6590         MethodType bodyType = body.type();
6591         Class<?> returnType = bodyType.returnType();
6592         List<Class<?>> innerList = bodyType.parameterList();
6593         List<Class<?>> outerList = innerList;
6594         if (returnType == void.class) {
6595             // OK
6596         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
6597             // leading V argument missing => error
6598             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6599             throw misMatchedTypes("body function", bodyType, expected);
6600         } else {
6601             outerList = innerList.subList(1, innerList.size());
6602         }
6603         MethodType predType = pred.type();
6604         if (predType.returnType() != boolean.class ||
6605                 !predType.effectivelyIdenticalParameters(0, innerList)) {
6606             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
6607         }
6608         if (init != null) {
6609             MethodType initType = init.type();
6610             if (initType.returnType() != returnType ||
6611                     !initType.effectivelyIdenticalParameters(0, outerList)) {
6612                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6613             }
6614         }
6615     }
6616 
6617     /**
6618      * Constructs a loop that runs a given number of iterations.
6619      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6620      * <p>
6621      * The number of iterations is determined by the {@code iterations} handle evaluation result.
6622      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
6623      * It will be initialized to 0 and incremented by 1 in each iteration.
6624      * <p>
6625      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6626      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6627      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6628      * <p>
6629      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6630      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6631      * iteration variable.
6632      * The result of the loop handle execution will be the final {@code V} value of that variable
6633      * (or {@code void} if there is no {@code V} variable).
6634      * <p>
6635      * The following rules hold for the argument handles:<ul>
6636      * <li>The {@code iterations} handle must not be {@code null}, and must return
6637      * the type {@code int}, referred to here as {@code I} in parameter type lists.
6638      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6639      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6640      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6641      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6642      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6643      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6644      * of types called the <em>internal parameter list</em>.
6645      * It will constrain the parameter lists of the other loop parts.
6646      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6647      * with no additional {@code A} types, then the internal parameter list is extended by
6648      * the argument types {@code A...} of the {@code iterations} handle.
6649      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6650      * list {@code (A...)} is called the <em>external parameter list</em>.
6651      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6652      * additional state variable of the loop.
6653      * The body must both accept a leading parameter and return a value of this type {@code V}.
6654      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6655      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6656      * <a href="MethodHandles.html#effid">effectively identical</a>
6657      * to the external parameter list {@code (A...)}.
6658      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6659      * {@linkplain #empty default value}.
6660      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
6661      * effectively identical to the external parameter list {@code (A...)}.
6662      * </ul>
6663      * <p>
6664      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6665      * <li>The loop handle's result type is the result type {@code V} of the body.
6666      * <li>The loop handle's parameter types are the types {@code (A...)},
6667      * from the external parameter list.
6668      * </ul>
6669      * <p>
6670      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6671      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6672      * arguments passed to the loop.
6673      * <blockquote><pre>{@code
6674      * int iterations(A...);
6675      * V init(A...);
6676      * V body(V, int, A...);
6677      * V countedLoop(A... a...) {
6678      *   int end = iterations(a...);
6679      *   V v = init(a...);
6680      *   for (int i = 0; i < end; ++i) {
6681      *     v = body(v, i, a...);
6682      *   }
6683      *   return v;
6684      * }
6685      * }</pre></blockquote>
6686      *
6687      * @apiNote Example with a fully conformant body method:
6688      * <blockquote><pre>{@code
6689      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6690      * // => a variation on a well known theme
6691      * static String step(String v, int counter, String init) { return "na " + v; }
6692      * // assume MH_step is a handle to the method above
6693      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
6694      * MethodHandle start = MethodHandles.identity(String.class);
6695      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
6696      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
6697      * }</pre></blockquote>
6698      *
6699      * @apiNote Example with the simplest possible body method type,
6700      * and passing the number of iterations to the loop invocation:
6701      * <blockquote><pre>{@code
6702      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6703      * // => a variation on a well known theme
6704      * static String step(String v, int counter ) { return "na " + v; }
6705      * // assume MH_step is a handle to the method above
6706      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
6707      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
6708      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
6709      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
6710      * }</pre></blockquote>
6711      *
6712      * @apiNote Example that treats the number of iterations, string to append to, and string to append
6713      * as loop parameters:
6714      * <blockquote><pre>{@code
6715      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6716      * // => a variation on a well known theme
6717      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
6718      * // assume MH_step is a handle to the method above
6719      * MethodHandle count = MethodHandles.identity(int.class);
6720      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
6721      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
6722      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
6723      * }</pre></blockquote>
6724      *
6725      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
6726      * to enforce a loop type:
6727      * <blockquote><pre>{@code
6728      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6729      * // => a variation on a well known theme
6730      * static String step(String v, int counter, String pre) { return pre + " " + v; }
6731      * // assume MH_step is a handle to the method above
6732      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
6733      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
6734      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
6735      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
6736      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
6737      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
6738      * }</pre></blockquote>
6739      *
6740      * @apiNote The implementation of this method can be expressed as follows:
6741      * <blockquote><pre>{@code
6742      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6743      *     return countedLoop(empty(iterations.type()), iterations, init, body);
6744      * }
6745      * }</pre></blockquote>
6746      *
6747      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
6748      *                   result type must be {@code int}. See above for other constraints.
6749      * @param init optional initializer, providing the initial value of the loop variable.
6750      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6751      * @param body body of the loop, which may not be {@code null}.
6752      *             It controls the loop parameters and result type in the standard case (see above for details).
6753      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6754      *             and may accept any number of additional types.
6755      *             See above for other constraints.
6756      *
6757      * @return a method handle representing the loop.
6758      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
6759      * @throws IllegalArgumentException if any argument violates the rules formulated above.
6760      *
6761      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
6762      * @since 9
6763      */
6764     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6765         return countedLoop(empty(iterations.type()), iterations, init, body);
6766     }
6767 
6768     /**
6769      * Constructs a loop that counts over a range of numbers.
6770      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6771      * <p>
6772      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
6773      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
6774      * values of the loop counter.
6775      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
6776      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
6777      * <p>
6778      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6779      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6780      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6781      * <p>
6782      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6783      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6784      * iteration variable.
6785      * The result of the loop handle execution will be the final {@code V} value of that variable
6786      * (or {@code void} if there is no {@code V} variable).
6787      * <p>
6788      * The following rules hold for the argument handles:<ul>
6789      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
6790      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
6791      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6792      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6793      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6794      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6795      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6796      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6797      * of types called the <em>internal parameter list</em>.
6798      * It will constrain the parameter lists of the other loop parts.
6799      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6800      * with no additional {@code A} types, then the internal parameter list is extended by
6801      * the argument types {@code A...} of the {@code end} handle.
6802      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6803      * list {@code (A...)} is called the <em>external parameter list</em>.
6804      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6805      * additional state variable of the loop.
6806      * The body must both accept a leading parameter and return a value of this type {@code V}.
6807      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6808      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6809      * <a href="MethodHandles.html#effid">effectively identical</a>
6810      * to the external parameter list {@code (A...)}.
6811      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6812      * {@linkplain #empty default value}.
6813      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
6814      * effectively identical to the external parameter list {@code (A...)}.
6815      * <li>Likewise, the parameter list of {@code end} must be effectively identical
6816      * to the external parameter list.
6817      * </ul>
6818      * <p>
6819      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6820      * <li>The loop handle's result type is the result type {@code V} of the body.
6821      * <li>The loop handle's parameter types are the types {@code (A...)},
6822      * from the external parameter list.
6823      * </ul>
6824      * <p>
6825      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6826      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6827      * arguments passed to the loop.
6828      * <blockquote><pre>{@code
6829      * int start(A...);
6830      * int end(A...);
6831      * V init(A...);
6832      * V body(V, int, A...);
6833      * V countedLoop(A... a...) {
6834      *   int e = end(a...);
6835      *   int s = start(a...);
6836      *   V v = init(a...);
6837      *   for (int i = s; i < e; ++i) {
6838      *     v = body(v, i, a...);
6839      *   }
6840      *   return v;
6841      * }
6842      * }</pre></blockquote>
6843      *
6844      * @apiNote The implementation of this method can be expressed as follows:
6845      * <blockquote><pre>{@code
6846      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6847      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
6848      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
6849      *     // the following semantics:
6850      *     // MH_increment: (int limit, int counter) -> counter + 1
6851      *     // MH_predicate: (int limit, int counter) -> counter < limit
6852      *     Class<?> counterType = start.type().returnType();  // int
6853      *     Class<?> returnType = body.type().returnType();
6854      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
6855      *     if (returnType != void.class) {  // ignore the V variable
6856      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
6857      *         pred = dropArguments(pred, 1, returnType);  // ditto
6858      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
6859      *     }
6860      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
6861      *     MethodHandle[]
6862      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
6863      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
6864      *         indexVar   = { start, incr };           // i = start(); i = i + 1
6865      *     return loop(loopLimit, bodyClause, indexVar);
6866      * }
6867      * }</pre></blockquote>
6868      *
6869      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
6870      *              See above for other constraints.
6871      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
6872      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
6873      * @param init optional initializer, providing the initial value of the loop variable.
6874      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6875      * @param body body of the loop, which may not be {@code null}.
6876      *             It controls the loop parameters and result type in the standard case (see above for details).
6877      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6878      *             and may accept any number of additional types.
6879      *             See above for other constraints.
6880      *
6881      * @return a method handle representing the loop.
6882      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
6883      * @throws IllegalArgumentException if any argument violates the rules formulated above.
6884      *
6885      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
6886      * @since 9
6887      */
6888     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6889         countedLoopChecks(start, end, init, body);
6890         Class<?> counterType = start.type().returnType();  // int, but who's counting?
6891         Class<?> limitType   = end.type().returnType();    // yes, int again
6892         Class<?> returnType  = body.type().returnType();
6893         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
6894         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
6895         MethodHandle retv = null;
6896         if (returnType != void.class) {
6897             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
6898             pred = dropArguments(pred, 1, returnType);  // ditto
6899             retv = dropArguments(identity(returnType), 0, counterType);
6900         }
6901         body = dropArguments(body, 0, counterType);  // ignore the limit variable
6902         MethodHandle[]
6903             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
6904             bodyClause = { init, body },            // v = init(); v = body(v, i)
6905             indexVar   = { start, incr };           // i = start(); i = i + 1
6906         return loop(loopLimit, bodyClause, indexVar);
6907     }
6908 
6909     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
6910         Objects.requireNonNull(start);
6911         Objects.requireNonNull(end);
6912         Objects.requireNonNull(body);
6913         Class<?> counterType = start.type().returnType();
6914         if (counterType != int.class) {
6915             MethodType expected = start.type().changeReturnType(int.class);
6916             throw misMatchedTypes("start function", start.type(), expected);
6917         } else if (end.type().returnType() != counterType) {
6918             MethodType expected = end.type().changeReturnType(counterType);
6919             throw misMatchedTypes("end function", end.type(), expected);
6920         }
6921         MethodType bodyType = body.type();
6922         Class<?> returnType = bodyType.returnType();
6923         List<Class<?>> innerList = bodyType.parameterList();
6924         // strip leading V value if present
6925         int vsize = (returnType == void.class ? 0 : 1);
6926         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
6927             // argument list has no "V" => error
6928             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6929             throw misMatchedTypes("body function", bodyType, expected);
6930         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
6931             // missing I type => error
6932             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
6933             throw misMatchedTypes("body function", bodyType, expected);
6934         }
6935         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
6936         if (outerList.isEmpty()) {
6937             // special case; take lists from end handle
6938             outerList = end.type().parameterList();
6939             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
6940         }
6941         MethodType expected = methodType(counterType, outerList);
6942         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
6943             throw misMatchedTypes("start parameter types", start.type(), expected);
6944         }
6945         if (end.type() != start.type() &&
6946             !end.type().effectivelyIdenticalParameters(0, outerList)) {
6947             throw misMatchedTypes("end parameter types", end.type(), expected);
6948         }
6949         if (init != null) {
6950             MethodType initType = init.type();
6951             if (initType.returnType() != returnType ||
6952                 !initType.effectivelyIdenticalParameters(0, outerList)) {
6953                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6954             }
6955         }
6956     }
6957 
6958     /**
6959      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
6960      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6961      * <p>
6962      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
6963      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
6964      * <p>
6965      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6966      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6967      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6968      * <p>
6969      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6970      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6971      * iteration variable.
6972      * The result of the loop handle execution will be the final {@code V} value of that variable
6973      * (or {@code void} if there is no {@code V} variable).
6974      * <p>
6975      * The following rules hold for the argument handles:<ul>
6976      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6977      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
6978      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6979      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
6980      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
6981      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
6982      * of types called the <em>internal parameter list</em>.
6983      * It will constrain the parameter lists of the other loop parts.
6984      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
6985      * with no additional {@code A} types, then the internal parameter list is extended by
6986      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
6987      * single type {@code Iterable} is added and constitutes the {@code A...} list.
6988      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
6989      * list {@code (A...)} is called the <em>external parameter list</em>.
6990      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6991      * additional state variable of the loop.
6992      * The body must both accept a leading parameter and return a value of this type {@code V}.
6993      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6994      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6995      * <a href="MethodHandles.html#effid">effectively identical</a>
6996      * to the external parameter list {@code (A...)}.
6997      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6998      * {@linkplain #empty default value}.
6999      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7000      * type {@code java.util.Iterator} or a subtype thereof.
7001      * The iterator it produces when the loop is executed will be assumed
7002      * to yield values which can be converted to type {@code T}.
7003      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7004      * effectively identical to the external parameter list {@code (A...)}.
7005      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7006      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7007      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7008      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7009      * the {@link MethodHandle#asType asType} conversion method.
7010      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7011      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7012      * </ul>
7013      * <p>
7014      * The type {@code T} may be either a primitive or reference.
7015      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7016      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7017      * as if by the {@link MethodHandle#asType asType} conversion method.
7018      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7019      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7020      * <p>
7021      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7022      * <li>The loop handle's result type is the result type {@code V} of the body.
7023      * <li>The loop handle's parameter types are the types {@code (A...)},
7024      * from the external parameter list.
7025      * </ul>
7026      * <p>
7027      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7028      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7029      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7030      * <blockquote><pre>{@code
7031      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7032      * V init(A...);
7033      * V body(V,T,A...);
7034      * V iteratedLoop(A... a...) {
7035      *   Iterator<T> it = iterator(a...);
7036      *   V v = init(a...);
7037      *   while (it.hasNext()) {
7038      *     T t = it.next();
7039      *     v = body(v, t, a...);
7040      *   }
7041      *   return v;
7042      * }
7043      * }</pre></blockquote>
7044      *
7045      * @apiNote Example:
7046      * <blockquote><pre>{@code
7047      * // get an iterator from a list
7048      * static List<String> reverseStep(List<String> r, String e) {
7049      *   r.add(0, e);
7050      *   return r;
7051      * }
7052      * static List<String> newArrayList() { return new ArrayList<>(); }
7053      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7054      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7055      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7056      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7057      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7058      * }</pre></blockquote>
7059      *
7060      * @apiNote The implementation of this method can be expressed approximately as follows:
7061      * <blockquote><pre>{@code
7062      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7063      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7064      *     Class<?> returnType = body.type().returnType();
7065      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7066      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7067      *     MethodHandle retv = null, step = body, startIter = iterator;
7068      *     if (returnType != void.class) {
7069      *         // the simple thing first:  in (I V A...), drop the I to get V
7070      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7071      *         // body type signature (V T A...), internal loop types (I V A...)
7072      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7073      *     }
7074      *     if (startIter == null)  startIter = MH_getIter;
7075      *     MethodHandle[]
7076      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7077      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7078      *     return loop(iterVar, bodyClause);
7079      * }
7080      * }</pre></blockquote>
7081      *
7082      * @param iterator an optional handle to return the iterator to start the loop.
7083      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7084      *                 See above for other constraints.
7085      * @param init optional initializer, providing the initial value of the loop variable.
7086      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7087      * @param body body of the loop, which may not be {@code null}.
7088      *             It controls the loop parameters and result type in the standard case (see above for details).
7089      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7090      *             and may accept any number of additional types.
7091      *             See above for other constraints.
7092      *
7093      * @return a method handle embodying the iteration loop functionality.
7094      * @throws NullPointerException if the {@code body} handle is {@code null}.
7095      * @throws IllegalArgumentException if any argument violates the above requirements.
7096      *
7097      * @since 9
7098      */
7099     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7100         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7101         Class<?> returnType = body.type().returnType();
7102         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7103         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7104         MethodHandle startIter;
7105         MethodHandle nextVal;
7106         {
7107             MethodType iteratorType;
7108             if (iterator == null) {
7109                 // derive argument type from body, if available, else use Iterable
7110                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7111                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7112             } else {
7113                 // force return type to the internal iterator class
7114                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7115                 startIter = iterator;
7116             }
7117             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7118             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7119 
7120             // perform the asType transforms under an exception transformer, as per spec.:
7121             try {
7122                 startIter = startIter.asType(iteratorType);
7123                 nextVal = nextRaw.asType(nextValType);
7124             } catch (WrongMethodTypeException ex) {
7125                 throw new IllegalArgumentException(ex);
7126             }
7127         }
7128 
7129         MethodHandle retv = null, step = body;
7130         if (returnType != void.class) {
7131             // the simple thing first:  in (I V A...), drop the I to get V
7132             retv = dropArguments(identity(returnType), 0, Iterator.class);
7133             // body type signature (V T A...), internal loop types (I V A...)
7134             step = swapArguments(body, 0, 1);  // swap V <-> T
7135         }
7136 
7137         MethodHandle[]
7138             iterVar    = { startIter, null, hasNext, retv },
7139             bodyClause = { init, filterArgument(step, 0, nextVal) };
7140         return loop(iterVar, bodyClause);
7141     }
7142 
7143     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7144         Objects.requireNonNull(body);
7145         MethodType bodyType = body.type();
7146         Class<?> returnType = bodyType.returnType();
7147         List<Class<?>> internalParamList = bodyType.parameterList();
7148         // strip leading V value if present
7149         int vsize = (returnType == void.class ? 0 : 1);
7150         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
7151             // argument list has no "V" => error
7152             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7153             throw misMatchedTypes("body function", bodyType, expected);
7154         } else if (internalParamList.size() <= vsize) {
7155             // missing T type => error
7156             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7157             throw misMatchedTypes("body function", bodyType, expected);
7158         }
7159         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7160         Class<?> iterableType = null;
7161         if (iterator != null) {
7162             // special case; if the body handle only declares V and T then
7163             // the external parameter list is obtained from iterator handle
7164             if (externalParamList.isEmpty()) {
7165                 externalParamList = iterator.type().parameterList();
7166             }
7167             MethodType itype = iterator.type();
7168             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7169                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7170             }
7171             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7172                 MethodType expected = methodType(itype.returnType(), externalParamList);
7173                 throw misMatchedTypes("iterator parameters", itype, expected);
7174             }
7175         } else {
7176             if (externalParamList.isEmpty()) {
7177                 // special case; if the iterator handle is null and the body handle
7178                 // only declares V and T then the external parameter list consists
7179                 // of Iterable
7180                 externalParamList = Arrays.asList(Iterable.class);
7181                 iterableType = Iterable.class;
7182             } else {
7183                 // special case; if the iterator handle is null and the external
7184                 // parameter list is not empty then the first parameter must be
7185                 // assignable to Iterable
7186                 iterableType = externalParamList.get(0);
7187                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7188                     throw newIllegalArgumentException(
7189                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7190                 }
7191             }
7192         }
7193         if (init != null) {
7194             MethodType initType = init.type();
7195             if (initType.returnType() != returnType ||
7196                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7197                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7198             }
7199         }
7200         return iterableType;  // help the caller a bit
7201     }
7202 
7203     /*non-public*/
7204     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7205         // there should be a better way to uncross my wires
7206         int arity = mh.type().parameterCount();
7207         int[] order = new int[arity];
7208         for (int k = 0; k < arity; k++)  order[k] = k;
7209         order[i] = j; order[j] = i;
7210         Class<?>[] types = mh.type().parameterArray();
7211         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7212         MethodType swapType = methodType(mh.type().returnType(), types);
7213         return permuteArguments(mh, swapType, order);
7214     }
7215 
7216     /**
7217      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7218      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7219      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7220      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7221      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7222      * {@code try-finally} handle.
7223      * <p>
7224      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7225      * The first is the exception thrown during the
7226      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7227      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7228      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7229      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7230      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7231      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7232      * <p>
7233      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7234      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7235      * two extra leading parameters:<ul>
7236      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7237      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7238      * the result from the execution of the {@code target} handle.
7239      * This parameter is not present if the {@code target} returns {@code void}.
7240      * </ul>
7241      * <p>
7242      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7243      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7244      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7245      * the cleanup.
7246      * <blockquote><pre>{@code
7247      * V target(A..., B...);
7248      * V cleanup(Throwable, V, A...);
7249      * V adapter(A... a, B... b) {
7250      *   V result = (zero value for V);
7251      *   Throwable throwable = null;
7252      *   try {
7253      *     result = target(a..., b...);
7254      *   } catch (Throwable t) {
7255      *     throwable = t;
7256      *     throw t;
7257      *   } finally {
7258      *     result = cleanup(throwable, result, a...);
7259      *   }
7260      *   return result;
7261      * }
7262      * }</pre></blockquote>
7263      * <p>
7264      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7265      * be modified by execution of the target, and so are passed unchanged
7266      * from the caller to the cleanup, if it is invoked.
7267      * <p>
7268      * The target and cleanup must return the same type, even if the cleanup
7269      * always throws.
7270      * To create such a throwing cleanup, compose the cleanup logic
7271      * with {@link #throwException throwException},
7272      * in order to create a method handle of the correct return type.
7273      * <p>
7274      * Note that {@code tryFinally} never converts exceptions into normal returns.
7275      * In rare cases where exceptions must be converted in that way, first wrap
7276      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7277      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7278      * <p>
7279      * It is recommended that the first parameter type of {@code cleanup} be
7280      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7281      * {@code cleanup} will always be invoked with whatever exception that
7282      * {@code target} throws.  Declaring a narrower type may result in a
7283      * {@code ClassCastException} being thrown by the {@code try-finally}
7284      * handle if the type of the exception thrown by {@code target} is not
7285      * assignable to the first parameter type of {@code cleanup}.  Note that
7286      * various exception types of {@code VirtualMachineError},
7287      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7288      * thrown by almost any kind of Java code, and a finally clause that
7289      * catches (say) only {@code IOException} would mask any of the others
7290      * behind a {@code ClassCastException}.
7291      *
7292      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7293      * @param cleanup the handle that is invoked in the finally block.
7294      *
7295      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7296      * @throws NullPointerException if any argument is null
7297      * @throws IllegalArgumentException if {@code cleanup} does not accept
7298      *          the required leading arguments, or if the method handle types do
7299      *          not match in their return types and their
7300      *          corresponding trailing parameters
7301      *
7302      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7303      * @since 9
7304      */
7305     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7306         List<Class<?>> targetParamTypes = target.type().parameterList();
7307         Class<?> rtype = target.type().returnType();
7308 
7309         tryFinallyChecks(target, cleanup);
7310 
7311         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7312         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7313         // target parameter list.
7314         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
7315 
7316         // Ensure that the intrinsic type checks the instance thrown by the
7317         // target against the first parameter of cleanup
7318         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7319 
7320         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7321         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7322     }
7323 
7324     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7325         Class<?> rtype = target.type().returnType();
7326         if (rtype != cleanup.type().returnType()) {
7327             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7328         }
7329         MethodType cleanupType = cleanup.type();
7330         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7331             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7332         }
7333         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7334             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7335         }
7336         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7337         // target parameter list.
7338         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7339         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7340             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7341                     cleanup.type(), target.type());
7342         }
7343     }
7344 
7345 }