1 /*
   2  * Copyright (c) 2008, 2018, 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.misc.JavaLangAccess;
  29 import jdk.internal.misc.SharedSecrets;
  30 import jdk.internal.module.IllegalAccessLogger;
  31 import jdk.internal.org.objectweb.asm.ClassReader;
  32 import jdk.internal.reflect.CallerSensitive;
  33 import jdk.internal.reflect.Reflection;
  34 import jdk.internal.vm.annotation.ForceInline;
  35 import sun.invoke.util.ValueConversions;
  36 import sun.invoke.util.VerifyAccess;
  37 import sun.invoke.util.Wrapper;
  38 import sun.reflect.misc.ReflectUtil;
  39 import sun.security.util.SecurityConstants;
  40 
  41 import java.lang.invoke.LambdaForm.BasicType;
  42 import java.lang.reflect.Constructor;
  43 import java.lang.reflect.Field;
  44 import java.lang.reflect.Member;
  45 import java.lang.reflect.Method;
  46 import java.lang.reflect.Modifier;
  47 import java.lang.reflect.ReflectPermission;
  48 import java.nio.ByteOrder;
  49 import java.security.ProtectionDomain;
  50 import java.util.ArrayList;
  51 import java.util.Arrays;
  52 import java.util.BitSet;
  53 import java.util.Iterator;
  54 import java.util.List;
  55 import java.util.Objects;
  56 import java.util.Set;
  57 import java.util.WeakHashMap;
  58 import java.util.concurrent.ConcurrentHashMap;
  59 import java.util.stream.Collectors;
  60 import java.util.stream.Stream;
  61 
  62 import static java.lang.invoke.MethodHandles.Lookup.ClassProperty.*;
  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 <a href="MethodHandles.Lookup.html#privacc">private access</a> 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 private access
 107      */
 108     @CallerSensitive
 109     @ForceInline // to ensure Reflection.getCallerClass optimization
 110     public static Lookup lookup() {
 111         return new Lookup(Reflection.getCallerClass());
 112     }
 113 
 114     /**
 115      * This reflected$lookup method is the alternate implementation of
 116      * the lookup method when being invoked by reflection.
 117      */
 118     @CallerSensitive
 119     private static Lookup reflected$lookup() {
 120         Class<?> caller = Reflection.getCallerClass();
 121         if (caller.getClassLoader() == null) {
 122             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 123         }
 124         return new Lookup(caller);
 125     }
 126 
 127     /**
 128      * Returns a {@link Lookup lookup object} which is trusted minimally.
 129      * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes.
 130      * It can only be used to create method handles to public members of
 131      * public classes in packages that are exported unconditionally.
 132      * <p>
 133      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 134      * of this lookup object will be {@link java.lang.Object}.
 135      *
 136      * @apiNote The use of Object is conventional, and because the lookup modes are
 137      * limited, there is no special access provided to the internals of Object, its package
 138      * or its module. Consequently, the lookup context of this lookup object will be the
 139      * bootstrap class loader, which means it cannot find user classes.
 140      *
 141      * <p style="font-size:smaller;">
 142      * <em>Discussion:</em>
 143      * The lookup class can be changed to any other class {@code C} using an expression of the form
 144      * {@link Lookup#in publicLookup().in(C.class)}.
 145      * but may change the lookup context by virtue of changing the class loader.
 146      * A public lookup object is always subject to
 147      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 148      * Also, it cannot access
 149      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 150      * @return a lookup object which is trusted minimally
 151      *
 152      * @revised 9
 153      * @spec JPMS
 154      */
 155     public static Lookup publicLookup() {
 156         return Lookup.PUBLIC_LOOKUP;
 157     }
 158 
 159     /**
 160      * Returns a {@link Lookup lookup object} with full capabilities to emulate all
 161      * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">
 162      * private access</a>, on a target class.
 163      * This method checks that a caller, specified as a {@code Lookup} object, is allowed to
 164      * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing
 165      * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing
 166      * the target class, then this check ensures that
 167      * <ul>
 168      *     <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li>
 169      *     <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing
 170      *     the target class to at least {@code m1}.</li>
 171      *     <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li>
 172      * </ul>
 173      * <p>
 174      * If there is a security manager, its {@code checkPermission} method is called to
 175      * check {@code ReflectPermission("suppressAccessChecks")}.
 176      * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object
 177      * was created by code in the caller module (or derived from a lookup object originally
 178      * created by the caller). A lookup object with the {@code MODULE} lookup mode can be
 179      * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE}
 180      * access to the caller.
 181      * @param targetClass the target class
 182      * @param lookup the caller lookup object
 183      * @return a lookup object for the target class, with private access
 184      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or array class
 185      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 186      * @throws IllegalAccessException if the access check specified above fails
 187      * @throws SecurityException if denied by the security manager
 188      * @since 9
 189      * @spec JPMS
 190      * @see Lookup#dropLookupMode
 191      */
 192     public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException {
 193         if (lookup.allowedModes == Lookup.TRUSTED) {
 194             return new Lookup(targetClass);
 195         }
 196 
 197         SecurityManager sm = System.getSecurityManager();
 198         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 199         if (targetClass.isPrimitive())
 200             throw new IllegalArgumentException(targetClass + " is a primitive class");
 201         if (targetClass.isArray())
 202             throw new IllegalArgumentException(targetClass + " is an array class");
 203         Module targetModule = targetClass.getModule();
 204         Module callerModule = lookup.lookupClass().getModule();
 205         if (!callerModule.canRead(targetModule))
 206             throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 207         if (targetModule.isNamed()) {
 208             String pn = targetClass.getPackageName();
 209             assert pn.length() > 0 : "unnamed package cannot be in named module";
 210             if (!targetModule.isOpen(pn, callerModule))
 211                 throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 212         }
 213         if ((lookup.lookupModes() & Lookup.MODULE) == 0)
 214             throw new IllegalAccessException("lookup does not have MODULE lookup mode");
 215         if (!callerModule.isNamed() && targetModule.isNamed()) {
 216             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 217             if (logger != null) {
 218                 logger.logIfOpenedForIllegalAccess(lookup, targetClass);
 219             }
 220         }
 221         return new Lookup(targetClass);
 222     }
 223 
 224     /**
 225      * Performs an unchecked "crack" of a
 226      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 227      * The result is as if the user had obtained a lookup object capable enough
 228      * to crack the target method handle, called
 229      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 230      * on the target to obtain its symbolic reference, and then called
 231      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 232      * to resolve the symbolic reference to a member.
 233      * <p>
 234      * If there is a security manager, its {@code checkPermission} method
 235      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 236      * @param <T> the desired type of the result, either {@link Member} or a subtype
 237      * @param target a direct method handle to crack into symbolic reference components
 238      * @param expected a class object representing the desired result type {@code T}
 239      * @return a reference to the method, constructor, or field object
 240      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 241      * @exception NullPointerException if either argument is {@code null}
 242      * @exception IllegalArgumentException if the target is not a direct method handle
 243      * @exception ClassCastException if the member is not of the expected type
 244      * @since 1.8
 245      */
 246     public static <T extends Member> T
 247     reflectAs(Class<T> expected, MethodHandle target) {
 248         SecurityManager smgr = System.getSecurityManager();
 249         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 250         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 251         return lookup.revealDirect(target).reflectAs(expected, lookup);
 252     }
 253     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 254     private static final java.security.Permission ACCESS_PERMISSION =
 255         new ReflectPermission("suppressAccessChecks");
 256 
 257     /**
 258      * A <em>lookup object</em> is a factory for creating method handles,
 259      * when the creation requires access checking.
 260      * Method handles do not perform
 261      * access checks when they are called, but rather when they are created.
 262      * Therefore, method handle access
 263      * restrictions must be enforced when a method handle is created.
 264      * The caller class against which those restrictions are enforced
 265      * is known as the {@linkplain #lookupClass() lookup class}.
 266      * <p>
 267      * A lookup class which needs to create method handles will call
 268      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 269      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 270      * determined, and securely stored in the {@code Lookup} object.
 271      * The lookup class (or its delegates) may then use factory methods
 272      * on the {@code Lookup} object to create method handles for access-checked members.
 273      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 274      * even private ones.
 275      *
 276      * <h1><a id="lookups"></a>Lookup Factory Methods</h1>
 277      * The factory methods on a {@code Lookup} object correspond to all major
 278      * use cases for methods, constructors, and fields.
 279      * Each method handle created by a factory method is the functional
 280      * equivalent of a particular <em>bytecode behavior</em>.
 281      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 282      * Here is a summary of the correspondence between these factory methods and
 283      * the behavior of the resulting method handles:
 284      * <table class="striped">
 285      * <caption style="display:none">lookup method behaviors</caption>
 286      * <thead>
 287      * <tr>
 288      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 289      *     <th scope="col">member</th>
 290      *     <th scope="col">bytecode behavior</th>
 291      * </tr>
 292      * </thead>
 293      * <tbody>
 294      * <tr>
 295      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 296      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 297      * </tr>
 298      * <tr>
 299      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 300      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 301      * </tr>
 302      * <tr>
 303      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 304      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 305      * </tr>
 306      * <tr>
 307      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 308      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 309      * </tr>
 310      * <tr>
 311      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 312      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 313      * </tr>
 314      * <tr>
 315      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 316      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 317      * </tr>
 318      * <tr>
 319      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 320      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 321      * </tr>
 322      * <tr>
 323      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 324      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 325      * </tr>
 326      * <tr>
 327      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 328      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 329      * </tr>
 330      * <tr>
 331      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 332      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 333      * </tr>
 334      * <tr>
 335      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 336      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 337      * </tr>
 338      * <tr>
 339      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 340      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 341      * </tr>
 342      * <tr>
 343      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 344      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 345      * </tr>
 346      * <tr>
 347      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 348      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 349      * </tr>
 350      * </tbody>
 351      * </table>
 352      *
 353      * Here, the type {@code C} is the class or interface being searched for a member,
 354      * documented as a parameter named {@code refc} in the lookup methods.
 355      * The method type {@code MT} is composed from the return type {@code T}
 356      * and the sequence of argument types {@code A*}.
 357      * The constructor also has a sequence of argument types {@code A*} and
 358      * is deemed to return the newly-created object of type {@code C}.
 359      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 360      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 361      * if it is present, it is always the leading argument to the method handle invocation.
 362      * (In the case of some {@code protected} members, {@code this} may be
 363      * restricted in type to the lookup class; see below.)
 364      * The name {@code arg} stands for all the other method handle arguments.
 365      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 366      * stands for a null reference if the accessed method or field is static,
 367      * and {@code this} otherwise.
 368      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 369      * for reflective objects corresponding to the given members.
 370      * <p>
 371      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 372      * as if by {@code ldc CONSTANT_Class}.
 373      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 374      * <p>
 375      * In cases where the given member is of variable arity (i.e., a method or constructor)
 376      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 377      * In all other cases, the returned method handle will be of fixed arity.
 378      * <p style="font-size:smaller;">
 379      * <em>Discussion:</em>
 380      * The equivalence between looked-up method handles and underlying
 381      * class members and bytecode behaviors
 382      * can break down in a few ways:
 383      * <ul style="font-size:smaller;">
 384      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 385      * the lookup can still succeed, even when there is no equivalent
 386      * Java expression or bytecoded constant.
 387      * <li>Likewise, if {@code T} or {@code MT}
 388      * is not symbolically accessible from the lookup class's loader,
 389      * the lookup can still succeed.
 390      * For example, lookups for {@code MethodHandle.invokeExact} and
 391      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 392      * <li>If there is a security manager installed, it can forbid the lookup
 393      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 394      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 395      * constant is not subject to security manager checks.
 396      * <li>If the looked-up method has a
 397      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 398      * the method handle creation may fail with an
 399      * {@code IllegalArgumentException}, due to the method handle type having
 400      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 401      * </ul>
 402      *
 403      * <h1><a id="access"></a>Access checking</h1>
 404      * Access checks are applied in the factory methods of {@code Lookup},
 405      * when a method handle is created.
 406      * This is a key difference from the Core Reflection API, since
 407      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 408      * performs access checking against every caller, on every call.
 409      * <p>
 410      * All access checks start from a {@code Lookup} object, which
 411      * compares its recorded lookup class against all requests to
 412      * create method handles.
 413      * A single {@code Lookup} object can be used to create any number
 414      * of access-checked method handles, all checked against a single
 415      * lookup class.
 416      * <p>
 417      * A {@code Lookup} object can be shared with other trusted code,
 418      * such as a metaobject protocol.
 419      * A shared {@code Lookup} object delegates the capability
 420      * to create method handles on private members of the lookup class.
 421      * Even if privileged code uses the {@code Lookup} object,
 422      * the access checking is confined to the privileges of the
 423      * original lookup class.
 424      * <p>
 425      * A lookup can fail, because
 426      * the containing class is not accessible to the lookup class, or
 427      * because the desired class member is missing, or because the
 428      * desired class member is not accessible to the lookup class, or
 429      * because the lookup object is not trusted enough to access the member.
 430      * In any of these cases, a {@code ReflectiveOperationException} will be
 431      * thrown from the attempted lookup.  The exact class will be one of
 432      * the following:
 433      * <ul>
 434      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 435      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 436      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 437      * </ul>
 438      * <p>
 439      * In general, the conditions under which a method handle may be
 440      * looked up for a method {@code M} are no more restrictive than the conditions
 441      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 442      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 443      * a method handle lookup will generally raise a corresponding
 444      * checked exception, such as {@code NoSuchMethodException}.
 445      * And the effect of invoking the method handle resulting from the lookup
 446      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 447      * to executing the compiled, verified, and resolved call to {@code M}.
 448      * The same point is true of fields and constructors.
 449      * <p style="font-size:smaller;">
 450      * <em>Discussion:</em>
 451      * Access checks only apply to named and reflected methods,
 452      * constructors, and fields.
 453      * Other method handle creation methods, such as
 454      * {@link MethodHandle#asType MethodHandle.asType},
 455      * do not require any access checks, and are used
 456      * independently of any {@code Lookup} object.
 457      * <p>
 458      * If the desired member is {@code protected}, the usual JVM rules apply,
 459      * including the requirement that the lookup class must either be in the
 460      * same package as the desired member, or must inherit that member.
 461      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 462      * In addition, if the desired member is a non-static field or method
 463      * in a different package, the resulting method handle may only be applied
 464      * to objects of the lookup class or one of its subclasses.
 465      * This requirement is enforced by narrowing the type of the leading
 466      * {@code this} parameter from {@code C}
 467      * (which will necessarily be a superclass of the lookup class)
 468      * to the lookup class itself.
 469      * <p>
 470      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 471      * that the receiver argument must match both the resolved method <em>and</em>
 472      * the current class.  Again, this requirement is enforced by narrowing the
 473      * type of the leading parameter to the resulting method handle.
 474      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 475      * <p>
 476      * The JVM represents constructors and static initializer blocks as internal methods
 477      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 478      * The internal syntax of invocation instructions allows them to refer to such internal
 479      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 480      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 481      * <p>
 482      * If the relationship between nested types is expressed directly through the
 483      * {@code NestHost} and {@code NestMembers} attributes
 484      * (see the Java Virtual Machine Specification, sections 4.7.28 and 4.7.29),
 485      * then the associated {@code Lookup} object provides direct access to
 486      * the lookup class and all of its nestmates
 487      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 488      * Otherwise, access between nested classes is obtained by the Java compiler creating
 489      * a wrapper method to access a private method of another class in the same nest.
 490      * For example, a nested class {@code C.D}
 491      * can access private members within other related classes such as
 492      * {@code C}, {@code C.D.E}, or {@code C.B},
 493      * but the Java compiler may need to generate wrapper methods in
 494      * those related classes.  In such cases, a {@code Lookup} object on
 495      * {@code C.E} would be unable to access those private members.
 496      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 497      * which can transform a lookup on {@code C.E} into one on any of those other
 498      * classes, without special elevation of privilege.
 499      * <p>
 500      * The accesses permitted to a given lookup object may be limited,
 501      * according to its set of {@link #lookupModes lookupModes},
 502      * to a subset of members normally accessible to the lookup class.
 503      * For example, the {@link MethodHandles#publicLookup publicLookup}
 504      * method produces a lookup object which is only allowed to access
 505      * public members in public classes of exported packages.
 506      * The caller sensitive method {@link MethodHandles#lookup lookup}
 507      * produces a lookup object with full capabilities relative to
 508      * its caller class, to emulate all supported bytecode behaviors.
 509      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 510      * with fewer access modes than the original lookup object.
 511      *
 512      * <p style="font-size:smaller;">
 513      * <a id="privacc"></a>
 514      * <em>Discussion of private access:</em>
 515      * We say that a lookup has <em>private access</em>
 516      * if its {@linkplain #lookupModes lookup modes}
 517      * include the possibility of accessing {@code private} members
 518      * (which includes the private members of nestmates).
 519      * As documented in the relevant methods elsewhere,
 520      * only lookups with private access possess the following capabilities:
 521      * <ul style="font-size:smaller;">
 522      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 523      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 524      *     such as {@code Class.forName}
 525      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 526      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 527      *     for classes accessible to the lookup class
 528      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 529      *     within the same package member
 530      * </ul>
 531      * <p style="font-size:smaller;">
 532      * Each of these permissions is a consequence of the fact that a lookup object
 533      * with private access can be securely traced back to an originating class,
 534      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 535      * can be reliably determined and emulated by method handles.
 536      *
 537      * <h1><a id="secmgr"></a>Security manager interactions</h1>
 538      * Although bytecode instructions can only refer to classes in
 539      * a related class loader, this API can search for methods in any
 540      * class, as long as a reference to its {@code Class} object is
 541      * available.  Such cross-loader references are also possible with the
 542      * Core Reflection API, and are impossible to bytecode instructions
 543      * such as {@code invokestatic} or {@code getfield}.
 544      * There is a {@linkplain java.lang.SecurityManager security manager API}
 545      * to allow applications to check such cross-loader references.
 546      * These checks apply to both the {@code MethodHandles.Lookup} API
 547      * and the Core Reflection API
 548      * (as found on {@link java.lang.Class Class}).
 549      * <p>
 550      * If a security manager is present, member and class lookups are subject to
 551      * additional checks.
 552      * From one to three calls are made to the security manager.
 553      * Any of these calls can refuse access by throwing a
 554      * {@link java.lang.SecurityException SecurityException}.
 555      * Define {@code smgr} as the security manager,
 556      * {@code lookc} as the lookup class of the current lookup object,
 557      * {@code refc} as the containing class in which the member
 558      * is being sought, and {@code defc} as the class in which the
 559      * member is actually defined.
 560      * (If a class or other type is being accessed,
 561      * the {@code refc} and {@code defc} values are the class itself.)
 562      * The value {@code lookc} is defined as <em>not present</em>
 563      * if the current lookup object does not have
 564      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 565      * The calls are made according to the following rules:
 566      * <ul>
 567      * <li><b>Step 1:</b>
 568      *     If {@code lookc} is not present, or if its class loader is not
 569      *     the same as or an ancestor of the class loader of {@code refc},
 570      *     then {@link SecurityManager#checkPackageAccess
 571      *     smgr.checkPackageAccess(refcPkg)} is called,
 572      *     where {@code refcPkg} is the package of {@code refc}.
 573      * <li><b>Step 2a:</b>
 574      *     If the retrieved member is not public and
 575      *     {@code lookc} is not present, then
 576      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 577      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 578      * <li><b>Step 2b:</b>
 579      *     If the retrieved class has a {@code null} class loader,
 580      *     and {@code lookc} is not present, then
 581      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 582      *     with {@code RuntimePermission("getClassLoader")} is called.
 583      * <li><b>Step 3:</b>
 584      *     If the retrieved member is not public,
 585      *     and if {@code lookc} is not present,
 586      *     and if {@code defc} and {@code refc} are different,
 587      *     then {@link SecurityManager#checkPackageAccess
 588      *     smgr.checkPackageAccess(defcPkg)} is called,
 589      *     where {@code defcPkg} is the package of {@code defc}.
 590      * </ul>
 591      * Security checks are performed after other access checks have passed.
 592      * Therefore, the above rules presuppose a member or class that is public,
 593      * or else that is being accessed from a lookup class that has
 594      * rights to access the member or class.
 595      *
 596      * <h1><a id="callsens"></a>Caller sensitive methods</h1>
 597      * A small number of Java methods have a special property called caller sensitivity.
 598      * A <em>caller-sensitive</em> method can behave differently depending on the
 599      * identity of its immediate caller.
 600      * <p>
 601      * If a method handle for a caller-sensitive method is requested,
 602      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 603      * but they take account of the lookup class in a special way.
 604      * The resulting method handle behaves as if it were called
 605      * from an instruction contained in the lookup class,
 606      * so that the caller-sensitive method detects the lookup class.
 607      * (By contrast, the invoker of the method handle is disregarded.)
 608      * Thus, in the case of caller-sensitive methods,
 609      * different lookup classes may give rise to
 610      * differently behaving method handles.
 611      * <p>
 612      * In cases where the lookup object is
 613      * {@link MethodHandles#publicLookup() publicLookup()},
 614      * or some other lookup object without
 615      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 616      * the lookup class is disregarded.
 617      * In such cases, no caller-sensitive method handle can be created,
 618      * access is forbidden, and the lookup fails with an
 619      * {@code IllegalAccessException}.
 620      * <p style="font-size:smaller;">
 621      * <em>Discussion:</em>
 622      * For example, the caller-sensitive method
 623      * {@link java.lang.Class#forName(String) Class.forName(x)}
 624      * can return varying classes or throw varying exceptions,
 625      * depending on the class loader of the class that calls it.
 626      * A public lookup of {@code Class.forName} will fail, because
 627      * there is no reasonable way to determine its bytecode behavior.
 628      * <p style="font-size:smaller;">
 629      * If an application caches method handles for broad sharing,
 630      * it should use {@code publicLookup()} to create them.
 631      * If there is a lookup of {@code Class.forName}, it will fail,
 632      * and the application must take appropriate action in that case.
 633      * It may be that a later lookup, perhaps during the invocation of a
 634      * bootstrap method, can incorporate the specific identity
 635      * of the caller, making the method accessible.
 636      * <p style="font-size:smaller;">
 637      * The function {@code MethodHandles.lookup} is caller sensitive
 638      * so that there can be a secure foundation for lookups.
 639      * Nearly all other methods in the JSR 292 API rely on lookup
 640      * objects to check access requests.
 641      *
 642      * @revised 9
 643      */
 644     public static final
 645     class Lookup {
 646         /** The class on behalf of whom the lookup is being performed. */
 647         private final Class<?> lookupClass;
 648 
 649         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 650         private final int allowedModes;
 651 
 652         static {
 653             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
 654         }
 655 
 656         /** A single-bit mask representing {@code public} access,
 657          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 658          *  The value, {@code 0x01}, happens to be the same as the value of the
 659          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 660          */
 661         public static final int PUBLIC = Modifier.PUBLIC;
 662 
 663         /** A single-bit mask representing {@code private} access,
 664          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 665          *  The value, {@code 0x02}, happens to be the same as the value of the
 666          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 667          */
 668         public static final int PRIVATE = Modifier.PRIVATE;
 669 
 670         /** A single-bit mask representing {@code protected} access,
 671          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 672          *  The value, {@code 0x04}, happens to be the same as the value of the
 673          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 674          */
 675         public static final int PROTECTED = Modifier.PROTECTED;
 676 
 677         /** A single-bit mask representing {@code package} access (default access),
 678          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 679          *  The value is {@code 0x08}, which does not correspond meaningfully to
 680          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 681          */
 682         public static final int PACKAGE = Modifier.STATIC;
 683 
 684         /** A single-bit mask representing {@code module} access (default access),
 685          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 686          *  The value is {@code 0x10}, which does not correspond meaningfully to
 687          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 688          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 689          *  with this lookup mode can access all public types in the module of the
 690          *  lookup class and public types in packages exported by other modules
 691          *  to the module of the lookup class.
 692          *  @since 9
 693          *  @spec JPMS
 694          */
 695         public static final int MODULE = PACKAGE << 1;
 696 
 697         /** A single-bit mask representing {@code unconditional} access
 698          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 699          *  The value is {@code 0x20}, which does not correspond meaningfully to
 700          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 701          *  A {@code Lookup} with this lookup mode assumes {@linkplain
 702          *  java.lang.Module#canRead(java.lang.Module) readability}.
 703          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 704          *  with this lookup mode can access all public members of public types
 705          *  of all modules where the type is in a package that is {@link
 706          *  java.lang.Module#isExported(String) exported unconditionally}.
 707          *  @since 9
 708          *  @spec JPMS
 709          *  @see #publicLookup()
 710          */
 711         public static final int UNCONDITIONAL = PACKAGE << 2;
 712 
 713         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL);
 714         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);
 715         private static final int TRUSTED   = -1;
 716 
 717         private static int fixmods(int mods) {
 718             mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL);
 719             return (mods != 0) ? mods : (PACKAGE | MODULE | UNCONDITIONAL);
 720         }
 721 
 722         /** Tells which class is performing the lookup.  It is this class against
 723          *  which checks are performed for visibility and access permissions.
 724          *  <p>
 725          *  The class implies a maximum level of access permission,
 726          *  but the permissions may be additionally limited by the bitmask
 727          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 728          *  can be accessed.
 729          *  @return the lookup class, on behalf of which this lookup object finds members
 730          */
 731         public Class<?> lookupClass() {
 732             return lookupClass;
 733         }
 734 
 735         // This is just for calling out to MethodHandleImpl.
 736         private Class<?> lookupClassOrNull() {
 737             return (allowedModes == TRUSTED) ? null : lookupClass;
 738         }
 739 
 740         /** Tells which access-protection classes of members this lookup object can produce.
 741          *  The result is a bit-mask of the bits
 742          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 743          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 744          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 745          *  {@linkplain #PACKAGE PACKAGE (0x08)},
 746          *  {@linkplain #MODULE MODULE (0x10)},
 747          *  and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}.
 748          *  <p>
 749          *  A freshly-created lookup object
 750          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
 751          *  all possible bits set, except {@code UNCONDITIONAL}.
 752          *  A lookup object on a new lookup class
 753          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 754          *  may have some mode bits set to zero.
 755          *  Mode bits can also be
 756          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
 757          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
 758          *  The purpose of this is to restrict access via the new lookup object,
 759          *  so that it can access only names which can be reached by the original
 760          *  lookup object, and also by the new lookup class.
 761          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 762          *  @see #in
 763          *  @see #dropLookupMode
 764          *
 765          *  @revised 9
 766          *  @spec JPMS
 767          */
 768         public int lookupModes() {
 769             return allowedModes & ALL_MODES;
 770         }
 771 
 772         /** Embody the current class (the lookupClass) as a lookup class
 773          * for method handle creation.
 774          * Must be called by from a method in this package,
 775          * which in turn is called by a method not in this package.
 776          */
 777         Lookup(Class<?> lookupClass) {
 778             this(lookupClass, FULL_POWER_MODES);
 779         }
 780 
 781         private Lookup(Class<?> lookupClass, int allowedModes) {
 782             this.lookupClass = lookupClass;
 783             this.allowedModes = allowedModes;
 784             assert !lookupClass.isPrimitive() && !lookupClass.isArray();
 785         }
 786 
 787         /**
 788          * Creates a lookup on the specified new lookup class.
 789          * The resulting object will report the specified
 790          * class as its own {@link #lookupClass() lookupClass}.
 791          * <p>
 792          * However, the resulting {@code Lookup} object is guaranteed
 793          * to have no more access capabilities than the original.
 794          * In particular, access capabilities can be lost as follows:<ul>
 795          * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and
 796          * the new lookup class is in a different module {@code M}, then no members, not
 797          * even public members in {@code M}'s exported packages, will be accessible.
 798          * The exception to this is when this lookup is {@link #publicLookup()
 799          * publicLookup}, in which case {@code PUBLIC} access is not lost.
 800          * <li>If the old lookup class is in an unnamed module, and the new lookup class
 801          * is a different module then {@link #MODULE MODULE} access is lost.
 802          * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost.
 803          * <li>If the new lookup class is in a different package
 804          * than the old one, protected and default (package) members will not be accessible.
 805          * <li>If the new lookup class is not within the same package member
 806          * as the old one, private members will not be accessible, and protected members
 807          * will not be accessible by virtue of inheritance.
 808          * (Protected members may continue to be accessible because of package sharing.)
 809          * <li>If the new lookup class is not accessible to the old lookup class,
 810          * then no members, not even public members, will be accessible.
 811          * (In all other cases, public members will continue to be accessible.)
 812          * </ul>
 813          * <p>
 814          * The resulting lookup's capabilities for loading classes
 815          * (used during {@link #findClass} invocations)
 816          * are determined by the lookup class' loader,
 817          * which may change due to this operation.
 818          *
 819          * @param requestedLookupClass the desired lookup class for the new lookup object
 820          * @return a lookup object which reports the desired lookup class, or the same object
 821          * if there is no change
 822          * @throws IllegalArgumentException if {@code requestedLookupClass} is
 823          * a primitive type or array class
 824          * @throws NullPointerException if the argument is null
 825          *
 826          * @revised 9
 827          * @spec JPMS
 828          */
 829         public Lookup in(Class<?> requestedLookupClass) {
 830             Objects.requireNonNull(requestedLookupClass);
 831             if (requestedLookupClass.isPrimitive())
 832                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
 833             if (requestedLookupClass.isArray())
 834                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
 835 
 836             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 837                 return new Lookup(requestedLookupClass, FULL_POWER_MODES);
 838             if (requestedLookupClass == this.lookupClass)
 839                 return this;  // keep same capabilities
 840             int newModes = (allowedModes & FULL_POWER_MODES);
 841             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 842                 // Need to drop all access when teleporting from a named module to another
 843                 // module. The exception is publicLookup where PUBLIC is not lost.
 844                 if (this.lookupClass.getModule().isNamed()
 845                     && (this.allowedModes & UNCONDITIONAL) == 0)
 846                     newModes = 0;
 847                 else
 848                     newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
 849             }
 850             if ((newModes & PACKAGE) != 0
 851                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 852                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
 853             }
 854             // Allow nestmate lookups to be created without special privilege:
 855             if ((newModes & PRIVATE) != 0
 856                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 857                 newModes &= ~(PRIVATE|PROTECTED);
 858             }
 859             if ((newModes & PUBLIC) != 0
 860                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 861                 // The requested class it not accessible from the lookup class.
 862                 // No permissions.
 863                 newModes = 0;
 864             }
 865 
 866             checkUnprivilegedlookupClass(requestedLookupClass);
 867             return new Lookup(requestedLookupClass, newModes);
 868         }
 869 
 870 
 871         /**
 872          * Creates a lookup on the same lookup class which this lookup object
 873          * finds members, but with a lookup mode that has lost the given lookup mode.
 874          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
 875          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}.
 876          * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always
 877          * dropped and so the resulting lookup mode will never have these access capabilities.
 878          * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE}
 879          * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will
 880          * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC}
 881          * is dropped then the resulting lookup has no access.
 882          * @param modeToDrop the lookup mode to drop
 883          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
 884          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
 885          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
 886          * @see MethodHandles#privateLookupIn
 887          * @since 9
 888          */
 889         public Lookup dropLookupMode(int modeToDrop) {
 890             int oldModes = lookupModes();
 891             int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL);
 892             switch (modeToDrop) {
 893                 case PUBLIC: newModes &= ~(ALL_MODES); break;
 894                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
 895                 case PACKAGE: newModes &= ~(PRIVATE); break;
 896                 case PROTECTED:
 897                 case PRIVATE:
 898                 case UNCONDITIONAL: break;
 899                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
 900             }
 901             if (newModes == oldModes) return this;  // return self if no change
 902             return new Lookup(lookupClass(), newModes);
 903         }
 904 
 905         /**
 906          * Defines a class to the same class loader and in the same runtime package and
 907          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
 908          * {@linkplain #lookupClass() lookup class}.
 909          *
 910          * This method is equivalent to calling
 911          * {@link #defineClass(byte[], ClassProperty[])
 912          * defineClass(bytes, (ClassProperty[])null)}.
 913          *
 914          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
 915          * {@link #PACKAGE PACKAGE} access as default (package) members will be
 916          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
 917          * that the lookup object was created by a caller in the runtime package (or derived
 918          * from a lookup originally created by suitably privileged code to a target class in
 919          * the runtime package). </p>
 920          *
 921          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
 922          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
 923          * same package as the lookup class. </p>
 924          *
 925          * <p> This method does not run the class initializer. The class initializer may
 926          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
 927          * Specification</em>. </p>
 928          *
 929          * <p> If there is a security manager, its {@code checkPermission} method is first called
 930          * to check {@code RuntimePermission("defineClass")}. </p>
 931          *
 932          * @param bytes the class bytes
 933          * @return the {@code Class} object for the class
 934          * @throws IllegalArgumentException the bytes are for a class in a different package
 935          * to the lookup class
 936          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
 937          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
 938          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
 939          * @throws SecurityException if denied by the security manager
 940          * @throws NullPointerException if {@code bytes} is {@code null}
 941          * @since 9
 942          * @spec JPMS
 943          * @see Lookup#privateLookupIn
 944          * @see Lookup#dropLookupMode
 945          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
 946          */
 947         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
 948             return defineClass(bytes, (ClassProperty[])null);
 949         }
 950 
 951         /**
 952          * Defines a class to the same class loader and in the same runtime package
 953          * and {@linkplain java.security.ProtectionDomain protection domain} as
 954          * this lookup's {@linkplain #lookupClass() lookup class}.
 955          * The {@code props} parameter specifies the properties of the class.
 956          *
 957          * <p> A class can be defined with the following properties:
 958          * <ul>
 959          * <li>A {@linkplain ClassProperty#NESTMATE <em>nestmate</em>} of the lookup class,
 960          *     i.e. in the same {@linkplain Class#getNestHost nest}
 961          *     of the lookup class.  The class will have access to the private members
 962          *     of all classes and interfaces in the same nest.
 963          *     </li>
 964          * <li>A {@linkplain ClassProperty#HIDDEN <em>hidden</em>} class,
 965          *     i.e. a class cannot be referenced by other classes.
 966          *     A hidden class has the following properties:
 967          *     <ul>
 968          *     <li>Naming:
 969          *     The name of this class is derived from the name of
 970          *     the class in the class bytes so that the class name does not
 971          *     collide with other classes defined to the same class loader.
 972          *     <li>Class resolution:
 973          *     The Java virtual machine does not find a hidden class with
 974          *     its name.  A hidden class can reference its members
 975          *     locally with the name of the class in the class bytes as if
 976          *     a non-hidden class. The name returned by {@link Class#getName()}
 977          *     is not known when the class bytes are generated.
 978          *     <li>Class retransformation:
 979          *     The class is not modifiable by Java agents or tool agents using
 980          *     the <a href="{@docRoot}/../specs/jvmti.html">JVM Tool Interface</a>.
 981          *     </ul>
 982          *     </li>
 983          * <li>A {@linkplain ClassProperty#WEAK <em>weak</em>} class,
 984          *     i.e. a class may be unloaded even if its defining class loader is
 985          *     <a href="../ref/package.html#reachability">reachable</a>,
 986          *     as if the defining class loader would only hold a
 987          *     {@linkplain java.lang.ref.WeakReference weak reference} of
 988          *     the class.
 989          *     A weak class is hidden.  If the {@code WEAK} property is set,
 990          *     then it implies that {@code HIDDEN} property is also set.</li>
 991          * </ul>
 992          *
 993          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must
 994          * include {@link #PACKAGE PACKAGE} access as default (package) members
 995          * will be accessible to the class. The {@code PACKAGE} lookup mode serves
 996          * to authenticate that the lookup object was created by a caller in
 997          * the runtime package (or derived from a lookup originally created by
 998          * suitably privileged code to a target class in the runtime package).
 999          * If the class is defined as a {@linkplain ClassProperty#NESTMATE nestmate}
1000          * then the {@linkplain #lookupModes() lookup modes} for this lookup must
1001          * include {@link #PRIVATE PRIVATE} access. </p>
1002          *
1003          * <p> The {@code bytes} parameter is the class bytes of a valid class file
1004          * (as defined by the <em>The Java Virtual Machine Specification</em>)
1005          * with a class name in the same package as the lookup class.
1006          * The class bytes of a nestmate class must not contain
1007          * the {@code NestHost} attribute nor the {@code NestMembers} attribute. </p>
1008          *
1009          * <p> If there is a security manager, its {@code checkPermission} method is first called
1010          * to check {@code RuntimePermission("defineClass")}. </p>
1011          *
1012          * <p> This method does not run the class initializer. The class initializer
1013          * may run at a later time, as detailed in section 12.4 of the The Java Language Specification.
1014          *
1015          * <p> The class can obtain {@code classData} by calling
1016          * the {@link Lookup#classData()} method of its {@code Lookup} object.
1017          *
1018          * @apiNote  An implementation of the Java Progamming Language may
1019          * unload classes as specified in section 12.7 of the Java Language Specification.
1020          * A class or interface may be unloaded if and only if
1021          * its defining class loader may be reclaimed by the garbage collector.
1022          * If the implementation supports class loading, a weak class
1023          * may become weakly reachable as if the defining class loader would
1024          * only hold a {@linkplain java.lang.ref.WeakReference weak reference}
1025          * of the class.
1026          *
1027          * @param bytes      the class bytes
1028          * @param props {@linkplain ClassProperty class properties}
1029          * @return the {@code Class} object for the class
1030          *
1031          * @throws IllegalArgumentException the bytes are for a class in a different package
1032          *                                  to the lookup class
1033          * @throws IllegalAccessException   if this lookup does not have {@code PACKAGE} access, or
1034          *                                  if {@code properties} contains {@code NESTMATE} but this lookup
1035          *                                  does not have {@code PRIVATE} access
1036          * @throws LinkageError             if the class is malformed ({@code ClassFormatError}), cannot be
1037          *                                  verified ({@code VerifyError}), is already defined,
1038          *                                  or another linkage error occurs
1039          * @throws SecurityException        if denied by the security manager
1040          * @throws NullPointerException     if {@code bytes} is {@code null}
1041          *
1042          * @since 12
1043          * @jls 12.7 Unloading of Classes and Interfaces
1044          * @see Lookup#privateLookupIn(Class, Lookup)
1045          * @see Lookup#dropLookupMode(int)
1046          */
1047         public Class<?> defineClass(byte[] bytes, ClassProperty... props) throws IllegalAccessException {
1048             Objects.requireNonNull(bytes);
1049 
1050             // clone the properties before access
1051             Set<ClassProperty> properties;
1052             if (props == null || props.length == 0) {
1053                 properties = EMPTY_PROPS;
1054             } else {
1055                 properties = Set.of(props);
1056             }
1057 
1058             // Is it ever possible to create Lookup for int.class or Object[].class?
1059             assert !lookupClass.isPrimitive() && !lookupClass.isArray();
1060 
1061             if ((lookupModes() & PACKAGE) == 0){
1062                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1063             }
1064 
1065             if (properties.contains(NESTMATE) && (lookupModes() & PRIVATE) == 0){
1066                 throw new IllegalAccessException("Lookup does not have PRIVATE access");
1067             }
1068 
1069             assert (lookupModes() & (MODULE | PUBLIC)) != 0;
1070 
1071             SecurityManager sm = System.getSecurityManager();
1072             if (sm != null)
1073                 sm.checkPermission(new RuntimePermission("defineClass"));
1074 
1075             return defineClassWithNoCheck(bytes, classPropertiesToFlags(properties));
1076         }
1077 
1078         /**
1079          * Defines a class to the same class loader and in the same runtime package
1080          * and {@linkplain java.security.ProtectionDomain protection domain} as
1081          * this lookup's {@linkplain #lookupClass() lookup class} with
1082          * the given class properties and {@code classData}.
1083          *
1084          * <p> This method defines a class as if calling
1085          * {@link #defineClass(byte[], ClassProperty...) defineClass(bytes, props)}
1086          * and then the class initializer with an injected the {@code classData}
1087          * as a pre-initialized static unnamed field.
1088          * The injected pre-initialized static unnamed field can be
1089          * obtained by calling the {@link Lookup#classData()} method of
1090          * its {@code Lookup} object.
1091          *
1092          * <p> If there is a security manager, its {@code checkPermission} method is first called
1093          * to check {@code RuntimePermission("defineClass")}. </p>
1094          *
1095          * @apiNote
1096          * This method initializes the class, as opposed to the {@link #defineClass(byte[], ClassProperty...)}
1097          * method which does not invoke {@code <clinit>}, because the returned {@code Class}
1098          * is as if it contains a private static unnamed field that is initialized to
1099          * the given {@code classData} along with other declared static fields
1100          * via {@code <clinit>}.
1101          *
1102          * @param bytes      the class bytes
1103          * @param classData pre-initialized class data
1104          * @param props {@linkplain ClassProperty class properties}
1105          * @return the {@code Class} object for the class
1106          *
1107          * @throws IllegalArgumentException the bytes are for a class in a different package
1108          *                                  to the lookup class
1109          * @throws IllegalAccessException   if this lookup does not have {@code PACKAGE} access, or
1110          *                                  if {@code properties} contains {@code NESTMATE} but this lookup
1111          *                                  does not have {@code PRIVATE} access
1112          * @throws LinkageError             if the class is malformed ({@code ClassFormatError}), cannot be
1113          *                                  verified ({@code VerifyError}), is already defined,
1114          *                                  or another linkage error occurs
1115          * @throws SecurityException        if denied by the security manager
1116          * @throws NullPointerException     if {@code bytes} or {@code classData} is {@code null}
1117          *
1118          * @since 12
1119          * @jls 12.7 Unloading of Classes and Interfaces
1120          * @see Lookup#privateLookupIn(Class, Lookup)
1121          * @see Lookup#dropLookupMode(int)
1122          */
1123         public Class<?> defineClassWithClassData(byte[] bytes, Object classData, ClassProperty... props)
1124                 throws IllegalAccessException
1125         {
1126             Objects.requireNonNull(bytes);
1127             Objects.requireNonNull(classData);
1128 
1129             // Is it ever possible to create Lookup for int.class or Object[].class?
1130             assert !lookupClass.isPrimitive() && !lookupClass.isArray();
1131 
1132             if ((lookupModes() & PACKAGE) == 0){
1133                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1134             }
1135 
1136             Set<ClassProperty> properties;
1137             if (props == null || props.length == 0) {
1138                 properties = EMPTY_PROPS;
1139             } else {
1140                 properties = Set.of(props);
1141             }
1142 
1143             if (properties.contains(NESTMATE) && (lookupModes() & PRIVATE) == 0){
1144                 throw new IllegalAccessException("Lookup does not have PRIVATE access");
1145             }
1146 
1147             assert (lookupModes() & (MODULE | PUBLIC)) != 0;
1148 
1149             SecurityManager sm = System.getSecurityManager();
1150             if (sm != null)
1151                 sm.checkPermission(new RuntimePermission("defineClass"));
1152 
1153             return defineClassWithNoCheck(bytes, classPropertiesToFlags(properties), classData);
1154         }
1155 
1156         private static int classPropertiesToFlags(Set<ClassProperty> props) {
1157             if (props.isEmpty()) return 0;
1158 
1159             int flags = 0;
1160             for (ClassProperty cp : props) {
1161                 flags |= cp.flag;
1162                 if (cp == WEAK) {
1163                     // weak class property implies hidden
1164                     flags |= HIDDEN.flag;
1165                 }
1166             }
1167             return flags;
1168         }
1169 
1170         /**
1171          * Returns the class data associated with this lookup class.
1172          * If this lookup class was defined via
1173          * {@link #defineClassWithClassData(byte[], Object, ClassProperty...)
1174          * defineClassWithClassData(bytes, classData, properties)}
1175          * then the supplied {@code classData} object is returned; otherwise,
1176          * {@code null}.
1177          *
1178          * <p> This method will invoke the static class initializer of
1179          * this lookup class if it has not been initialized.
1180          *
1181          * @apiNote
1182          * A class data can be considered as
1183          * private static unnamed field that has been pre-initialized
1184          * and supplied at define class time.
1185          *
1186          * <p> For example a class can pack one or more pre-initialized objects
1187          * in a {@code List} as the class data and at class initialization
1188          * time unpack them for subsequent access.
1189          * The class data is {@code List.of(o1, o2, o3....)}
1190          * passed to {@link #defineClassWithClassData(byte[], Object, ClassProperty...)} where
1191          * {@code <clinit>} of the class bytes does the following:
1192          *
1193          * <pre>{@code
1194          *     private static final T t;
1195          *     private static final R r;
1196          *     static {
1197          *        List<Object> data = (List<Object>) MethodHandles.lookup().classData();
1198          *        t = (T)data.get(0);
1199          *        r = (R)data.get(1);
1200          *     }
1201          *}</pre>
1202          *
1203          * @return the class data if this lookup class was defined via
1204          * {@link #defineClassWithClassData(byte[], Object, ClassProperty...)}; otherwise {@code null}.
1205          *
1206          * @throws IllegalAccessException if this lookup does not have {@code PRIVATE} access
1207          * @since 12
1208          */
1209         public Object classData() throws IllegalAccessException {
1210             if ((lookupModes() & PRIVATE) == 0){
1211                 throw new IllegalAccessException("Lookup does not have PRIVATE access");
1212             }
1213 
1214             // should we allow clearing?  getAndClearClassData
1215             return CLASS_DATA_MAP.get(lookupClass);
1216         }
1217 
1218         // package-private
1219         static final int HIDDEN_NESTMATE = NESTMATE_CLASS|NONFINDABLE_CLASS|ACCESS_VM_ANNOTATIONS;
1220         static final int WEAK_HIDDEN_NESTMATE =  WEAK_CLASS|NESTMATE_CLASS|NONFINDABLE_CLASS|ACCESS_VM_ANNOTATIONS;
1221         static final Set<ClassProperty> EMPTY_PROPS = Set.of();
1222 
1223         Class<?> defineClassWithNoCheck(byte[] bytes, int flags) {
1224             return defineClassWithNoCheck(bytes, flags, null);
1225         }
1226 
1227         Class<?> defineClassWithNoCheck(byte[] bytes, int flags, Object classData) {
1228             // Can't use lambda during bootstrapping
1229             // parse class bytes to get class name (in internal form)
1230             bytes = bytes.clone();
1231             String name;
1232             try {
1233                 ClassReader reader = new ClassReader(bytes);
1234                 name = reader.getClassName();
1235             } catch (RuntimeException e) {
1236                 // ASM exceptions are poorly specified
1237                 ClassFormatError cfe = new ClassFormatError();
1238                 cfe.initCause(e);
1239                 throw cfe;
1240             }
1241 
1242             // get package and class name in binary form
1243             String cn, pn;
1244             int index = name.lastIndexOf('/');
1245             if (index == -1) {
1246                 cn = name;
1247                 pn = "";
1248             } else {
1249                 cn = name.replace('/', '.');
1250                 pn = cn.substring(0, index);
1251             }
1252             if (!pn.equals(lookupClass.getPackageName())) {
1253                 throw new IllegalArgumentException(cn + " not in same package as lookup class: " + lookupClass.getName());
1254             }
1255 






1256             if ((flags & NONFINDABLE_CLASS) != 0) {
1257                 // assign a new name
1258                 // cn = cn + "\\" + ++seq;
1259                 cn = null;
1260             }
1261 
1262             // invoke the class loader's defineClass method
1263             ClassLoader loader = lookupClass.getClassLoader();
1264             ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null;
1265             Class<?> clazz = JLA.defineClass(loader, lookupClass, cn, bytes, pd, flags, classData);
1266             assert clazz.getClassLoader() == lookupClass.getClassLoader()
1267                    && clazz.getPackageName().equals(lookupClass.getPackageName());
1268 
1269             // ## TBD what if multiple threads defining this same class??
1270             // may need VM to inject the classData in a Class itself at define class time
1271             if (classData != null) {
1272                 CLASS_DATA_MAP.putIfAbsent(clazz, classData);
1273             }
1274             return clazz;
1275         }
1276 
1277         private static volatile int seq = 0;
1278 
1279         private ProtectionDomain lookupClassProtectionDomain() {
1280             ProtectionDomain pd = cachedProtectionDomain;
1281             if (pd == null) {
1282                 cachedProtectionDomain = pd = JLA.protectionDomain(lookupClass);
1283             }
1284             return pd;
1285         }
1286 
1287         // cached protection domain
1288         private volatile ProtectionDomain cachedProtectionDomain;
1289 
1290         // don't see the need to use ClassValue
1291         private static final WeakHashMap<Class<?>, Object> CLASS_DATA_MAP = new WeakHashMap<>();
1292 
1293         // Make sure outer class is initialized first.
1294         static { IMPL_NAMES.getClass(); }
1295 
1296         /** Package-private version of lookup which is trusted. */
1297         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
1298 
1299         /** Version of lookup which is trusted minimally.
1300          *  It can only be used to create method handles to publicly accessible
1301          *  members in packages that are exported unconditionally.
1302          */
1303         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC|UNCONDITIONAL));
1304 
1305         static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
1306 
1307         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
1308             String name = lookupClass.getName();
1309             if (name.startsWith("java.lang.invoke."))
1310                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
1311         }
1312 
1313         /**
1314          * Displays the name of the class from which lookups are to be made.
1315          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
1316          * If there are restrictions on the access permitted to this lookup,
1317          * this is indicated by adding a suffix to the class name, consisting
1318          * of a slash and a keyword.  The keyword represents the strongest
1319          * allowed access, and is chosen as follows:
1320          * <ul>
1321          * <li>If no access is allowed, the suffix is "/noaccess".
1322          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
1323          * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup".
1324          * <li>If only public and module access are allowed, the suffix is "/module".
1325          * <li>If only public, module and package access are allowed, the suffix is "/package".
1326          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
1327          * </ul>
1328          * If none of the above cases apply, it is the case that full
1329          * access (public, module, package, private, and protected) is allowed.
1330          * In this case, no suffix is added.
1331          * This is true only of an object obtained originally from
1332          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
1333          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
1334          * always have restricted access, and will display a suffix.
1335          * <p>
1336          * (It may seem strange that protected access should be
1337          * stronger than private access.  Viewed independently from
1338          * package access, protected access is the first to be lost,
1339          * because it requires a direct subclass relationship between
1340          * caller and callee.)
1341          * @see #in
1342          *
1343          * @revised 9
1344          * @spec JPMS
1345          */
1346         @Override
1347         public String toString() {
1348             String cname = lookupClass.getName();
1349             switch (allowedModes) {
1350             case 0:  // no privileges
1351                 return cname + "/noaccess";
1352             case PUBLIC:
1353                 return cname + "/public";
1354             case PUBLIC|UNCONDITIONAL:
1355                 return cname  + "/publicLookup";
1356             case PUBLIC|MODULE:
1357                 return cname + "/module";
1358             case PUBLIC|MODULE|PACKAGE:
1359                 return cname + "/package";
1360             case FULL_POWER_MODES & ~PROTECTED:
1361                 return cname + "/private";
1362             case FULL_POWER_MODES:
1363                 return cname;
1364             case TRUSTED:
1365                 return "/trusted";  // internal only; not exported
1366             default:  // Should not happen, but it's a bitfield...
1367                 cname = cname + "/" + Integer.toHexString(allowedModes);
1368                 assert(false) : cname;
1369                 return cname;
1370             }
1371         }
1372 
1373         /**
1374          * Produces a method handle for a static method.
1375          * The type of the method handle will be that of the method.
1376          * (Since static methods do not take receivers, there is no
1377          * additional receiver argument inserted into the method handle type,
1378          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
1379          * The method and all its argument types must be accessible to the lookup object.
1380          * <p>
1381          * The returned method handle will have
1382          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1383          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1384          * <p>
1385          * If the returned method handle is invoked, the method's class will
1386          * be initialized, if it has not already been initialized.
1387          * <p><b>Example:</b>
1388          * <blockquote><pre>{@code
1389 import static java.lang.invoke.MethodHandles.*;
1390 import static java.lang.invoke.MethodType.*;
1391 ...
1392 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
1393   "asList", methodType(List.class, Object[].class));
1394 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
1395          * }</pre></blockquote>
1396          * @param refc the class from which the method is accessed
1397          * @param name the name of the method
1398          * @param type the type of the method
1399          * @return the desired method handle
1400          * @throws NoSuchMethodException if the method does not exist
1401          * @throws IllegalAccessException if access checking fails,
1402          *                                or if the method is not {@code static},
1403          *                                or if the method's variable arity modifier bit
1404          *                                is set and {@code asVarargsCollector} fails
1405          * @exception SecurityException if a security manager is present and it
1406          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1407          * @throws NullPointerException if any argument is null
1408          */
1409         public
1410         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1411             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
1412             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
1413         }
1414 
1415         /**
1416          * Produces a method handle for a virtual method.
1417          * The type of the method handle will be that of the method,
1418          * with the receiver type (usually {@code refc}) prepended.
1419          * The method and all its argument types must be accessible to the lookup object.
1420          * <p>
1421          * When called, the handle will treat the first argument as a receiver
1422          * and, for non-private methods, dispatch on the receiver's type to determine which method
1423          * implementation to enter.
1424          * For private methods the named method in {@code refc} will be invoked on the receiver.
1425          * (The dispatching action is identical with that performed by an
1426          * {@code invokevirtual} or {@code invokeinterface} instruction.)
1427          * <p>
1428          * The first argument will be of type {@code refc} if the lookup
1429          * class has full privileges to access the member.  Otherwise
1430          * the member must be {@code protected} and the first argument
1431          * will be restricted in type to the lookup class.
1432          * <p>
1433          * The returned method handle will have
1434          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1435          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1436          * <p>
1437          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
1438          * instructions and method handles produced by {@code findVirtual},
1439          * if the class is {@code MethodHandle} and the name string is
1440          * {@code invokeExact} or {@code invoke}, the resulting
1441          * method handle is equivalent to one produced by
1442          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
1443          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
1444          * with the same {@code type} argument.
1445          * <p>
1446          * If the class is {@code VarHandle} and the name string corresponds to
1447          * the name of a signature-polymorphic access mode method, the resulting
1448          * method handle is equivalent to one produced by
1449          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
1450          * the access mode corresponding to the name string and with the same
1451          * {@code type} arguments.
1452          * <p>
1453          * <b>Example:</b>
1454          * <blockquote><pre>{@code
1455 import static java.lang.invoke.MethodHandles.*;
1456 import static java.lang.invoke.MethodType.*;
1457 ...
1458 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
1459   "concat", methodType(String.class, String.class));
1460 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
1461   "hashCode", methodType(int.class));
1462 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
1463   "hashCode", methodType(int.class));
1464 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
1465 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
1466 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
1467 // interface method:
1468 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
1469   "subSequence", methodType(CharSequence.class, int.class, int.class));
1470 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
1471 // constructor "internal method" must be accessed differently:
1472 MethodType MT_newString = methodType(void.class); //()V for new String()
1473 try { assertEquals("impossible", lookup()
1474         .findVirtual(String.class, "<init>", MT_newString));
1475  } catch (NoSuchMethodException ex) { } // OK
1476 MethodHandle MH_newString = publicLookup()
1477   .findConstructor(String.class, MT_newString);
1478 assertEquals("", (String) MH_newString.invokeExact());
1479          * }</pre></blockquote>
1480          *
1481          * @param refc the class or interface from which the method is accessed
1482          * @param name the name of the method
1483          * @param type the type of the method, with the receiver argument omitted
1484          * @return the desired method handle
1485          * @throws NoSuchMethodException if the method does not exist
1486          * @throws IllegalAccessException if access checking fails,
1487          *                                or if the method is {@code static},
1488          *                                or if the method's variable arity modifier bit
1489          *                                is set and {@code asVarargsCollector} fails
1490          * @exception SecurityException if a security manager is present and it
1491          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1492          * @throws NullPointerException if any argument is null
1493          */
1494         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1495             if (refc == MethodHandle.class) {
1496                 MethodHandle mh = findVirtualForMH(name, type);
1497                 if (mh != null)  return mh;
1498             } else if (refc == VarHandle.class) {
1499                 MethodHandle mh = findVirtualForVH(name, type);
1500                 if (mh != null)  return mh;
1501             }
1502             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
1503             MemberName method = resolveOrFail(refKind, refc, name, type);
1504             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
1505         }
1506         private MethodHandle findVirtualForMH(String name, MethodType type) {
1507             // these names require special lookups because of the implicit MethodType argument
1508             if ("invoke".equals(name))
1509                 return invoker(type);
1510             if ("invokeExact".equals(name))
1511                 return exactInvoker(type);
1512             assert(!MemberName.isMethodHandleInvokeName(name));
1513             return null;
1514         }
1515         private MethodHandle findVirtualForVH(String name, MethodType type) {
1516             try {
1517                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
1518             } catch (IllegalArgumentException e) {
1519                 return null;
1520             }
1521         }
1522 
1523         /**
1524          * Produces a method handle which creates an object and initializes it, using
1525          * the constructor of the specified type.
1526          * The parameter types of the method handle will be those of the constructor,
1527          * while the return type will be a reference to the constructor's class.
1528          * The constructor and all its argument types must be accessible to the lookup object.
1529          * <p>
1530          * The requested type must have a return type of {@code void}.
1531          * (This is consistent with the JVM's treatment of constructor type descriptors.)
1532          * <p>
1533          * The returned method handle will have
1534          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1535          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1536          * <p>
1537          * If the returned method handle is invoked, the constructor's class will
1538          * be initialized, if it has not already been initialized.
1539          * <p><b>Example:</b>
1540          * <blockquote><pre>{@code
1541 import static java.lang.invoke.MethodHandles.*;
1542 import static java.lang.invoke.MethodType.*;
1543 ...
1544 MethodHandle MH_newArrayList = publicLookup().findConstructor(
1545   ArrayList.class, methodType(void.class, Collection.class));
1546 Collection orig = Arrays.asList("x", "y");
1547 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
1548 assert(orig != copy);
1549 assertEquals(orig, copy);
1550 // a variable-arity constructor:
1551 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
1552   ProcessBuilder.class, methodType(void.class, String[].class));
1553 ProcessBuilder pb = (ProcessBuilder)
1554   MH_newProcessBuilder.invoke("x", "y", "z");
1555 assertEquals("[x, y, z]", pb.command().toString());
1556          * }</pre></blockquote>
1557          * @param refc the class or interface from which the method is accessed
1558          * @param type the type of the method, with the receiver argument omitted, and a void return type
1559          * @return the desired method handle
1560          * @throws NoSuchMethodException if the constructor does not exist
1561          * @throws IllegalAccessException if access checking fails
1562          *                                or if the method's variable arity modifier bit
1563          *                                is set and {@code asVarargsCollector} fails
1564          * @exception SecurityException if a security manager is present and it
1565          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1566          * @throws NullPointerException if any argument is null
1567          */
1568         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1569             if (refc.isArray()) {
1570                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
1571             }
1572             String name = "<init>";
1573             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1574             return getDirectConstructor(refc, ctor);
1575         }
1576 
1577         /**
1578          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1579          * initializer of the class is not run.
1580          * <p>
1581          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1582          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1583          * load the requested class, and then determines whether the class is accessible to this lookup object.
1584          *
1585          * @param targetName the fully qualified name of the class to be looked up.
1586          * @return the requested class.
1587          * @exception SecurityException if a security manager is present and it
1588          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1589          * @throws LinkageError if the linkage fails
1590          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader
1591          *                                or the class is not {@linkplain Class#isHidden hidden}
1592          * @throws IllegalAccessException if the class is not accessible, using the allowed access modes.
1593 
1594          * @since 9
1595          * @see Class#isHidden
1596          */
1597         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1598             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1599             return accessClass(targetClass);
1600         }
1601 
1602         /**
1603          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1604          * static initializer of the class is not run.
1605          * <p>
1606          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1607          * {@linkplain #lookupModes() lookup modes}.
1608          *
1609          * @param targetClass the class to be access-checked
1610          *
1611          * @return the class that has been access-checked
1612          *
1613          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1614          * modes.
1615          * @exception SecurityException if a security manager is present and it
1616          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1617          * @since 9
1618          */
1619         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1620             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1621                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1622             }
1623             checkSecurityManager(targetClass, null);
1624             return targetClass;
1625         }
1626 
1627         /**
1628          * Produces an early-bound method handle for a virtual method.
1629          * It will bypass checks for overriding methods on the receiver,
1630          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1631          * instruction from within the explicitly specified {@code specialCaller}.
1632          * The type of the method handle will be that of the method,
1633          * with a suitably restricted receiver type prepended.
1634          * (The receiver type will be {@code specialCaller} or a subtype.)
1635          * The method and all its argument types must be accessible
1636          * to the lookup object.
1637          * <p>
1638          * Before method resolution,
1639          * if the explicitly specified caller class is not identical with the
1640          * lookup class, or if this lookup object does not have
1641          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1642          * privileges, the access fails.
1643          * <p>
1644          * The returned method handle will have
1645          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1646          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1647          * <p style="font-size:smaller;">
1648          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1649          * even though the {@code invokespecial} instruction can refer to them
1650          * in special circumstances.  Use {@link #findConstructor findConstructor}
1651          * to access instance initialization methods in a safe manner.)</em>
1652          * <p><b>Example:</b>
1653          * <blockquote><pre>{@code
1654 import static java.lang.invoke.MethodHandles.*;
1655 import static java.lang.invoke.MethodType.*;
1656 ...
1657 static class Listie extends ArrayList {
1658   public String toString() { return "[wee Listie]"; }
1659   static Lookup lookup() { return MethodHandles.lookup(); }
1660 }
1661 ...
1662 // no access to constructor via invokeSpecial:
1663 MethodHandle MH_newListie = Listie.lookup()
1664   .findConstructor(Listie.class, methodType(void.class));
1665 Listie l = (Listie) MH_newListie.invokeExact();
1666 try { assertEquals("impossible", Listie.lookup().findSpecial(
1667         Listie.class, "<init>", methodType(void.class), Listie.class));
1668  } catch (NoSuchMethodException ex) { } // OK
1669 // access to super and self methods via invokeSpecial:
1670 MethodHandle MH_super = Listie.lookup().findSpecial(
1671   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1672 MethodHandle MH_this = Listie.lookup().findSpecial(
1673   Listie.class, "toString" , methodType(String.class), Listie.class);
1674 MethodHandle MH_duper = Listie.lookup().findSpecial(
1675   Object.class, "toString" , methodType(String.class), Listie.class);
1676 assertEquals("[]", (String) MH_super.invokeExact(l));
1677 assertEquals(""+l, (String) MH_this.invokeExact(l));
1678 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1679 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1680         String.class, "toString", methodType(String.class), Listie.class));
1681  } catch (IllegalAccessException ex) { } // OK
1682 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1683 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1684          * }</pre></blockquote>
1685          *
1686          * @param refc the class or interface from which the method is accessed
1687          * @param name the name of the method (which must not be "&lt;init&gt;")
1688          * @param type the type of the method, with the receiver argument omitted
1689          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1690          * @return the desired method handle
1691          * @throws NoSuchMethodException if the method does not exist
1692          * @throws IllegalAccessException if access checking fails,
1693          *                                or if the method is {@code static},
1694          *                                or if the method's variable arity modifier bit
1695          *                                is set and {@code asVarargsCollector} fails
1696          * @exception SecurityException if a security manager is present and it
1697          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1698          * @throws NullPointerException if any argument is null
1699          */
1700         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1701                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1702             checkSpecialCaller(specialCaller, refc);
1703             Lookup specialLookup = this.in(specialCaller);
1704             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1705             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1706         }
1707 
1708         /**
1709          * Produces a method handle giving read access to a non-static field.
1710          * The type of the method handle will have a return type of the field's
1711          * value type.
1712          * The method handle's single argument will be the instance containing
1713          * the field.
1714          * Access checking is performed immediately on behalf of the lookup class.
1715          * @param refc the class or interface from which the method is accessed
1716          * @param name the field's name
1717          * @param type the field's type
1718          * @return a method handle which can load values from the field
1719          * @throws NoSuchFieldException if the field does not exist
1720          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1721          * @exception SecurityException if a security manager is present and it
1722          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1723          * @throws NullPointerException if any argument is null
1724          * @see #findVarHandle(Class, String, Class)
1725          */
1726         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1727             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1728             return getDirectField(REF_getField, refc, field);
1729         }
1730 
1731         /**
1732          * Produces a method handle giving write access to a non-static field.
1733          * The type of the method handle will have a void return type.
1734          * The method handle will take two arguments, the instance containing
1735          * the field, and the value to be stored.
1736          * The second argument will be of the field's value type.
1737          * Access checking is performed immediately on behalf of the lookup class.
1738          * @param refc the class or interface from which the method is accessed
1739          * @param name the field's name
1740          * @param type the field's type
1741          * @return a method handle which can store values into the field
1742          * @throws NoSuchFieldException if the field does not exist
1743          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1744          * @exception SecurityException if a security manager is present and it
1745          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1746          * @throws NullPointerException if any argument is null
1747          * @see #findVarHandle(Class, String, Class)
1748          */
1749         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1750             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1751             return getDirectField(REF_putField, refc, field);
1752         }
1753 
1754         /**
1755          * Produces a VarHandle giving access to a non-static field {@code name}
1756          * of type {@code type} declared in a class of type {@code recv}.
1757          * The VarHandle's variable type is {@code type} and it has one
1758          * coordinate type, {@code recv}.
1759          * <p>
1760          * Access checking is performed immediately on behalf of the lookup
1761          * class.
1762          * <p>
1763          * Certain access modes of the returned VarHandle are unsupported under
1764          * the following conditions:
1765          * <ul>
1766          * <li>if the field is declared {@code final}, then the write, atomic
1767          *     update, numeric atomic update, and bitwise atomic update access
1768          *     modes are unsupported.
1769          * <li>if the field type is anything other than {@code byte},
1770          *     {@code short}, {@code char}, {@code int}, {@code long},
1771          *     {@code float}, or {@code double} then numeric atomic update
1772          *     access modes are unsupported.
1773          * <li>if the field type is anything other than {@code boolean},
1774          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1775          *     {@code long} then bitwise atomic update access modes are
1776          *     unsupported.
1777          * </ul>
1778          * <p>
1779          * If the field is declared {@code volatile} then the returned VarHandle
1780          * will override access to the field (effectively ignore the
1781          * {@code volatile} declaration) in accordance to its specified
1782          * access modes.
1783          * <p>
1784          * If the field type is {@code float} or {@code double} then numeric
1785          * and atomic update access modes compare values using their bitwise
1786          * representation (see {@link Float#floatToRawIntBits} and
1787          * {@link Double#doubleToRawLongBits}, respectively).
1788          * @apiNote
1789          * Bitwise comparison of {@code float} values or {@code double} values,
1790          * as performed by the numeric and atomic update access modes, differ
1791          * from the primitive {@code ==} operator and the {@link Float#equals}
1792          * and {@link Double#equals} methods, specifically with respect to
1793          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1794          * Care should be taken when performing a compare and set or a compare
1795          * and exchange operation with such values since the operation may
1796          * unexpectedly fail.
1797          * There are many possible NaN values that are considered to be
1798          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1799          * provided by Java can distinguish between them.  Operation failure can
1800          * occur if the expected or witness value is a NaN value and it is
1801          * transformed (perhaps in a platform specific manner) into another NaN
1802          * value, and thus has a different bitwise representation (see
1803          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1804          * details).
1805          * The values {@code -0.0} and {@code +0.0} have different bitwise
1806          * representations but are considered equal when using the primitive
1807          * {@code ==} operator.  Operation failure can occur if, for example, a
1808          * numeric algorithm computes an expected value to be say {@code -0.0}
1809          * and previously computed the witness value to be say {@code +0.0}.
1810          * @param recv the receiver class, of type {@code R}, that declares the
1811          * non-static field
1812          * @param name the field's name
1813          * @param type the field's type, of type {@code T}
1814          * @return a VarHandle giving access to non-static fields.
1815          * @throws NoSuchFieldException if the field does not exist
1816          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1817          * @exception SecurityException if a security manager is present and it
1818          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1819          * @throws NullPointerException if any argument is null
1820          * @since 9
1821          */
1822         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1823             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
1824             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
1825             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
1826         }
1827 
1828         /**
1829          * Produces a method handle giving read access to a static field.
1830          * The type of the method handle will have a return type of the field's
1831          * value type.
1832          * The method handle will take no arguments.
1833          * Access checking is performed immediately on behalf of the lookup class.
1834          * <p>
1835          * If the returned method handle is invoked, the field's class will
1836          * be initialized, if it has not already been initialized.
1837          * @param refc the class or interface from which the method is accessed
1838          * @param name the field's name
1839          * @param type the field's type
1840          * @return a method handle which can load values from the field
1841          * @throws NoSuchFieldException if the field does not exist
1842          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1843          * @exception SecurityException if a security manager is present and it
1844          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1845          * @throws NullPointerException if any argument is null
1846          */
1847         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1848             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1849             return getDirectField(REF_getStatic, refc, field);
1850         }
1851 
1852         /**
1853          * Produces a method handle giving write access to a static field.
1854          * The type of the method handle will have a void return type.
1855          * The method handle will take a single
1856          * argument, of the field's value type, the value to be stored.
1857          * Access checking is performed immediately on behalf of the lookup class.
1858          * <p>
1859          * If the returned method handle is invoked, the field's class will
1860          * be initialized, if it has not already been initialized.
1861          * @param refc the class or interface from which the method is accessed
1862          * @param name the field's name
1863          * @param type the field's type
1864          * @return a method handle which can store values into the field
1865          * @throws NoSuchFieldException if the field does not exist
1866          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1867          * @exception SecurityException if a security manager is present and it
1868          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1869          * @throws NullPointerException if any argument is null
1870          */
1871         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1872             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1873             return getDirectField(REF_putStatic, refc, field);
1874         }
1875 
1876         /**
1877          * Produces a VarHandle giving access to a static field {@code name} of
1878          * type {@code type} declared in a class of type {@code decl}.
1879          * The VarHandle's variable type is {@code type} and it has no
1880          * coordinate types.
1881          * <p>
1882          * Access checking is performed immediately on behalf of the lookup
1883          * class.
1884          * <p>
1885          * If the returned VarHandle is operated on, the declaring class will be
1886          * initialized, if it has not already been initialized.
1887          * <p>
1888          * Certain access modes of the returned VarHandle are unsupported under
1889          * the following conditions:
1890          * <ul>
1891          * <li>if the field is declared {@code final}, then the write, atomic
1892          *     update, numeric atomic update, and bitwise atomic update access
1893          *     modes are unsupported.
1894          * <li>if the field type is anything other than {@code byte},
1895          *     {@code short}, {@code char}, {@code int}, {@code long},
1896          *     {@code float}, or {@code double}, then numeric atomic update
1897          *     access modes are unsupported.
1898          * <li>if the field type is anything other than {@code boolean},
1899          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1900          *     {@code long} then bitwise atomic update access modes are
1901          *     unsupported.
1902          * </ul>
1903          * <p>
1904          * If the field is declared {@code volatile} then the returned VarHandle
1905          * will override access to the field (effectively ignore the
1906          * {@code volatile} declaration) in accordance to its specified
1907          * access modes.
1908          * <p>
1909          * If the field type is {@code float} or {@code double} then numeric
1910          * and atomic update access modes compare values using their bitwise
1911          * representation (see {@link Float#floatToRawIntBits} and
1912          * {@link Double#doubleToRawLongBits}, respectively).
1913          * @apiNote
1914          * Bitwise comparison of {@code float} values or {@code double} values,
1915          * as performed by the numeric and atomic update access modes, differ
1916          * from the primitive {@code ==} operator and the {@link Float#equals}
1917          * and {@link Double#equals} methods, specifically with respect to
1918          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1919          * Care should be taken when performing a compare and set or a compare
1920          * and exchange operation with such values since the operation may
1921          * unexpectedly fail.
1922          * There are many possible NaN values that are considered to be
1923          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1924          * provided by Java can distinguish between them.  Operation failure can
1925          * occur if the expected or witness value is a NaN value and it is
1926          * transformed (perhaps in a platform specific manner) into another NaN
1927          * value, and thus has a different bitwise representation (see
1928          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1929          * details).
1930          * The values {@code -0.0} and {@code +0.0} have different bitwise
1931          * representations but are considered equal when using the primitive
1932          * {@code ==} operator.  Operation failure can occur if, for example, a
1933          * numeric algorithm computes an expected value to be say {@code -0.0}
1934          * and previously computed the witness value to be say {@code +0.0}.
1935          * @param decl the class that declares the static field
1936          * @param name the field's name
1937          * @param type the field's type, of type {@code T}
1938          * @return a VarHandle giving access to a static field
1939          * @throws NoSuchFieldException if the field does not exist
1940          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1941          * @exception SecurityException if a security manager is present and it
1942          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1943          * @throws NullPointerException if any argument is null
1944          * @since 9
1945          */
1946         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1947             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
1948             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
1949             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
1950         }
1951 
1952         /**
1953          * Produces an early-bound method handle for a non-static method.
1954          * The receiver must have a supertype {@code defc} in which a method
1955          * of the given name and type is accessible to the lookup class.
1956          * The method and all its argument types must be accessible to the lookup object.
1957          * The type of the method handle will be that of the method,
1958          * without any insertion of an additional receiver parameter.
1959          * The given receiver will be bound into the method handle,
1960          * so that every call to the method handle will invoke the
1961          * requested method on the given receiver.
1962          * <p>
1963          * The returned method handle will have
1964          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1965          * the method's variable arity modifier bit ({@code 0x0080}) is set
1966          * <em>and</em> the trailing array argument is not the only argument.
1967          * (If the trailing array argument is the only argument,
1968          * the given receiver value will be bound to it.)
1969          * <p>
1970          * This is almost equivalent to the following code, with some differences noted below:
1971          * <blockquote><pre>{@code
1972 import static java.lang.invoke.MethodHandles.*;
1973 import static java.lang.invoke.MethodType.*;
1974 ...
1975 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1976 MethodHandle mh1 = mh0.bindTo(receiver);
1977 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
1978 return mh1;
1979          * }</pre></blockquote>
1980          * where {@code defc} is either {@code receiver.getClass()} or a super
1981          * type of that class, in which the requested method is accessible
1982          * to the lookup class.
1983          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
1984          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
1985          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
1986          * the receiver is restricted by {@code findVirtual} to the lookup class.)
1987          * @param receiver the object from which the method is accessed
1988          * @param name the name of the method
1989          * @param type the type of the method, with the receiver argument omitted
1990          * @return the desired method handle
1991          * @throws NoSuchMethodException if the method does not exist
1992          * @throws IllegalAccessException if access checking fails
1993          *                                or if the method's variable arity modifier bit
1994          *                                is set and {@code asVarargsCollector} fails
1995          * @exception SecurityException if a security manager is present and it
1996          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1997          * @throws NullPointerException if any argument is null
1998          * @see MethodHandle#bindTo
1999          * @see #findVirtual
2000          */
2001         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2002             Class<? extends Object> refc = receiver.getClass(); // may get NPE
2003             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
2004             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerClass(method));
2005             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
2006                 throw new IllegalAccessException("The restricted defining class " +
2007                                                  mh.type().leadingReferenceParameter().getName() +
2008                                                  " is not assignable from receiver class " +
2009                                                  receiver.getClass().getName());
2010             }
2011             return mh.bindArgumentL(0, receiver).setVarargs(method);
2012         }
2013 
2014         /**
2015          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
2016          * to <i>m</i>, if the lookup class has permission.
2017          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
2018          * If <i>m</i> is virtual, overriding is respected on every call.
2019          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
2020          * The type of the method handle will be that of the method,
2021          * with the receiver type prepended (but only if it is non-static).
2022          * If the method's {@code accessible} flag is not set,
2023          * access checking is performed immediately on behalf of the lookup class.
2024          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
2025          * <p>
2026          * The returned method handle will have
2027          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2028          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2029          * <p>
2030          * If <i>m</i> is static, and
2031          * if the returned method handle is invoked, the method's class will
2032          * be initialized, if it has not already been initialized.
2033          * @param m the reflected method
2034          * @return a method handle which can invoke the reflected method
2035          * @throws IllegalAccessException if access checking fails
2036          *                                or if the method's variable arity modifier bit
2037          *                                is set and {@code asVarargsCollector} fails
2038          * @throws NullPointerException if the argument is null
2039          */
2040         public MethodHandle unreflect(Method m) throws IllegalAccessException {
2041             if (m.getDeclaringClass() == MethodHandle.class) {
2042                 MethodHandle mh = unreflectForMH(m);
2043                 if (mh != null)  return mh;
2044             }
2045             if (m.getDeclaringClass() == VarHandle.class) {
2046                 MethodHandle mh = unreflectForVH(m);
2047                 if (mh != null)  return mh;
2048             }
2049             MemberName method = new MemberName(m);
2050             byte refKind = method.getReferenceKind();
2051             if (refKind == REF_invokeSpecial)
2052                 refKind = REF_invokeVirtual;
2053             assert(method.isMethod());
2054             @SuppressWarnings("deprecation")
2055             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
2056             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
2057         }
2058         private MethodHandle unreflectForMH(Method m) {
2059             // these names require special lookups because they throw UnsupportedOperationException
2060             if (MemberName.isMethodHandleInvokeName(m.getName()))
2061                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
2062             return null;
2063         }
2064         private MethodHandle unreflectForVH(Method m) {
2065             // these names require special lookups because they throw UnsupportedOperationException
2066             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
2067                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
2068             return null;
2069         }
2070 
2071         /**
2072          * Produces a method handle for a reflected method.
2073          * It will bypass checks for overriding methods on the receiver,
2074          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2075          * instruction from within the explicitly specified {@code specialCaller}.
2076          * The type of the method handle will be that of the method,
2077          * with a suitably restricted receiver type prepended.
2078          * (The receiver type will be {@code specialCaller} or a subtype.)
2079          * If the method's {@code accessible} flag is not set,
2080          * access checking is performed immediately on behalf of the lookup class,
2081          * as if {@code invokespecial} instruction were being linked.
2082          * <p>
2083          * Before method resolution,
2084          * if the explicitly specified caller class is not identical with the
2085          * lookup class, or if this lookup object does not have
2086          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2087          * privileges, the access fails.
2088          * <p>
2089          * The returned method handle will have
2090          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2091          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2092          * @param m the reflected method
2093          * @param specialCaller the class nominally calling the method
2094          * @return a method handle which can invoke the reflected method
2095          * @throws IllegalAccessException if access checking fails,
2096          *                                or if the method is {@code static},
2097          *                                or if the method's variable arity modifier bit
2098          *                                is set and {@code asVarargsCollector} fails
2099          * @throws NullPointerException if any argument is null
2100          */
2101         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
2102             checkSpecialCaller(specialCaller, null);
2103             Lookup specialLookup = this.in(specialCaller);
2104             MemberName method = new MemberName(m, true);
2105             assert(method.isMethod());
2106             // ignore m.isAccessible:  this is a new kind of access
2107             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
2108         }
2109 
2110         /**
2111          * Produces a method handle for a reflected constructor.
2112          * The type of the method handle will be that of the constructor,
2113          * with the return type changed to the declaring class.
2114          * The method handle will perform a {@code newInstance} operation,
2115          * creating a new instance of the constructor's class on the
2116          * arguments passed to the method handle.
2117          * <p>
2118          * If the constructor's {@code accessible} flag is not set,
2119          * access checking is performed immediately on behalf of the lookup class.
2120          * <p>
2121          * The returned method handle will have
2122          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2123          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2124          * <p>
2125          * If the returned method handle is invoked, the constructor's class will
2126          * be initialized, if it has not already been initialized.
2127          * @param c the reflected constructor
2128          * @return a method handle which can invoke the reflected constructor
2129          * @throws IllegalAccessException if access checking fails
2130          *                                or if the method's variable arity modifier bit
2131          *                                is set and {@code asVarargsCollector} fails
2132          * @throws NullPointerException if the argument is null
2133          */
2134         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
2135             MemberName ctor = new MemberName(c);
2136             assert(ctor.isConstructor());
2137             @SuppressWarnings("deprecation")
2138             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
2139             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
2140         }
2141 
2142         /**
2143          * Produces a method handle giving read access to a reflected field.
2144          * The type of the method handle will have a return type of the field's
2145          * value type.
2146          * If the field is static, the method handle will take no arguments.
2147          * Otherwise, its single argument will be the instance containing
2148          * the field.
2149          * If the field's {@code accessible} flag is not set,
2150          * access checking is performed immediately on behalf of the lookup class.
2151          * <p>
2152          * If the field is static, and
2153          * if the returned method handle is invoked, the field's class will
2154          * be initialized, if it has not already been initialized.
2155          * @param f the reflected field
2156          * @return a method handle which can load values from the reflected field
2157          * @throws IllegalAccessException if access checking fails
2158          * @throws NullPointerException if the argument is null
2159          */
2160         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
2161             return unreflectField(f, false);
2162         }
2163         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
2164             MemberName field = new MemberName(f, isSetter);
2165             assert(isSetter
2166                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
2167                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
2168             @SuppressWarnings("deprecation")
2169             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
2170             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
2171         }
2172 
2173         /**
2174          * Produces a method handle giving write access to a reflected field.
2175          * The type of the method handle will have a void return type.
2176          * If the field is static, the method handle will take a single
2177          * argument, of the field's value type, the value to be stored.
2178          * Otherwise, the two arguments will be the instance containing
2179          * the field, and the value to be stored.
2180          * If the field's {@code accessible} flag is not set,
2181          * access checking is performed immediately on behalf of the lookup class.
2182          * <p>
2183          * If the field is static, and
2184          * if the returned method handle is invoked, the field's class will
2185          * be initialized, if it has not already been initialized.
2186          * @param f the reflected field
2187          * @return a method handle which can store values into the reflected field
2188          * @throws IllegalAccessException if access checking fails
2189          * @throws NullPointerException if the argument is null
2190          */
2191         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
2192             return unreflectField(f, true);
2193         }
2194 
2195         /**
2196          * Produces a VarHandle giving access to a reflected field {@code f}
2197          * of type {@code T} declared in a class of type {@code R}.
2198          * The VarHandle's variable type is {@code T}.
2199          * If the field is non-static the VarHandle has one coordinate type,
2200          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
2201          * coordinate types.
2202          * <p>
2203          * Access checking is performed immediately on behalf of the lookup
2204          * class, regardless of the value of the field's {@code accessible}
2205          * flag.
2206          * <p>
2207          * If the field is static, and if the returned VarHandle is operated
2208          * on, the field's declaring class will be initialized, if it has not
2209          * already been initialized.
2210          * <p>
2211          * Certain access modes of the returned VarHandle are unsupported under
2212          * the following conditions:
2213          * <ul>
2214          * <li>if the field is declared {@code final}, then the write, atomic
2215          *     update, numeric atomic update, and bitwise atomic update access
2216          *     modes are unsupported.
2217          * <li>if the field type is anything other than {@code byte},
2218          *     {@code short}, {@code char}, {@code int}, {@code long},
2219          *     {@code float}, or {@code double} then numeric atomic update
2220          *     access modes are unsupported.
2221          * <li>if the field type is anything other than {@code boolean},
2222          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2223          *     {@code long} then bitwise atomic update access modes are
2224          *     unsupported.
2225          * </ul>
2226          * <p>
2227          * If the field is declared {@code volatile} then the returned VarHandle
2228          * will override access to the field (effectively ignore the
2229          * {@code volatile} declaration) in accordance to its specified
2230          * access modes.
2231          * <p>
2232          * If the field type is {@code float} or {@code double} then numeric
2233          * and atomic update access modes compare values using their bitwise
2234          * representation (see {@link Float#floatToRawIntBits} and
2235          * {@link Double#doubleToRawLongBits}, respectively).
2236          * @apiNote
2237          * Bitwise comparison of {@code float} values or {@code double} values,
2238          * as performed by the numeric and atomic update access modes, differ
2239          * from the primitive {@code ==} operator and the {@link Float#equals}
2240          * and {@link Double#equals} methods, specifically with respect to
2241          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2242          * Care should be taken when performing a compare and set or a compare
2243          * and exchange operation with such values since the operation may
2244          * unexpectedly fail.
2245          * There are many possible NaN values that are considered to be
2246          * {@code NaN} in Java, although no IEEE 754 floating-point operation
2247          * provided by Java can distinguish between them.  Operation failure can
2248          * occur if the expected or witness value is a NaN value and it is
2249          * transformed (perhaps in a platform specific manner) into another NaN
2250          * value, and thus has a different bitwise representation (see
2251          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2252          * details).
2253          * The values {@code -0.0} and {@code +0.0} have different bitwise
2254          * representations but are considered equal when using the primitive
2255          * {@code ==} operator.  Operation failure can occur if, for example, a
2256          * numeric algorithm computes an expected value to be say {@code -0.0}
2257          * and previously computed the witness value to be say {@code +0.0}.
2258          * @param f the reflected field, with a field of type {@code T}, and
2259          * a declaring class of type {@code R}
2260          * @return a VarHandle giving access to non-static fields or a static
2261          * field
2262          * @throws IllegalAccessException if access checking fails
2263          * @throws NullPointerException if the argument is null
2264          * @since 9
2265          */
2266         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
2267             MemberName getField = new MemberName(f, false);
2268             MemberName putField = new MemberName(f, true);
2269             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
2270                                                       f.getDeclaringClass(), getField, putField);
2271         }
2272 
2273         /**
2274          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
2275          * created by this lookup object or a similar one.
2276          * Security and access checks are performed to ensure that this lookup object
2277          * is capable of reproducing the target method handle.
2278          * This means that the cracking may fail if target is a direct method handle
2279          * but was created by an unrelated lookup object.
2280          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
2281          * and was created by a lookup object for a different class.
2282          * @param target a direct method handle to crack into symbolic reference components
2283          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
2284          * @exception SecurityException if a security manager is present and it
2285          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2286          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
2287          * @exception NullPointerException if the target is {@code null}
2288          * @see MethodHandleInfo
2289          * @since 1.8
2290          */
2291         public MethodHandleInfo revealDirect(MethodHandle target) {
2292             MemberName member = target.internalMemberName();
2293             if (member == null || (!member.isResolved() &&
2294                                    !member.isMethodHandleInvoke() &&
2295                                    !member.isVarHandleMethodInvoke()))
2296                 throw newIllegalArgumentException("not a direct method handle");
2297             Class<?> defc = member.getDeclaringClass();
2298             byte refKind = member.getReferenceKind();
2299             assert(MethodHandleNatives.refKindIsValid(refKind));
2300             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
2301                 // Devirtualized method invocation is usually formally virtual.
2302                 // To avoid creating extra MemberName objects for this common case,
2303                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
2304                 refKind = REF_invokeVirtual;
2305             if (refKind == REF_invokeVirtual && defc.isInterface())
2306                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
2307                 refKind = REF_invokeInterface;
2308             // Check SM permissions and member access before cracking.
2309             try {
2310                 checkAccess(refKind, defc, member);
2311                 checkSecurityManager(defc, member);
2312             } catch (IllegalAccessException ex) {
2313                 throw new IllegalArgumentException(ex);
2314             }
2315             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
2316                 Class<?> callerClass = target.internalCallerClass();
2317                 if (!hasPrivateAccess() || callerClass != lookupClass())
2318                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
2319             }
2320             // Produce the handle to the results.
2321             return new InfoFromMemberName(this, member, refKind);
2322         }
2323 
2324         /// Helper methods, all package-private.
2325 
2326         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2327             checkSymbolicClass(refc);  // do this before attempting to resolve
2328             Objects.requireNonNull(name);
2329             Objects.requireNonNull(type);
2330             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2331                                             NoSuchFieldException.class);
2332         }
2333 
2334         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2335             checkSymbolicClass(refc);  // do this before attempting to resolve
2336             Objects.requireNonNull(name);
2337             Objects.requireNonNull(type);
2338             checkMethodName(refKind, name);  // NPE check on name
2339             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2340                                             NoSuchMethodException.class);
2341         }
2342 
2343         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
2344             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
2345             Objects.requireNonNull(member.getName());
2346             Objects.requireNonNull(member.getType());
2347             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
2348                                             ReflectiveOperationException.class);
2349         }
2350 
2351         MemberName resolveOrNull(byte refKind, MemberName member) {
2352             // do this before attempting to resolve
2353             if (!isClassAccessible(member.getDeclaringClass())) {
2354                 return null;
2355             }
2356             Objects.requireNonNull(member.getName());
2357             Objects.requireNonNull(member.getType());
2358             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull());
2359         }
2360 
2361         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
2362             if (!isClassAccessible(refc)) {
2363                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
2364             }
2365         }
2366 
2367         boolean isClassAccessible(Class<?> refc) {
2368             Objects.requireNonNull(refc);
2369             Class<?> caller = lookupClassOrNull();
2370             return caller == null || VerifyAccess.isClassAccessible(refc, caller, allowedModes);
2371         }
2372 
2373         /** Check name for an illegal leading "&lt;" character. */
2374         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
2375             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
2376                 throw new NoSuchMethodException("illegal method name: "+name);
2377         }
2378 
2379 
2380         /**
2381          * Find my trustable caller class if m is a caller sensitive method.
2382          * If this lookup object has private access, then the caller class is the lookupClass.
2383          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
2384          */
2385         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
2386             Class<?> callerClass = null;
2387             if (MethodHandleNatives.isCallerSensitive(m)) {
2388                 // Only lookups with private access are allowed to resolve caller-sensitive methods
2389                 if (hasPrivateAccess()) {
2390                     callerClass = lookupClass;
2391                 } else {
2392                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
2393                 }
2394             }
2395             return callerClass;
2396         }
2397 
2398         /**
2399          * Returns {@code true} if this lookup has {@code PRIVATE} access.
2400          * @return {@code true} if this lookup has {@code PRIVATE} access.
2401          * @since 9
2402          */
2403         public boolean hasPrivateAccess() {
2404             return (allowedModes & PRIVATE) != 0;
2405         }
2406 
2407         /**
2408          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
2409          * Determines a trustable caller class to compare with refc, the symbolic reference class.
2410          * If this lookup object has private access, then the caller class is the lookupClass.
2411          */
2412         void checkSecurityManager(Class<?> refc, MemberName m) {
2413             SecurityManager smgr = System.getSecurityManager();
2414             if (smgr == null)  return;
2415             if (allowedModes == TRUSTED)  return;
2416 
2417             // Step 1:
2418             boolean fullPowerLookup = hasPrivateAccess();
2419             if (!fullPowerLookup ||
2420                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
2421                 ReflectUtil.checkPackageAccess(refc);
2422             }
2423 
2424             if (m == null) {  // findClass or accessClass
2425                 // Step 2b:
2426                 if (!fullPowerLookup) {
2427                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
2428                 }
2429                 return;
2430             }
2431 
2432             // Step 2a:
2433             if (m.isPublic()) return;
2434             if (!fullPowerLookup) {
2435                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
2436             }
2437 
2438             // Step 3:
2439             Class<?> defc = m.getDeclaringClass();
2440             if (!fullPowerLookup && defc != refc) {
2441                 ReflectUtil.checkPackageAccess(defc);
2442             }
2443         }
2444 
2445         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2446             boolean wantStatic = (refKind == REF_invokeStatic);
2447             String message;
2448             if (m.isConstructor())
2449                 message = "expected a method, not a constructor";
2450             else if (!m.isMethod())
2451                 message = "expected a method";
2452             else if (wantStatic != m.isStatic())
2453                 message = wantStatic ? "expected a static method" : "expected a non-static method";
2454             else
2455                 { checkAccess(refKind, refc, m); return; }
2456             throw m.makeAccessException(message, this);
2457         }
2458 
2459         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2460             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
2461             String message;
2462             if (wantStatic != m.isStatic())
2463                 message = wantStatic ? "expected a static field" : "expected a non-static field";
2464             else
2465                 { checkAccess(refKind, refc, m); return; }
2466             throw m.makeAccessException(message, this);
2467         }
2468 
2469         /** Check public/protected/private bits on the symbolic reference class and its member. */
2470         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2471             assert(m.referenceKindIsConsistentWith(refKind) &&
2472                    MethodHandleNatives.refKindIsValid(refKind) &&
2473                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
2474             int allowedModes = this.allowedModes;
2475             if (allowedModes == TRUSTED)  return;
2476             int mods = m.getModifiers();
2477             if (Modifier.isProtected(mods) &&
2478                     refKind == REF_invokeVirtual &&
2479                     m.getDeclaringClass() == Object.class &&
2480                     m.getName().equals("clone") &&
2481                     refc.isArray()) {
2482                 // The JVM does this hack also.
2483                 // (See ClassVerifier::verify_invoke_instructions
2484                 // and LinkResolver::check_method_accessability.)
2485                 // Because the JVM does not allow separate methods on array types,
2486                 // there is no separate method for int[].clone.
2487                 // All arrays simply inherit Object.clone.
2488                 // But for access checking logic, we make Object.clone
2489                 // (normally protected) appear to be public.
2490                 // Later on, when the DirectMethodHandle is created,
2491                 // its leading argument will be restricted to the
2492                 // requested array type.
2493                 // N.B. The return type is not adjusted, because
2494                 // that is *not* the bytecode behavior.
2495                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
2496             }
2497             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
2498                 // cannot "new" a protected ctor in a different package
2499                 mods ^= Modifier.PROTECTED;
2500             }
2501             if (Modifier.isFinal(mods) &&
2502                     MethodHandleNatives.refKindIsSetter(refKind))
2503                 throw m.makeAccessException("unexpected set of a final field", this);
2504             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
2505             if ((requestedModes & allowedModes) != 0) {
2506                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
2507                                                     mods, lookupClass(), allowedModes))
2508                     return;
2509             } else {
2510                 // Protected members can also be checked as if they were package-private.
2511                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
2512                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
2513                     return;
2514             }
2515             throw m.makeAccessException(accessFailedMessage(refc, m), this);
2516         }
2517 
2518         String accessFailedMessage(Class<?> refc, MemberName m) {
2519             Class<?> defc = m.getDeclaringClass();
2520             int mods = m.getModifiers();
2521             // check the class first:
2522             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
2523                                (defc == refc ||
2524                                 Modifier.isPublic(refc.getModifiers())));
2525             if (!classOK && (allowedModes & PACKAGE) != 0) {
2526                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), FULL_POWER_MODES) &&
2527                            (defc == refc ||
2528                             VerifyAccess.isClassAccessible(refc, lookupClass(), FULL_POWER_MODES)));
2529             }
2530             if (!classOK)
2531                 return "class is not public";
2532             if (Modifier.isPublic(mods))
2533                 return "access to public member failed";  // (how?, module not readable?)
2534             if (Modifier.isPrivate(mods))
2535                 return "member is private";
2536             if (Modifier.isProtected(mods))
2537                 return "member is protected";
2538             return "member is private to package";
2539         }
2540 
2541         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
2542             int allowedModes = this.allowedModes;
2543             if (allowedModes == TRUSTED)  return;
2544             if (!hasPrivateAccess()
2545                 || (specialCaller != lookupClass()
2546                        // ensure non-abstract methods in superinterfaces can be special-invoked
2547                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
2548                 throw new MemberName(specialCaller).
2549                     makeAccessException("no private access for invokespecial", this);
2550         }
2551 
2552         private boolean restrictProtectedReceiver(MemberName method) {
2553             // The accessing class only has the right to use a protected member
2554             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
2555             if (!method.isProtected() || method.isStatic()
2556                 || allowedModes == TRUSTED
2557                 || method.getDeclaringClass() == lookupClass()
2558                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
2559                 return false;
2560             return true;
2561         }
2562         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
2563             assert(!method.isStatic());
2564             // receiver type of mh is too wide; narrow to caller
2565             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
2566                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
2567             }
2568             MethodType rawType = mh.type();
2569             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
2570             MethodType narrowType = rawType.changeParameterType(0, caller);
2571             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
2572             assert(mh.viewAsTypeChecks(narrowType, true));
2573             return mh.copyWith(narrowType, mh.form);
2574         }
2575 
2576         /** Check access and get the requested method. */
2577         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2578             final boolean doRestrict    = true;
2579             final boolean checkSecurity = true;
2580             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2581         }
2582         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
2583         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2584             final boolean doRestrict    = false;
2585             final boolean checkSecurity = true;
2586             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, boundCallerClass);
2587         }
2588         /** Check access and get the requested method, eliding security manager checks. */
2589         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2590             final boolean doRestrict    = true;
2591             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2592             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2593         }
2594         /** Common code for all methods; do not call directly except from immediately above. */
2595         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
2596                                                    boolean checkSecurity,
2597                                                    boolean doRestrict, Class<?> boundCallerClass) throws IllegalAccessException {
2598 
2599             checkMethod(refKind, refc, method);
2600             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2601             if (checkSecurity)
2602                 checkSecurityManager(refc, method);
2603             assert(!method.isMethodHandleInvoke());
2604 
2605             if (refKind == REF_invokeSpecial &&
2606                 refc != lookupClass() &&
2607                 !refc.isInterface() &&
2608                 refc != lookupClass().getSuperclass() &&
2609                 refc.isAssignableFrom(lookupClass())) {
2610                 assert(!method.getName().equals("<init>"));  // not this code path
2611 
2612                 // Per JVMS 6.5, desc. of invokespecial instruction:
2613                 // If the method is in a superclass of the LC,
2614                 // and if our original search was above LC.super,
2615                 // repeat the search (symbolic lookup) from LC.super
2616                 // and continue with the direct superclass of that class,
2617                 // and so forth, until a match is found or no further superclasses exist.
2618                 // FIXME: MemberName.resolve should handle this instead.
2619                 Class<?> refcAsSuper = lookupClass();
2620                 MemberName m2;
2621                 do {
2622                     refcAsSuper = refcAsSuper.getSuperclass();
2623                     m2 = new MemberName(refcAsSuper,
2624                                         method.getName(),
2625                                         method.getMethodType(),
2626                                         REF_invokeSpecial);
2627                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
2628                 } while (m2 == null &&         // no method is found yet
2629                          refc != refcAsSuper); // search up to refc
2630                 if (m2 == null)  throw new InternalError(method.toString());
2631                 method = m2;
2632                 refc = refcAsSuper;
2633                 // redo basic checks
2634                 checkMethod(refKind, refc, method);
2635             }
2636 
2637             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
2638             MethodHandle mh = dmh;
2639             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
2640             if ((doRestrict && refKind == REF_invokeSpecial) ||
2641                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
2642                 mh = restrictReceiver(method, dmh, lookupClass());
2643             }
2644             mh = maybeBindCaller(method, mh, boundCallerClass);
2645             mh = mh.setVarargs(method);
2646             return mh;
2647         }
2648         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
2649                                              Class<?> boundCallerClass)
2650                                              throws IllegalAccessException {
2651             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
2652                 return mh;
2653             Class<?> hostClass = lookupClass;
2654             if (!hasPrivateAccess())  // caller must have private access
2655                 hostClass = boundCallerClass;  // boundCallerClass came from a security manager style stack walk
2656             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
2657             // Note: caller will apply varargs after this step happens.
2658             return cbmh;
2659         }
2660         /** Check access and get the requested field. */
2661         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2662             final boolean checkSecurity = true;
2663             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2664         }
2665         /** Check access and get the requested field, eliding security manager checks. */
2666         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2667             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2668             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2669         }
2670         /** Common code for all fields; do not call directly except from immediately above. */
2671         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
2672                                                   boolean checkSecurity) throws IllegalAccessException {
2673             checkField(refKind, refc, field);
2674             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2675             if (checkSecurity)
2676                 checkSecurityManager(refc, field);
2677             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
2678             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
2679                                     restrictProtectedReceiver(field));
2680             if (doRestrict)
2681                 return restrictReceiver(field, dmh, lookupClass());
2682             return dmh;
2683         }
2684         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
2685                                             Class<?> refc, MemberName getField, MemberName putField)
2686                 throws IllegalAccessException {
2687             final boolean checkSecurity = true;
2688             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2689         }
2690         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
2691                                                              Class<?> refc, MemberName getField, MemberName putField)
2692                 throws IllegalAccessException {
2693             final boolean checkSecurity = false;
2694             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2695         }
2696         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
2697                                                   Class<?> refc, MemberName getField, MemberName putField,
2698                                                   boolean checkSecurity) throws IllegalAccessException {
2699             assert getField.isStatic() == putField.isStatic();
2700             assert getField.isGetter() && putField.isSetter();
2701             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
2702             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
2703 
2704             checkField(getRefKind, refc, getField);
2705             if (checkSecurity)
2706                 checkSecurityManager(refc, getField);
2707 
2708             if (!putField.isFinal()) {
2709                 // A VarHandle does not support updates to final fields, any
2710                 // such VarHandle to a final field will be read-only and
2711                 // therefore the following write-based accessibility checks are
2712                 // only required for non-final fields
2713                 checkField(putRefKind, refc, putField);
2714                 if (checkSecurity)
2715                     checkSecurityManager(refc, putField);
2716             }
2717 
2718             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
2719                                   restrictProtectedReceiver(getField));
2720             if (doRestrict) {
2721                 assert !getField.isStatic();
2722                 // receiver type of VarHandle is too wide; narrow to caller
2723                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
2724                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
2725                 }
2726                 refc = lookupClass();
2727             }
2728             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED);
2729         }
2730         /** Check access and get the requested constructor. */
2731         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2732             final boolean checkSecurity = true;
2733             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2734         }
2735         /** Check access and get the requested constructor, eliding security manager checks. */
2736         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2737             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2738             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2739         }
2740         /** Common code for all constructors; do not call directly except from immediately above. */
2741         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
2742                                                   boolean checkSecurity) throws IllegalAccessException {
2743             assert(ctor.isConstructor());
2744             checkAccess(REF_newInvokeSpecial, refc, ctor);
2745             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2746             if (checkSecurity)
2747                 checkSecurityManager(refc, ctor);
2748             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
2749             return DirectMethodHandle.make(ctor).setVarargs(ctor);
2750         }
2751 
2752         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
2753          */
2754         /*non-public*/
2755         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
2756             if (!(type instanceof Class || type instanceof MethodType))
2757                 throw new InternalError("unresolved MemberName");
2758             MemberName member = new MemberName(refKind, defc, name, type);
2759             MethodHandle mh = LOOKASIDE_TABLE.get(member);
2760             if (mh != null) {
2761                 checkSymbolicClass(defc);
2762                 return mh;
2763             }
2764             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
2765                 // Treat MethodHandle.invoke and invokeExact specially.
2766                 mh = findVirtualForMH(member.getName(), member.getMethodType());
2767                 if (mh != null) {
2768                     return mh;
2769                 }
2770             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
2771                 // Treat signature-polymorphic methods on VarHandle specially.
2772                 mh = findVirtualForVH(member.getName(), member.getMethodType());
2773                 if (mh != null) {
2774                     return mh;
2775                 }
2776             }
2777             MemberName resolved = resolveOrFail(refKind, member);
2778             mh = getDirectMethodForConstant(refKind, defc, resolved);
2779             if (mh instanceof DirectMethodHandle
2780                     && canBeCached(refKind, defc, resolved)) {
2781                 MemberName key = mh.internalMemberName();
2782                 if (key != null) {
2783                     key = key.asNormalOriginal();
2784                 }
2785                 if (member.equals(key)) {  // better safe than sorry
2786                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
2787                 }
2788             }
2789             return mh;
2790         }
2791         private
2792         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
2793             if (refKind == REF_invokeSpecial) {
2794                 return false;
2795             }
2796             if (!Modifier.isPublic(defc.getModifiers()) ||
2797                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
2798                     !member.isPublic() ||
2799                     member.isCallerSensitive()) {
2800                 return false;
2801             }
2802             ClassLoader loader = defc.getClassLoader();
2803             if (loader != null) {
2804                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
2805                 boolean found = false;
2806                 while (sysl != null) {
2807                     if (loader == sysl) { found = true; break; }
2808                     sysl = sysl.getParent();
2809                 }
2810                 if (!found) {
2811                     return false;
2812                 }
2813             }
2814             try {
2815                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
2816                     new MemberName(refKind, defc, member.getName(), member.getType()));
2817                 if (resolved2 == null) {
2818                     return false;
2819                 }
2820                 checkSecurityManager(defc, resolved2);
2821             } catch (SecurityException ex) {
2822                 return false;
2823             }
2824             return true;
2825         }
2826         private
2827         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
2828                 throws ReflectiveOperationException {
2829             if (MethodHandleNatives.refKindIsField(refKind)) {
2830                 return getDirectFieldNoSecurityManager(refKind, defc, member);
2831             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
2832                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
2833             } else if (refKind == REF_newInvokeSpecial) {
2834                 return getDirectConstructorNoSecurityManager(defc, member);
2835             }
2836             // oops
2837             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
2838         }
2839 
2840         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
2841 
2842         /**
2843          * Property of a class to be defined via the
2844          * {@link Lookup#defineClass(byte[], ClassProperty[]) Lookup.defineClass} method.
2845          *
2846          * @since 12
2847          * @see Lookup#defineClass(byte[], ClassProperty[])
2848          * @see Lookup#defineClassWithClassData(byte[], Object, ClassProperty[])
2849          */
2850         public enum ClassProperty {
2851             /**
2852              * A nestmate is a class that is in the same {@linkplain Class#getNestHost nest}
2853              * of a lookup class.  It has access to the private members of all
2854              * classes and interfaces in the same nest.
2855              *
2856              * @see Class#getNestHost()
2857              */
2858             NESTMATE(NESTMATE_CLASS),
2859 
2860             /**
2861              * A hidden class is a class that cannot be referenced by other
2862              * classes.  A Java Virtual Machine implementation may hide
2863              * the hidden frames from {@linkplain Throwable#getStackTrace()
2864              * stack traces}.
2865              *
2866              * @see Class#isHidden()
2867              * @see StackWalker.Option#SHOW_HIDDEN_FRAMES
2868              */
2869             HIDDEN(NONFINDABLE_CLASS),
2870 
2871             /**
2872              * A weak class is a class that may be unloaded even if
2873              * its defining class loader is
2874              * <a href="../ref/package.html#reachability">reachable</a>.
2875              * A weak class is {@linkplain #HIDDEN hidden}.
2876              *
2877              * @jls 12.7 Unloading of Classes and Interfaces
2878              */
2879             WEAK(WEAK_CLASS);
2880 
2881             /* the flag value is used by VM at define class time */
2882             final int flag;
2883             ClassProperty(int flag) {
2884                 this.flag = flag;
2885             }
2886         }
2887     }
2888 
2889     /**
2890      * Produces a method handle constructing arrays of a desired type,
2891      * as if by the {@code anewarray} bytecode.
2892      * The return type of the method handle will be the array type.
2893      * The type of its sole argument will be {@code int}, which specifies the size of the array.
2894      *
2895      * <p> If the returned method handle is invoked with a negative
2896      * array size, a {@code NegativeArraySizeException} will be thrown.
2897      *
2898      * @param arrayClass an array type
2899      * @return a method handle which can create arrays of the given type
2900      * @throws NullPointerException if the argument is {@code null}
2901      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
2902      * @see java.lang.reflect.Array#newInstance(Class, int)
2903      * @jvms 6.5 {@code anewarray} Instruction
2904      * @since 9
2905      */
2906     public static
2907     MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
2908         if (!arrayClass.isArray()) {
2909             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
2910         }
2911         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
2912                 bindTo(arrayClass.getComponentType());
2913         return ani.asType(ani.type().changeReturnType(arrayClass));
2914     }
2915 
2916     /**
2917      * Produces a method handle returning the length of an array,
2918      * as if by the {@code arraylength} bytecode.
2919      * The type of the method handle will have {@code int} as return type,
2920      * and its sole argument will be the array type.
2921      *
2922      * <p> If the returned method handle is invoked with a {@code null}
2923      * array reference, a {@code NullPointerException} will be thrown.
2924      *
2925      * @param arrayClass an array type
2926      * @return a method handle which can retrieve the length of an array of the given array type
2927      * @throws NullPointerException if the argument is {@code null}
2928      * @throws IllegalArgumentException if arrayClass is not an array type
2929      * @jvms 6.5 {@code arraylength} Instruction
2930      * @since 9
2931      */
2932     public static
2933     MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
2934         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
2935     }
2936 
2937     /**
2938      * Produces a method handle giving read access to elements of an array,
2939      * as if by the {@code aaload} bytecode.
2940      * The type of the method handle will have a return type of the array's
2941      * element type.  Its first argument will be the array type,
2942      * and the second will be {@code int}.
2943      *
2944      * <p> When the returned method handle is invoked,
2945      * the array reference and array index are checked.
2946      * A {@code NullPointerException} will be thrown if the array reference
2947      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2948      * thrown if the index is negative or if it is greater than or equal to
2949      * the length of the array.
2950      *
2951      * @param arrayClass an array type
2952      * @return a method handle which can load values from the given array type
2953      * @throws NullPointerException if the argument is null
2954      * @throws  IllegalArgumentException if arrayClass is not an array type
2955      * @jvms 6.5 {@code aaload} Instruction
2956      */
2957     public static
2958     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
2959         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
2960     }
2961 
2962     /**
2963      * Produces a method handle giving write access to elements of an array,
2964      * as if by the {@code astore} bytecode.
2965      * The type of the method handle will have a void return type.
2966      * Its last argument will be the array's element type.
2967      * The first and second arguments will be the array type and int.
2968      *
2969      * <p> When the returned method handle is invoked,
2970      * the array reference and array index are checked.
2971      * A {@code NullPointerException} will be thrown if the array reference
2972      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2973      * thrown if the index is negative or if it is greater than or equal to
2974      * the length of the array.
2975      *
2976      * @param arrayClass the class of an array
2977      * @return a method handle which can store values into the array type
2978      * @throws NullPointerException if the argument is null
2979      * @throws IllegalArgumentException if arrayClass is not an array type
2980      * @jvms 6.5 {@code aastore} Instruction
2981      */
2982     public static
2983     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
2984         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
2985     }
2986 
2987     /**
2988      * Produces a VarHandle giving access to elements of an array of type
2989      * {@code arrayClass}.  The VarHandle's variable type is the component type
2990      * of {@code arrayClass} and the list of coordinate types is
2991      * {@code (arrayClass, int)}, where the {@code int} coordinate type
2992      * corresponds to an argument that is an index into an array.
2993      * <p>
2994      * Certain access modes of the returned VarHandle are unsupported under
2995      * the following conditions:
2996      * <ul>
2997      * <li>if the component type is anything other than {@code byte},
2998      *     {@code short}, {@code char}, {@code int}, {@code long},
2999      *     {@code float}, or {@code double} then numeric atomic update access
3000      *     modes are unsupported.
3001      * <li>if the field type is anything other than {@code boolean},
3002      *     {@code byte}, {@code short}, {@code char}, {@code int} or
3003      *     {@code long} then bitwise atomic update access modes are
3004      *     unsupported.
3005      * </ul>
3006      * <p>
3007      * If the component type is {@code float} or {@code double} then numeric
3008      * and atomic update access modes compare values using their bitwise
3009      * representation (see {@link Float#floatToRawIntBits} and
3010      * {@link Double#doubleToRawLongBits}, respectively).
3011      *
3012      * <p> When the returned {@code VarHandle} is invoked,
3013      * the array reference and array index are checked.
3014      * A {@code NullPointerException} will be thrown if the array reference
3015      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
3016      * thrown if the index is negative or if it is greater than or equal to
3017      * the length of the array.
3018      *
3019      * @apiNote
3020      * Bitwise comparison of {@code float} values or {@code double} values,
3021      * as performed by the numeric and atomic update access modes, differ
3022      * from the primitive {@code ==} operator and the {@link Float#equals}
3023      * and {@link Double#equals} methods, specifically with respect to
3024      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3025      * Care should be taken when performing a compare and set or a compare
3026      * and exchange operation with such values since the operation may
3027      * unexpectedly fail.
3028      * There are many possible NaN values that are considered to be
3029      * {@code NaN} in Java, although no IEEE 754 floating-point operation
3030      * provided by Java can distinguish between them.  Operation failure can
3031      * occur if the expected or witness value is a NaN value and it is
3032      * transformed (perhaps in a platform specific manner) into another NaN
3033      * value, and thus has a different bitwise representation (see
3034      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3035      * details).
3036      * The values {@code -0.0} and {@code +0.0} have different bitwise
3037      * representations but are considered equal when using the primitive
3038      * {@code ==} operator.  Operation failure can occur if, for example, a
3039      * numeric algorithm computes an expected value to be say {@code -0.0}
3040      * and previously computed the witness value to be say {@code +0.0}.
3041      * @param arrayClass the class of an array, of type {@code T[]}
3042      * @return a VarHandle giving access to elements of an array
3043      * @throws NullPointerException if the arrayClass is null
3044      * @throws IllegalArgumentException if arrayClass is not an array type
3045      * @since 9
3046      */
3047     public static
3048     VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
3049         return VarHandles.makeArrayElementHandle(arrayClass);
3050     }
3051 
3052     /**
3053      * Produces a VarHandle giving access to elements of a {@code byte[]} array
3054      * viewed as if it were a different primitive array type, such as
3055      * {@code int[]} or {@code long[]}.
3056      * The VarHandle's variable type is the component type of
3057      * {@code viewArrayClass} and the list of coordinate types is
3058      * {@code (byte[], int)}, where the {@code int} coordinate type
3059      * corresponds to an argument that is an index into a {@code byte[]} array.
3060      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
3061      * array, composing bytes to or from a value of the component type of
3062      * {@code viewArrayClass} according to the given endianness.
3063      * <p>
3064      * The supported component types (variables types) are {@code short},
3065      * {@code char}, {@code int}, {@code long}, {@code float} and
3066      * {@code double}.
3067      * <p>
3068      * Access of bytes at a given index will result in an
3069      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
3070      * or greater than the {@code byte[]} array length minus the size (in bytes)
3071      * of {@code T}.
3072      * <p>
3073      * Access of bytes at an index may be aligned or misaligned for {@code T},
3074      * with respect to the underlying memory address, {@code A} say, associated
3075      * with the array and index.
3076      * If access is misaligned then access for anything other than the
3077      * {@code get} and {@code set} access modes will result in an
3078      * {@code IllegalStateException}.  In such cases atomic access is only
3079      * guaranteed with respect to the largest power of two that divides the GCD
3080      * of {@code A} and the size (in bytes) of {@code T}.
3081      * If access is aligned then following access modes are supported and are
3082      * guaranteed to support atomic access:
3083      * <ul>
3084      * <li>read write access modes for all {@code T}, with the exception of
3085      *     access modes {@code get} and {@code set} for {@code long} and
3086      *     {@code double} on 32-bit platforms.
3087      * <li>atomic update access modes for {@code int}, {@code long},
3088      *     {@code float} or {@code double}.
3089      *     (Future major platform releases of the JDK may support additional
3090      *     types for certain currently unsupported access modes.)
3091      * <li>numeric atomic update access modes for {@code int} and {@code long}.
3092      *     (Future major platform releases of the JDK may support additional
3093      *     numeric types for certain currently unsupported access modes.)
3094      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
3095      *     (Future major platform releases of the JDK may support additional
3096      *     numeric types for certain currently unsupported access modes.)
3097      * </ul>
3098      * <p>
3099      * Misaligned access, and therefore atomicity guarantees, may be determined
3100      * for {@code byte[]} arrays without operating on a specific array.  Given
3101      * an {@code index}, {@code T} and it's corresponding boxed type,
3102      * {@code T_BOX}, misalignment may be determined as follows:
3103      * <pre>{@code
3104      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
3105      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
3106      *     alignmentOffset(0, sizeOfT);
3107      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
3108      * boolean isMisaligned = misalignedAtIndex != 0;
3109      * }</pre>
3110      * <p>
3111      * If the variable type is {@code float} or {@code double} then atomic
3112      * update access modes compare values using their bitwise representation
3113      * (see {@link Float#floatToRawIntBits} and
3114      * {@link Double#doubleToRawLongBits}, respectively).
3115      * @param viewArrayClass the view array class, with a component type of
3116      * type {@code T}
3117      * @param byteOrder the endianness of the view array elements, as
3118      * stored in the underlying {@code byte} array
3119      * @return a VarHandle giving access to elements of a {@code byte[]} array
3120      * viewed as if elements corresponding to the components type of the view
3121      * array class
3122      * @throws NullPointerException if viewArrayClass or byteOrder is null
3123      * @throws IllegalArgumentException if viewArrayClass is not an array type
3124      * @throws UnsupportedOperationException if the component type of
3125      * viewArrayClass is not supported as a variable type
3126      * @since 9
3127      */
3128     public static
3129     VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
3130                                      ByteOrder byteOrder) throws IllegalArgumentException {
3131         Objects.requireNonNull(byteOrder);
3132         return VarHandles.byteArrayViewHandle(viewArrayClass,
3133                                               byteOrder == ByteOrder.BIG_ENDIAN);
3134     }
3135 
3136     /**
3137      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
3138      * viewed as if it were an array of elements of a different primitive
3139      * component type to that of {@code byte}, such as {@code int[]} or
3140      * {@code long[]}.
3141      * The VarHandle's variable type is the component type of
3142      * {@code viewArrayClass} and the list of coordinate types is
3143      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
3144      * corresponds to an argument that is an index into a {@code byte[]} array.
3145      * The returned VarHandle accesses bytes at an index in a
3146      * {@code ByteBuffer}, composing bytes to or from a value of the component
3147      * type of {@code viewArrayClass} according to the given endianness.
3148      * <p>
3149      * The supported component types (variables types) are {@code short},
3150      * {@code char}, {@code int}, {@code long}, {@code float} and
3151      * {@code double}.
3152      * <p>
3153      * Access will result in a {@code ReadOnlyBufferException} for anything
3154      * other than the read access modes if the {@code ByteBuffer} is read-only.
3155      * <p>
3156      * Access of bytes at a given index will result in an
3157      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
3158      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
3159      * {@code T}.
3160      * <p>
3161      * Access of bytes at an index may be aligned or misaligned for {@code T},
3162      * with respect to the underlying memory address, {@code A} say, associated
3163      * with the {@code ByteBuffer} and index.
3164      * If access is misaligned then access for anything other than the
3165      * {@code get} and {@code set} access modes will result in an
3166      * {@code IllegalStateException}.  In such cases atomic access is only
3167      * guaranteed with respect to the largest power of two that divides the GCD
3168      * of {@code A} and the size (in bytes) of {@code T}.
3169      * If access is aligned then following access modes are supported and are
3170      * guaranteed to support atomic access:
3171      * <ul>
3172      * <li>read write access modes for all {@code T}, with the exception of
3173      *     access modes {@code get} and {@code set} for {@code long} and
3174      *     {@code double} on 32-bit platforms.
3175      * <li>atomic update access modes for {@code int}, {@code long},
3176      *     {@code float} or {@code double}.
3177      *     (Future major platform releases of the JDK may support additional
3178      *     types for certain currently unsupported access modes.)
3179      * <li>numeric atomic update access modes for {@code int} and {@code long}.
3180      *     (Future major platform releases of the JDK may support additional
3181      *     numeric types for certain currently unsupported access modes.)
3182      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
3183      *     (Future major platform releases of the JDK may support additional
3184      *     numeric types for certain currently unsupported access modes.)
3185      * </ul>
3186      * <p>
3187      * Misaligned access, and therefore atomicity guarantees, may be determined
3188      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
3189      * {@code index}, {@code T} and it's corresponding boxed type,
3190      * {@code T_BOX}, as follows:
3191      * <pre>{@code
3192      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
3193      * ByteBuffer bb = ...
3194      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
3195      * boolean isMisaligned = misalignedAtIndex != 0;
3196      * }</pre>
3197      * <p>
3198      * If the variable type is {@code float} or {@code double} then atomic
3199      * update access modes compare values using their bitwise representation
3200      * (see {@link Float#floatToRawIntBits} and
3201      * {@link Double#doubleToRawLongBits}, respectively).
3202      * @param viewArrayClass the view array class, with a component type of
3203      * type {@code T}
3204      * @param byteOrder the endianness of the view array elements, as
3205      * stored in the underlying {@code ByteBuffer} (Note this overrides the
3206      * endianness of a {@code ByteBuffer})
3207      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
3208      * viewed as if elements corresponding to the components type of the view
3209      * array class
3210      * @throws NullPointerException if viewArrayClass or byteOrder is null
3211      * @throws IllegalArgumentException if viewArrayClass is not an array type
3212      * @throws UnsupportedOperationException if the component type of
3213      * viewArrayClass is not supported as a variable type
3214      * @since 9
3215      */
3216     public static
3217     VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
3218                                       ByteOrder byteOrder) throws IllegalArgumentException {
3219         Objects.requireNonNull(byteOrder);
3220         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
3221                                                    byteOrder == ByteOrder.BIG_ENDIAN);
3222     }
3223 
3224 
3225     /// method handle invocation (reflective style)
3226 
3227     /**
3228      * Produces a method handle which will invoke any method handle of the
3229      * given {@code type}, with a given number of trailing arguments replaced by
3230      * a single trailing {@code Object[]} array.
3231      * The resulting invoker will be a method handle with the following
3232      * arguments:
3233      * <ul>
3234      * <li>a single {@code MethodHandle} target
3235      * <li>zero or more leading values (counted by {@code leadingArgCount})
3236      * <li>an {@code Object[]} array containing trailing arguments
3237      * </ul>
3238      * <p>
3239      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
3240      * the indicated {@code type}.
3241      * That is, if the target is exactly of the given {@code type}, it will behave
3242      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
3243      * is used to convert the target to the required {@code type}.
3244      * <p>
3245      * The type of the returned invoker will not be the given {@code type}, but rather
3246      * will have all parameters except the first {@code leadingArgCount}
3247      * replaced by a single array of type {@code Object[]}, which will be
3248      * the final parameter.
3249      * <p>
3250      * Before invoking its target, the invoker will spread the final array, apply
3251      * reference casts as necessary, and unbox and widen primitive arguments.
3252      * If, when the invoker is called, the supplied array argument does
3253      * not have the correct number of elements, the invoker will throw
3254      * an {@link IllegalArgumentException} instead of invoking the target.
3255      * <p>
3256      * This method is equivalent to the following code (though it may be more efficient):
3257      * <blockquote><pre>{@code
3258 MethodHandle invoker = MethodHandles.invoker(type);
3259 int spreadArgCount = type.parameterCount() - leadingArgCount;
3260 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
3261 return invoker;
3262      * }</pre></blockquote>
3263      * This method throws no reflective or security exceptions.
3264      * @param type the desired target type
3265      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
3266      * @return a method handle suitable for invoking any method handle of the given type
3267      * @throws NullPointerException if {@code type} is null
3268      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
3269      *                  the range from 0 to {@code type.parameterCount()} inclusive,
3270      *                  or if the resulting method handle's type would have
3271      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3272      */
3273     public static
3274     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
3275         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
3276             throw newIllegalArgumentException("bad argument count", leadingArgCount);
3277         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
3278         return type.invokers().spreadInvoker(leadingArgCount);
3279     }
3280 
3281     /**
3282      * Produces a special <em>invoker method handle</em> which can be used to
3283      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
3284      * The resulting invoker will have a type which is
3285      * exactly equal to the desired type, except that it will accept
3286      * an additional leading argument of type {@code MethodHandle}.
3287      * <p>
3288      * This method is equivalent to the following code (though it may be more efficient):
3289      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
3290      *
3291      * <p style="font-size:smaller;">
3292      * <em>Discussion:</em>
3293      * Invoker method handles can be useful when working with variable method handles
3294      * of unknown types.
3295      * For example, to emulate an {@code invokeExact} call to a variable method
3296      * handle {@code M}, extract its type {@code T},
3297      * look up the invoker method {@code X} for {@code T},
3298      * and call the invoker method, as {@code X.invoke(T, A...)}.
3299      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
3300      * is unknown.)
3301      * If spreading, collecting, or other argument transformations are required,
3302      * they can be applied once to the invoker {@code X} and reused on many {@code M}
3303      * method handle values, as long as they are compatible with the type of {@code X}.
3304      * <p style="font-size:smaller;">
3305      * <em>(Note:  The invoker method is not available via the Core Reflection API.
3306      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
3307      * on the declared {@code invokeExact} or {@code invoke} method will raise an
3308      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
3309      * <p>
3310      * This method throws no reflective or security exceptions.
3311      * @param type the desired target type
3312      * @return a method handle suitable for invoking any method handle of the given type
3313      * @throws IllegalArgumentException if the resulting method handle's type would have
3314      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3315      */
3316     public static
3317     MethodHandle exactInvoker(MethodType type) {
3318         return type.invokers().exactInvoker();
3319     }
3320 
3321     /**
3322      * Produces a special <em>invoker method handle</em> which can be used to
3323      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
3324      * The resulting invoker will have a type which is
3325      * exactly equal to the desired type, except that it will accept
3326      * an additional leading argument of type {@code MethodHandle}.
3327      * <p>
3328      * Before invoking its target, if the target differs from the expected type,
3329      * the invoker will apply reference casts as
3330      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
3331      * Similarly, the return value will be converted as necessary.
3332      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
3333      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
3334      * <p>
3335      * This method is equivalent to the following code (though it may be more efficient):
3336      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
3337      * <p style="font-size:smaller;">
3338      * <em>Discussion:</em>
3339      * A {@linkplain MethodType#genericMethodType general method type} is one which
3340      * mentions only {@code Object} arguments and return values.
3341      * An invoker for such a type is capable of calling any method handle
3342      * of the same arity as the general type.
3343      * <p style="font-size:smaller;">
3344      * <em>(Note:  The invoker method is not available via the Core Reflection API.
3345      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
3346      * on the declared {@code invokeExact} or {@code invoke} method will raise an
3347      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
3348      * <p>
3349      * This method throws no reflective or security exceptions.
3350      * @param type the desired target type
3351      * @return a method handle suitable for invoking any method handle convertible to the given type
3352      * @throws IllegalArgumentException if the resulting method handle's type would have
3353      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3354      */
3355     public static
3356     MethodHandle invoker(MethodType type) {
3357         return type.invokers().genericInvoker();
3358     }
3359 
3360     /**
3361      * Produces a special <em>invoker method handle</em> which can be used to
3362      * invoke a signature-polymorphic access mode method on any VarHandle whose
3363      * associated access mode type is compatible with the given type.
3364      * The resulting invoker will have a type which is exactly equal to the
3365      * desired given type, except that it will accept an additional leading
3366      * argument of type {@code VarHandle}.
3367      *
3368      * @param accessMode the VarHandle access mode
3369      * @param type the desired target type
3370      * @return a method handle suitable for invoking an access mode method of
3371      *         any VarHandle whose access mode type is of the given type.
3372      * @since 9
3373      */
3374     static public
3375     MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3376         return type.invokers().varHandleMethodExactInvoker(accessMode);
3377     }
3378 
3379     /**
3380      * Produces a special <em>invoker method handle</em> which can be used to
3381      * invoke a signature-polymorphic access mode method on any VarHandle whose
3382      * associated access mode type is compatible with the given type.
3383      * The resulting invoker will have a type which is exactly equal to the
3384      * desired given type, except that it will accept an additional leading
3385      * argument of type {@code VarHandle}.
3386      * <p>
3387      * Before invoking its target, if the access mode type differs from the
3388      * desired given type, the invoker will apply reference casts as necessary
3389      * and box, unbox, or widen primitive values, as if by
3390      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
3391      * converted as necessary.
3392      * <p>
3393      * This method is equivalent to the following code (though it may be more
3394      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
3395      *
3396      * @param accessMode the VarHandle access mode
3397      * @param type the desired target type
3398      * @return a method handle suitable for invoking an access mode method of
3399      *         any VarHandle whose access mode type is convertible to the given
3400      *         type.
3401      * @since 9
3402      */
3403     static public
3404     MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3405         return type.invokers().varHandleMethodInvoker(accessMode);
3406     }
3407 
3408     static /*non-public*/
3409     MethodHandle basicInvoker(MethodType type) {
3410         return type.invokers().basicInvoker();
3411     }
3412 
3413      /// method handle modification (creation from other method handles)
3414 
3415     /**
3416      * Produces a method handle which adapts the type of the
3417      * given method handle to a new type by pairwise argument and return type conversion.
3418      * The original type and new type must have the same number of arguments.
3419      * The resulting method handle is guaranteed to report a type
3420      * which is equal to the desired new type.
3421      * <p>
3422      * If the original type and new type are equal, returns target.
3423      * <p>
3424      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
3425      * and some additional conversions are also applied if those conversions fail.
3426      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
3427      * if possible, before or instead of any conversions done by {@code asType}:
3428      * <ul>
3429      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
3430      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
3431      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
3432      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
3433      *     the boolean is converted to a byte value, 1 for true, 0 for false.
3434      *     (This treatment follows the usage of the bytecode verifier.)
3435      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
3436      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
3437      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
3438      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
3439      *     then a Java casting conversion (JLS 5.5) is applied.
3440      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
3441      *     widening and/or narrowing.)
3442      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
3443      *     conversion will be applied at runtime, possibly followed
3444      *     by a Java casting conversion (JLS 5.5) on the primitive value,
3445      *     possibly followed by a conversion from byte to boolean by testing
3446      *     the low-order bit.
3447      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
3448      *     and if the reference is null at runtime, a zero value is introduced.
3449      * </ul>
3450      * @param target the method handle to invoke after arguments are retyped
3451      * @param newType the expected type of the new method handle
3452      * @return a method handle which delegates to the target after performing
3453      *           any necessary argument conversions, and arranges for any
3454      *           necessary return value conversions
3455      * @throws NullPointerException if either argument is null
3456      * @throws WrongMethodTypeException if the conversion cannot be made
3457      * @see MethodHandle#asType
3458      */
3459     public static
3460     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
3461         explicitCastArgumentsChecks(target, newType);
3462         // use the asTypeCache when possible:
3463         MethodType oldType = target.type();
3464         if (oldType == newType)  return target;
3465         if (oldType.explicitCastEquivalentToAsType(newType)) {
3466             return target.asFixedArity().asType(newType);
3467         }
3468         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
3469     }
3470 
3471     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
3472         if (target.type().parameterCount() != newType.parameterCount()) {
3473             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
3474         }
3475     }
3476 
3477     /**
3478      * Produces a method handle which adapts the calling sequence of the
3479      * given method handle to a new type, by reordering the arguments.
3480      * The resulting method handle is guaranteed to report a type
3481      * which is equal to the desired new type.
3482      * <p>
3483      * The given array controls the reordering.
3484      * Call {@code #I} the number of incoming parameters (the value
3485      * {@code newType.parameterCount()}, and call {@code #O} the number
3486      * of outgoing parameters (the value {@code target.type().parameterCount()}).
3487      * Then the length of the reordering array must be {@code #O},
3488      * and each element must be a non-negative number less than {@code #I}.
3489      * For every {@code N} less than {@code #O}, the {@code N}-th
3490      * outgoing argument will be taken from the {@code I}-th incoming
3491      * argument, where {@code I} is {@code reorder[N]}.
3492      * <p>
3493      * No argument or return value conversions are applied.
3494      * The type of each incoming argument, as determined by {@code newType},
3495      * must be identical to the type of the corresponding outgoing parameter
3496      * or parameters in the target method handle.
3497      * The return type of {@code newType} must be identical to the return
3498      * type of the original target.
3499      * <p>
3500      * The reordering array need not specify an actual permutation.
3501      * An incoming argument will be duplicated if its index appears
3502      * more than once in the array, and an incoming argument will be dropped
3503      * if its index does not appear in the array.
3504      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
3505      * incoming arguments which are not mentioned in the reordering array
3506      * may be of any type, as determined only by {@code newType}.
3507      * <blockquote><pre>{@code
3508 import static java.lang.invoke.MethodHandles.*;
3509 import static java.lang.invoke.MethodType.*;
3510 ...
3511 MethodType intfn1 = methodType(int.class, int.class);
3512 MethodType intfn2 = methodType(int.class, int.class, int.class);
3513 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
3514 assert(sub.type().equals(intfn2));
3515 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
3516 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
3517 assert((int)rsub.invokeExact(1, 100) == 99);
3518 MethodHandle add = ... (int x, int y) -> (x+y) ...;
3519 assert(add.type().equals(intfn2));
3520 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
3521 assert(twice.type().equals(intfn1));
3522 assert((int)twice.invokeExact(21) == 42);
3523      * }</pre></blockquote>
3524      * <p>
3525      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3526      * variable-arity method handle}, even if the original target method handle was.
3527      * @param target the method handle to invoke after arguments are reordered
3528      * @param newType the expected type of the new method handle
3529      * @param reorder an index array which controls the reordering
3530      * @return a method handle which delegates to the target after it
3531      *           drops unused arguments and moves and/or duplicates the other arguments
3532      * @throws NullPointerException if any argument is null
3533      * @throws IllegalArgumentException if the index array length is not equal to
3534      *                  the arity of the target, or if any index array element
3535      *                  not a valid index for a parameter of {@code newType},
3536      *                  or if two corresponding parameter types in
3537      *                  {@code target.type()} and {@code newType} are not identical,
3538      */
3539     public static
3540     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
3541         reorder = reorder.clone();  // get a private copy
3542         MethodType oldType = target.type();
3543         permuteArgumentChecks(reorder, newType, oldType);
3544         // first detect dropped arguments and handle them separately
3545         int[] originalReorder = reorder;
3546         BoundMethodHandle result = target.rebind();
3547         LambdaForm form = result.form;
3548         int newArity = newType.parameterCount();
3549         // Normalize the reordering into a real permutation,
3550         // by removing duplicates and adding dropped elements.
3551         // This somewhat improves lambda form caching, as well
3552         // as simplifying the transform by breaking it up into steps.
3553         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
3554             if (ddIdx > 0) {
3555                 // We found a duplicated entry at reorder[ddIdx].
3556                 // Example:  (x,y,z)->asList(x,y,z)
3557                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
3558                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
3559                 // The starred element corresponds to the argument
3560                 // deleted by the dupArgumentForm transform.
3561                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
3562                 boolean killFirst = false;
3563                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
3564                     // Set killFirst if the dup is larger than an intervening position.
3565                     // This will remove at least one inversion from the permutation.
3566                     if (dupVal > val) killFirst = true;
3567                 }
3568                 if (!killFirst) {
3569                     srcPos = dstPos;
3570                     dstPos = ddIdx;
3571                 }
3572                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
3573                 assert (reorder[srcPos] == reorder[dstPos]);
3574                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
3575                 // contract the reordering by removing the element at dstPos
3576                 int tailPos = dstPos + 1;
3577                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
3578                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
3579             } else {
3580                 int dropVal = ~ddIdx, insPos = 0;
3581                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
3582                     // Find first element of reorder larger than dropVal.
3583                     // This is where we will insert the dropVal.
3584                     insPos += 1;
3585                 }
3586                 Class<?> ptype = newType.parameterType(dropVal);
3587                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
3588                 oldType = oldType.insertParameterTypes(insPos, ptype);
3589                 // expand the reordering by inserting an element at insPos
3590                 int tailPos = insPos + 1;
3591                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
3592                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
3593                 reorder[insPos] = dropVal;
3594             }
3595             assert (permuteArgumentChecks(reorder, newType, oldType));
3596         }
3597         assert (reorder.length == newArity);  // a perfect permutation
3598         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
3599         form = form.editor().permuteArgumentsForm(1, reorder);
3600         if (newType == result.type() && form == result.internalForm())
3601             return result;
3602         return result.copyWith(newType, form);
3603     }
3604 
3605     /**
3606      * Return an indication of any duplicate or omission in reorder.
3607      * If the reorder contains a duplicate entry, return the index of the second occurrence.
3608      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
3609      * Otherwise, return zero.
3610      * If an element not in [0..newArity-1] is encountered, return reorder.length.
3611      */
3612     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
3613         final int BIT_LIMIT = 63;  // max number of bits in bit mask
3614         if (newArity < BIT_LIMIT) {
3615             long mask = 0;
3616             for (int i = 0; i < reorder.length; i++) {
3617                 int arg = reorder[i];
3618                 if (arg >= newArity) {
3619                     return reorder.length;
3620                 }
3621                 long bit = 1L << arg;
3622                 if ((mask & bit) != 0) {
3623                     return i;  // >0 indicates a dup
3624                 }
3625                 mask |= bit;
3626             }
3627             if (mask == (1L << newArity) - 1) {
3628                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
3629                 return 0;
3630             }
3631             // find first zero
3632             long zeroBit = Long.lowestOneBit(~mask);
3633             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
3634             assert(zeroPos <= newArity);
3635             if (zeroPos == newArity) {
3636                 return 0;
3637             }
3638             return ~zeroPos;
3639         } else {
3640             // same algorithm, different bit set
3641             BitSet mask = new BitSet(newArity);
3642             for (int i = 0; i < reorder.length; i++) {
3643                 int arg = reorder[i];
3644                 if (arg >= newArity) {
3645                     return reorder.length;
3646                 }
3647                 if (mask.get(arg)) {
3648                     return i;  // >0 indicates a dup
3649                 }
3650                 mask.set(arg);
3651             }
3652             int zeroPos = mask.nextClearBit(0);
3653             assert(zeroPos <= newArity);
3654             if (zeroPos == newArity) {
3655                 return 0;
3656             }
3657             return ~zeroPos;
3658         }
3659     }
3660 
3661     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
3662         if (newType.returnType() != oldType.returnType())
3663             throw newIllegalArgumentException("return types do not match",
3664                     oldType, newType);
3665         if (reorder.length == oldType.parameterCount()) {
3666             int limit = newType.parameterCount();
3667             boolean bad = false;
3668             for (int j = 0; j < reorder.length; j++) {
3669                 int i = reorder[j];
3670                 if (i < 0 || i >= limit) {
3671                     bad = true; break;
3672                 }
3673                 Class<?> src = newType.parameterType(i);
3674                 Class<?> dst = oldType.parameterType(j);
3675                 if (src != dst)
3676                     throw newIllegalArgumentException("parameter types do not match after reorder",
3677                             oldType, newType);
3678             }
3679             if (!bad)  return true;
3680         }
3681         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
3682     }
3683 
3684     /**
3685      * Produces a method handle of the requested return type which returns the given
3686      * constant value every time it is invoked.
3687      * <p>
3688      * Before the method handle is returned, the passed-in value is converted to the requested type.
3689      * If the requested type is primitive, widening primitive conversions are attempted,
3690      * else reference conversions are attempted.
3691      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
3692      * @param type the return type of the desired method handle
3693      * @param value the value to return
3694      * @return a method handle of the given return type and no arguments, which always returns the given value
3695      * @throws NullPointerException if the {@code type} argument is null
3696      * @throws ClassCastException if the value cannot be converted to the required return type
3697      * @throws IllegalArgumentException if the given type is {@code void.class}
3698      */
3699     public static
3700     MethodHandle constant(Class<?> type, Object value) {
3701         if (type.isPrimitive()) {
3702             if (type == void.class)
3703                 throw newIllegalArgumentException("void type");
3704             Wrapper w = Wrapper.forPrimitiveType(type);
3705             value = w.convert(value, type);
3706             if (w.zero().equals(value))
3707                 return zero(w, type);
3708             return insertArguments(identity(type), 0, value);
3709         } else {
3710             if (value == null)
3711                 return zero(Wrapper.OBJECT, type);
3712             return identity(type).bindTo(value);
3713         }
3714     }
3715 
3716     /**
3717      * Produces a method handle which returns its sole argument when invoked.
3718      * @param type the type of the sole parameter and return value of the desired method handle
3719      * @return a unary method handle which accepts and returns the given type
3720      * @throws NullPointerException if the argument is null
3721      * @throws IllegalArgumentException if the given type is {@code void.class}
3722      */
3723     public static
3724     MethodHandle identity(Class<?> type) {
3725         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
3726         int pos = btw.ordinal();
3727         MethodHandle ident = IDENTITY_MHS[pos];
3728         if (ident == null) {
3729             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
3730         }
3731         if (ident.type().returnType() == type)
3732             return ident;
3733         // something like identity(Foo.class); do not bother to intern these
3734         assert (btw == Wrapper.OBJECT);
3735         return makeIdentity(type);
3736     }
3737 
3738     /**
3739      * Produces a constant method handle of the requested return type which
3740      * returns the default value for that type every time it is invoked.
3741      * The resulting constant method handle will have no side effects.
3742      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
3743      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
3744      * since {@code explicitCastArguments} converts {@code null} to default values.
3745      * @param type the expected return type of the desired method handle
3746      * @return a constant method handle that takes no arguments
3747      *         and returns the default value of the given type (or void, if the type is void)
3748      * @throws NullPointerException if the argument is null
3749      * @see MethodHandles#constant
3750      * @see MethodHandles#empty
3751      * @see MethodHandles#explicitCastArguments
3752      * @since 9
3753      */
3754     public static MethodHandle zero(Class<?> type) {
3755         Objects.requireNonNull(type);
3756         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
3757     }
3758 
3759     private static MethodHandle identityOrVoid(Class<?> type) {
3760         return type == void.class ? zero(type) : identity(type);
3761     }
3762 
3763     /**
3764      * Produces a method handle of the requested type which ignores any arguments, does nothing,
3765      * and returns a suitable default depending on the return type.
3766      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
3767      * <p>The returned method handle is equivalent to
3768      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
3769      *
3770      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
3771      * {@code guardWithTest(pred, target, empty(target.type())}.
3772      * @param type the type of the desired method handle
3773      * @return a constant method handle of the given type, which returns a default value of the given return type
3774      * @throws NullPointerException if the argument is null
3775      * @see MethodHandles#zero
3776      * @see MethodHandles#constant
3777      * @since 9
3778      */
3779     public static  MethodHandle empty(MethodType type) {
3780         Objects.requireNonNull(type);
3781         return dropArguments(zero(type.returnType()), 0, type.parameterList());
3782     }
3783 
3784     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
3785     private static MethodHandle makeIdentity(Class<?> ptype) {
3786         MethodType mtype = methodType(ptype, ptype);
3787         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
3788         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
3789     }
3790 
3791     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
3792         int pos = btw.ordinal();
3793         MethodHandle zero = ZERO_MHS[pos];
3794         if (zero == null) {
3795             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
3796         }
3797         if (zero.type().returnType() == rtype)
3798             return zero;
3799         assert(btw == Wrapper.OBJECT);
3800         return makeZero(rtype);
3801     }
3802     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
3803     private static MethodHandle makeZero(Class<?> rtype) {
3804         MethodType mtype = methodType(rtype);
3805         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
3806         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
3807     }
3808 
3809     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
3810         // Simulate a CAS, to avoid racy duplication of results.
3811         MethodHandle prev = cache[pos];
3812         if (prev != null) return prev;
3813         return cache[pos] = value;
3814     }
3815 
3816     /**
3817      * Provides a target method handle with one or more <em>bound arguments</em>
3818      * in advance of the method handle's invocation.
3819      * The formal parameters to the target corresponding to the bound
3820      * arguments are called <em>bound parameters</em>.
3821      * Returns a new method handle which saves away the bound arguments.
3822      * When it is invoked, it receives arguments for any non-bound parameters,
3823      * binds the saved arguments to their corresponding parameters,
3824      * and calls the original target.
3825      * <p>
3826      * The type of the new method handle will drop the types for the bound
3827      * parameters from the original target type, since the new method handle
3828      * will no longer require those arguments to be supplied by its callers.
3829      * <p>
3830      * Each given argument object must match the corresponding bound parameter type.
3831      * If a bound parameter type is a primitive, the argument object
3832      * must be a wrapper, and will be unboxed to produce the primitive value.
3833      * <p>
3834      * The {@code pos} argument selects which parameters are to be bound.
3835      * It may range between zero and <i>N-L</i> (inclusively),
3836      * where <i>N</i> is the arity of the target method handle
3837      * and <i>L</i> is the length of the values array.
3838      * <p>
3839      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3840      * variable-arity method handle}, even if the original target method handle was.
3841      * @param target the method handle to invoke after the argument is inserted
3842      * @param pos where to insert the argument (zero for the first)
3843      * @param values the series of arguments to insert
3844      * @return a method handle which inserts an additional argument,
3845      *         before calling the original method handle
3846      * @throws NullPointerException if the target or the {@code values} array is null
3847      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
3848      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
3849      *         is the length of the values array.
3850      * @throws ClassCastException if an argument does not match the corresponding bound parameter
3851      *         type.
3852      * @see MethodHandle#bindTo
3853      */
3854     public static
3855     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
3856         int insCount = values.length;
3857         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
3858         if (insCount == 0)  return target;
3859         BoundMethodHandle result = target.rebind();
3860         for (int i = 0; i < insCount; i++) {
3861             Object value = values[i];
3862             Class<?> ptype = ptypes[pos+i];
3863             if (ptype.isPrimitive()) {
3864                 result = insertArgumentPrimitive(result, pos, ptype, value);
3865             } else {
3866                 value = ptype.cast(value);  // throw CCE if needed
3867                 result = result.bindArgumentL(pos, value);
3868             }
3869         }
3870         return result;
3871     }
3872 
3873     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
3874                                                              Class<?> ptype, Object value) {
3875         Wrapper w = Wrapper.forPrimitiveType(ptype);
3876         // perform unboxing and/or primitive conversion
3877         value = w.convert(value, ptype);
3878         switch (w) {
3879         case INT:     return result.bindArgumentI(pos, (int)value);
3880         case LONG:    return result.bindArgumentJ(pos, (long)value);
3881         case FLOAT:   return result.bindArgumentF(pos, (float)value);
3882         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
3883         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
3884         }
3885     }
3886 
3887     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
3888         MethodType oldType = target.type();
3889         int outargs = oldType.parameterCount();
3890         int inargs  = outargs - insCount;
3891         if (inargs < 0)
3892             throw newIllegalArgumentException("too many values to insert");
3893         if (pos < 0 || pos > inargs)
3894             throw newIllegalArgumentException("no argument type to append");
3895         return oldType.ptypes();
3896     }
3897 
3898     /**
3899      * Produces a method handle which will discard some dummy arguments
3900      * before calling some other specified <i>target</i> method handle.
3901      * The type of the new method handle will be the same as the target's type,
3902      * except it will also include the dummy argument types,
3903      * at some given position.
3904      * <p>
3905      * The {@code pos} argument may range between zero and <i>N</i>,
3906      * where <i>N</i> is the arity of the target.
3907      * If {@code pos} is zero, the dummy arguments will precede
3908      * the target's real arguments; if {@code pos} is <i>N</i>
3909      * they will come after.
3910      * <p>
3911      * <b>Example:</b>
3912      * <blockquote><pre>{@code
3913 import static java.lang.invoke.MethodHandles.*;
3914 import static java.lang.invoke.MethodType.*;
3915 ...
3916 MethodHandle cat = lookup().findVirtual(String.class,
3917   "concat", methodType(String.class, String.class));
3918 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3919 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
3920 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
3921 assertEquals(bigType, d0.type());
3922 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
3923      * }</pre></blockquote>
3924      * <p>
3925      * This method is also equivalent to the following code:
3926      * <blockquote><pre>
3927      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
3928      * </pre></blockquote>
3929      * @param target the method handle to invoke after the arguments are dropped
3930      * @param valueTypes the type(s) of the argument(s) to drop
3931      * @param pos position of first argument to drop (zero for the leftmost)
3932      * @return a method handle which drops arguments of the given types,
3933      *         before calling the original method handle
3934      * @throws NullPointerException if the target is null,
3935      *                              or if the {@code valueTypes} list or any of its elements is null
3936      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3937      *                  or if {@code pos} is negative or greater than the arity of the target,
3938      *                  or if the new method handle's type would have too many parameters
3939      */
3940     public static
3941     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3942         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
3943     }
3944 
3945     private static List<Class<?>> copyTypes(Object[] array) {
3946         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
3947     }
3948 
3949     private static
3950     MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3951         MethodType oldType = target.type();  // get NPE
3952         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
3953         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
3954         if (dropped == 0)  return target;
3955         BoundMethodHandle result = target.rebind();
3956         LambdaForm lform = result.form;
3957         int insertFormArg = 1 + pos;
3958         for (Class<?> ptype : valueTypes) {
3959             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
3960         }
3961         result = result.copyWith(newType, lform);
3962         return result;
3963     }
3964 
3965     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
3966         int dropped = valueTypes.size();
3967         MethodType.checkSlotCount(dropped);
3968         int outargs = oldType.parameterCount();
3969         int inargs  = outargs + dropped;
3970         if (pos < 0 || pos > outargs)
3971             throw newIllegalArgumentException("no argument type to remove"
3972                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
3973                     );
3974         return dropped;
3975     }
3976 
3977     /**
3978      * Produces a method handle which will discard some dummy arguments
3979      * before calling some other specified <i>target</i> method handle.
3980      * The type of the new method handle will be the same as the target's type,
3981      * except it will also include the dummy argument types,
3982      * at some given position.
3983      * <p>
3984      * The {@code pos} argument may range between zero and <i>N</i>,
3985      * where <i>N</i> is the arity of the target.
3986      * If {@code pos} is zero, the dummy arguments will precede
3987      * the target's real arguments; if {@code pos} is <i>N</i>
3988      * they will come after.
3989      * @apiNote
3990      * <blockquote><pre>{@code
3991 import static java.lang.invoke.MethodHandles.*;
3992 import static java.lang.invoke.MethodType.*;
3993 ...
3994 MethodHandle cat = lookup().findVirtual(String.class,
3995   "concat", methodType(String.class, String.class));
3996 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3997 MethodHandle d0 = dropArguments(cat, 0, String.class);
3998 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
3999 MethodHandle d1 = dropArguments(cat, 1, String.class);
4000 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
4001 MethodHandle d2 = dropArguments(cat, 2, String.class);
4002 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
4003 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
4004 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
4005      * }</pre></blockquote>
4006      * <p>
4007      * This method is also equivalent to the following code:
4008      * <blockquote><pre>
4009      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
4010      * </pre></blockquote>
4011      * @param target the method handle to invoke after the arguments are dropped
4012      * @param valueTypes the type(s) of the argument(s) to drop
4013      * @param pos position of first argument to drop (zero for the leftmost)
4014      * @return a method handle which drops arguments of the given types,
4015      *         before calling the original method handle
4016      * @throws NullPointerException if the target is null,
4017      *                              or if the {@code valueTypes} array or any of its elements is null
4018      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
4019      *                  or if {@code pos} is negative or greater than the arity of the target,
4020      *                  or if the new method handle's type would have
4021      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
4022      */
4023     public static
4024     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
4025         return dropArguments0(target, pos, copyTypes(valueTypes));
4026     }
4027 
4028     // private version which allows caller some freedom with error handling
4029     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
4030                                       boolean nullOnFailure) {
4031         newTypes = copyTypes(newTypes.toArray());
4032         List<Class<?>> oldTypes = target.type().parameterList();
4033         int match = oldTypes.size();
4034         if (skip != 0) {
4035             if (skip < 0 || skip > match) {
4036                 throw newIllegalArgumentException("illegal skip", skip, target);
4037             }
4038             oldTypes = oldTypes.subList(skip, match);
4039             match -= skip;
4040         }
4041         List<Class<?>> addTypes = newTypes;
4042         int add = addTypes.size();
4043         if (pos != 0) {
4044             if (pos < 0 || pos > add) {
4045                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
4046             }
4047             addTypes = addTypes.subList(pos, add);
4048             add -= pos;
4049             assert(addTypes.size() == add);
4050         }
4051         // Do not add types which already match the existing arguments.
4052         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
4053             if (nullOnFailure) {
4054                 return null;
4055             }
4056             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
4057         }
4058         addTypes = addTypes.subList(match, add);
4059         add -= match;
4060         assert(addTypes.size() == add);
4061         // newTypes:     (   P*[pos], M*[match], A*[add] )
4062         // target: ( S*[skip],        M*[match]  )
4063         MethodHandle adapter = target;
4064         if (add > 0) {
4065             adapter = dropArguments0(adapter, skip+ match, addTypes);
4066         }
4067         // adapter: (S*[skip],        M*[match], A*[add] )
4068         if (pos > 0) {
4069             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
4070         }
4071         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
4072         return adapter;
4073     }
4074 
4075     /**
4076      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
4077      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
4078      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
4079      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
4080      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
4081      * {@link #dropArguments(MethodHandle, int, Class[])}.
4082      * <p>
4083      * The resulting handle will have the same return type as the target handle.
4084      * <p>
4085      * In more formal terms, assume these two type lists:<ul>
4086      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
4087      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
4088      * {@code newTypes}.
4089      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
4090      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
4091      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
4092      * sub-list.
4093      * </ul>
4094      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
4095      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
4096      * {@link #dropArguments(MethodHandle, int, Class[])}.
4097      *
4098      * @apiNote
4099      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
4100      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
4101      * <blockquote><pre>{@code
4102 import static java.lang.invoke.MethodHandles.*;
4103 import static java.lang.invoke.MethodType.*;
4104 ...
4105 ...
4106 MethodHandle h0 = constant(boolean.class, true);
4107 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
4108 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
4109 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
4110 if (h1.type().parameterCount() < h2.type().parameterCount())
4111     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
4112 else
4113     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
4114 MethodHandle h3 = guardWithTest(h0, h1, h2);
4115 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
4116      * }</pre></blockquote>
4117      * @param target the method handle to adapt
4118      * @param skip number of targets parameters to disregard (they will be unchanged)
4119      * @param newTypes the list of types to match {@code target}'s parameter type list to
4120      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
4121      * @return a possibly adapted method handle
4122      * @throws NullPointerException if either argument is null
4123      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
4124      *         or if {@code skip} is negative or greater than the arity of the target,
4125      *         or if {@code pos} is negative or greater than the newTypes list size,
4126      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
4127      *         {@code pos}.
4128      * @since 9
4129      */
4130     public static
4131     MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
4132         Objects.requireNonNull(target);
4133         Objects.requireNonNull(newTypes);
4134         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
4135     }
4136 
4137     /**
4138      * Adapts a target method handle by pre-processing
4139      * one or more of its arguments, each with its own unary filter function,
4140      * and then calling the target with each pre-processed argument
4141      * replaced by the result of its corresponding filter function.
4142      * <p>
4143      * The pre-processing is performed by one or more method handles,
4144      * specified in the elements of the {@code filters} array.
4145      * The first element of the filter array corresponds to the {@code pos}
4146      * argument of the target, and so on in sequence.
4147      * The filter functions are invoked in left to right order.
4148      * <p>
4149      * Null arguments in the array are treated as identity functions,
4150      * and the corresponding arguments left unchanged.
4151      * (If there are no non-null elements in the array, the original target is returned.)
4152      * Each filter is applied to the corresponding argument of the adapter.
4153      * <p>
4154      * If a filter {@code F} applies to the {@code N}th argument of
4155      * the target, then {@code F} must be a method handle which
4156      * takes exactly one argument.  The type of {@code F}'s sole argument
4157      * replaces the corresponding argument type of the target
4158      * in the resulting adapted method handle.
4159      * The return type of {@code F} must be identical to the corresponding
4160      * parameter type of the target.
4161      * <p>
4162      * It is an error if there are elements of {@code filters}
4163      * (null or not)
4164      * which do not correspond to argument positions in the target.
4165      * <p><b>Example:</b>
4166      * <blockquote><pre>{@code
4167 import static java.lang.invoke.MethodHandles.*;
4168 import static java.lang.invoke.MethodType.*;
4169 ...
4170 MethodHandle cat = lookup().findVirtual(String.class,
4171   "concat", methodType(String.class, String.class));
4172 MethodHandle upcase = lookup().findVirtual(String.class,
4173   "toUpperCase", methodType(String.class));
4174 assertEquals("xy", (String) cat.invokeExact("x", "y"));
4175 MethodHandle f0 = filterArguments(cat, 0, upcase);
4176 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
4177 MethodHandle f1 = filterArguments(cat, 1, upcase);
4178 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
4179 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
4180 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
4181      * }</pre></blockquote>
4182      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4183      * denotes the return type of both the {@code target} and resulting adapter.
4184      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
4185      * of the parameters and arguments that precede and follow the filter position
4186      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
4187      * values of the filtered parameters and arguments; they also represent the
4188      * return types of the {@code filter[i]} handles. The latter accept arguments
4189      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
4190      * the resulting adapter.
4191      * <blockquote><pre>{@code
4192      * T target(P... p, A[i]... a[i], B... b);
4193      * A[i] filter[i](V[i]);
4194      * T adapter(P... p, V[i]... v[i], B... b) {
4195      *   return target(p..., filter[i](v[i])..., b...);
4196      * }
4197      * }</pre></blockquote>
4198      * <p>
4199      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4200      * variable-arity method handle}, even if the original target method handle was.
4201      *
4202      * @param target the method handle to invoke after arguments are filtered
4203      * @param pos the position of the first argument to filter
4204      * @param filters method handles to call initially on filtered arguments
4205      * @return method handle which incorporates the specified argument filtering logic
4206      * @throws NullPointerException if the target is null
4207      *                              or if the {@code filters} array is null
4208      * @throws IllegalArgumentException if a non-null element of {@code filters}
4209      *          does not match a corresponding argument type of target as described above,
4210      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
4211      *          or if the resulting method handle's type would have
4212      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4213      */
4214     public static
4215     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
4216         filterArgumentsCheckArity(target, pos, filters);
4217         MethodHandle adapter = target;
4218         // process filters in reverse order so that the invocation of
4219         // the resulting adapter will invoke the filters in left-to-right order
4220         for (int i = filters.length - 1; i >= 0; --i) {
4221             MethodHandle filter = filters[i];
4222             if (filter == null)  continue;  // ignore null elements of filters
4223             adapter = filterArgument(adapter, pos + i, filter);
4224         }
4225         return adapter;
4226     }
4227 
4228     /*non-public*/ static
4229     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
4230         filterArgumentChecks(target, pos, filter);
4231         MethodType targetType = target.type();
4232         MethodType filterType = filter.type();
4233         BoundMethodHandle result = target.rebind();
4234         Class<?> newParamType = filterType.parameterType(0);
4235         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
4236         MethodType newType = targetType.changeParameterType(pos, newParamType);
4237         result = result.copyWithExtendL(newType, lform, filter);
4238         return result;
4239     }
4240 
4241     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
4242         MethodType targetType = target.type();
4243         int maxPos = targetType.parameterCount();
4244         if (pos + filters.length > maxPos)
4245             throw newIllegalArgumentException("too many filters");
4246     }
4247 
4248     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
4249         MethodType targetType = target.type();
4250         MethodType filterType = filter.type();
4251         if (filterType.parameterCount() != 1
4252             || filterType.returnType() != targetType.parameterType(pos))
4253             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4254     }
4255 
4256     /**
4257      * Adapts a target method handle by pre-processing
4258      * a sub-sequence of its arguments with a filter (another method handle).
4259      * The pre-processed arguments are replaced by the result (if any) of the
4260      * filter function.
4261      * The target is then called on the modified (usually shortened) argument list.
4262      * <p>
4263      * If the filter returns a value, the target must accept that value as
4264      * its argument in position {@code pos}, preceded and/or followed by
4265      * any arguments not passed to the filter.
4266      * If the filter returns void, the target must accept all arguments
4267      * not passed to the filter.
4268      * No arguments are reordered, and a result returned from the filter
4269      * replaces (in order) the whole subsequence of arguments originally
4270      * passed to the adapter.
4271      * <p>
4272      * The argument types (if any) of the filter
4273      * replace zero or one argument types of the target, at position {@code pos},
4274      * in the resulting adapted method handle.
4275      * The return type of the filter (if any) must be identical to the
4276      * argument type of the target at position {@code pos}, and that target argument
4277      * is supplied by the return value of the filter.
4278      * <p>
4279      * In all cases, {@code pos} must be greater than or equal to zero, and
4280      * {@code pos} must also be less than or equal to the target's arity.
4281      * <p><b>Example:</b>
4282      * <blockquote><pre>{@code
4283 import static java.lang.invoke.MethodHandles.*;
4284 import static java.lang.invoke.MethodType.*;
4285 ...
4286 MethodHandle deepToString = publicLookup()
4287   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
4288 
4289 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
4290 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
4291 
4292 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
4293 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
4294 
4295 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
4296 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
4297 assertEquals("[top, [up, down], strange]",
4298              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
4299 
4300 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
4301 assertEquals("[top, [up, down], [strange]]",
4302              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
4303 
4304 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
4305 assertEquals("[top, [[up, down, strange], charm], bottom]",
4306              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
4307      * }</pre></blockquote>
4308      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4309      * represents the return type of the {@code target} and resulting adapter.
4310      * {@code V}/{@code v} stand for the return type and value of the
4311      * {@code filter}, which are also found in the signature and arguments of
4312      * the {@code target}, respectively, unless {@code V} is {@code void}.
4313      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
4314      * and values preceding and following the collection position, {@code pos},
4315      * in the {@code target}'s signature. They also turn up in the resulting
4316      * adapter's signature and arguments, where they surround
4317      * {@code B}/{@code b}, which represent the parameter types and arguments
4318      * to the {@code filter} (if any).
4319      * <blockquote><pre>{@code
4320      * T target(A...,V,C...);
4321      * V filter(B...);
4322      * T adapter(A... a,B... b,C... c) {
4323      *   V v = filter(b...);
4324      *   return target(a...,v,c...);
4325      * }
4326      * // and if the filter has no arguments:
4327      * T target2(A...,V,C...);
4328      * V filter2();
4329      * T adapter2(A... a,C... c) {
4330      *   V v = filter2();
4331      *   return target2(a...,v,c...);
4332      * }
4333      * // and if the filter has a void return:
4334      * T target3(A...,C...);
4335      * void filter3(B...);
4336      * T adapter3(A... a,B... b,C... c) {
4337      *   filter3(b...);
4338      *   return target3(a...,c...);
4339      * }
4340      * }</pre></blockquote>
4341      * <p>
4342      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
4343      * one which first "folds" the affected arguments, and then drops them, in separate
4344      * steps as follows:
4345      * <blockquote><pre>{@code
4346      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
4347      * mh = MethodHandles.foldArguments(mh, coll); //step 1
4348      * }</pre></blockquote>
4349      * If the target method handle consumes no arguments besides than the result
4350      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
4351      * is equivalent to {@code filterReturnValue(coll, mh)}.
4352      * If the filter method handle {@code coll} consumes one argument and produces
4353      * a non-void result, then {@code collectArguments(mh, N, coll)}
4354      * is equivalent to {@code filterArguments(mh, N, coll)}.
4355      * Other equivalences are possible but would require argument permutation.
4356      * <p>
4357      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4358      * variable-arity method handle}, even if the original target method handle was.
4359      *
4360      * @param target the method handle to invoke after filtering the subsequence of arguments
4361      * @param pos the position of the first adapter argument to pass to the filter,
4362      *            and/or the target argument which receives the result of the filter
4363      * @param filter method handle to call on the subsequence of arguments
4364      * @return method handle which incorporates the specified argument subsequence filtering logic
4365      * @throws NullPointerException if either argument is null
4366      * @throws IllegalArgumentException if the return type of {@code filter}
4367      *          is non-void and is not the same as the {@code pos} argument of the target,
4368      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
4369      *          or if the resulting method handle's type would have
4370      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4371      * @see MethodHandles#foldArguments
4372      * @see MethodHandles#filterArguments
4373      * @see MethodHandles#filterReturnValue
4374      */
4375     public static
4376     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
4377         MethodType newType = collectArgumentsChecks(target, pos, filter);
4378         MethodType collectorType = filter.type();
4379         BoundMethodHandle result = target.rebind();
4380         LambdaForm lform;
4381         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
4382             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
4383             if (lform != null) {
4384                 return result.copyWith(newType, lform);
4385             }
4386         }
4387         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
4388         return result.copyWithExtendL(newType, lform, filter);
4389     }
4390 
4391     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
4392         MethodType targetType = target.type();
4393         MethodType filterType = filter.type();
4394         Class<?> rtype = filterType.returnType();
4395         List<Class<?>> filterArgs = filterType.parameterList();
4396         if (rtype == void.class) {
4397             return targetType.insertParameterTypes(pos, filterArgs);
4398         }
4399         if (rtype != targetType.parameterType(pos)) {
4400             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4401         }
4402         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
4403     }
4404 
4405     /**
4406      * Adapts a target method handle by post-processing
4407      * its return value (if any) with a filter (another method handle).
4408      * The result of the filter is returned from the adapter.
4409      * <p>
4410      * If the target returns a value, the filter must accept that value as
4411      * its only argument.
4412      * If the target returns void, the filter must accept no arguments.
4413      * <p>
4414      * The return type of the filter
4415      * replaces the return type of the target
4416      * in the resulting adapted method handle.
4417      * The argument type of the filter (if any) must be identical to the
4418      * return type of the target.
4419      * <p><b>Example:</b>
4420      * <blockquote><pre>{@code
4421 import static java.lang.invoke.MethodHandles.*;
4422 import static java.lang.invoke.MethodType.*;
4423 ...
4424 MethodHandle cat = lookup().findVirtual(String.class,
4425   "concat", methodType(String.class, String.class));
4426 MethodHandle length = lookup().findVirtual(String.class,
4427   "length", methodType(int.class));
4428 System.out.println((String) cat.invokeExact("x", "y")); // xy
4429 MethodHandle f0 = filterReturnValue(cat, length);
4430 System.out.println((int) f0.invokeExact("x", "y")); // 2
4431      * }</pre></blockquote>
4432      * <p>Here is pseudocode for the resulting adapter. In the code,
4433      * {@code T}/{@code t} represent the result type and value of the
4434      * {@code target}; {@code V}, the result type of the {@code filter}; and
4435      * {@code A}/{@code a}, the types and values of the parameters and arguments
4436      * of the {@code target} as well as the resulting adapter.
4437      * <blockquote><pre>{@code
4438      * T target(A...);
4439      * V filter(T);
4440      * V adapter(A... a) {
4441      *   T t = target(a...);
4442      *   return filter(t);
4443      * }
4444      * // and if the target has a void return:
4445      * void target2(A...);
4446      * V filter2();
4447      * V adapter2(A... a) {
4448      *   target2(a...);
4449      *   return filter2();
4450      * }
4451      * // and if the filter has a void return:
4452      * T target3(A...);
4453      * void filter3(V);
4454      * void adapter3(A... a) {
4455      *   T t = target3(a...);
4456      *   filter3(t);
4457      * }
4458      * }</pre></blockquote>
4459      * <p>
4460      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4461      * variable-arity method handle}, even if the original target method handle was.
4462      * @param target the method handle to invoke before filtering the return value
4463      * @param filter method handle to call on the return value
4464      * @return method handle which incorporates the specified return value filtering logic
4465      * @throws NullPointerException if either argument is null
4466      * @throws IllegalArgumentException if the argument list of {@code filter}
4467      *          does not match the return type of target as described above
4468      */
4469     public static
4470     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
4471         MethodType targetType = target.type();
4472         MethodType filterType = filter.type();
4473         filterReturnValueChecks(targetType, filterType);
4474         BoundMethodHandle result = target.rebind();
4475         BasicType rtype = BasicType.basicType(filterType.returnType());
4476         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
4477         MethodType newType = targetType.changeReturnType(filterType.returnType());
4478         result = result.copyWithExtendL(newType, lform, filter);
4479         return result;
4480     }
4481 
4482     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
4483         Class<?> rtype = targetType.returnType();
4484         int filterValues = filterType.parameterCount();
4485         if (filterValues == 0
4486                 ? (rtype != void.class)
4487                 : (rtype != filterType.parameterType(0) || filterValues != 1))
4488             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4489     }
4490 
4491     /**
4492      * Adapts a target method handle by pre-processing
4493      * some of its arguments, and then calling the target with
4494      * the result of the pre-processing, inserted into the original
4495      * sequence of arguments.
4496      * <p>
4497      * The pre-processing is performed by {@code combiner}, a second method handle.
4498      * Of the arguments passed to the adapter, the first {@code N} arguments
4499      * are copied to the combiner, which is then called.
4500      * (Here, {@code N} is defined as the parameter count of the combiner.)
4501      * After this, control passes to the target, with any result
4502      * from the combiner inserted before the original {@code N} incoming
4503      * arguments.
4504      * <p>
4505      * If the combiner returns a value, the first parameter type of the target
4506      * must be identical with the return type of the combiner, and the next
4507      * {@code N} parameter types of the target must exactly match the parameters
4508      * of the combiner.
4509      * <p>
4510      * If the combiner has a void return, no result will be inserted,
4511      * and the first {@code N} parameter types of the target
4512      * must exactly match the parameters of the combiner.
4513      * <p>
4514      * The resulting adapter is the same type as the target, except that the
4515      * first parameter type is dropped,
4516      * if it corresponds to the result of the combiner.
4517      * <p>
4518      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
4519      * that either the combiner or the target does not wish to receive.
4520      * If some of the incoming arguments are destined only for the combiner,
4521      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
4522      * arguments will not need to be live on the stack on entry to the
4523      * target.)
4524      * <p><b>Example:</b>
4525      * <blockquote><pre>{@code
4526 import static java.lang.invoke.MethodHandles.*;
4527 import static java.lang.invoke.MethodType.*;
4528 ...
4529 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4530   "println", methodType(void.class, String.class))
4531     .bindTo(System.out);
4532 MethodHandle cat = lookup().findVirtual(String.class,
4533   "concat", methodType(String.class, String.class));
4534 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4535 MethodHandle catTrace = foldArguments(cat, trace);
4536 // also prints "boo":
4537 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4538      * }</pre></blockquote>
4539      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4540      * represents the result type of the {@code target} and resulting adapter.
4541      * {@code V}/{@code v} represent the type and value of the parameter and argument
4542      * of {@code target} that precedes the folding position; {@code V} also is
4543      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4544      * types and values of the {@code N} parameters and arguments at the folding
4545      * position. {@code B}/{@code b} represent the types and values of the
4546      * {@code target} parameters and arguments that follow the folded parameters
4547      * and arguments.
4548      * <blockquote><pre>{@code
4549      * // there are N arguments in A...
4550      * T target(V, A[N]..., B...);
4551      * V combiner(A...);
4552      * T adapter(A... a, B... b) {
4553      *   V v = combiner(a...);
4554      *   return target(v, a..., b...);
4555      * }
4556      * // and if the combiner has a void return:
4557      * T target2(A[N]..., B...);
4558      * void combiner2(A...);
4559      * T adapter2(A... a, B... b) {
4560      *   combiner2(a...);
4561      *   return target2(a..., b...);
4562      * }
4563      * }</pre></blockquote>
4564      * <p>
4565      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4566      * variable-arity method handle}, even if the original target method handle was.
4567      * @param target the method handle to invoke after arguments are combined
4568      * @param combiner method handle to call initially on the incoming arguments
4569      * @return method handle which incorporates the specified argument folding logic
4570      * @throws NullPointerException if either argument is null
4571      * @throws IllegalArgumentException if {@code combiner}'s return type
4572      *          is non-void and not the same as the first argument type of
4573      *          the target, or if the initial {@code N} argument types
4574      *          of the target
4575      *          (skipping one matching the {@code combiner}'s return type)
4576      *          are not identical with the argument types of {@code combiner}
4577      */
4578     public static
4579     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
4580         return foldArguments(target, 0, combiner);
4581     }
4582 
4583     /**
4584      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
4585      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
4586      * before the folded arguments.
4587      * <p>
4588      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
4589      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
4590      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
4591      * 0.
4592      *
4593      * @apiNote Example:
4594      * <blockquote><pre>{@code
4595     import static java.lang.invoke.MethodHandles.*;
4596     import static java.lang.invoke.MethodType.*;
4597     ...
4598     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4599     "println", methodType(void.class, String.class))
4600     .bindTo(System.out);
4601     MethodHandle cat = lookup().findVirtual(String.class,
4602     "concat", methodType(String.class, String.class));
4603     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4604     MethodHandle catTrace = foldArguments(cat, 1, trace);
4605     // also prints "jum":
4606     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4607      * }</pre></blockquote>
4608      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4609      * represents the result type of the {@code target} and resulting adapter.
4610      * {@code V}/{@code v} represent the type and value of the parameter and argument
4611      * of {@code target} that precedes the folding position; {@code V} also is
4612      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4613      * types and values of the {@code N} parameters and arguments at the folding
4614      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
4615      * and values of the {@code target} parameters and arguments that precede and
4616      * follow the folded parameters and arguments starting at {@code pos},
4617      * respectively.
4618      * <blockquote><pre>{@code
4619      * // there are N arguments in A...
4620      * T target(Z..., V, A[N]..., B...);
4621      * V combiner(A...);
4622      * T adapter(Z... z, A... a, B... b) {
4623      *   V v = combiner(a...);
4624      *   return target(z..., v, a..., b...);
4625      * }
4626      * // and if the combiner has a void return:
4627      * T target2(Z..., A[N]..., B...);
4628      * void combiner2(A...);
4629      * T adapter2(Z... z, A... a, B... b) {
4630      *   combiner2(a...);
4631      *   return target2(z..., a..., b...);
4632      * }
4633      * }</pre></blockquote>
4634      * <p>
4635      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4636      * variable-arity method handle}, even if the original target method handle was.
4637      *
4638      * @param target the method handle to invoke after arguments are combined
4639      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
4640      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4641      * @param combiner method handle to call initially on the incoming arguments
4642      * @return method handle which incorporates the specified argument folding logic
4643      * @throws NullPointerException if either argument is null
4644      * @throws IllegalArgumentException if either of the following two conditions holds:
4645      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
4646      *              {@code pos} of the target signature;
4647      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
4648      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
4649      *
4650      * @see #foldArguments(MethodHandle, MethodHandle)
4651      * @since 9
4652      */
4653     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
4654         MethodType targetType = target.type();
4655         MethodType combinerType = combiner.type();
4656         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
4657         BoundMethodHandle result = target.rebind();
4658         boolean dropResult = rtype == void.class;
4659         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
4660         MethodType newType = targetType;
4661         if (!dropResult) {
4662             newType = newType.dropParameterTypes(pos, pos + 1);
4663         }
4664         result = result.copyWithExtendL(newType, lform, combiner);
4665         return result;
4666     }
4667 
4668     /**
4669      * As {@see foldArguments(MethodHandle, int, MethodHandle)}, but with the
4670      * added capability of selecting the arguments from the targets parameters
4671      * to call the combiner with. This allows us to avoid some simple cases of
4672      * permutations and padding the combiner with dropArguments to select the
4673      * right argument, which may ultimately produce fewer intermediaries.
4674      */
4675     static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner, int ... argPositions) {
4676         MethodType targetType = target.type();
4677         MethodType combinerType = combiner.type();
4678         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType, argPositions);
4679         BoundMethodHandle result = target.rebind();
4680         boolean dropResult = rtype == void.class;
4681         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType(), argPositions);
4682         MethodType newType = targetType;
4683         if (!dropResult) {
4684             newType = newType.dropParameterTypes(pos, pos + 1);
4685         }
4686         result = result.copyWithExtendL(newType, lform, combiner);
4687         return result;
4688     }
4689 
4690     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
4691         int foldArgs   = combinerType.parameterCount();
4692         Class<?> rtype = combinerType.returnType();
4693         int foldVals = rtype == void.class ? 0 : 1;
4694         int afterInsertPos = foldPos + foldVals;
4695         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
4696         if (ok) {
4697             for (int i = 0; i < foldArgs; i++) {
4698                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
4699                     ok = false;
4700                     break;
4701                 }
4702             }
4703         }
4704         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
4705             ok = false;
4706         if (!ok)
4707             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4708         return rtype;
4709     }
4710 
4711     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType, int ... argPos) {
4712         int foldArgs = combinerType.parameterCount();
4713         if (argPos.length != foldArgs) {
4714             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
4715         }
4716         Class<?> rtype = combinerType.returnType();
4717         int foldVals = rtype == void.class ? 0 : 1;
4718         boolean ok = true;
4719         for (int i = 0; i < foldArgs; i++) {
4720             int arg = argPos[i];
4721             if (arg < 0 || arg > targetType.parameterCount()) {
4722                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
4723             }
4724             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
4725                 throw newIllegalArgumentException("target argument type at position " + arg
4726                         + " must match combiner argument type at index " + i + ": " + targetType
4727                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
4728             }
4729         }
4730         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos)) {
4731             ok = false;
4732         }
4733         if (!ok)
4734             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4735         return rtype;
4736     }
4737 
4738     /**
4739      * Makes a method handle which adapts a target method handle,
4740      * by guarding it with a test, a boolean-valued method handle.
4741      * If the guard fails, a fallback handle is called instead.
4742      * All three method handles must have the same corresponding
4743      * argument and return types, except that the return type
4744      * of the test must be boolean, and the test is allowed
4745      * to have fewer arguments than the other two method handles.
4746      * <p>
4747      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4748      * represents the uniform result type of the three involved handles;
4749      * {@code A}/{@code a}, the types and values of the {@code target}
4750      * parameters and arguments that are consumed by the {@code test}; and
4751      * {@code B}/{@code b}, those types and values of the {@code target}
4752      * parameters and arguments that are not consumed by the {@code test}.
4753      * <blockquote><pre>{@code
4754      * boolean test(A...);
4755      * T target(A...,B...);
4756      * T fallback(A...,B...);
4757      * T adapter(A... a,B... b) {
4758      *   if (test(a...))
4759      *     return target(a..., b...);
4760      *   else
4761      *     return fallback(a..., b...);
4762      * }
4763      * }</pre></blockquote>
4764      * Note that the test arguments ({@code a...} in the pseudocode) cannot
4765      * be modified by execution of the test, and so are passed unchanged
4766      * from the caller to the target or fallback as appropriate.
4767      * @param test method handle used for test, must return boolean
4768      * @param target method handle to call if test passes
4769      * @param fallback method handle to call if test fails
4770      * @return method handle which incorporates the specified if/then/else logic
4771      * @throws NullPointerException if any argument is null
4772      * @throws IllegalArgumentException if {@code test} does not return boolean,
4773      *          or if all three method types do not match (with the return
4774      *          type of {@code test} changed to match that of the target).
4775      */
4776     public static
4777     MethodHandle guardWithTest(MethodHandle test,
4778                                MethodHandle target,
4779                                MethodHandle fallback) {
4780         MethodType gtype = test.type();
4781         MethodType ttype = target.type();
4782         MethodType ftype = fallback.type();
4783         if (!ttype.equals(ftype))
4784             throw misMatchedTypes("target and fallback types", ttype, ftype);
4785         if (gtype.returnType() != boolean.class)
4786             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
4787         List<Class<?>> targs = ttype.parameterList();
4788         test = dropArgumentsToMatch(test, 0, targs, 0, true);
4789         if (test == null) {
4790             throw misMatchedTypes("target and test types", ttype, gtype);
4791         }
4792         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
4793     }
4794 
4795     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
4796         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
4797     }
4798 
4799     /**
4800      * Makes a method handle which adapts a target method handle,
4801      * by running it inside an exception handler.
4802      * If the target returns normally, the adapter returns that value.
4803      * If an exception matching the specified type is thrown, the fallback
4804      * handle is called instead on the exception, plus the original arguments.
4805      * <p>
4806      * The target and handler must have the same corresponding
4807      * argument and return types, except that handler may omit trailing arguments
4808      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
4809      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
4810      * <p>
4811      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4812      * represents the return type of the {@code target} and {@code handler},
4813      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
4814      * the types and values of arguments to the resulting handle consumed by
4815      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
4816      * resulting handle discarded by {@code handler}.
4817      * <blockquote><pre>{@code
4818      * T target(A..., B...);
4819      * T handler(ExType, A...);
4820      * T adapter(A... a, B... b) {
4821      *   try {
4822      *     return target(a..., b...);
4823      *   } catch (ExType ex) {
4824      *     return handler(ex, a...);
4825      *   }
4826      * }
4827      * }</pre></blockquote>
4828      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
4829      * be modified by execution of the target, and so are passed unchanged
4830      * from the caller to the handler, if the handler is invoked.
4831      * <p>
4832      * The target and handler must return the same type, even if the handler
4833      * always throws.  (This might happen, for instance, because the handler
4834      * is simulating a {@code finally} clause).
4835      * To create such a throwing handler, compose the handler creation logic
4836      * with {@link #throwException throwException},
4837      * in order to create a method handle of the correct return type.
4838      * @param target method handle to call
4839      * @param exType the type of exception which the handler will catch
4840      * @param handler method handle to call if a matching exception is thrown
4841      * @return method handle which incorporates the specified try/catch logic
4842      * @throws NullPointerException if any argument is null
4843      * @throws IllegalArgumentException if {@code handler} does not accept
4844      *          the given exception type, or if the method handle types do
4845      *          not match in their return types and their
4846      *          corresponding parameters
4847      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
4848      */
4849     public static
4850     MethodHandle catchException(MethodHandle target,
4851                                 Class<? extends Throwable> exType,
4852                                 MethodHandle handler) {
4853         MethodType ttype = target.type();
4854         MethodType htype = handler.type();
4855         if (!Throwable.class.isAssignableFrom(exType))
4856             throw new ClassCastException(exType.getName());
4857         if (htype.parameterCount() < 1 ||
4858             !htype.parameterType(0).isAssignableFrom(exType))
4859             throw newIllegalArgumentException("handler does not accept exception type "+exType);
4860         if (htype.returnType() != ttype.returnType())
4861             throw misMatchedTypes("target and handler return types", ttype, htype);
4862         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
4863         if (handler == null) {
4864             throw misMatchedTypes("target and handler types", ttype, htype);
4865         }
4866         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
4867     }
4868 
4869     /**
4870      * Produces a method handle which will throw exceptions of the given {@code exType}.
4871      * The method handle will accept a single argument of {@code exType},
4872      * and immediately throw it as an exception.
4873      * The method type will nominally specify a return of {@code returnType}.
4874      * The return type may be anything convenient:  It doesn't matter to the
4875      * method handle's behavior, since it will never return normally.
4876      * @param returnType the return type of the desired method handle
4877      * @param exType the parameter type of the desired method handle
4878      * @return method handle which can throw the given exceptions
4879      * @throws NullPointerException if either argument is null
4880      */
4881     public static
4882     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
4883         if (!Throwable.class.isAssignableFrom(exType))
4884             throw new ClassCastException(exType.getName());
4885         return MethodHandleImpl.throwException(methodType(returnType, exType));
4886     }
4887 
4888     /**
4889      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
4890      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
4891      * delivers the loop's result, which is the return value of the resulting handle.
4892      * <p>
4893      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
4894      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
4895      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
4896      * terms of method handles, each clause will specify up to four independent actions:<ul>
4897      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
4898      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
4899      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
4900      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
4901      * </ul>
4902      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
4903      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
4904      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
4905      * <p>
4906      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
4907      * this case. See below for a detailed description.
4908      * <p>
4909      * <em>Parameters optional everywhere:</em>
4910      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
4911      * As an exception, the init functions cannot take any {@code v} parameters,
4912      * because those values are not yet computed when the init functions are executed.
4913      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
4914      * In fact, any clause function may take no arguments at all.
4915      * <p>
4916      * <em>Loop parameters:</em>
4917      * A clause function may take all the iteration variable values it is entitled to, in which case
4918      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
4919      * with their types and values notated as {@code (A...)} and {@code (a...)}.
4920      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
4921      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
4922      * init function is automatically a loop parameter {@code a}.)
4923      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
4924      * These loop parameters act as loop-invariant values visible across the whole loop.
4925      * <p>
4926      * <em>Parameters visible everywhere:</em>
4927      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
4928      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
4929      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
4930      * Most clause functions will not need all of this information, but they will be formally connected to it
4931      * as if by {@link #dropArguments}.
4932      * <a id="astar"></a>
4933      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
4934      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
4935      * In that notation, the general form of an init function parameter list
4936      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
4937      * <p>
4938      * <em>Checking clause structure:</em>
4939      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
4940      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
4941      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
4942      * met by the inputs to the loop combinator.
4943      * <p>
4944      * <em>Effectively identical sequences:</em>
4945      * <a id="effid"></a>
4946      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
4947      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
4948      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
4949      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
4950      * that longest list.
4951      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
4952      * and the same is true if more sequences of the form {@code (V... A*)} are added.
4953      * <p>
4954      * <em>Step 0: Determine clause structure.</em><ol type="a">
4955      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
4956      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
4957      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
4958      * four. Padding takes place by appending elements to the array.
4959      * <li>Clauses with all {@code null}s are disregarded.
4960      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
4961      * </ol>
4962      * <p>
4963      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
4964      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
4965      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
4966      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
4967      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
4968      * iteration variable type.
4969      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
4970      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
4971      * </ol>
4972      * <p>
4973      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
4974      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
4975      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
4976      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
4977      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
4978      * (These types will checked in step 2, along with all the clause function types.)
4979      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
4980      * <li>All of the collected parameter lists must be effectively identical.
4981      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
4982      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
4983      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
4984      * the "internal parameter list".
4985      * </ul>
4986      * <p>
4987      * <em>Step 1C: Determine loop return type.</em><ol type="a">
4988      * <li>Examine fini function return types, disregarding omitted fini functions.
4989      * <li>If there are no fini functions, the loop return type is {@code void}.
4990      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
4991      * type.
4992      * </ol>
4993      * <p>
4994      * <em>Step 1D: Check other types.</em><ol type="a">
4995      * <li>There must be at least one non-omitted pred function.
4996      * <li>Every non-omitted pred function must have a {@code boolean} return type.
4997      * </ol>
4998      * <p>
4999      * <em>Step 2: Determine parameter lists.</em><ol type="a">
5000      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
5001      * <li>The parameter list for init functions will be adjusted to the external parameter list.
5002      * (Note that their parameter lists are already effectively identical to this list.)
5003      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
5004      * effectively identical to the internal parameter list {@code (V... A...)}.
5005      * </ol>
5006      * <p>
5007      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
5008      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
5009      * type.
5010      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
5011      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
5012      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
5013      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
5014      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
5015      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
5016      * loop return type.
5017      * </ol>
5018      * <p>
5019      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
5020      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
5021      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
5022      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
5023      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
5024      * pad out the end of the list.
5025      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
5026      * </ol>
5027      * <p>
5028      * <em>Final observations.</em><ol type="a">
5029      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
5030      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
5031      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
5032      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
5033      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
5034      * <li>Each pair of init and step functions agrees in their return type {@code V}.
5035      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
5036      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
5037      * </ol>
5038      * <p>
5039      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
5040      * <ul>
5041      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
5042      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
5043      * (Only one {@code Pn} has to be non-{@code null}.)
5044      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
5045      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
5046      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
5047      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
5048      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
5049      * the resulting loop handle's parameter types {@code (A...)}.
5050      * </ul>
5051      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
5052      * which is natural if most of the loop computation happens in the steps.  For some loops,
5053      * the burden of computation might be heaviest in the pred functions, and so the pred functions
5054      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
5055      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
5056      * where the init functions will need the extra parameters.  For such reasons, the rules for
5057      * determining these parameters are as symmetric as possible, across all clause parts.
5058      * In general, the loop parameters function as common invariant values across the whole
5059      * loop, while the iteration variables function as common variant values, or (if there is
5060      * no step function) as internal loop invariant temporaries.
5061      * <p>
5062      * <em>Loop execution.</em><ol type="a">
5063      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
5064      * every clause function. These locals are loop invariant.
5065      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
5066      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
5067      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
5068      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
5069      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
5070      * (in argument order).
5071      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
5072      * returns {@code false}.
5073      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
5074      * sequence {@code (v...)} of loop variables.
5075      * The updated value is immediately visible to all subsequent function calls.
5076      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
5077      * (of type {@code R}) is returned from the loop as a whole.
5078      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
5079      * except by throwing an exception.
5080      * </ol>
5081      * <p>
5082      * <em>Usage tips.</em>
5083      * <ul>
5084      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
5085      * sometimes a step function only needs to observe the current value of its own variable.
5086      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
5087      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
5088      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
5089      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
5090      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
5091      * <li>If some of the clause functions are virtual methods on an instance, the instance
5092      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
5093      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
5094      * will be the first iteration variable value, and it will be easy to use virtual
5095      * methods as clause parts, since all of them will take a leading instance reference matching that value.
5096      * </ul>
5097      * <p>
5098      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
5099      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
5100      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
5101      * <blockquote><pre>{@code
5102      * V... init...(A...);
5103      * boolean pred...(V..., A...);
5104      * V... step...(V..., A...);
5105      * R fini...(V..., A...);
5106      * R loop(A... a) {
5107      *   V... v... = init...(a...);
5108      *   for (;;) {
5109      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
5110      *       v = s(v..., a...);
5111      *       if (!p(v..., a...)) {
5112      *         return f(v..., a...);
5113      *       }
5114      *     }
5115      *   }
5116      * }
5117      * }</pre></blockquote>
5118      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
5119      * to their full length, even though individual clause functions may neglect to take them all.
5120      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
5121      *
5122      * @apiNote Example:
5123      * <blockquote><pre>{@code
5124      * // iterative implementation of the factorial function as a loop handle
5125      * static int one(int k) { return 1; }
5126      * static int inc(int i, int acc, int k) { return i + 1; }
5127      * static int mult(int i, int acc, int k) { return i * acc; }
5128      * static boolean pred(int i, int acc, int k) { return i < k; }
5129      * static int fin(int i, int acc, int k) { return acc; }
5130      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
5131      * // null initializer for counter, should initialize to 0
5132      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
5133      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
5134      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
5135      * assertEquals(120, loop.invoke(5));
5136      * }</pre></blockquote>
5137      * The same example, dropping arguments and using combinators:
5138      * <blockquote><pre>{@code
5139      * // simplified implementation of the factorial function as a loop handle
5140      * static int inc(int i) { return i + 1; } // drop acc, k
5141      * static int mult(int i, int acc) { return i * acc; } //drop k
5142      * static boolean cmp(int i, int k) { return i < k; }
5143      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
5144      * // null initializer for counter, should initialize to 0
5145      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
5146      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
5147      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
5148      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
5149      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
5150      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
5151      * assertEquals(720, loop.invoke(6));
5152      * }</pre></blockquote>
5153      * A similar example, using a helper object to hold a loop parameter:
5154      * <blockquote><pre>{@code
5155      * // instance-based implementation of the factorial function as a loop handle
5156      * static class FacLoop {
5157      *   final int k;
5158      *   FacLoop(int k) { this.k = k; }
5159      *   int inc(int i) { return i + 1; }
5160      *   int mult(int i, int acc) { return i * acc; }
5161      *   boolean pred(int i) { return i < k; }
5162      *   int fin(int i, int acc) { return acc; }
5163      * }
5164      * // assume MH_FacLoop is a handle to the constructor
5165      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
5166      * // null initializer for counter, should initialize to 0
5167      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
5168      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
5169      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
5170      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
5171      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
5172      * assertEquals(5040, loop.invoke(7));
5173      * }</pre></blockquote>
5174      *
5175      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
5176      *
5177      * @return a method handle embodying the looping behavior as defined by the arguments.
5178      *
5179      * @throws IllegalArgumentException in case any of the constraints described above is violated.
5180      *
5181      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
5182      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
5183      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
5184      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
5185      * @since 9
5186      */
5187     public static MethodHandle loop(MethodHandle[]... clauses) {
5188         // Step 0: determine clause structure.
5189         loopChecks0(clauses);
5190 
5191         List<MethodHandle> init = new ArrayList<>();
5192         List<MethodHandle> step = new ArrayList<>();
5193         List<MethodHandle> pred = new ArrayList<>();
5194         List<MethodHandle> fini = new ArrayList<>();
5195 
5196         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
5197             init.add(clause[0]); // all clauses have at least length 1
5198             step.add(clause.length <= 1 ? null : clause[1]);
5199             pred.add(clause.length <= 2 ? null : clause[2]);
5200             fini.add(clause.length <= 3 ? null : clause[3]);
5201         });
5202 
5203         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
5204         final int nclauses = init.size();
5205 
5206         // Step 1A: determine iteration variables (V...).
5207         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
5208         for (int i = 0; i < nclauses; ++i) {
5209             MethodHandle in = init.get(i);
5210             MethodHandle st = step.get(i);
5211             if (in == null && st == null) {
5212                 iterationVariableTypes.add(void.class);
5213             } else if (in != null && st != null) {
5214                 loopChecks1a(i, in, st);
5215                 iterationVariableTypes.add(in.type().returnType());
5216             } else {
5217                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
5218             }
5219         }
5220         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
5221                 collect(Collectors.toList());
5222 
5223         // Step 1B: determine loop parameters (A...).
5224         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
5225         loopChecks1b(init, commonSuffix);
5226 
5227         // Step 1C: determine loop return type.
5228         // Step 1D: check other types.
5229         final Class<?> loopReturnType = fini.stream().filter(Objects::nonNull).map(MethodHandle::type).
5230                 map(MethodType::returnType).findFirst().orElse(void.class);
5231         loopChecks1cd(pred, fini, loopReturnType);
5232 
5233         // Step 2: determine parameter lists.
5234         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
5235         commonParameterSequence.addAll(commonSuffix);
5236         loopChecks2(step, pred, fini, commonParameterSequence);
5237 
5238         // Step 3: fill in omitted functions.
5239         for (int i = 0; i < nclauses; ++i) {
5240             Class<?> t = iterationVariableTypes.get(i);
5241             if (init.get(i) == null) {
5242                 init.set(i, empty(methodType(t, commonSuffix)));
5243             }
5244             if (step.get(i) == null) {
5245                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
5246             }
5247             if (pred.get(i) == null) {
5248                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
5249             }
5250             if (fini.get(i) == null) {
5251                 fini.set(i, empty(methodType(t, commonParameterSequence)));
5252             }
5253         }
5254 
5255         // Step 4: fill in missing parameter types.
5256         // Also convert all handles to fixed-arity handles.
5257         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
5258         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
5259         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
5260         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
5261 
5262         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
5263                 allMatch(pl -> pl.equals(commonSuffix));
5264         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
5265                 allMatch(pl -> pl.equals(commonParameterSequence));
5266 
5267         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
5268     }
5269 
5270     private static void loopChecks0(MethodHandle[][] clauses) {
5271         if (clauses == null || clauses.length == 0) {
5272             throw newIllegalArgumentException("null or no clauses passed");
5273         }
5274         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
5275             throw newIllegalArgumentException("null clauses are not allowed");
5276         }
5277         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
5278             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
5279         }
5280     }
5281 
5282     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
5283         if (in.type().returnType() != st.type().returnType()) {
5284             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
5285                     st.type().returnType());
5286         }
5287     }
5288 
5289     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
5290         final List<Class<?>> empty = List.of();
5291         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
5292                 // take only those that can contribute to a common suffix because they are longer than the prefix
5293                         map(MethodHandle::type).
5294                         filter(t -> t.parameterCount() > skipSize).
5295                         map(MethodType::parameterList).
5296                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
5297         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
5298     }
5299 
5300     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
5301         final List<Class<?>> empty = List.of();
5302         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
5303     }
5304 
5305     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
5306         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
5307         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
5308         return longestParameterList(Arrays.asList(longest1, longest2));
5309     }
5310 
5311     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
5312         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
5313                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
5314             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
5315                     " (common suffix: " + commonSuffix + ")");
5316         }
5317     }
5318 
5319     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
5320         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
5321                 anyMatch(t -> t != loopReturnType)) {
5322             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
5323                     loopReturnType + ")");
5324         }
5325 
5326         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
5327             throw newIllegalArgumentException("no predicate found", pred);
5328         }
5329         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
5330                 anyMatch(t -> t != boolean.class)) {
5331             throw newIllegalArgumentException("predicates must have boolean return type", pred);
5332         }
5333     }
5334 
5335     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
5336         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
5337                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
5338             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
5339                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
5340         }
5341     }
5342 
5343     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
5344         return hs.stream().map(h -> {
5345             int pc = h.type().parameterCount();
5346             int tpsize = targetParams.size();
5347             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
5348         }).collect(Collectors.toList());
5349     }
5350 
5351     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
5352         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
5353     }
5354 
5355     /**
5356      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
5357      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5358      * <p>
5359      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5360      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
5361      * evaluates to {@code true}).
5362      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
5363      * <p>
5364      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5365      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5366      * and updated with the value returned from its invocation. The result of loop execution will be
5367      * the final value of the additional loop-local variable (if present).
5368      * <p>
5369      * The following rules hold for these argument handles:<ul>
5370      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5371      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5372      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5373      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5374      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5375      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5376      * It will constrain the parameter lists of the other loop parts.
5377      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5378      * list {@code (A...)} is called the <em>external parameter list</em>.
5379      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5380      * additional state variable of the loop.
5381      * The body must both accept and return a value of this type {@code V}.
5382      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5383      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5384      * <a href="MethodHandles.html#effid">effectively identical</a>
5385      * to the external parameter list {@code (A...)}.
5386      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5387      * {@linkplain #empty default value}.
5388      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5389      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5390      * effectively identical to the internal parameter list.
5391      * </ul>
5392      * <p>
5393      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5394      * <li>The loop handle's result type is the result type {@code V} of the body.
5395      * <li>The loop handle's parameter types are the types {@code (A...)},
5396      * from the external parameter list.
5397      * </ul>
5398      * <p>
5399      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5400      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5401      * passed to the loop.
5402      * <blockquote><pre>{@code
5403      * V init(A...);
5404      * boolean pred(V, A...);
5405      * V body(V, A...);
5406      * V whileLoop(A... a...) {
5407      *   V v = init(a...);
5408      *   while (pred(v, a...)) {
5409      *     v = body(v, a...);
5410      *   }
5411      *   return v;
5412      * }
5413      * }</pre></blockquote>
5414      *
5415      * @apiNote Example:
5416      * <blockquote><pre>{@code
5417      * // implement the zip function for lists as a loop handle
5418      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
5419      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
5420      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
5421      *   zip.add(a.next());
5422      *   zip.add(b.next());
5423      *   return zip;
5424      * }
5425      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
5426      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
5427      * List<String> a = Arrays.asList("a", "b", "c", "d");
5428      * List<String> b = Arrays.asList("e", "f", "g", "h");
5429      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
5430      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
5431      * }</pre></blockquote>
5432      *
5433      *
5434      * @apiNote The implementation of this method can be expressed as follows:
5435      * <blockquote><pre>{@code
5436      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5437      *     MethodHandle fini = (body.type().returnType() == void.class
5438      *                         ? null : identity(body.type().returnType()));
5439      *     MethodHandle[]
5440      *         checkExit = { null, null, pred, fini },
5441      *         varBody   = { init, body };
5442      *     return loop(checkExit, varBody);
5443      * }
5444      * }</pre></blockquote>
5445      *
5446      * @param init optional initializer, providing the initial value of the loop variable.
5447      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5448      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5449      *             above for other constraints.
5450      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5451      *             See above for other constraints.
5452      *
5453      * @return a method handle implementing the {@code while} loop as described by the arguments.
5454      * @throws IllegalArgumentException if the rules for the arguments are violated.
5455      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5456      *
5457      * @see #loop(MethodHandle[][])
5458      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
5459      * @since 9
5460      */
5461     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5462         whileLoopChecks(init, pred, body);
5463         MethodHandle fini = identityOrVoid(body.type().returnType());
5464         MethodHandle[] checkExit = { null, null, pred, fini };
5465         MethodHandle[] varBody = { init, body };
5466         return loop(checkExit, varBody);
5467     }
5468 
5469     /**
5470      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
5471      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5472      * <p>
5473      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5474      * method will, in each iteration, first execute its body and then evaluate the predicate.
5475      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
5476      * <p>
5477      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5478      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5479      * and updated with the value returned from its invocation. The result of loop execution will be
5480      * the final value of the additional loop-local variable (if present).
5481      * <p>
5482      * The following rules hold for these argument handles:<ul>
5483      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5484      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5485      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5486      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5487      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5488      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5489      * It will constrain the parameter lists of the other loop parts.
5490      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5491      * list {@code (A...)} is called the <em>external parameter list</em>.
5492      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5493      * additional state variable of the loop.
5494      * The body must both accept and return a value of this type {@code V}.
5495      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5496      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5497      * <a href="MethodHandles.html#effid">effectively identical</a>
5498      * to the external parameter list {@code (A...)}.
5499      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5500      * {@linkplain #empty default value}.
5501      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5502      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5503      * effectively identical to the internal parameter list.
5504      * </ul>
5505      * <p>
5506      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5507      * <li>The loop handle's result type is the result type {@code V} of the body.
5508      * <li>The loop handle's parameter types are the types {@code (A...)},
5509      * from the external parameter list.
5510      * </ul>
5511      * <p>
5512      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5513      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5514      * passed to the loop.
5515      * <blockquote><pre>{@code
5516      * V init(A...);
5517      * boolean pred(V, A...);
5518      * V body(V, A...);
5519      * V doWhileLoop(A... a...) {
5520      *   V v = init(a...);
5521      *   do {
5522      *     v = body(v, a...);
5523      *   } while (pred(v, a...));
5524      *   return v;
5525      * }
5526      * }</pre></blockquote>
5527      *
5528      * @apiNote Example:
5529      * <blockquote><pre>{@code
5530      * // int i = 0; while (i < limit) { ++i; } return i; => limit
5531      * static int zero(int limit) { return 0; }
5532      * static int step(int i, int limit) { return i + 1; }
5533      * static boolean pred(int i, int limit) { return i < limit; }
5534      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
5535      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
5536      * assertEquals(23, loop.invoke(23));
5537      * }</pre></blockquote>
5538      *
5539      *
5540      * @apiNote The implementation of this method can be expressed as follows:
5541      * <blockquote><pre>{@code
5542      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5543      *     MethodHandle fini = (body.type().returnType() == void.class
5544      *                         ? null : identity(body.type().returnType()));
5545      *     MethodHandle[] clause = { init, body, pred, fini };
5546      *     return loop(clause);
5547      * }
5548      * }</pre></blockquote>
5549      *
5550      * @param init optional initializer, providing the initial value of the loop variable.
5551      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5552      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5553      *             See above for other constraints.
5554      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5555      *             above for other constraints.
5556      *
5557      * @return a method handle implementing the {@code while} loop as described by the arguments.
5558      * @throws IllegalArgumentException if the rules for the arguments are violated.
5559      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5560      *
5561      * @see #loop(MethodHandle[][])
5562      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
5563      * @since 9
5564      */
5565     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5566         whileLoopChecks(init, pred, body);
5567         MethodHandle fini = identityOrVoid(body.type().returnType());
5568         MethodHandle[] clause = {init, body, pred, fini };
5569         return loop(clause);
5570     }
5571 
5572     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
5573         Objects.requireNonNull(pred);
5574         Objects.requireNonNull(body);
5575         MethodType bodyType = body.type();
5576         Class<?> returnType = bodyType.returnType();
5577         List<Class<?>> innerList = bodyType.parameterList();
5578         List<Class<?>> outerList = innerList;
5579         if (returnType == void.class) {
5580             // OK
5581         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
5582             // leading V argument missing => error
5583             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5584             throw misMatchedTypes("body function", bodyType, expected);
5585         } else {
5586             outerList = innerList.subList(1, innerList.size());
5587         }
5588         MethodType predType = pred.type();
5589         if (predType.returnType() != boolean.class ||
5590                 !predType.effectivelyIdenticalParameters(0, innerList)) {
5591             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
5592         }
5593         if (init != null) {
5594             MethodType initType = init.type();
5595             if (initType.returnType() != returnType ||
5596                     !initType.effectivelyIdenticalParameters(0, outerList)) {
5597                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5598             }
5599         }
5600     }
5601 
5602     /**
5603      * Constructs a loop that runs a given number of iterations.
5604      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5605      * <p>
5606      * The number of iterations is determined by the {@code iterations} handle evaluation result.
5607      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
5608      * It will be initialized to 0 and incremented by 1 in each iteration.
5609      * <p>
5610      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5611      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5612      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5613      * <p>
5614      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5615      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5616      * iteration variable.
5617      * The result of the loop handle execution will be the final {@code V} value of that variable
5618      * (or {@code void} if there is no {@code V} variable).
5619      * <p>
5620      * The following rules hold for the argument handles:<ul>
5621      * <li>The {@code iterations} handle must not be {@code null}, and must return
5622      * the type {@code int}, referred to here as {@code I} in parameter type lists.
5623      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5624      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5625      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5626      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5627      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5628      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5629      * of types called the <em>internal parameter list</em>.
5630      * It will constrain the parameter lists of the other loop parts.
5631      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5632      * with no additional {@code A} types, then the internal parameter list is extended by
5633      * the argument types {@code A...} of the {@code iterations} handle.
5634      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5635      * list {@code (A...)} is called the <em>external parameter list</em>.
5636      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5637      * additional state variable of the loop.
5638      * The body must both accept a leading parameter and return a value of this type {@code V}.
5639      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5640      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5641      * <a href="MethodHandles.html#effid">effectively identical</a>
5642      * to the external parameter list {@code (A...)}.
5643      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5644      * {@linkplain #empty default value}.
5645      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
5646      * effectively identical to the external parameter list {@code (A...)}.
5647      * </ul>
5648      * <p>
5649      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5650      * <li>The loop handle's result type is the result type {@code V} of the body.
5651      * <li>The loop handle's parameter types are the types {@code (A...)},
5652      * from the external parameter list.
5653      * </ul>
5654      * <p>
5655      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5656      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5657      * arguments passed to the loop.
5658      * <blockquote><pre>{@code
5659      * int iterations(A...);
5660      * V init(A...);
5661      * V body(V, int, A...);
5662      * V countedLoop(A... a...) {
5663      *   int end = iterations(a...);
5664      *   V v = init(a...);
5665      *   for (int i = 0; i < end; ++i) {
5666      *     v = body(v, i, a...);
5667      *   }
5668      *   return v;
5669      * }
5670      * }</pre></blockquote>
5671      *
5672      * @apiNote Example with a fully conformant body method:
5673      * <blockquote><pre>{@code
5674      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5675      * // => a variation on a well known theme
5676      * static String step(String v, int counter, String init) { return "na " + v; }
5677      * // assume MH_step is a handle to the method above
5678      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
5679      * MethodHandle start = MethodHandles.identity(String.class);
5680      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
5681      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
5682      * }</pre></blockquote>
5683      *
5684      * @apiNote Example with the simplest possible body method type,
5685      * and passing the number of iterations to the loop invocation:
5686      * <blockquote><pre>{@code
5687      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5688      * // => a variation on a well known theme
5689      * static String step(String v, int counter ) { return "na " + v; }
5690      * // assume MH_step is a handle to the method above
5691      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
5692      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
5693      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
5694      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
5695      * }</pre></blockquote>
5696      *
5697      * @apiNote Example that treats the number of iterations, string to append to, and string to append
5698      * as loop parameters:
5699      * <blockquote><pre>{@code
5700      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5701      * // => a variation on a well known theme
5702      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
5703      * // assume MH_step is a handle to the method above
5704      * MethodHandle count = MethodHandles.identity(int.class);
5705      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
5706      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
5707      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
5708      * }</pre></blockquote>
5709      *
5710      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
5711      * to enforce a loop type:
5712      * <blockquote><pre>{@code
5713      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5714      * // => a variation on a well known theme
5715      * static String step(String v, int counter, String pre) { return pre + " " + v; }
5716      * // assume MH_step is a handle to the method above
5717      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
5718      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
5719      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
5720      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
5721      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
5722      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
5723      * }</pre></blockquote>
5724      *
5725      * @apiNote The implementation of this method can be expressed as follows:
5726      * <blockquote><pre>{@code
5727      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5728      *     return countedLoop(empty(iterations.type()), iterations, init, body);
5729      * }
5730      * }</pre></blockquote>
5731      *
5732      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
5733      *                   result type must be {@code int}. See above for other constraints.
5734      * @param init optional initializer, providing the initial value of the loop variable.
5735      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5736      * @param body body of the loop, which may not be {@code null}.
5737      *             It controls the loop parameters and result type in the standard case (see above for details).
5738      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5739      *             and may accept any number of additional types.
5740      *             See above for other constraints.
5741      *
5742      * @return a method handle representing the loop.
5743      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
5744      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5745      *
5746      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
5747      * @since 9
5748      */
5749     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5750         return countedLoop(empty(iterations.type()), iterations, init, body);
5751     }
5752 
5753     /**
5754      * Constructs a loop that counts over a range of numbers.
5755      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5756      * <p>
5757      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
5758      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
5759      * values of the loop counter.
5760      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
5761      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
5762      * <p>
5763      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5764      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5765      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5766      * <p>
5767      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5768      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5769      * iteration variable.
5770      * The result of the loop handle execution will be the final {@code V} value of that variable
5771      * (or {@code void} if there is no {@code V} variable).
5772      * <p>
5773      * The following rules hold for the argument handles:<ul>
5774      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
5775      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
5776      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5777      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5778      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5779      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5780      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5781      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5782      * of types called the <em>internal parameter list</em>.
5783      * It will constrain the parameter lists of the other loop parts.
5784      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5785      * with no additional {@code A} types, then the internal parameter list is extended by
5786      * the argument types {@code A...} of the {@code end} handle.
5787      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5788      * list {@code (A...)} is called the <em>external parameter list</em>.
5789      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5790      * additional state variable of the loop.
5791      * The body must both accept a leading parameter and return a value of this type {@code V}.
5792      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5793      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5794      * <a href="MethodHandles.html#effid">effectively identical</a>
5795      * to the external parameter list {@code (A...)}.
5796      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5797      * {@linkplain #empty default value}.
5798      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
5799      * effectively identical to the external parameter list {@code (A...)}.
5800      * <li>Likewise, the parameter list of {@code end} must be effectively identical
5801      * to the external parameter list.
5802      * </ul>
5803      * <p>
5804      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5805      * <li>The loop handle's result type is the result type {@code V} of the body.
5806      * <li>The loop handle's parameter types are the types {@code (A...)},
5807      * from the external parameter list.
5808      * </ul>
5809      * <p>
5810      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5811      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5812      * arguments passed to the loop.
5813      * <blockquote><pre>{@code
5814      * int start(A...);
5815      * int end(A...);
5816      * V init(A...);
5817      * V body(V, int, A...);
5818      * V countedLoop(A... a...) {
5819      *   int e = end(a...);
5820      *   int s = start(a...);
5821      *   V v = init(a...);
5822      *   for (int i = s; i < e; ++i) {
5823      *     v = body(v, i, a...);
5824      *   }
5825      *   return v;
5826      * }
5827      * }</pre></blockquote>
5828      *
5829      * @apiNote The implementation of this method can be expressed as follows:
5830      * <blockquote><pre>{@code
5831      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5832      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
5833      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
5834      *     // the following semantics:
5835      *     // MH_increment: (int limit, int counter) -> counter + 1
5836      *     // MH_predicate: (int limit, int counter) -> counter < limit
5837      *     Class<?> counterType = start.type().returnType();  // int
5838      *     Class<?> returnType = body.type().returnType();
5839      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
5840      *     if (returnType != void.class) {  // ignore the V variable
5841      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5842      *         pred = dropArguments(pred, 1, returnType);  // ditto
5843      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
5844      *     }
5845      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
5846      *     MethodHandle[]
5847      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5848      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
5849      *         indexVar   = { start, incr };           // i = start(); i = i + 1
5850      *     return loop(loopLimit, bodyClause, indexVar);
5851      * }
5852      * }</pre></blockquote>
5853      *
5854      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
5855      *              See above for other constraints.
5856      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
5857      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
5858      * @param init optional initializer, providing the initial value of the loop variable.
5859      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5860      * @param body body of the loop, which may not be {@code null}.
5861      *             It controls the loop parameters and result type in the standard case (see above for details).
5862      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5863      *             and may accept any number of additional types.
5864      *             See above for other constraints.
5865      *
5866      * @return a method handle representing the loop.
5867      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
5868      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5869      *
5870      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
5871      * @since 9
5872      */
5873     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5874         countedLoopChecks(start, end, init, body);
5875         Class<?> counterType = start.type().returnType();  // int, but who's counting?
5876         Class<?> limitType   = end.type().returnType();    // yes, int again
5877         Class<?> returnType  = body.type().returnType();
5878         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
5879         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
5880         MethodHandle retv = null;
5881         if (returnType != void.class) {
5882             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5883             pred = dropArguments(pred, 1, returnType);  // ditto
5884             retv = dropArguments(identity(returnType), 0, counterType);
5885         }
5886         body = dropArguments(body, 0, counterType);  // ignore the limit variable
5887         MethodHandle[]
5888             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5889             bodyClause = { init, body },            // v = init(); v = body(v, i)
5890             indexVar   = { start, incr };           // i = start(); i = i + 1
5891         return loop(loopLimit, bodyClause, indexVar);
5892     }
5893 
5894     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5895         Objects.requireNonNull(start);
5896         Objects.requireNonNull(end);
5897         Objects.requireNonNull(body);
5898         Class<?> counterType = start.type().returnType();
5899         if (counterType != int.class) {
5900             MethodType expected = start.type().changeReturnType(int.class);
5901             throw misMatchedTypes("start function", start.type(), expected);
5902         } else if (end.type().returnType() != counterType) {
5903             MethodType expected = end.type().changeReturnType(counterType);
5904             throw misMatchedTypes("end function", end.type(), expected);
5905         }
5906         MethodType bodyType = body.type();
5907         Class<?> returnType = bodyType.returnType();
5908         List<Class<?>> innerList = bodyType.parameterList();
5909         // strip leading V value if present
5910         int vsize = (returnType == void.class ? 0 : 1);
5911         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
5912             // argument list has no "V" => error
5913             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5914             throw misMatchedTypes("body function", bodyType, expected);
5915         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
5916             // missing I type => error
5917             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
5918             throw misMatchedTypes("body function", bodyType, expected);
5919         }
5920         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
5921         if (outerList.isEmpty()) {
5922             // special case; take lists from end handle
5923             outerList = end.type().parameterList();
5924             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
5925         }
5926         MethodType expected = methodType(counterType, outerList);
5927         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
5928             throw misMatchedTypes("start parameter types", start.type(), expected);
5929         }
5930         if (end.type() != start.type() &&
5931             !end.type().effectivelyIdenticalParameters(0, outerList)) {
5932             throw misMatchedTypes("end parameter types", end.type(), expected);
5933         }
5934         if (init != null) {
5935             MethodType initType = init.type();
5936             if (initType.returnType() != returnType ||
5937                 !initType.effectivelyIdenticalParameters(0, outerList)) {
5938                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5939             }
5940         }
5941     }
5942 
5943     /**
5944      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
5945      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5946      * <p>
5947      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
5948      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
5949      * <p>
5950      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5951      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5952      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5953      * <p>
5954      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5955      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5956      * iteration variable.
5957      * The result of the loop handle execution will be the final {@code V} value of that variable
5958      * (or {@code void} if there is no {@code V} variable).
5959      * <p>
5960      * The following rules hold for the argument handles:<ul>
5961      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5962      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
5963      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5964      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
5965      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
5966      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
5967      * of types called the <em>internal parameter list</em>.
5968      * It will constrain the parameter lists of the other loop parts.
5969      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
5970      * with no additional {@code A} types, then the internal parameter list is extended by
5971      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
5972      * single type {@code Iterable} is added and constitutes the {@code A...} list.
5973      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
5974      * list {@code (A...)} is called the <em>external parameter list</em>.
5975      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5976      * additional state variable of the loop.
5977      * The body must both accept a leading parameter and return a value of this type {@code V}.
5978      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5979      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5980      * <a href="MethodHandles.html#effid">effectively identical</a>
5981      * to the external parameter list {@code (A...)}.
5982      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5983      * {@linkplain #empty default value}.
5984      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
5985      * type {@code java.util.Iterator} or a subtype thereof.
5986      * The iterator it produces when the loop is executed will be assumed
5987      * to yield values which can be converted to type {@code T}.
5988      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
5989      * effectively identical to the external parameter list {@code (A...)}.
5990      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
5991      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
5992      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
5993      * handle parameter is adjusted to accept the leading {@code A} type, as if by
5994      * the {@link MethodHandle#asType asType} conversion method.
5995      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
5996      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
5997      * </ul>
5998      * <p>
5999      * The type {@code T} may be either a primitive or reference.
6000      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
6001      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
6002      * as if by the {@link MethodHandle#asType asType} conversion method.
6003      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
6004      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
6005      * <p>
6006      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6007      * <li>The loop handle's result type is the result type {@code V} of the body.
6008      * <li>The loop handle's parameter types are the types {@code (A...)},
6009      * from the external parameter list.
6010      * </ul>
6011      * <p>
6012      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6013      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
6014      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
6015      * <blockquote><pre>{@code
6016      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
6017      * V init(A...);
6018      * V body(V,T,A...);
6019      * V iteratedLoop(A... a...) {
6020      *   Iterator<T> it = iterator(a...);
6021      *   V v = init(a...);
6022      *   while (it.hasNext()) {
6023      *     T t = it.next();
6024      *     v = body(v, t, a...);
6025      *   }
6026      *   return v;
6027      * }
6028      * }</pre></blockquote>
6029      *
6030      * @apiNote Example:
6031      * <blockquote><pre>{@code
6032      * // get an iterator from a list
6033      * static List<String> reverseStep(List<String> r, String e) {
6034      *   r.add(0, e);
6035      *   return r;
6036      * }
6037      * static List<String> newArrayList() { return new ArrayList<>(); }
6038      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
6039      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
6040      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
6041      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
6042      * assertEquals(reversedList, (List<String>) loop.invoke(list));
6043      * }</pre></blockquote>
6044      *
6045      * @apiNote The implementation of this method can be expressed approximately as follows:
6046      * <blockquote><pre>{@code
6047      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
6048      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
6049      *     Class<?> returnType = body.type().returnType();
6050      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
6051      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
6052      *     MethodHandle retv = null, step = body, startIter = iterator;
6053      *     if (returnType != void.class) {
6054      *         // the simple thing first:  in (I V A...), drop the I to get V
6055      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
6056      *         // body type signature (V T A...), internal loop types (I V A...)
6057      *         step = swapArguments(body, 0, 1);  // swap V <-> T
6058      *     }
6059      *     if (startIter == null)  startIter = MH_getIter;
6060      *     MethodHandle[]
6061      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
6062      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
6063      *     return loop(iterVar, bodyClause);
6064      * }
6065      * }</pre></blockquote>
6066      *
6067      * @param iterator an optional handle to return the iterator to start the loop.
6068      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
6069      *                 See above for other constraints.
6070      * @param init optional initializer, providing the initial value of the loop variable.
6071      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6072      * @param body body of the loop, which may not be {@code null}.
6073      *             It controls the loop parameters and result type in the standard case (see above for details).
6074      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
6075      *             and may accept any number of additional types.
6076      *             See above for other constraints.
6077      *
6078      * @return a method handle embodying the iteration loop functionality.
6079      * @throws NullPointerException if the {@code body} handle is {@code null}.
6080      * @throws IllegalArgumentException if any argument violates the above requirements.
6081      *
6082      * @since 9
6083      */
6084     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
6085         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
6086         Class<?> returnType = body.type().returnType();
6087         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
6088         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
6089         MethodHandle startIter;
6090         MethodHandle nextVal;
6091         {
6092             MethodType iteratorType;
6093             if (iterator == null) {
6094                 // derive argument type from body, if available, else use Iterable
6095                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
6096                 iteratorType = startIter.type().changeParameterType(0, iterableType);
6097             } else {
6098                 // force return type to the internal iterator class
6099                 iteratorType = iterator.type().changeReturnType(Iterator.class);
6100                 startIter = iterator;
6101             }
6102             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
6103             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
6104 
6105             // perform the asType transforms under an exception transformer, as per spec.:
6106             try {
6107                 startIter = startIter.asType(iteratorType);
6108                 nextVal = nextRaw.asType(nextValType);
6109             } catch (WrongMethodTypeException ex) {
6110                 throw new IllegalArgumentException(ex);
6111             }
6112         }
6113 
6114         MethodHandle retv = null, step = body;
6115         if (returnType != void.class) {
6116             // the simple thing first:  in (I V A...), drop the I to get V
6117             retv = dropArguments(identity(returnType), 0, Iterator.class);
6118             // body type signature (V T A...), internal loop types (I V A...)
6119             step = swapArguments(body, 0, 1);  // swap V <-> T
6120         }
6121 
6122         MethodHandle[]
6123             iterVar    = { startIter, null, hasNext, retv },
6124             bodyClause = { init, filterArgument(step, 0, nextVal) };
6125         return loop(iterVar, bodyClause);
6126     }
6127 
6128     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
6129         Objects.requireNonNull(body);
6130         MethodType bodyType = body.type();
6131         Class<?> returnType = bodyType.returnType();
6132         List<Class<?>> internalParamList = bodyType.parameterList();
6133         // strip leading V value if present
6134         int vsize = (returnType == void.class ? 0 : 1);
6135         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
6136             // argument list has no "V" => error
6137             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6138             throw misMatchedTypes("body function", bodyType, expected);
6139         } else if (internalParamList.size() <= vsize) {
6140             // missing T type => error
6141             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
6142             throw misMatchedTypes("body function", bodyType, expected);
6143         }
6144         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
6145         Class<?> iterableType = null;
6146         if (iterator != null) {
6147             // special case; if the body handle only declares V and T then
6148             // the external parameter list is obtained from iterator handle
6149             if (externalParamList.isEmpty()) {
6150                 externalParamList = iterator.type().parameterList();
6151             }
6152             MethodType itype = iterator.type();
6153             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
6154                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
6155             }
6156             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
6157                 MethodType expected = methodType(itype.returnType(), externalParamList);
6158                 throw misMatchedTypes("iterator parameters", itype, expected);
6159             }
6160         } else {
6161             if (externalParamList.isEmpty()) {
6162                 // special case; if the iterator handle is null and the body handle
6163                 // only declares V and T then the external parameter list consists
6164                 // of Iterable
6165                 externalParamList = Arrays.asList(Iterable.class);
6166                 iterableType = Iterable.class;
6167             } else {
6168                 // special case; if the iterator handle is null and the external
6169                 // parameter list is not empty then the first parameter must be
6170                 // assignable to Iterable
6171                 iterableType = externalParamList.get(0);
6172                 if (!Iterable.class.isAssignableFrom(iterableType)) {
6173                     throw newIllegalArgumentException(
6174                             "inferred first loop argument must inherit from Iterable: " + iterableType);
6175                 }
6176             }
6177         }
6178         if (init != null) {
6179             MethodType initType = init.type();
6180             if (initType.returnType() != returnType ||
6181                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
6182                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
6183             }
6184         }
6185         return iterableType;  // help the caller a bit
6186     }
6187 
6188     /*non-public*/ static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
6189         // there should be a better way to uncross my wires
6190         int arity = mh.type().parameterCount();
6191         int[] order = new int[arity];
6192         for (int k = 0; k < arity; k++)  order[k] = k;
6193         order[i] = j; order[j] = i;
6194         Class<?>[] types = mh.type().parameterArray();
6195         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
6196         MethodType swapType = methodType(mh.type().returnType(), types);
6197         return permuteArguments(mh, swapType, order);
6198     }
6199 
6200     /**
6201      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
6202      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
6203      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
6204      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
6205      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
6206      * {@code try-finally} handle.
6207      * <p>
6208      * The {@code cleanup} handle will be passed one or two additional leading arguments.
6209      * The first is the exception thrown during the
6210      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
6211      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
6212      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
6213      * The second argument is not present if the {@code target} handle has a {@code void} return type.
6214      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
6215      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
6216      * <p>
6217      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
6218      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
6219      * two extra leading parameters:<ul>
6220      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
6221      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
6222      * the result from the execution of the {@code target} handle.
6223      * This parameter is not present if the {@code target} returns {@code void}.
6224      * </ul>
6225      * <p>
6226      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
6227      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
6228      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
6229      * the cleanup.
6230      * <blockquote><pre>{@code
6231      * V target(A..., B...);
6232      * V cleanup(Throwable, V, A...);
6233      * V adapter(A... a, B... b) {
6234      *   V result = (zero value for V);
6235      *   Throwable throwable = null;
6236      *   try {
6237      *     result = target(a..., b...);
6238      *   } catch (Throwable t) {
6239      *     throwable = t;
6240      *     throw t;
6241      *   } finally {
6242      *     result = cleanup(throwable, result, a...);
6243      *   }
6244      *   return result;
6245      * }
6246      * }</pre></blockquote>
6247      * <p>
6248      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6249      * be modified by execution of the target, and so are passed unchanged
6250      * from the caller to the cleanup, if it is invoked.
6251      * <p>
6252      * The target and cleanup must return the same type, even if the cleanup
6253      * always throws.
6254      * To create such a throwing cleanup, compose the cleanup logic
6255      * with {@link #throwException throwException},
6256      * in order to create a method handle of the correct return type.
6257      * <p>
6258      * Note that {@code tryFinally} never converts exceptions into normal returns.
6259      * In rare cases where exceptions must be converted in that way, first wrap
6260      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
6261      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
6262      * <p>
6263      * It is recommended that the first parameter type of {@code cleanup} be
6264      * declared {@code Throwable} rather than a narrower subtype.  This ensures
6265      * {@code cleanup} will always be invoked with whatever exception that
6266      * {@code target} throws.  Declaring a narrower type may result in a
6267      * {@code ClassCastException} being thrown by the {@code try-finally}
6268      * handle if the type of the exception thrown by {@code target} is not
6269      * assignable to the first parameter type of {@code cleanup}.  Note that
6270      * various exception types of {@code VirtualMachineError},
6271      * {@code LinkageError}, and {@code RuntimeException} can in principle be
6272      * thrown by almost any kind of Java code, and a finally clause that
6273      * catches (say) only {@code IOException} would mask any of the others
6274      * behind a {@code ClassCastException}.
6275      *
6276      * @param target the handle whose execution is to be wrapped in a {@code try} block.
6277      * @param cleanup the handle that is invoked in the finally block.
6278      *
6279      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
6280      * @throws NullPointerException if any argument is null
6281      * @throws IllegalArgumentException if {@code cleanup} does not accept
6282      *          the required leading arguments, or if the method handle types do
6283      *          not match in their return types and their
6284      *          corresponding trailing parameters
6285      *
6286      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
6287      * @since 9
6288      */
6289     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
6290         List<Class<?>> targetParamTypes = target.type().parameterList();
6291         Class<?> rtype = target.type().returnType();
6292 
6293         tryFinallyChecks(target, cleanup);
6294 
6295         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
6296         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
6297         // target parameter list.
6298         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
6299 
6300         // Ensure that the intrinsic type checks the instance thrown by the
6301         // target against the first parameter of cleanup
6302         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
6303 
6304         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
6305         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
6306     }
6307 
6308     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
6309         Class<?> rtype = target.type().returnType();
6310         if (rtype != cleanup.type().returnType()) {
6311             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
6312         }
6313         MethodType cleanupType = cleanup.type();
6314         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
6315             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
6316         }
6317         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
6318             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
6319         }
6320         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
6321         // target parameter list.
6322         int cleanupArgIndex = rtype == void.class ? 1 : 2;
6323         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
6324             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
6325                     cleanup.type(), target.type());
6326         }
6327     }
6328 
6329 }
--- EOF ---