1 /*
   2  * Copyright (c) 2008, 2017, 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.SharedSecrets;
  29 import jdk.internal.module.IllegalAccessLogger;
  30 import jdk.internal.org.objectweb.asm.ClassReader;
  31 import jdk.internal.reflect.CallerSensitive;
  32 import jdk.internal.reflect.Reflection;
  33 import jdk.internal.vm.annotation.ForceInline;
  34 import sun.invoke.util.ValueConversions;
  35 import sun.invoke.util.VerifyAccess;
  36 import sun.invoke.util.Wrapper;
  37 import sun.reflect.misc.ReflectUtil;
  38 import sun.security.util.SecurityConstants;
  39 
  40 import java.lang.invoke.LambdaForm.BasicType;
  41 import java.lang.reflect.Constructor;
  42 import java.lang.reflect.Field;
  43 import java.lang.reflect.Member;
  44 import java.lang.reflect.Method;
  45 import java.lang.reflect.Modifier;
  46 import java.lang.reflect.ReflectPermission;
  47 import java.nio.ByteOrder;
  48 import java.security.AccessController;
  49 import java.security.PrivilegedAction;
  50 import java.security.ProtectionDomain;
  51 import java.util.ArrayList;
  52 import java.util.Arrays;
  53 import java.util.BitSet;
  54 import java.util.Iterator;
  55 import java.util.List;
  56 import java.util.Objects;
  57 import java.util.concurrent.ConcurrentHashMap;
  58 import java.util.stream.Collectors;
  59 import java.util.stream.Stream;
  60 
  61 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  62 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  63 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  64 import static java.lang.invoke.MethodType.methodType;
  65 
  66 /**
  67  * This class consists exclusively of static methods that operate on or return
  68  * method handles. They fall into several categories:
  69  * <ul>
  70  * <li>Lookup methods which help create method handles for methods and fields.
  71  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  72  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  73  * </ul>
  74  *
  75  * @author John Rose, JSR 292 EG
  76  * @since 1.7
  77  */
  78 public class MethodHandles {
  79 
  80     private MethodHandles() { }  // do not instantiate
  81 
  82     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  83 
  84     // See IMPL_LOOKUP below.
  85 
  86     //// Method handle creation from ordinary methods.
  87 
  88     /**
  89      * Returns a {@link Lookup lookup object} with
  90      * full capabilities to emulate all supported bytecode behaviors of the caller.
  91      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  92      * Factory methods on the lookup object can create
  93      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  94      * for any member that the caller has access to via bytecodes,
  95      * including protected and private fields and methods.
  96      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
  97      * Do not store it in place where untrusted code can access it.
  98      * <p>
  99      * This method is caller sensitive, which means that it may return different
 100      * values to different callers.
 101      * <p>
 102      * For any given caller class {@code C}, the lookup object returned by this call
 103      * has equivalent capabilities to any lookup object
 104      * supplied by the JVM to the bootstrap method of an
 105      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
 106      * executing in the same caller class {@code C}.
 107      * @return a lookup object for the caller of this method, with private access
 108      */
 109     @CallerSensitive
 110     @ForceInline // to ensure Reflection.getCallerClass optimization
 111     public static Lookup lookup() {
 112         return new Lookup(Reflection.getCallerClass());
 113     }
 114 
 115     /**
 116      * This reflected$lookup method is the alternate implementation of
 117      * the lookup method when being invoked by reflection.
 118      */
 119     @CallerSensitive
 120     private static Lookup reflected$lookup() {
 121         Class<?> caller = Reflection.getCallerClass();
 122         if (caller.getClassLoader() == null) {
 123             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 124         }
 125         return new Lookup(caller);
 126     }
 127 
 128     /**
 129      * Returns a {@link Lookup lookup object} which is trusted minimally.
 130      * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes.
 131      * It can only be used to create method handles to public members of
 132      * public classes in packages that are exported unconditionally.
 133      * <p>
 134      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 135      * of this lookup object will be {@link java.lang.Object}.
 136      *
 137      * @apiNote The use of Object is conventional, and because the lookup modes are
 138      * limited, there is no special access provided to the internals of Object, its package
 139      * or its module. Consequently, the lookup context of this lookup object will be the
 140      * bootstrap class loader, which means it cannot find user classes.
 141      *
 142      * <p style="font-size:smaller;">
 143      * <em>Discussion:</em>
 144      * The lookup class can be changed to any other class {@code C} using an expression of the form
 145      * {@link Lookup#in publicLookup().in(C.class)}.
 146      * but may change the lookup context by virtue of changing the class loader.
 147      * A public lookup object is always subject to
 148      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 149      * Also, it cannot access
 150      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 151      * @return a lookup object which is trusted minimally
 152      *
 153      * @revised 9
 154      * @spec JPMS
 155      */
 156     public static Lookup publicLookup() {
 157         return Lookup.PUBLIC_LOOKUP;
 158     }
 159 
 160     /**
 161      * Returns a {@link Lookup lookup object} with full capabilities to emulate all
 162      * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">
 163      * private access</a>, on a target class.
 164      * This method checks that a caller, specified as a {@code Lookup} object, is allowed to
 165      * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing
 166      * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing
 167      * the target class, then this check ensures that
 168      * <ul>
 169      *     <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li>
 170      *     <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing
 171      *     the target class to at least {@code m1}.</li>
 172      *     <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li>
 173      * </ul>
 174      * <p>
 175      * If there is a security manager, its {@code checkPermission} method is called to
 176      * check {@code ReflectPermission("suppressAccessChecks")}.
 177      * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object
 178      * was created by code in the caller module (or derived from a lookup object originally
 179      * created by the caller). A lookup object with the {@code MODULE} lookup mode can be
 180      * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE}
 181      * access to the caller.
 182      * @param targetClass the target class
 183      * @param lookup the caller lookup object
 184      * @return a lookup object for the target class, with private access
 185      * @throws IllegalArgumentException if {@code targetClass} is a primitve type or array class
 186      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 187      * @throws IllegalAccessException if the access check specified above fails
 188      * @throws SecurityException if denied by the security manager
 189      * @since 9
 190      * @spec JPMS
 191      * @see Lookup#dropLookupMode
 192      */
 193     public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException {
 194         SecurityManager sm = System.getSecurityManager();
 195         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 196         if (targetClass.isPrimitive())
 197             throw new IllegalArgumentException(targetClass + " is a primitive class");
 198         if (targetClass.isArray())
 199             throw new IllegalArgumentException(targetClass + " is an array class");
 200         Module targetModule = targetClass.getModule();
 201         Module callerModule = lookup.lookupClass().getModule();
 202         if (!callerModule.canRead(targetModule))
 203             throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 204         if (targetModule.isNamed()) {
 205             String pn = targetClass.getPackageName();
 206             assert pn.length() > 0 : "unnamed package cannot be in named module";
 207             if (!targetModule.isOpen(pn, callerModule))
 208                 throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 209         }
 210         if ((lookup.lookupModes() & Lookup.MODULE) == 0)
 211             throw new IllegalAccessException("lookup does not have MODULE lookup mode");
 212         if (!callerModule.isNamed() && targetModule.isNamed()) {
 213             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 214             if (logger != null) {
 215                 logger.logIfOpenedForIllegalAccess(lookup, targetClass);
 216             }
 217         }
 218         return new Lookup(targetClass);
 219     }
 220 
 221     /**
 222      * Performs an unchecked "crack" of a
 223      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 224      * The result is as if the user had obtained a lookup object capable enough
 225      * to crack the target method handle, called
 226      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 227      * on the target to obtain its symbolic reference, and then called
 228      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 229      * to resolve the symbolic reference to a member.
 230      * <p>
 231      * If there is a security manager, its {@code checkPermission} method
 232      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 233      * @param <T> the desired type of the result, either {@link Member} or a subtype
 234      * @param target a direct method handle to crack into symbolic reference components
 235      * @param expected a class object representing the desired result type {@code T}
 236      * @return a reference to the method, constructor, or field object
 237      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 238      * @exception NullPointerException if either argument is {@code null}
 239      * @exception IllegalArgumentException if the target is not a direct method handle
 240      * @exception ClassCastException if the member is not of the expected type
 241      * @since 1.8
 242      */
 243     public static <T extends Member> T
 244     reflectAs(Class<T> expected, MethodHandle target) {
 245         SecurityManager smgr = System.getSecurityManager();
 246         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 247         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 248         return lookup.revealDirect(target).reflectAs(expected, lookup);
 249     }
 250     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 251     private static final java.security.Permission ACCESS_PERMISSION =
 252         new ReflectPermission("suppressAccessChecks");
 253 
 254     /**
 255      * A <em>lookup object</em> is a factory for creating method handles,
 256      * when the creation requires access checking.
 257      * Method handles do not perform
 258      * access checks when they are called, but rather when they are created.
 259      * Therefore, method handle access
 260      * restrictions must be enforced when a method handle is created.
 261      * The caller class against which those restrictions are enforced
 262      * is known as the {@linkplain #lookupClass() lookup class}.
 263      * <p>
 264      * A lookup class which needs to create method handles will call
 265      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 266      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 267      * determined, and securely stored in the {@code Lookup} object.
 268      * The lookup class (or its delegates) may then use factory methods
 269      * on the {@code Lookup} object to create method handles for access-checked members.
 270      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 271      * even private ones.
 272      *
 273      * <h1><a id="lookups"></a>Lookup Factory Methods</h1>
 274      * The factory methods on a {@code Lookup} object correspond to all major
 275      * use cases for methods, constructors, and fields.
 276      * Each method handle created by a factory method is the functional
 277      * equivalent of a particular <em>bytecode behavior</em>.
 278      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 279      * Here is a summary of the correspondence between these factory methods and
 280      * the behavior of the resulting method handles:
 281      * <table class="striped">
 282      * <caption style="display:none">lookup method behaviors</caption>
 283      * <thead>
 284      * <tr>
 285      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 286      *     <th scope="col">member</th>
 287      *     <th scope="col">bytecode behavior</th>
 288      * </tr>
 289      * </thead>
 290      * <tbody>
 291      * <tr>
 292      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 293      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 294      * </tr>
 295      * <tr>
 296      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 297      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 298      * </tr>
 299      * <tr>
 300      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 301      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 302      * </tr>
 303      * <tr>
 304      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 305      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 306      * </tr>
 307      * <tr>
 308      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 309      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 310      * </tr>
 311      * <tr>
 312      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 313      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 314      * </tr>
 315      * <tr>
 316      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 317      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 318      * </tr>
 319      * <tr>
 320      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 321      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 322      * </tr>
 323      * <tr>
 324      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 325      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 326      * </tr>
 327      * <tr>
 328      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 329      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 330      * </tr>
 331      * <tr>
 332      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 333      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 334      * </tr>
 335      * <tr>
 336      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 337      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 338      * </tr>
 339      * <tr>
 340      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 341      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 342      * </tr>
 343      * <tr>
 344      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 345      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 346      * </tr>
 347      * </tbody>
 348      * </table>
 349      *
 350      * Here, the type {@code C} is the class or interface being searched for a member,
 351      * documented as a parameter named {@code refc} in the lookup methods.
 352      * The method type {@code MT} is composed from the return type {@code T}
 353      * and the sequence of argument types {@code A*}.
 354      * The constructor also has a sequence of argument types {@code A*} and
 355      * is deemed to return the newly-created object of type {@code C}.
 356      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 357      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 358      * if it is present, it is always the leading argument to the method handle invocation.
 359      * (In the case of some {@code protected} members, {@code this} may be
 360      * restricted in type to the lookup class; see below.)
 361      * The name {@code arg} stands for all the other method handle arguments.
 362      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 363      * stands for a null reference if the accessed method or field is static,
 364      * and {@code this} otherwise.
 365      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 366      * for reflective objects corresponding to the given members.
 367      * <p>
 368      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 369      * as if by {@code ldc CONSTANT_Class}.
 370      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 371      * <p>
 372      * In cases where the given member is of variable arity (i.e., a method or constructor)
 373      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 374      * In all other cases, the returned method handle will be of fixed arity.
 375      * <p style="font-size:smaller;">
 376      * <em>Discussion:</em>
 377      * The equivalence between looked-up method handles and underlying
 378      * class members and bytecode behaviors
 379      * can break down in a few ways:
 380      * <ul style="font-size:smaller;">
 381      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 382      * the lookup can still succeed, even when there is no equivalent
 383      * Java expression or bytecoded constant.
 384      * <li>Likewise, if {@code T} or {@code MT}
 385      * is not symbolically accessible from the lookup class's loader,
 386      * the lookup can still succeed.
 387      * For example, lookups for {@code MethodHandle.invokeExact} and
 388      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 389      * <li>If there is a security manager installed, it can forbid the lookup
 390      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 391      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 392      * constant is not subject to security manager checks.
 393      * <li>If the looked-up method has a
 394      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 395      * the method handle creation may fail, due to the method handle
 396      * type having too many parameters.
 397      * </ul>
 398      *
 399      * <h1><a id="access"></a>Access checking</h1>
 400      * Access checks are applied in the factory methods of {@code Lookup},
 401      * when a method handle is created.
 402      * This is a key difference from the Core Reflection API, since
 403      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 404      * performs access checking against every caller, on every call.
 405      * <p>
 406      * All access checks start from a {@code Lookup} object, which
 407      * compares its recorded lookup class against all requests to
 408      * create method handles.
 409      * A single {@code Lookup} object can be used to create any number
 410      * of access-checked method handles, all checked against a single
 411      * lookup class.
 412      * <p>
 413      * A {@code Lookup} object can be shared with other trusted code,
 414      * such as a metaobject protocol.
 415      * A shared {@code Lookup} object delegates the capability
 416      * to create method handles on private members of the lookup class.
 417      * Even if privileged code uses the {@code Lookup} object,
 418      * the access checking is confined to the privileges of the
 419      * original lookup class.
 420      * <p>
 421      * A lookup can fail, because
 422      * the containing class is not accessible to the lookup class, or
 423      * because the desired class member is missing, or because the
 424      * desired class member is not accessible to the lookup class, or
 425      * because the lookup object is not trusted enough to access the member.
 426      * In any of these cases, a {@code ReflectiveOperationException} will be
 427      * thrown from the attempted lookup.  The exact class will be one of
 428      * the following:
 429      * <ul>
 430      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 431      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 432      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 433      * </ul>
 434      * <p>
 435      * In general, the conditions under which a method handle may be
 436      * looked up for a method {@code M} are no more restrictive than the conditions
 437      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 438      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 439      * a method handle lookup will generally raise a corresponding
 440      * checked exception, such as {@code NoSuchMethodException}.
 441      * And the effect of invoking the method handle resulting from the lookup
 442      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 443      * to executing the compiled, verified, and resolved call to {@code M}.
 444      * The same point is true of fields and constructors.
 445      * <p style="font-size:smaller;">
 446      * <em>Discussion:</em>
 447      * Access checks only apply to named and reflected methods,
 448      * constructors, and fields.
 449      * Other method handle creation methods, such as
 450      * {@link MethodHandle#asType MethodHandle.asType},
 451      * do not require any access checks, and are used
 452      * independently of any {@code Lookup} object.
 453      * <p>
 454      * If the desired member is {@code protected}, the usual JVM rules apply,
 455      * including the requirement that the lookup class must be either be in the
 456      * same package as the desired member, or must inherit that member.
 457      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 458      * In addition, if the desired member is a non-static field or method
 459      * in a different package, the resulting method handle may only be applied
 460      * to objects of the lookup class or one of its subclasses.
 461      * This requirement is enforced by narrowing the type of the leading
 462      * {@code this} parameter from {@code C}
 463      * (which will necessarily be a superclass of the lookup class)
 464      * to the lookup class itself.
 465      * <p>
 466      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 467      * that the receiver argument must match both the resolved method <em>and</em>
 468      * the current class.  Again, this requirement is enforced by narrowing the
 469      * type of the leading parameter to the resulting method handle.
 470      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 471      * <p>
 472      * The JVM represents constructors and static initializer blocks as internal methods
 473      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 474      * The internal syntax of invocation instructions allows them to refer to such internal
 475      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 476      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 477      * <p>
 478      * In some cases, access between nested classes is obtained by the Java compiler by creating
 479      * an wrapper method to access a private method of another class
 480      * in the same top-level declaration.
 481      * For example, a nested class {@code C.D}
 482      * can access private members within other related classes such as
 483      * {@code C}, {@code C.D.E}, or {@code C.B},
 484      * but the Java compiler may need to generate wrapper methods in
 485      * those related classes.  In such cases, a {@code Lookup} object on
 486      * {@code C.E} would be unable to those private members.
 487      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 488      * which can transform a lookup on {@code C.E} into one on any of those other
 489      * classes, without special elevation of privilege.
 490      * <p>
 491      * The accesses permitted to a given lookup object may be limited,
 492      * according to its set of {@link #lookupModes lookupModes},
 493      * to a subset of members normally accessible to the lookup class.
 494      * For example, the {@link MethodHandles#publicLookup publicLookup}
 495      * method produces a lookup object which is only allowed to access
 496      * public members in public classes of exported packages.
 497      * The caller sensitive method {@link MethodHandles#lookup lookup}
 498      * produces a lookup object with full capabilities relative to
 499      * its caller class, to emulate all supported bytecode behaviors.
 500      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 501      * with fewer access modes than the original lookup object.
 502      *
 503      * <p style="font-size:smaller;">
 504      * <a id="privacc"></a>
 505      * <em>Discussion of private access:</em>
 506      * We say that a lookup has <em>private access</em>
 507      * if its {@linkplain #lookupModes lookup modes}
 508      * include the possibility of accessing {@code private} members.
 509      * As documented in the relevant methods elsewhere,
 510      * only lookups with private access possess the following capabilities:
 511      * <ul style="font-size:smaller;">
 512      * <li>access private fields, methods, and constructors of the lookup class
 513      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 514      *     such as {@code Class.forName}
 515      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 516      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 517      *     for classes accessible to the lookup class
 518      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 519      *     within the same package member
 520      * </ul>
 521      * <p style="font-size:smaller;">
 522      * Each of these permissions is a consequence of the fact that a lookup object
 523      * with private access can be securely traced back to an originating class,
 524      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 525      * can be reliably determined and emulated by method handles.
 526      *
 527      * <h1><a id="secmgr"></a>Security manager interactions</h1>
 528      * Although bytecode instructions can only refer to classes in
 529      * a related class loader, this API can search for methods in any
 530      * class, as long as a reference to its {@code Class} object is
 531      * available.  Such cross-loader references are also possible with the
 532      * Core Reflection API, and are impossible to bytecode instructions
 533      * such as {@code invokestatic} or {@code getfield}.
 534      * There is a {@linkplain java.lang.SecurityManager security manager API}
 535      * to allow applications to check such cross-loader references.
 536      * These checks apply to both the {@code MethodHandles.Lookup} API
 537      * and the Core Reflection API
 538      * (as found on {@link java.lang.Class Class}).
 539      * <p>
 540      * If a security manager is present, member and class lookups are subject to
 541      * additional checks.
 542      * From one to three calls are made to the security manager.
 543      * Any of these calls can refuse access by throwing a
 544      * {@link java.lang.SecurityException SecurityException}.
 545      * Define {@code smgr} as the security manager,
 546      * {@code lookc} as the lookup class of the current lookup object,
 547      * {@code refc} as the containing class in which the member
 548      * is being sought, and {@code defc} as the class in which the
 549      * member is actually defined.
 550      * (If a class or other type is being accessed,
 551      * the {@code refc} and {@code defc} values are the class itself.)
 552      * The value {@code lookc} is defined as <em>not present</em>
 553      * if the current lookup object does not have
 554      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 555      * The calls are made according to the following rules:
 556      * <ul>
 557      * <li><b>Step 1:</b>
 558      *     If {@code lookc} is not present, or if its class loader is not
 559      *     the same as or an ancestor of the class loader of {@code refc},
 560      *     then {@link SecurityManager#checkPackageAccess
 561      *     smgr.checkPackageAccess(refcPkg)} is called,
 562      *     where {@code refcPkg} is the package of {@code refc}.
 563      * <li><b>Step 2a:</b>
 564      *     If the retrieved member is not public and
 565      *     {@code lookc} is not present, then
 566      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 567      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 568      * <li><b>Step 2b:</b>
 569      *     If the retrieved class has a {@code null} class loader,
 570      *     and {@code lookc} is not present, then
 571      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 572      *     with {@code RuntimePermission("getClassLoader")} is called.
 573      * <li><b>Step 3:</b>
 574      *     If the retrieved member is not public,
 575      *     and if {@code lookc} is not present,
 576      *     and if {@code defc} and {@code refc} are different,
 577      *     then {@link SecurityManager#checkPackageAccess
 578      *     smgr.checkPackageAccess(defcPkg)} is called,
 579      *     where {@code defcPkg} is the package of {@code defc}.
 580      * </ul>
 581      * Security checks are performed after other access checks have passed.
 582      * Therefore, the above rules presuppose a member or class that is public,
 583      * or else that is being accessed from a lookup class that has
 584      * rights to access the member or class.
 585      *
 586      * <h1><a id="callsens"></a>Caller sensitive methods</h1>
 587      * A small number of Java methods have a special property called caller sensitivity.
 588      * A <em>caller-sensitive</em> method can behave differently depending on the
 589      * identity of its immediate caller.
 590      * <p>
 591      * If a method handle for a caller-sensitive method is requested,
 592      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 593      * but they take account of the lookup class in a special way.
 594      * The resulting method handle behaves as if it were called
 595      * from an instruction contained in the lookup class,
 596      * so that the caller-sensitive method detects the lookup class.
 597      * (By contrast, the invoker of the method handle is disregarded.)
 598      * Thus, in the case of caller-sensitive methods,
 599      * different lookup classes may give rise to
 600      * differently behaving method handles.
 601      * <p>
 602      * In cases where the lookup object is
 603      * {@link MethodHandles#publicLookup() publicLookup()},
 604      * or some other lookup object without
 605      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 606      * the lookup class is disregarded.
 607      * In such cases, no caller-sensitive method handle can be created,
 608      * access is forbidden, and the lookup fails with an
 609      * {@code IllegalAccessException}.
 610      * <p style="font-size:smaller;">
 611      * <em>Discussion:</em>
 612      * For example, the caller-sensitive method
 613      * {@link java.lang.Class#forName(String) Class.forName(x)}
 614      * can return varying classes or throw varying exceptions,
 615      * depending on the class loader of the class that calls it.
 616      * A public lookup of {@code Class.forName} will fail, because
 617      * there is no reasonable way to determine its bytecode behavior.
 618      * <p style="font-size:smaller;">
 619      * If an application caches method handles for broad sharing,
 620      * it should use {@code publicLookup()} to create them.
 621      * If there is a lookup of {@code Class.forName}, it will fail,
 622      * and the application must take appropriate action in that case.
 623      * It may be that a later lookup, perhaps during the invocation of a
 624      * bootstrap method, can incorporate the specific identity
 625      * of the caller, making the method accessible.
 626      * <p style="font-size:smaller;">
 627      * The function {@code MethodHandles.lookup} is caller sensitive
 628      * so that there can be a secure foundation for lookups.
 629      * Nearly all other methods in the JSR 292 API rely on lookup
 630      * objects to check access requests.
 631      *
 632      * @revised 9
 633      */
 634     public static final
 635     class Lookup {
 636         /** The class on behalf of whom the lookup is being performed. */
 637         private final Class<?> lookupClass;
 638 
 639         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 640         private final int allowedModes;
 641 
 642         /** A single-bit mask representing {@code public} access,
 643          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 644          *  The value, {@code 0x01}, happens to be the same as the value of the
 645          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 646          */
 647         public static final int PUBLIC = Modifier.PUBLIC;
 648 
 649         /** A single-bit mask representing {@code private} access,
 650          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 651          *  The value, {@code 0x02}, happens to be the same as the value of the
 652          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 653          */
 654         public static final int PRIVATE = Modifier.PRIVATE;
 655 
 656         /** A single-bit mask representing {@code protected} access,
 657          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 658          *  The value, {@code 0x04}, happens to be the same as the value of the
 659          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 660          */
 661         public static final int PROTECTED = Modifier.PROTECTED;
 662 
 663         /** A single-bit mask representing {@code package} access (default access),
 664          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 665          *  The value is {@code 0x08}, which does not correspond meaningfully to
 666          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 667          */
 668         public static final int PACKAGE = Modifier.STATIC;
 669 
 670         /** A single-bit mask representing {@code module} access (default access),
 671          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 672          *  The value is {@code 0x10}, which does not correspond meaningfully to
 673          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 674          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 675          *  with this lookup mode can access all public types in the module of the
 676          *  lookup class and public types in packages exported by other modules
 677          *  to the module of the lookup class.
 678          *  @since 9
 679          *  @spec JPMS
 680          */
 681         public static final int MODULE = PACKAGE << 1;
 682 
 683         /** A single-bit mask representing {@code unconditional} access
 684          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 685          *  The value is {@code 0x20}, which does not correspond meaningfully to
 686          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 687          *  A {@code Lookup} with this lookup mode assumes {@linkplain
 688          *  java.lang.Module#canRead(java.lang.Module) readability}.
 689          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 690          *  with this lookup mode can access all public members of public types
 691          *  of all modules where the type is in a package that is {@link
 692          *  java.lang.Module#isExported(String) exported unconditionally}.
 693          *  @since 9
 694          *  @spec JPMS
 695          *  @see #publicLookup()
 696          */
 697         public static final int UNCONDITIONAL = PACKAGE << 2;
 698 
 699         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL);
 700         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);
 701         private static final int TRUSTED   = -1;
 702 
 703         private static int fixmods(int mods) {
 704             mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL);
 705             return (mods != 0) ? mods : (PACKAGE | MODULE | UNCONDITIONAL);
 706         }
 707 
 708         /** Tells which class is performing the lookup.  It is this class against
 709          *  which checks are performed for visibility and access permissions.
 710          *  <p>
 711          *  The class implies a maximum level of access permission,
 712          *  but the permissions may be additionally limited by the bitmask
 713          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 714          *  can be accessed.
 715          *  @return the lookup class, on behalf of which this lookup object finds members
 716          */
 717         public Class<?> lookupClass() {
 718             return lookupClass;
 719         }
 720 
 721         // This is just for calling out to MethodHandleImpl.
 722         private Class<?> lookupClassOrNull() {
 723             return (allowedModes == TRUSTED) ? null : lookupClass;
 724         }
 725 
 726         /** Tells which access-protection classes of members this lookup object can produce.
 727          *  The result is a bit-mask of the bits
 728          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 729          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 730          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 731          *  {@linkplain #PACKAGE PACKAGE (0x08)},
 732          *  {@linkplain #MODULE MODULE (0x10)},
 733          *  and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}.
 734          *  <p>
 735          *  A freshly-created lookup object
 736          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
 737          *  all possible bits set, except {@code UNCONDITIONAL}. The lookup can be used to
 738          *  access all members of the caller's class, all public types in the caller's module,
 739          *  and all public types in packages exported by other modules to the caller's module.
 740          *  A lookup object on a new lookup class
 741          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 742          *  may have some mode bits set to zero.
 743          *  Mode bits can also be
 744          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
 745          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
 746          *  The purpose of this is to restrict access via the new lookup object,
 747          *  so that it can access only names which can be reached by the original
 748          *  lookup object, and also by the new lookup class.
 749          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 750          *  @see #in
 751          *  @see #dropLookupMode
 752          *
 753          *  @revised 9
 754          *  @spec JPMS
 755          */
 756         public int lookupModes() {
 757             return allowedModes & ALL_MODES;
 758         }
 759 
 760         /** Embody the current class (the lookupClass) as a lookup class
 761          * for method handle creation.
 762          * Must be called by from a method in this package,
 763          * which in turn is called by a method not in this package.
 764          */
 765         Lookup(Class<?> lookupClass) {
 766             this(lookupClass, FULL_POWER_MODES);
 767             // make sure we haven't accidentally picked up a privileged class:
 768             checkUnprivilegedlookupClass(lookupClass);
 769         }
 770 
 771         private Lookup(Class<?> lookupClass, int allowedModes) {
 772             this.lookupClass = lookupClass;
 773             this.allowedModes = allowedModes;
 774         }
 775 
 776         /**
 777          * Creates a lookup on the specified new lookup class.
 778          * The resulting object will report the specified
 779          * class as its own {@link #lookupClass() lookupClass}.
 780          * <p>
 781          * However, the resulting {@code Lookup} object is guaranteed
 782          * to have no more access capabilities than the original.
 783          * In particular, access capabilities can be lost as follows:<ul>
 784          * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and
 785          * the new lookup class is in a different module {@code M}, then no members, not
 786          * even public members in {@code M}'s exported packages, will be accessible.
 787          * The exception to this is when this lookup is {@link #publicLookup()
 788          * publicLookup}, in which case {@code PUBLIC} access is not lost.
 789          * <li>If the old lookup class is in an unnamed module, and the new lookup class
 790          * is a different module then {@link #MODULE MODULE} access is lost.
 791          * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost.
 792          * <li>If the new lookup class is in a different package
 793          * than the old one, protected and default (package) members will not be accessible.
 794          * <li>If the new lookup class is not within the same package member
 795          * as the old one, private members will not be accessible, and protected members
 796          * will not be accessible by virtue of inheritance.
 797          * (Protected members may continue to be accessible because of package sharing.)
 798          * <li>If the new lookup class is not accessible to the old lookup class,
 799          * then no members, not even public members, will be accessible.
 800          * (In all other cases, public members will continue to be accessible.)
 801          * </ul>
 802          * <p>
 803          * The resulting lookup's capabilities for loading classes
 804          * (used during {@link #findClass} invocations)
 805          * are determined by the lookup class' loader,
 806          * which may change due to this operation.
 807          *
 808          * @param requestedLookupClass the desired lookup class for the new lookup object
 809          * @return a lookup object which reports the desired lookup class, or the same object
 810          * if there is no change
 811          * @throws NullPointerException if the argument is null
 812          *
 813          * @revised 9
 814          * @spec JPMS
 815          */
 816         public Lookup in(Class<?> requestedLookupClass) {
 817             Objects.requireNonNull(requestedLookupClass);
 818             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 819                 return new Lookup(requestedLookupClass, FULL_POWER_MODES);
 820             if (requestedLookupClass == this.lookupClass)
 821                 return this;  // keep same capabilities
 822             int newModes = (allowedModes & FULL_POWER_MODES);
 823             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 824                 // Need to drop all access when teleporting from a named module to another
 825                 // module. The exception is publicLookup where PUBLIC is not lost.
 826                 if (this.lookupClass.getModule().isNamed()
 827                     && (this.allowedModes & UNCONDITIONAL) == 0)
 828                     newModes = 0;
 829                 else
 830                     newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
 831             }
 832             if ((newModes & PACKAGE) != 0
 833                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 834                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
 835             }
 836             // Allow nestmate lookups to be created without special privilege:
 837             if ((newModes & PRIVATE) != 0
 838                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 839                 newModes &= ~(PRIVATE|PROTECTED);
 840             }
 841             if ((newModes & PUBLIC) != 0
 842                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 843                 // The requested class it not accessible from the lookup class.
 844                 // No permissions.
 845                 newModes = 0;
 846             }
 847 
 848             checkUnprivilegedlookupClass(requestedLookupClass);
 849             return new Lookup(requestedLookupClass, newModes);
 850         }
 851 
 852 
 853         /**
 854          * Creates a lookup on the same lookup class which this lookup object
 855          * finds members, but with a lookup mode that has lost the given lookup mode.
 856          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
 857          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}.
 858          * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always
 859          * dropped and so the resulting lookup mode will never have these access capabilities.
 860          * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE}
 861          * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will
 862          * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC}
 863          * is dropped then the resulting lookup has no access.
 864          * @param modeToDrop the lookup mode to drop
 865          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
 866          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
 867          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
 868          * @see MethodHandles#privateLookupIn
 869          * @since 9
 870          */
 871         public Lookup dropLookupMode(int modeToDrop) {
 872             int oldModes = lookupModes();
 873             int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL);
 874             switch (modeToDrop) {
 875                 case PUBLIC: newModes &= ~(ALL_MODES); break;
 876                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
 877                 case PACKAGE: newModes &= ~(PRIVATE); break;
 878                 case PROTECTED:
 879                 case PRIVATE:
 880                 case UNCONDITIONAL: break;
 881                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
 882             }
 883             if (newModes == oldModes) return this;  // return self if no change
 884             return new Lookup(lookupClass(), newModes);
 885         }
 886 
 887         /**
 888          * Defines a class to the same class loader and in the same runtime package and
 889          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
 890          * {@linkplain #lookupClass() lookup class}.
 891          *
 892          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
 893          * {@link #PACKAGE PACKAGE} access as default (package) members will be
 894          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
 895          * that the lookup object was created by a caller in the runtime package (or derived
 896          * from a lookup originally created by suitably privileged code to a target class in
 897          * the runtime package). </p>
 898          *
 899          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
 900          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
 901          * same package as the lookup class. </p>
 902          *
 903          * <p> This method does not run the class initializer. The class initializer may
 904          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
 905          * Specification</em>. </p>
 906          *
 907          * <p> If there is a security manager, its {@code checkPermission} method is first called
 908          * to check {@code RuntimePermission("defineClass")}. </p>
 909          *
 910          * @param bytes the class bytes
 911          * @return the {@code Class} object for the class
 912          * @throws IllegalArgumentException the bytes are for a class in a different package
 913          * to the lookup class
 914          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
 915          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
 916          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
 917          * @throws SecurityException if denied by the security manager
 918          * @throws NullPointerException if {@code bytes} is {@code null}
 919          * @since 9
 920          * @spec JPMS
 921          * @see Lookup#privateLookupIn
 922          * @see Lookup#dropLookupMode
 923          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
 924          */
 925         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
 926             SecurityManager sm = System.getSecurityManager();
 927             if (sm != null)
 928                 sm.checkPermission(new RuntimePermission("defineClass"));
 929             if ((lookupModes() & PACKAGE) == 0)
 930                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
 931             assert (lookupModes() & (MODULE|PUBLIC)) != 0;
 932 
 933             // parse class bytes to get class name (in internal form)
 934             bytes = bytes.clone();
 935             String name;
 936             try {
 937                 ClassReader reader = new ClassReader(bytes);
 938                 name = reader.getClassName();
 939             } catch (RuntimeException e) {
 940                 // ASM exceptions are poorly specified
 941                 ClassFormatError cfe = new ClassFormatError();
 942                 cfe.initCause(e);
 943                 throw cfe;
 944             }
 945 
 946             // get package and class name in binary form
 947             String cn, pn;
 948             int index = name.lastIndexOf('/');
 949             if (index == -1) {
 950                 cn = name;
 951                 pn = "";
 952             } else {
 953                 cn = name.replace('/', '.');
 954                 pn = cn.substring(0, index);
 955             }
 956             if (!pn.equals(lookupClass.getPackageName())) {
 957                 throw new IllegalArgumentException("Class not in same package as lookup class");
 958             }
 959 
 960             // invoke the class loader's defineClass method
 961             ClassLoader loader = lookupClass.getClassLoader();
 962             ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null;
 963             String source = "__Lookup_defineClass__";
 964             Class<?> clazz = SharedSecrets.getJavaLangAccess().defineClass(loader, cn, bytes, pd, source);
 965             assert clazz.getClassLoader() == lookupClass.getClassLoader()
 966                     && clazz.getPackageName().equals(lookupClass.getPackageName())
 967                     && protectionDomain(clazz) == lookupClassProtectionDomain();
 968             return clazz;
 969         }
 970 
 971         private ProtectionDomain lookupClassProtectionDomain() {
 972             ProtectionDomain pd = cachedProtectionDomain;
 973             if (pd == null) {
 974                 cachedProtectionDomain = pd = protectionDomain(lookupClass);
 975             }
 976             return pd;
 977         }
 978 
 979         private ProtectionDomain protectionDomain(Class<?> clazz) {
 980             PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain;
 981             return AccessController.doPrivileged(pa);
 982         }
 983 
 984         // cached protection domain
 985         private volatile ProtectionDomain cachedProtectionDomain;
 986 
 987 
 988         // Make sure outer class is initialized first.
 989         static { IMPL_NAMES.getClass(); }
 990 
 991         /** Package-private version of lookup which is trusted. */
 992         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 993 
 994         /** Version of lookup which is trusted minimally.
 995          *  It can only be used to create method handles to publicly accessible
 996          *  members in packages that are exported unconditionally.
 997          */
 998         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC|UNCONDITIONAL));
 999 
1000         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
1001             String name = lookupClass.getName();
1002             if (name.startsWith("java.lang.invoke."))
1003                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
1004         }
1005 
1006         /**
1007          * Displays the name of the class from which lookups are to be made.
1008          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
1009          * If there are restrictions on the access permitted to this lookup,
1010          * this is indicated by adding a suffix to the class name, consisting
1011          * of a slash and a keyword.  The keyword represents the strongest
1012          * allowed access, and is chosen as follows:
1013          * <ul>
1014          * <li>If no access is allowed, the suffix is "/noaccess".
1015          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
1016          * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup".
1017          * <li>If only public and module access are allowed, the suffix is "/module".
1018          * <li>If only public, module and package access are allowed, the suffix is "/package".
1019          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
1020          * </ul>
1021          * If none of the above cases apply, it is the case that full
1022          * access (public, module, package, private, and protected) is allowed.
1023          * In this case, no suffix is added.
1024          * This is true only of an object obtained originally from
1025          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
1026          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
1027          * always have restricted access, and will display a suffix.
1028          * <p>
1029          * (It may seem strange that protected access should be
1030          * stronger than private access.  Viewed independently from
1031          * package access, protected access is the first to be lost,
1032          * because it requires a direct subclass relationship between
1033          * caller and callee.)
1034          * @see #in
1035          *
1036          * @revised 9
1037          * @spec JPMS
1038          */
1039         @Override
1040         public String toString() {
1041             String cname = lookupClass.getName();
1042             switch (allowedModes) {
1043             case 0:  // no privileges
1044                 return cname + "/noaccess";
1045             case PUBLIC:
1046                 return cname + "/public";
1047             case PUBLIC|UNCONDITIONAL:
1048                 return cname  + "/publicLookup";
1049             case PUBLIC|MODULE:
1050                 return cname + "/module";
1051             case PUBLIC|MODULE|PACKAGE:
1052                 return cname + "/package";
1053             case FULL_POWER_MODES & ~PROTECTED:
1054                 return cname + "/private";
1055             case FULL_POWER_MODES:
1056                 return cname;
1057             case TRUSTED:
1058                 return "/trusted";  // internal only; not exported
1059             default:  // Should not happen, but it's a bitfield...
1060                 cname = cname + "/" + Integer.toHexString(allowedModes);
1061                 assert(false) : cname;
1062                 return cname;
1063             }
1064         }
1065 
1066         /**
1067          * Produces a method handle for a static method.
1068          * The type of the method handle will be that of the method.
1069          * (Since static methods do not take receivers, there is no
1070          * additional receiver argument inserted into the method handle type,
1071          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
1072          * The method and all its argument types must be accessible to the lookup object.
1073          * <p>
1074          * The returned method handle will have
1075          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1076          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1077          * <p>
1078          * If the returned method handle is invoked, the method's class will
1079          * be initialized, if it has not already been initialized.
1080          * <p><b>Example:</b>
1081          * <blockquote><pre>{@code
1082 import static java.lang.invoke.MethodHandles.*;
1083 import static java.lang.invoke.MethodType.*;
1084 ...
1085 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
1086   "asList", methodType(List.class, Object[].class));
1087 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
1088          * }</pre></blockquote>
1089          * @param refc the class from which the method is accessed
1090          * @param name the name of the method
1091          * @param type the type of the method
1092          * @return the desired method handle
1093          * @throws NoSuchMethodException if the method does not exist
1094          * @throws IllegalAccessException if access checking fails,
1095          *                                or if the method is not {@code static},
1096          *                                or if the method's variable arity modifier bit
1097          *                                is set and {@code asVarargsCollector} fails
1098          * @exception SecurityException if a security manager is present and it
1099          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1100          * @throws NullPointerException if any argument is null
1101          */
1102         public
1103         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1104             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
1105             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
1106         }
1107 
1108         /**
1109          * Produces a method handle for a virtual method.
1110          * The type of the method handle will be that of the method,
1111          * with the receiver type (usually {@code refc}) prepended.
1112          * The method and all its argument types must be accessible to the lookup object.
1113          * <p>
1114          * When called, the handle will treat the first argument as a receiver
1115          * and dispatch on the receiver's type to determine which method
1116          * implementation to enter.
1117          * (The dispatching action is identical with that performed by an
1118          * {@code invokevirtual} or {@code invokeinterface} instruction.)
1119          * <p>
1120          * The first argument will be of type {@code refc} if the lookup
1121          * class has full privileges to access the member.  Otherwise
1122          * the member must be {@code protected} and the first argument
1123          * will be restricted in type to the lookup class.
1124          * <p>
1125          * The returned method handle will have
1126          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1127          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1128          * <p>
1129          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
1130          * instructions and method handles produced by {@code findVirtual},
1131          * if the class is {@code MethodHandle} and the name string is
1132          * {@code invokeExact} or {@code invoke}, the resulting
1133          * method handle is equivalent to one produced by
1134          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
1135          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
1136          * with the same {@code type} argument.
1137          * <p>
1138          * If the class is {@code VarHandle} and the name string corresponds to
1139          * the name of a signature-polymorphic access mode method, the resulting
1140          * method handle is equivalent to one produced by
1141          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
1142          * the access mode corresponding to the name string and with the same
1143          * {@code type} arguments.
1144          * <p>
1145          * <b>Example:</b>
1146          * <blockquote><pre>{@code
1147 import static java.lang.invoke.MethodHandles.*;
1148 import static java.lang.invoke.MethodType.*;
1149 ...
1150 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
1151   "concat", methodType(String.class, String.class));
1152 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
1153   "hashCode", methodType(int.class));
1154 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
1155   "hashCode", methodType(int.class));
1156 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
1157 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
1158 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
1159 // interface method:
1160 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
1161   "subSequence", methodType(CharSequence.class, int.class, int.class));
1162 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
1163 // constructor "internal method" must be accessed differently:
1164 MethodType MT_newString = methodType(void.class); //()V for new String()
1165 try { assertEquals("impossible", lookup()
1166         .findVirtual(String.class, "<init>", MT_newString));
1167  } catch (NoSuchMethodException ex) { } // OK
1168 MethodHandle MH_newString = publicLookup()
1169   .findConstructor(String.class, MT_newString);
1170 assertEquals("", (String) MH_newString.invokeExact());
1171          * }</pre></blockquote>
1172          *
1173          * @param refc the class or interface from which the method is accessed
1174          * @param name the name of the method
1175          * @param type the type of the method, with the receiver argument omitted
1176          * @return the desired method handle
1177          * @throws NoSuchMethodException if the method does not exist
1178          * @throws IllegalAccessException if access checking fails,
1179          *                                or if the method is {@code static},
1180          *                                or if the method is {@code private} method of interface,
1181          *                                or if the method's variable arity modifier bit
1182          *                                is set and {@code asVarargsCollector} fails
1183          * @exception SecurityException if a security manager is present and it
1184          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1185          * @throws NullPointerException if any argument is null
1186          */
1187         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1188             if (refc == MethodHandle.class) {
1189                 MethodHandle mh = findVirtualForMH(name, type);
1190                 if (mh != null)  return mh;
1191             } else if (refc == VarHandle.class) {
1192                 MethodHandle mh = findVirtualForVH(name, type);
1193                 if (mh != null)  return mh;
1194             }
1195             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
1196             MemberName method = resolveOrFail(refKind, refc, name, type);
1197             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
1198         }
1199         private MethodHandle findVirtualForMH(String name, MethodType type) {
1200             // these names require special lookups because of the implicit MethodType argument
1201             if ("invoke".equals(name))
1202                 return invoker(type);
1203             if ("invokeExact".equals(name))
1204                 return exactInvoker(type);
1205             assert(!MemberName.isMethodHandleInvokeName(name));
1206             return null;
1207         }
1208         private MethodHandle findVirtualForVH(String name, MethodType type) {
1209             try {
1210                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
1211             } catch (IllegalArgumentException e) {
1212                 return null;
1213             }
1214         }
1215 
1216         /**
1217          * Produces a method handle which creates an object and initializes it, using
1218          * the constructor of the specified type.
1219          * The parameter types of the method handle will be those of the constructor,
1220          * while the return type will be a reference to the constructor's class.
1221          * The constructor and all its argument types must be accessible to the lookup object.
1222          * <p>
1223          * The requested type must have a return type of {@code void}.
1224          * (This is consistent with the JVM's treatment of constructor type descriptors.)
1225          * <p>
1226          * The returned method handle will have
1227          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1228          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1229          * <p>
1230          * If the returned method handle is invoked, the constructor's class will
1231          * be initialized, if it has not already been initialized.
1232          * <p><b>Example:</b>
1233          * <blockquote><pre>{@code
1234 import static java.lang.invoke.MethodHandles.*;
1235 import static java.lang.invoke.MethodType.*;
1236 ...
1237 MethodHandle MH_newArrayList = publicLookup().findConstructor(
1238   ArrayList.class, methodType(void.class, Collection.class));
1239 Collection orig = Arrays.asList("x", "y");
1240 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
1241 assert(orig != copy);
1242 assertEquals(orig, copy);
1243 // a variable-arity constructor:
1244 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
1245   ProcessBuilder.class, methodType(void.class, String[].class));
1246 ProcessBuilder pb = (ProcessBuilder)
1247   MH_newProcessBuilder.invoke("x", "y", "z");
1248 assertEquals("[x, y, z]", pb.command().toString());
1249          * }</pre></blockquote>
1250          * @param refc the class or interface from which the method is accessed
1251          * @param type the type of the method, with the receiver argument omitted, and a void return type
1252          * @return the desired method handle
1253          * @throws NoSuchMethodException if the constructor does not exist
1254          * @throws IllegalAccessException if access checking fails
1255          *                                or if the method's variable arity modifier bit
1256          *                                is set and {@code asVarargsCollector} fails
1257          * @exception SecurityException if a security manager is present and it
1258          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1259          * @throws NullPointerException if any argument is null
1260          */
1261         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1262             if (refc.isArray()) {
1263                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
1264             }
1265             String name = "<init>";
1266             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1267             return getDirectConstructor(refc, ctor);
1268         }
1269 
1270         /**
1271          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1272          * initializer of the class is not run.
1273          * <p>
1274          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1275          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1276          * load the requested class, and then determines whether the class is accessible to this lookup object.
1277          *
1278          * @param targetName the fully qualified name of the class to be looked up.
1279          * @return the requested class.
1280          * @exception SecurityException if a security manager is present and it
1281          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1282          * @throws LinkageError if the linkage fails
1283          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
1284          * @throws IllegalAccessException if the class is not accessible, using the allowed access
1285          * modes.
1286          * @exception SecurityException if a security manager is present and it
1287          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1288          * @since 9
1289          */
1290         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1291             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1292             return accessClass(targetClass);
1293         }
1294 
1295         /**
1296          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1297          * static initializer of the class is not run.
1298          * <p>
1299          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1300          * {@linkplain #lookupModes() lookup modes}.
1301          *
1302          * @param targetClass the class to be access-checked
1303          *
1304          * @return the class that has been access-checked
1305          *
1306          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1307          * modes.
1308          * @exception SecurityException if a security manager is present and it
1309          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1310          * @since 9
1311          */
1312         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1313             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1314                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1315             }
1316             checkSecurityManager(targetClass, null);
1317             return targetClass;
1318         }
1319 
1320         /**
1321          * Produces an early-bound method handle for a virtual method.
1322          * It will bypass checks for overriding methods on the receiver,
1323          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1324          * instruction from within the explicitly specified {@code specialCaller}.
1325          * The type of the method handle will be that of the method,
1326          * with a suitably restricted receiver type prepended.
1327          * (The receiver type will be {@code specialCaller} or a subtype.)
1328          * The method and all its argument types must be accessible
1329          * to the lookup object.
1330          * <p>
1331          * Before method resolution,
1332          * if the explicitly specified caller class is not identical with the
1333          * lookup class, or if this lookup object does not have
1334          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1335          * privileges, the access fails.
1336          * <p>
1337          * The returned method handle will have
1338          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1339          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1340          * <p style="font-size:smaller;">
1341          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1342          * even though the {@code invokespecial} instruction can refer to them
1343          * in special circumstances.  Use {@link #findConstructor findConstructor}
1344          * to access instance initialization methods in a safe manner.)</em>
1345          * <p><b>Example:</b>
1346          * <blockquote><pre>{@code
1347 import static java.lang.invoke.MethodHandles.*;
1348 import static java.lang.invoke.MethodType.*;
1349 ...
1350 static class Listie extends ArrayList {
1351   public String toString() { return "[wee Listie]"; }
1352   static Lookup lookup() { return MethodHandles.lookup(); }
1353 }
1354 ...
1355 // no access to constructor via invokeSpecial:
1356 MethodHandle MH_newListie = Listie.lookup()
1357   .findConstructor(Listie.class, methodType(void.class));
1358 Listie l = (Listie) MH_newListie.invokeExact();
1359 try { assertEquals("impossible", Listie.lookup().findSpecial(
1360         Listie.class, "<init>", methodType(void.class), Listie.class));
1361  } catch (NoSuchMethodException ex) { } // OK
1362 // access to super and self methods via invokeSpecial:
1363 MethodHandle MH_super = Listie.lookup().findSpecial(
1364   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1365 MethodHandle MH_this = Listie.lookup().findSpecial(
1366   Listie.class, "toString" , methodType(String.class), Listie.class);
1367 MethodHandle MH_duper = Listie.lookup().findSpecial(
1368   Object.class, "toString" , methodType(String.class), Listie.class);
1369 assertEquals("[]", (String) MH_super.invokeExact(l));
1370 assertEquals(""+l, (String) MH_this.invokeExact(l));
1371 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1372 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1373         String.class, "toString", methodType(String.class), Listie.class));
1374  } catch (IllegalAccessException ex) { } // OK
1375 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1376 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1377          * }</pre></blockquote>
1378          *
1379          * @param refc the class or interface from which the method is accessed
1380          * @param name the name of the method (which must not be "&lt;init&gt;")
1381          * @param type the type of the method, with the receiver argument omitted
1382          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1383          * @return the desired method handle
1384          * @throws NoSuchMethodException if the method does not exist
1385          * @throws IllegalAccessException if access checking fails,
1386          *                                or if the method is {@code static},
1387          *                                or if the method's variable arity modifier bit
1388          *                                is set and {@code asVarargsCollector} fails
1389          * @exception SecurityException if a security manager is present and it
1390          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1391          * @throws NullPointerException if any argument is null
1392          */
1393         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1394                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1395             checkSpecialCaller(specialCaller, refc);
1396             Lookup specialLookup = this.in(specialCaller);
1397             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1398             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));






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