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