1 /*
   2  * Copyright (c) 2008, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.invoke;
  27 
  28 import jdk.internal.access.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      * @return a lookup object for the caller of this method, with private access
 102      */
 103     @CallerSensitive
 104     @ForceInline // to ensure Reflection.getCallerClass optimization
 105     public static Lookup lookup() {
 106         return new Lookup(Reflection.getCallerClass());
 107     }
 108 
 109     /**
 110      * This reflected$lookup method is the alternate implementation of
 111      * the lookup method when being invoked by reflection.
 112      */
 113     @CallerSensitive
 114     private static Lookup reflected$lookup() {
 115         Class<?> caller = Reflection.getCallerClass();
 116         if (caller.getClassLoader() == null) {
 117             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 118         }
 119         return new Lookup(caller);
 120     }
 121 
 122     /**
 123      * Returns a {@link Lookup lookup object} which is trusted minimally.
 124      * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes.
 125      * It can only be used to create method handles to public members of
 126      * public classes in packages that are exported unconditionally.
 127      * <p>
 128      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 129      * of this lookup object will be {@link java.lang.Object}.
 130      *
 131      * @apiNote The use of Object is conventional, and because the lookup modes are
 132      * limited, there is no special access provided to the internals of Object, its package
 133      * or its module. Consequently, the lookup context of this lookup object will be the
 134      * bootstrap class loader, which means it cannot find user classes.
 135      *
 136      * <p style="font-size:smaller;">
 137      * <em>Discussion:</em>
 138      * The lookup class can be changed to any other class {@code C} using an expression of the form
 139      * {@link Lookup#in publicLookup().in(C.class)}.
 140      * but may change the lookup context by virtue of changing the class loader.
 141      * A public lookup object is always subject to
 142      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 143      * Also, it cannot access
 144      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 145      * @return a lookup object which is trusted minimally
 146      *
 147      * @revised 9
 148      * @spec JPMS
 149      */
 150     public static Lookup publicLookup() {
 151         return Lookup.PUBLIC_LOOKUP;
 152     }
 153 
 154     /**
 155      * Returns a {@link Lookup lookup object} with full capabilities to emulate all
 156      * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">
 157      * private access</a>, on a target class.
 158      * This method checks that a caller, specified as a {@code Lookup} object, is allowed to
 159      * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing
 160      * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing
 161      * the target class, then this check ensures that
 162      * <ul>
 163      *     <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li>
 164      *     <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing
 165      *     the target class to at least {@code m1}.</li>
 166      *     <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li>
 167      * </ul>
 168      * <p>
 169      * If there is a security manager, its {@code checkPermission} method is called to
 170      * check {@code ReflectPermission("suppressAccessChecks")}.
 171      * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object
 172      * was created by code in the caller module (or derived from a lookup object originally
 173      * created by the caller). A lookup object with the {@code MODULE} lookup mode can be
 174      * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE}
 175      * access to the caller.
 176      * @param targetClass the target class
 177      * @param lookup the caller lookup object
 178      * @return a lookup object for the target class, with private access
 179      * @throws IllegalArgumentException if {@code targetClass} is a primitve type or array class
 180      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 181      * @throws IllegalAccessException if the access check specified above fails
 182      * @throws SecurityException if denied by the security manager
 183      * @since 9
 184      * @spec JPMS
 185      * @see Lookup#dropLookupMode
 186      */
 187     public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException {
 188         SecurityManager sm = System.getSecurityManager();
 189         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 190         if (targetClass.isPrimitive())
 191             throw new IllegalArgumentException(targetClass + " is a primitive class");
 192         if (targetClass.isArray())
 193             throw new IllegalArgumentException(targetClass + " is an array class");
 194         Module targetModule = targetClass.getModule();
 195         Module callerModule = lookup.lookupClass().getModule();
 196         if (!callerModule.canRead(targetModule))
 197             throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 198         if (targetModule.isNamed()) {
 199             String pn = targetClass.getPackageName();
 200             assert !pn.isEmpty() : "unnamed package cannot be in named module";
 201             if (!targetModule.isOpen(pn, callerModule))
 202                 throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 203         }
 204         if ((lookup.lookupModes() & Lookup.MODULE) == 0)
 205             throw new IllegalAccessException("lookup does not have MODULE lookup mode");
 206         if (!callerModule.isNamed() && targetModule.isNamed()) {
 207             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 208             if (logger != null) {
 209                 logger.logIfOpenedForIllegalAccess(lookup, targetClass);
 210             }
 211         }
 212         return new Lookup(targetClass);
 213     }
 214 
 215     /**
 216      * Performs an unchecked "crack" of a
 217      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 218      * The result is as if the user had obtained a lookup object capable enough
 219      * to crack the target method handle, called
 220      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 221      * on the target to obtain its symbolic reference, and then called
 222      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 223      * to resolve the symbolic reference to a member.
 224      * <p>
 225      * If there is a security manager, its {@code checkPermission} method
 226      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 227      * @param <T> the desired type of the result, either {@link Member} or a subtype
 228      * @param target a direct method handle to crack into symbolic reference components
 229      * @param expected a class object representing the desired result type {@code T}
 230      * @return a reference to the method, constructor, or field object
 231      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 232      * @exception NullPointerException if either argument is {@code null}
 233      * @exception IllegalArgumentException if the target is not a direct method handle
 234      * @exception ClassCastException if the member is not of the expected type
 235      * @since 1.8
 236      */
 237     public static <T extends Member> T
 238     reflectAs(Class<T> expected, MethodHandle target) {
 239         SecurityManager smgr = System.getSecurityManager();
 240         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 241         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 242         return lookup.revealDirect(target).reflectAs(expected, lookup);
 243     }
 244     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 245     private static final java.security.Permission ACCESS_PERMISSION =
 246         new ReflectPermission("suppressAccessChecks");
 247 
 248     /**
 249      * A <em>lookup object</em> is a factory for creating method handles,
 250      * when the creation requires access checking.
 251      * Method handles do not perform
 252      * access checks when they are called, but rather when they are created.
 253      * Therefore, method handle access
 254      * restrictions must be enforced when a method handle is created.
 255      * The caller class against which those restrictions are enforced
 256      * is known as the {@linkplain #lookupClass() lookup class}.
 257      * <p>
 258      * A lookup class which needs to create method handles will call
 259      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 260      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 261      * determined, and securely stored in the {@code Lookup} object.
 262      * The lookup class (or its delegates) may then use factory methods
 263      * on the {@code Lookup} object to create method handles for access-checked members.
 264      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 265      * even private ones.
 266      *
 267      * <h1><a id="lookups"></a>Lookup Factory Methods</h1>
 268      * The factory methods on a {@code Lookup} object correspond to all major
 269      * use cases for methods, constructors, and fields.
 270      * Each method handle created by a factory method is the functional
 271      * equivalent of a particular <em>bytecode behavior</em>.
 272      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 273      * Here is a summary of the correspondence between these factory methods and
 274      * the behavior of the resulting method handles:
 275      * <table class="striped">
 276      * <caption style="display:none">lookup method behaviors</caption>
 277      * <thead>
 278      * <tr>
 279      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 280      *     <th scope="col">member</th>
 281      *     <th scope="col">bytecode behavior</th>
 282      * </tr>
 283      * </thead>
 284      * <tbody>
 285      * <tr>
 286      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 287      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 288      * </tr>
 289      * <tr>
 290      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 291      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 292      * </tr>
 293      * <tr>
 294      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 295      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 296      * </tr>
 297      * <tr>
 298      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 299      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 300      * </tr>
 301      * <tr>
 302      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 303      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 304      * </tr>
 305      * <tr>
 306      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 307      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 308      * </tr>
 309      * <tr>
 310      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 311      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 312      * </tr>
 313      * <tr>
 314      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 315      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 316      * </tr>
 317      * <tr>
 318      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 319      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 320      * </tr>
 321      * <tr>
 322      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 323      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 324      * </tr>
 325      * <tr>
 326      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 327      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 328      * </tr>
 329      * <tr>
 330      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 331      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 332      * </tr>
 333      * <tr>
 334      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 335      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 336      * </tr>
 337      * <tr>
 338      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 339      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 340      * </tr>
 341      * </tbody>
 342      * </table>
 343      *
 344      * Here, the type {@code C} is the class or interface being searched for a member,
 345      * documented as a parameter named {@code refc} in the lookup methods.
 346      * The method type {@code MT} is composed from the return type {@code T}
 347      * and the sequence of argument types {@code A*}.
 348      * The constructor also has a sequence of argument types {@code A*} and
 349      * is deemed to return the newly-created object of type {@code C}.
 350      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 351      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 352      * if it is present, it is always the leading argument to the method handle invocation.
 353      * (In the case of some {@code protected} members, {@code this} may be
 354      * restricted in type to the lookup class; see below.)
 355      * The name {@code arg} stands for all the other method handle arguments.
 356      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 357      * stands for a null reference if the accessed method or field is static,
 358      * and {@code this} otherwise.
 359      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 360      * for reflective objects corresponding to the given members.
 361      * <p>
 362      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 363      * as if by {@code ldc CONSTANT_Class}.
 364      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 365      * <p>
 366      * In cases where the given member is of variable arity (i.e., a method or constructor)
 367      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 368      * In all other cases, the returned method handle will be of fixed arity.
 369      * <p style="font-size:smaller;">
 370      * <em>Discussion:</em>
 371      * The equivalence between looked-up method handles and underlying
 372      * class members and bytecode behaviors
 373      * can break down in a few ways:
 374      * <ul style="font-size:smaller;">
 375      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 376      * the lookup can still succeed, even when there is no equivalent
 377      * Java expression or bytecoded constant.
 378      * <li>Likewise, if {@code T} or {@code MT}
 379      * is not symbolically accessible from the lookup class's loader,
 380      * the lookup can still succeed.
 381      * For example, lookups for {@code MethodHandle.invokeExact} and
 382      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 383      * <li>If there is a security manager installed, it can forbid the lookup
 384      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 385      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 386      * constant is not subject to security manager checks.
 387      * <li>If the looked-up method has a
 388      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 389      * the method handle creation may fail, due to the method handle
 390      * type having too many parameters.
 391      * </ul>
 392      *
 393      * <h1><a id="access"></a>Access checking</h1>
 394      * Access checks are applied in the factory methods of {@code Lookup},
 395      * when a method handle is created.
 396      * This is a key difference from the Core Reflection API, since
 397      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 398      * performs access checking against every caller, on every call.
 399      * <p>
 400      * All access checks start from a {@code Lookup} object, which
 401      * compares its recorded lookup class against all requests to
 402      * create method handles.
 403      * A single {@code Lookup} object can be used to create any number
 404      * of access-checked method handles, all checked against a single
 405      * lookup class.
 406      * <p>
 407      * A {@code Lookup} object can be shared with other trusted code,
 408      * such as a metaobject protocol.
 409      * A shared {@code Lookup} object delegates the capability
 410      * to create method handles on private members of the lookup class.
 411      * Even if privileged code uses the {@code Lookup} object,
 412      * the access checking is confined to the privileges of the
 413      * original lookup class.
 414      * <p>
 415      * A lookup can fail, because
 416      * the containing class is not accessible to the lookup class, or
 417      * because the desired class member is missing, or because the
 418      * desired class member is not accessible to the lookup class, or
 419      * because the lookup object is not trusted enough to access the member.
 420      * In any of these cases, a {@code ReflectiveOperationException} will be
 421      * thrown from the attempted lookup.  The exact class will be one of
 422      * the following:
 423      * <ul>
 424      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 425      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 426      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 427      * </ul>
 428      * <p>
 429      * In general, the conditions under which a method handle may be
 430      * looked up for a method {@code M} are no more restrictive than the conditions
 431      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 432      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 433      * a method handle lookup will generally raise a corresponding
 434      * checked exception, such as {@code NoSuchMethodException}.
 435      * And the effect of invoking the method handle resulting from the lookup
 436      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 437      * to executing the compiled, verified, and resolved call to {@code M}.
 438      * The same point is true of fields and constructors.
 439      * <p style="font-size:smaller;">
 440      * <em>Discussion:</em>
 441      * Access checks only apply to named and reflected methods,
 442      * constructors, and fields.
 443      * Other method handle creation methods, such as
 444      * {@link MethodHandle#asType MethodHandle.asType},
 445      * do not require any access checks, and are used
 446      * independently of any {@code Lookup} object.
 447      * <p>
 448      * If the desired member is {@code protected}, the usual JVM rules apply,
 449      * including the requirement that the lookup class must be either be in the
 450      * same package as the desired member, or must inherit that member.
 451      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 452      * In addition, if the desired member is a non-static field or method
 453      * in a different package, the resulting method handle may only be applied
 454      * to objects of the lookup class or one of its subclasses.
 455      * This requirement is enforced by narrowing the type of the leading
 456      * {@code this} parameter from {@code C}
 457      * (which will necessarily be a superclass of the lookup class)
 458      * to the lookup class itself.
 459      * <p>
 460      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 461      * that the receiver argument must match both the resolved method <em>and</em>
 462      * the current class.  Again, this requirement is enforced by narrowing the
 463      * type of the leading parameter to the resulting method handle.
 464      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 465      * <p>
 466      * The JVM represents constructors and static initializer blocks as internal methods
 467      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 468      * The internal syntax of invocation instructions allows them to refer to such internal
 469      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 470      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 471      * <p>
 472      * If the relationship between nested types is expressed directly through the
 473      * {@code NestHost} and {@code NestMembers} attributes
 474      * (see the Java Virtual Machine Specification, sections 4.7.28 and 4.7.29),
 475      * then the associated {@code Lookup} object provides direct access to
 476      * the lookup class and all of its nestmates
 477      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 478      * Otherwise, access between nested classes is obtained by the Java compiler creating
 479      * a wrapper method to access a private method of another class in the same nest.
 480      * For example, a nested class {@code C.D}
 481      * can access private members within other related classes such as
 482      * {@code C}, {@code C.D.E}, or {@code C.B},
 483      * but the Java compiler may need to generate wrapper methods in
 484      * those related classes.  In such cases, a {@code Lookup} object on
 485      * {@code C.E} would be unable to access those private members.
 486      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 487      * which can transform a lookup on {@code C.E} into one on any of those other
 488      * classes, without special elevation of privilege.
 489      * <p>
 490      * The accesses permitted to a given lookup object may be limited,
 491      * according to its set of {@link #lookupModes lookupModes},
 492      * to a subset of members normally accessible to the lookup class.
 493      * For example, the {@link MethodHandles#publicLookup publicLookup}
 494      * method produces a lookup object which is only allowed to access
 495      * public members in public classes of exported packages.
 496      * The caller sensitive method {@link MethodHandles#lookup lookup}
 497      * produces a lookup object with full capabilities relative to
 498      * its caller class, to emulate all supported bytecode behaviors.
 499      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 500      * with fewer access modes than the original lookup object.
 501      *
 502      * <p style="font-size:smaller;">
 503      * <a id="privacc"></a>
 504      * <em>Discussion of private access:</em>
 505      * We say that a lookup has <em>private access</em>
 506      * if its {@linkplain #lookupModes lookup modes}
 507      * include the possibility of accessing {@code private} members
 508      * (which includes the private members of nestmates).
 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 and its nestmates
 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}.
 738          *  A lookup object on a new lookup class
 739          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 740          *  may have some mode bits set to zero.
 741          *  Mode bits can also be
 742          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
 743          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
 744          *  The purpose of this is to restrict access via the new lookup object,
 745          *  so that it can access only names which can be reached by the original
 746          *  lookup object, and also by the new lookup class.
 747          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 748          *  @see #in
 749          *  @see #dropLookupMode
 750          *
 751          *  @revised 9
 752          *  @spec JPMS
 753          */
 754         public int lookupModes() {
 755             return allowedModes & ALL_MODES;
 756         }
 757 
 758         /** Embody the current class (the lookupClass) as a lookup class
 759          * for method handle creation.
 760          * Must be called by from a method in this package,
 761          * which in turn is called by a method not in this package.
 762          */
 763         Lookup(Class<?> lookupClass) {
 764             this(lookupClass, FULL_POWER_MODES);
 765             // make sure we haven't accidentally picked up a privileged class:
 766             checkUnprivilegedlookupClass(lookupClass);
 767         }
 768 
 769         private Lookup(Class<?> lookupClass, int allowedModes) {
 770             this.lookupClass = lookupClass;
 771             this.allowedModes = allowedModes;
 772         }
 773 
 774         /**
 775          * Creates a lookup on the specified new lookup class.
 776          * The resulting object will report the specified
 777          * class as its own {@link #lookupClass() lookupClass}.
 778          * <p>
 779          * However, the resulting {@code Lookup} object is guaranteed
 780          * to have no more access capabilities than the original.
 781          * In particular, access capabilities can be lost as follows:<ul>
 782          * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and
 783          * the new lookup class is in a different module {@code M}, then no members, not
 784          * even public members in {@code M}'s exported packages, will be accessible.
 785          * The exception to this is when this lookup is {@link #publicLookup()
 786          * publicLookup}, in which case {@code PUBLIC} access is not lost.
 787          * <li>If the old lookup class is in an unnamed module, and the new lookup class
 788          * is a different module then {@link #MODULE MODULE} access is lost.
 789          * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost.
 790          * <li>If the new lookup class is in a different package
 791          * than the old one, protected and default (package) members will not be accessible.
 792          * <li>If the new lookup class is not within the same package member
 793          * as the old one, private members will not be accessible, and protected members
 794          * will not be accessible by virtue of inheritance.
 795          * (Protected members may continue to be accessible because of package sharing.)
 796          * <li>If the new lookup class is not accessible to the old lookup class,
 797          * then no members, not even public members, will be accessible.
 798          * (In all other cases, public members will continue to be accessible.)
 799          * </ul>
 800          * <p>
 801          * The resulting lookup's capabilities for loading classes
 802          * (used during {@link #findClass} invocations)
 803          * are determined by the lookup class' loader,
 804          * which may change due to this operation.
 805          *
 806          * @param requestedLookupClass the desired lookup class for the new lookup object
 807          * @return a lookup object which reports the desired lookup class, or the same object
 808          * if there is no change
 809          * @throws NullPointerException if the argument is null
 810          *
 811          * @revised 9
 812          * @spec JPMS
 813          */
 814         public Lookup in(Class<?> requestedLookupClass) {
 815             Objects.requireNonNull(requestedLookupClass);
 816             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 817                 return new Lookup(requestedLookupClass, FULL_POWER_MODES);
 818             if (requestedLookupClass == this.lookupClass)
 819                 return this;  // keep same capabilities
 820             int newModes = (allowedModes & FULL_POWER_MODES);
 821             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 822                 // Need to drop all access when teleporting from a named module to another
 823                 // module. The exception is publicLookup where PUBLIC is not lost.
 824                 if (this.lookupClass.getModule().isNamed()
 825                     && (this.allowedModes & UNCONDITIONAL) == 0)
 826                     newModes = 0;
 827                 else
 828                     newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
 829             }
 830             if ((newModes & PACKAGE) != 0
 831                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 832                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
 833             }
 834             // Allow nestmate lookups to be created without special privilege:
 835             if ((newModes & PRIVATE) != 0
 836                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 837                 newModes &= ~(PRIVATE|PROTECTED);
 838             }
 839             if ((newModes & PUBLIC) != 0
 840                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 841                 // The requested class it not accessible from the lookup class.
 842                 // No permissions.
 843                 newModes = 0;
 844             }
 845 
 846             checkUnprivilegedlookupClass(requestedLookupClass);
 847             return new Lookup(requestedLookupClass, newModes);
 848         }
 849 
 850 
 851         /**
 852          * Creates a lookup on the same lookup class which this lookup object
 853          * finds members, but with a lookup mode that has lost the given lookup mode.
 854          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
 855          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}.
 856          * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always
 857          * dropped and so the resulting lookup mode will never have these access capabilities.
 858          * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE}
 859          * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will
 860          * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC}
 861          * is dropped then the resulting lookup has no access.
 862          * @param modeToDrop the lookup mode to drop
 863          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
 864          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
 865          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
 866          * @see MethodHandles#privateLookupIn
 867          * @since 9
 868          */
 869         public Lookup dropLookupMode(int modeToDrop) {
 870             int oldModes = lookupModes();
 871             int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL);
 872             switch (modeToDrop) {
 873                 case PUBLIC: newModes &= ~(ALL_MODES); break;
 874                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
 875                 case PACKAGE: newModes &= ~(PRIVATE); break;
 876                 case PROTECTED:
 877                 case PRIVATE:
 878                 case UNCONDITIONAL: break;
 879                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
 880             }
 881             if (newModes == oldModes) return this;  // return self if no change
 882             return new Lookup(lookupClass(), newModes);
 883         }
 884 
 885         /**
 886          * Defines a class to the same class loader and in the same runtime package and
 887          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
 888          * {@linkplain #lookupClass() lookup class}.
 889          *
 890          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
 891          * {@link #PACKAGE PACKAGE} access as default (package) members will be
 892          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
 893          * that the lookup object was created by a caller in the runtime package (or derived
 894          * from a lookup originally created by suitably privileged code to a target class in
 895          * the runtime package). </p>
 896          *
 897          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
 898          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
 899          * same package as the lookup class. </p>
 900          *
 901          * <p> This method does not run the class initializer. The class initializer may
 902          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
 903          * Specification</em>. </p>
 904          *
 905          * <p> If there is a security manager, its {@code checkPermission} method is first called
 906          * to check {@code RuntimePermission("defineClass")}. </p>
 907          *
 908          * @param bytes the class bytes
 909          * @return the {@code Class} object for the class
 910          * @throws IllegalArgumentException the bytes are for a class in a different package
 911          * to the lookup class
 912          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
 913          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
 914          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
 915          * @throws SecurityException if denied by the security manager
 916          * @throws NullPointerException if {@code bytes} is {@code null}
 917          * @since 9
 918          * @spec JPMS
 919          * @see Lookup#privateLookupIn
 920          * @see Lookup#dropLookupMode
 921          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
 922          */
 923         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
 924             SecurityManager sm = System.getSecurityManager();
 925             if (sm != null)
 926                 sm.checkPermission(new RuntimePermission("defineClass"));
 927             if ((lookupModes() & PACKAGE) == 0)
 928                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
 929             assert (lookupModes() & (MODULE|PUBLIC)) != 0;
 930 
 931             // parse class bytes to get class name (in internal form)
 932             bytes = bytes.clone();
 933             String name;
 934             try {
 935                 ClassReader reader = new ClassReader(bytes);
 936                 name = reader.getClassName();
 937             } catch (RuntimeException e) {
 938                 // ASM exceptions are poorly specified
 939                 ClassFormatError cfe = new ClassFormatError();
 940                 cfe.initCause(e);
 941                 throw cfe;
 942             }
 943 
 944             // get package and class name in binary form
 945             String cn, pn;
 946             int index = name.lastIndexOf('/');
 947             if (index == -1) {
 948                 cn = name;
 949                 pn = "";
 950             } else {
 951                 cn = name.replace('/', '.');
 952                 pn = cn.substring(0, index);
 953             }
 954             if (!pn.equals(lookupClass.getPackageName())) {
 955                 throw new IllegalArgumentException("Class not in same package as lookup class");
 956             }
 957 
 958             // invoke the class loader's defineClass method
 959             ClassLoader loader = lookupClass.getClassLoader();
 960             ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null;
 961             String source = "__Lookup_defineClass__";
 962             Class<?> clazz = SharedSecrets.getJavaLangAccess().defineClass(loader, cn, bytes, pd, source);
 963             assert clazz.getClassLoader() == lookupClass.getClassLoader()
 964                     && clazz.getPackageName().equals(lookupClass.getPackageName())
 965                     && protectionDomain(clazz) == lookupClassProtectionDomain();
 966             return clazz;
 967         }
 968 
 969         private ProtectionDomain lookupClassProtectionDomain() {
 970             ProtectionDomain pd = cachedProtectionDomain;
 971             if (pd == null) {
 972                 cachedProtectionDomain = pd = protectionDomain(lookupClass);
 973             }
 974             return pd;
 975         }
 976 
 977         private ProtectionDomain protectionDomain(Class<?> clazz) {
 978             PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain;
 979             return AccessController.doPrivileged(pa);
 980         }
 981 
 982         // cached protection domain
 983         private volatile ProtectionDomain cachedProtectionDomain;
 984 
 985 
 986         // Make sure outer class is initialized first.
 987         static { IMPL_NAMES.getClass(); }
 988 
 989         /** Package-private version of lookup which is trusted. */
 990         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 991 
 992         /** Version of lookup which is trusted minimally.
 993          *  It can only be used to create method handles to publicly accessible
 994          *  members in packages that are exported unconditionally.
 995          */
 996         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC|UNCONDITIONAL));
 997 
 998         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
 999             String name = lookupClass.getName();
1000             if (name.startsWith("java.lang.invoke."))
1001                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
1002         }
1003 
1004         /**
1005          * Displays the name of the class from which lookups are to be made.
1006          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
1007          * If there are restrictions on the access permitted to this lookup,
1008          * this is indicated by adding a suffix to the class name, consisting
1009          * of a slash and a keyword.  The keyword represents the strongest
1010          * allowed access, and is chosen as follows:
1011          * <ul>
1012          * <li>If no access is allowed, the suffix is "/noaccess".
1013          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
1014          * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup".
1015          * <li>If only public and module access are allowed, the suffix is "/module".
1016          * <li>If only public, module and package access are allowed, the suffix is "/package".
1017          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
1018          * </ul>
1019          * If none of the above cases apply, it is the case that full
1020          * access (public, module, package, private, and protected) is allowed.
1021          * In this case, no suffix is added.
1022          * This is true only of an object obtained originally from
1023          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
1024          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
1025          * always have restricted access, and will display a suffix.
1026          * <p>
1027          * (It may seem strange that protected access should be
1028          * stronger than private access.  Viewed independently from
1029          * package access, protected access is the first to be lost,
1030          * because it requires a direct subclass relationship between
1031          * caller and callee.)
1032          * @see #in
1033          *
1034          * @revised 9
1035          * @spec JPMS
1036          */
1037         @Override
1038         public String toString() {
1039             String cname = lookupClass.getName();
1040             switch (allowedModes) {
1041             case 0:  // no privileges
1042                 return cname + "/noaccess";
1043             case PUBLIC:
1044                 return cname + "/public";
1045             case PUBLIC|UNCONDITIONAL:
1046                 return cname  + "/publicLookup";
1047             case PUBLIC|MODULE:
1048                 return cname + "/module";
1049             case PUBLIC|MODULE|PACKAGE:
1050                 return cname + "/package";
1051             case FULL_POWER_MODES & ~PROTECTED:
1052                 return cname + "/private";
1053             case FULL_POWER_MODES:
1054                 return cname;
1055             case TRUSTED:
1056                 return "/trusted";  // internal only; not exported
1057             default:  // Should not happen, but it's a bitfield...
1058                 cname = cname + "/" + Integer.toHexString(allowedModes);
1059                 assert(false) : cname;
1060                 return cname;
1061             }
1062         }
1063 
1064         /**
1065          * Produces a method handle for a static method.
1066          * The type of the method handle will be that of the method.
1067          * (Since static methods do not take receivers, there is no
1068          * additional receiver argument inserted into the method handle type,
1069          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
1070          * The method and all its argument types must be accessible to the lookup object.
1071          * <p>
1072          * The returned method handle will have
1073          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1074          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1075          * <p>
1076          * If the returned method handle is invoked, the method's class will
1077          * be initialized, if it has not already been initialized.
1078          * <p><b>Example:</b>
1079          * <blockquote><pre>{@code
1080 import static java.lang.invoke.MethodHandles.*;
1081 import static java.lang.invoke.MethodType.*;
1082 ...
1083 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
1084   "asList", methodType(List.class, Object[].class));
1085 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
1086          * }</pre></blockquote>
1087          * @param refc the class from which the method is accessed
1088          * @param name the name of the method
1089          * @param type the type of the method
1090          * @return the desired method handle
1091          * @throws NoSuchMethodException if the method does not exist
1092          * @throws IllegalAccessException if access checking fails,
1093          *                                or if the method is not {@code static},
1094          *                                or if the method's variable arity modifier bit
1095          *                                is set and {@code asVarargsCollector} fails
1096          * @exception SecurityException if a security manager is present and it
1097          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1098          * @throws NullPointerException if any argument is null
1099          */
1100         public
1101         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1102             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
1103             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
1104         }
1105 
1106         /**
1107          * Produces a method handle for a virtual method.
1108          * The type of the method handle will be that of the method,
1109          * with the receiver type (usually {@code refc}) prepended.
1110          * The method and all its argument types must be accessible to the lookup object.
1111          * <p>
1112          * When called, the handle will treat the first argument as a receiver
1113          * and, for non-private methods, dispatch on the receiver's type to determine which method
1114          * implementation to enter.
1115          * For private methods the named method in {@code refc} will be invoked on the receiver.
1116          * (The dispatching action is identical with that performed by an
1117          * {@code invokevirtual} or {@code invokeinterface} instruction.)
1118          * <p>
1119          * The first argument will be of type {@code refc} if the lookup
1120          * class has full privileges to access the member.  Otherwise
1121          * the member must be {@code protected} and the first argument
1122          * will be restricted in type to the lookup class.
1123          * <p>
1124          * The returned method handle will have
1125          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1126          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1127          * <p>
1128          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
1129          * instructions and method handles produced by {@code findVirtual},
1130          * if the class is {@code MethodHandle} and the name string is
1131          * {@code invokeExact} or {@code invoke}, the resulting
1132          * method handle is equivalent to one produced by
1133          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
1134          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
1135          * with the same {@code type} argument.
1136          * <p>
1137          * If the class is {@code VarHandle} and the name string corresponds to
1138          * the name of a signature-polymorphic access mode method, the resulting
1139          * method handle is equivalent to one produced by
1140          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
1141          * the access mode corresponding to the name string and with the same
1142          * {@code type} arguments.
1143          * <p>
1144          * <b>Example:</b>
1145          * <blockquote><pre>{@code
1146 import static java.lang.invoke.MethodHandles.*;
1147 import static java.lang.invoke.MethodType.*;
1148 ...
1149 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
1150   "concat", methodType(String.class, String.class));
1151 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
1152   "hashCode", methodType(int.class));
1153 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
1154   "hashCode", methodType(int.class));
1155 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
1156 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
1157 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
1158 // interface method:
1159 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
1160   "subSequence", methodType(CharSequence.class, int.class, int.class));
1161 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
1162 // constructor "internal method" must be accessed differently:
1163 MethodType MT_newString = methodType(void.class); //()V for new String()
1164 try { assertEquals("impossible", lookup()
1165         .findVirtual(String.class, "<init>", MT_newString));
1166  } catch (NoSuchMethodException ex) { } // OK
1167 MethodHandle MH_newString = publicLookup()
1168   .findConstructor(String.class, MT_newString);
1169 assertEquals("", (String) MH_newString.invokeExact());
1170          * }</pre></blockquote>
1171          *
1172          * @param refc the class or interface from which the method is accessed
1173          * @param name the name of the method
1174          * @param type the type of the method, with the receiver argument omitted
1175          * @return the desired method handle
1176          * @throws NoSuchMethodException if the method does not exist
1177          * @throws IllegalAccessException if access checking fails,
1178          *                                or if the method is {@code static},
1179          *                                or if the method's variable arity modifier bit
1180          *                                is set and {@code asVarargsCollector} fails
1181          * @exception SecurityException if a security manager is present and it
1182          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1183          * @throws NullPointerException if any argument is null
1184          */
1185         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1186             if (refc == MethodHandle.class) {
1187                 MethodHandle mh = findVirtualForMH(name, type);
1188                 if (mh != null)  return mh;
1189             } else if (refc == VarHandle.class) {
1190                 MethodHandle mh = findVirtualForVH(name, type);
1191                 if (mh != null)  return mh;
1192             }
1193             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
1194             MemberName method = resolveOrFail(refKind, refc, name, type);
1195             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
1196         }
1197         private MethodHandle findVirtualForMH(String name, MethodType type) {
1198             // these names require special lookups because of the implicit MethodType argument
1199             if ("invoke".equals(name))
1200                 return invoker(type);
1201             if ("invokeExact".equals(name))
1202                 return exactInvoker(type);
1203             assert(!MemberName.isMethodHandleInvokeName(name));
1204             return null;
1205         }
1206         private MethodHandle findVirtualForVH(String name, MethodType type) {
1207             try {
1208                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
1209             } catch (IllegalArgumentException e) {
1210                 return null;
1211             }
1212         }
1213 
1214         /**
1215          * Produces a method handle which creates an object and initializes it, using
1216          * the constructor of the specified type.
1217          * The parameter types of the method handle will be those of the constructor,
1218          * while the return type will be a reference to the constructor's class.
1219          * The constructor and all its argument types must be accessible to the lookup object.
1220          * <p>
1221          * The requested type must have a return type of {@code void}.
1222          * (This is consistent with the JVM's treatment of constructor type descriptors.)
1223          * <p>
1224          * The returned method handle will have
1225          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1226          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1227          * <p>
1228          * If the returned method handle is invoked, the constructor's class will
1229          * be initialized, if it has not already been initialized.
1230          * <p><b>Example:</b>
1231          * <blockquote><pre>{@code
1232 import static java.lang.invoke.MethodHandles.*;
1233 import static java.lang.invoke.MethodType.*;
1234 ...
1235 MethodHandle MH_newArrayList = publicLookup().findConstructor(
1236   ArrayList.class, methodType(void.class, Collection.class));
1237 Collection orig = Arrays.asList("x", "y");
1238 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
1239 assert(orig != copy);
1240 assertEquals(orig, copy);
1241 // a variable-arity constructor:
1242 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
1243   ProcessBuilder.class, methodType(void.class, String[].class));
1244 ProcessBuilder pb = (ProcessBuilder)
1245   MH_newProcessBuilder.invoke("x", "y", "z");
1246 assertEquals("[x, y, z]", pb.command().toString());
1247          * }</pre></blockquote>
1248          * @param refc the class or interface from which the method is accessed
1249          * @param type the type of the method, with the receiver argument omitted, and a void return type
1250          * @return the desired method handle
1251          * @throws NoSuchMethodException if the constructor does not exist
1252          * @throws IllegalAccessException if access checking fails
1253          *                                or if the method's variable arity modifier bit
1254          *                                is set and {@code asVarargsCollector} fails
1255          * @exception SecurityException if a security manager is present and it
1256          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1257          * @throws NullPointerException if any argument is null
1258          */
1259         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1260             if (refc.isArray()) {
1261                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
1262             }
1263             String name = "<init>";
1264             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1265             return getDirectConstructor(refc, ctor);
1266         }
1267 
1268         /**
1269          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1270          * initializer of the class is not run.
1271          * <p>
1272          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1273          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1274          * load the requested class, and then determines whether the class is accessible to this lookup object.
1275          *
1276          * @param targetName the fully qualified name of the class to be looked up.
1277          * @return the requested class.
1278          * @exception SecurityException if a security manager is present and it
1279          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1280          * @throws LinkageError if the linkage fails
1281          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
1282          * @throws IllegalAccessException if the class is not accessible, using the allowed access
1283          * modes.
1284          * @exception SecurityException if a security manager is present and it
1285          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1286          * @since 9
1287          */
1288         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1289             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1290             return accessClass(targetClass);
1291         }
1292 
1293         /**
1294          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1295          * static initializer of the class is not run.
1296          * <p>
1297          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1298          * {@linkplain #lookupModes() lookup modes}.
1299          *
1300          * @param targetClass the class to be access-checked
1301          *
1302          * @return the class that has been access-checked
1303          *
1304          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1305          * modes.
1306          * @exception SecurityException if a security manager is present and it
1307          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1308          * @since 9
1309          */
1310         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1311             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1312                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1313             }
1314             checkSecurityManager(targetClass, null);
1315             return targetClass;
1316         }
1317 
1318         /**
1319          * Produces an early-bound method handle for a virtual method.
1320          * It will bypass checks for overriding methods on the receiver,
1321          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1322          * instruction from within the explicitly specified {@code specialCaller}.
1323          * The type of the method handle will be that of the method,
1324          * with a suitably restricted receiver type prepended.
1325          * (The receiver type will be {@code specialCaller} or a subtype.)
1326          * The method and all its argument types must be accessible
1327          * to the lookup object.
1328          * <p>
1329          * Before method resolution,
1330          * if the explicitly specified caller class is not identical with the
1331          * lookup class, or if this lookup object does not have
1332          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1333          * privileges, the access fails.
1334          * <p>
1335          * The returned method handle will have
1336          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1337          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1338          * <p style="font-size:smaller;">
1339          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1340          * even though the {@code invokespecial} instruction can refer to them
1341          * in special circumstances.  Use {@link #findConstructor findConstructor}
1342          * to access instance initialization methods in a safe manner.)</em>
1343          * <p><b>Example:</b>
1344          * <blockquote><pre>{@code
1345 import static java.lang.invoke.MethodHandles.*;
1346 import static java.lang.invoke.MethodType.*;
1347 ...
1348 static class Listie extends ArrayList {
1349   public String toString() { return "[wee Listie]"; }
1350   static Lookup lookup() { return MethodHandles.lookup(); }
1351 }
1352 ...
1353 // no access to constructor via invokeSpecial:
1354 MethodHandle MH_newListie = Listie.lookup()
1355   .findConstructor(Listie.class, methodType(void.class));
1356 Listie l = (Listie) MH_newListie.invokeExact();
1357 try { assertEquals("impossible", Listie.lookup().findSpecial(
1358         Listie.class, "<init>", methodType(void.class), Listie.class));
1359  } catch (NoSuchMethodException ex) { } // OK
1360 // access to super and self methods via invokeSpecial:
1361 MethodHandle MH_super = Listie.lookup().findSpecial(
1362   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1363 MethodHandle MH_this = Listie.lookup().findSpecial(
1364   Listie.class, "toString" , methodType(String.class), Listie.class);
1365 MethodHandle MH_duper = Listie.lookup().findSpecial(
1366   Object.class, "toString" , methodType(String.class), Listie.class);
1367 assertEquals("[]", (String) MH_super.invokeExact(l));
1368 assertEquals(""+l, (String) MH_this.invokeExact(l));
1369 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1370 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1371         String.class, "toString", methodType(String.class), Listie.class));
1372  } catch (IllegalAccessException ex) { } // OK
1373 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1374 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1375          * }</pre></blockquote>
1376          *
1377          * @param refc the class or interface from which the method is accessed
1378          * @param name the name of the method (which must not be "&lt;init&gt;")
1379          * @param type the type of the method, with the receiver argument omitted
1380          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1381          * @return the desired method handle
1382          * @throws NoSuchMethodException if the method does not exist
1383          * @throws IllegalAccessException if access checking fails,
1384          *                                or if the method is {@code static},
1385          *                                or if the method's variable arity modifier bit
1386          *                                is set and {@code asVarargsCollector} fails
1387          * @exception SecurityException if a security manager is present and it
1388          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1389          * @throws NullPointerException if any argument is null
1390          */
1391         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1392                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1393             checkSpecialCaller(specialCaller, refc);
1394             Lookup specialLookup = this.in(specialCaller);
1395             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1396             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1397         }
1398 
1399         /**
1400          * Produces a method handle giving read access to a non-static field.
1401          * The type of the method handle will have a return type of the field's
1402          * value type.
1403          * The method handle's single argument will be the instance containing
1404          * the field.
1405          * Access checking is performed immediately on behalf of the lookup class.
1406          * @param refc the class or interface from which the method is accessed
1407          * @param name the field's name
1408          * @param type the field's type
1409          * @return a method handle which can load values from the field
1410          * @throws NoSuchFieldException if the field does not exist
1411          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1412          * @exception SecurityException if a security manager is present and it
1413          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1414          * @throws NullPointerException if any argument is null
1415          * @see #findVarHandle(Class, String, Class)
1416          */
1417         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1418             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1419             return getDirectField(REF_getField, refc, field);
1420         }
1421 
1422         /**
1423          * Produces a method handle giving write access to a non-static field.
1424          * The type of the method handle will have a void return type.
1425          * The method handle will take two arguments, the instance containing
1426          * the field, and the value to be stored.
1427          * The second argument will be of the field's value type.
1428          * Access checking is performed immediately on behalf of the lookup class.
1429          * @param refc the class or interface from which the method is accessed
1430          * @param name the field's name
1431          * @param type the field's type
1432          * @return a method handle which can store values into the field
1433          * @throws NoSuchFieldException if the field does not exist
1434          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1435          * @exception SecurityException if a security manager is present and it
1436          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1437          * @throws NullPointerException if any argument is null
1438          * @see #findVarHandle(Class, String, Class)
1439          */
1440         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1441             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1442             return getDirectField(REF_putField, refc, field);
1443         }
1444 
1445         /**
1446          * Produces a VarHandle giving access to a non-static field {@code name}
1447          * of type {@code type} declared in a class of type {@code recv}.
1448          * The VarHandle's variable type is {@code type} and it has one
1449          * coordinate type, {@code recv}.
1450          * <p>
1451          * Access checking is performed immediately on behalf of the lookup
1452          * class.
1453          * <p>
1454          * Certain access modes of the returned VarHandle are unsupported under
1455          * the following conditions:
1456          * <ul>
1457          * <li>if the field is declared {@code final}, then the write, atomic
1458          *     update, numeric atomic update, and bitwise atomic update access
1459          *     modes are unsupported.
1460          * <li>if the field type is anything other than {@code byte},
1461          *     {@code short}, {@code char}, {@code int}, {@code long},
1462          *     {@code float}, or {@code double} then numeric atomic update
1463          *     access modes are unsupported.
1464          * <li>if the field type is anything other than {@code boolean},
1465          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1466          *     {@code long} then bitwise atomic update access modes are
1467          *     unsupported.
1468          * </ul>
1469          * <p>
1470          * If the field is declared {@code volatile} then the returned VarHandle
1471          * will override access to the field (effectively ignore the
1472          * {@code volatile} declaration) in accordance to its specified
1473          * access modes.
1474          * <p>
1475          * If the field type is {@code float} or {@code double} then numeric
1476          * and atomic update access modes compare values using their bitwise
1477          * representation (see {@link Float#floatToRawIntBits} and
1478          * {@link Double#doubleToRawLongBits}, respectively).
1479          * @apiNote
1480          * Bitwise comparison of {@code float} values or {@code double} values,
1481          * as performed by the numeric and atomic update access modes, differ
1482          * from the primitive {@code ==} operator and the {@link Float#equals}
1483          * and {@link Double#equals} methods, specifically with respect to
1484          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1485          * Care should be taken when performing a compare and set or a compare
1486          * and exchange operation with such values since the operation may
1487          * unexpectedly fail.
1488          * There are many possible NaN values that are considered to be
1489          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1490          * provided by Java can distinguish between them.  Operation failure can
1491          * occur if the expected or witness value is a NaN value and it is
1492          * transformed (perhaps in a platform specific manner) into another NaN
1493          * value, and thus has a different bitwise representation (see
1494          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1495          * details).
1496          * The values {@code -0.0} and {@code +0.0} have different bitwise
1497          * representations but are considered equal when using the primitive
1498          * {@code ==} operator.  Operation failure can occur if, for example, a
1499          * numeric algorithm computes an expected value to be say {@code -0.0}
1500          * and previously computed the witness value to be say {@code +0.0}.
1501          * @param recv the receiver class, of type {@code R}, that declares the
1502          * non-static field
1503          * @param name the field's name
1504          * @param type the field's type, of type {@code T}
1505          * @return a VarHandle giving access to non-static fields.
1506          * @throws NoSuchFieldException if the field does not exist
1507          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1508          * @exception SecurityException if a security manager is present and it
1509          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1510          * @throws NullPointerException if any argument is null
1511          * @since 9
1512          */
1513         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1514             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
1515             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
1516             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
1517         }
1518 
1519         /**
1520          * Produces a method handle giving read access to a static field.
1521          * The type of the method handle will have a return type of the field's
1522          * value type.
1523          * The method handle will take no arguments.
1524          * Access checking is performed immediately on behalf of the lookup class.
1525          * <p>
1526          * If the returned method handle is invoked, the field's class will
1527          * be initialized, if it has not already been initialized.
1528          * @param refc the class or interface from which the method is accessed
1529          * @param name the field's name
1530          * @param type the field's type
1531          * @return a method handle which can load values from the field
1532          * @throws NoSuchFieldException if the field does not exist
1533          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1534          * @exception SecurityException if a security manager is present and it
1535          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1536          * @throws NullPointerException if any argument is null
1537          */
1538         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1539             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1540             return getDirectField(REF_getStatic, refc, field);
1541         }
1542 
1543         /**
1544          * Produces a method handle giving write access to a static field.
1545          * The type of the method handle will have a void return type.
1546          * The method handle will take a single
1547          * argument, of the field's value type, the value to be stored.
1548          * Access checking is performed immediately on behalf of the lookup class.
1549          * <p>
1550          * If the returned method handle is invoked, the field's class will
1551          * be initialized, if it has not already been initialized.
1552          * @param refc the class or interface from which the method is accessed
1553          * @param name the field's name
1554          * @param type the field's type
1555          * @return a method handle which can store values into the field
1556          * @throws NoSuchFieldException if the field does not exist
1557          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1558          * @exception SecurityException if a security manager is present and it
1559          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1560          * @throws NullPointerException if any argument is null
1561          */
1562         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1563             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1564             return getDirectField(REF_putStatic, refc, field);
1565         }
1566 
1567         /**
1568          * Produces a VarHandle giving access to a static field {@code name} of
1569          * type {@code type} declared in a class of type {@code decl}.
1570          * The VarHandle's variable type is {@code type} and it has no
1571          * coordinate types.
1572          * <p>
1573          * Access checking is performed immediately on behalf of the lookup
1574          * class.
1575          * <p>
1576          * If the returned VarHandle is operated on, the declaring class will be
1577          * initialized, if it has not already been initialized.
1578          * <p>
1579          * Certain access modes of the returned VarHandle are unsupported under
1580          * the following conditions:
1581          * <ul>
1582          * <li>if the field is declared {@code final}, then the write, atomic
1583          *     update, numeric atomic update, and bitwise atomic update access
1584          *     modes are unsupported.
1585          * <li>if the field type is anything other than {@code byte},
1586          *     {@code short}, {@code char}, {@code int}, {@code long},
1587          *     {@code float}, or {@code double}, then numeric atomic update
1588          *     access modes are unsupported.
1589          * <li>if the field type is anything other than {@code boolean},
1590          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1591          *     {@code long} then bitwise atomic update access modes are
1592          *     unsupported.
1593          * </ul>
1594          * <p>
1595          * If the field is declared {@code volatile} then the returned VarHandle
1596          * will override access to the field (effectively ignore the
1597          * {@code volatile} declaration) in accordance to its specified
1598          * access modes.
1599          * <p>
1600          * If the field type is {@code float} or {@code double} then numeric
1601          * and atomic update access modes compare values using their bitwise
1602          * representation (see {@link Float#floatToRawIntBits} and
1603          * {@link Double#doubleToRawLongBits}, respectively).
1604          * @apiNote
1605          * Bitwise comparison of {@code float} values or {@code double} values,
1606          * as performed by the numeric and atomic update access modes, differ
1607          * from the primitive {@code ==} operator and the {@link Float#equals}
1608          * and {@link Double#equals} methods, specifically with respect to
1609          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1610          * Care should be taken when performing a compare and set or a compare
1611          * and exchange operation with such values since the operation may
1612          * unexpectedly fail.
1613          * There are many possible NaN values that are considered to be
1614          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1615          * provided by Java can distinguish between them.  Operation failure can
1616          * occur if the expected or witness value is a NaN value and it is
1617          * transformed (perhaps in a platform specific manner) into another NaN
1618          * value, and thus has a different bitwise representation (see
1619          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1620          * details).
1621          * The values {@code -0.0} and {@code +0.0} have different bitwise
1622          * representations but are considered equal when using the primitive
1623          * {@code ==} operator.  Operation failure can occur if, for example, a
1624          * numeric algorithm computes an expected value to be say {@code -0.0}
1625          * and previously computed the witness value to be say {@code +0.0}.
1626          * @param decl the class that declares the static field
1627          * @param name the field's name
1628          * @param type the field's type, of type {@code T}
1629          * @return a VarHandle giving access to a static field
1630          * @throws NoSuchFieldException if the field does not exist
1631          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1632          * @exception SecurityException if a security manager is present and it
1633          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1634          * @throws NullPointerException if any argument is null
1635          * @since 9
1636          */
1637         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1638             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
1639             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
1640             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
1641         }
1642 
1643         /**
1644          * Produces an early-bound method handle for a non-static method.
1645          * The receiver must have a supertype {@code defc} in which a method
1646          * of the given name and type is accessible to the lookup class.
1647          * The method and all its argument types must be accessible to the lookup object.
1648          * The type of the method handle will be that of the method,
1649          * without any insertion of an additional receiver parameter.
1650          * The given receiver will be bound into the method handle,
1651          * so that every call to the method handle will invoke the
1652          * requested method on the given receiver.
1653          * <p>
1654          * The returned method handle will have
1655          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1656          * the method's variable arity modifier bit ({@code 0x0080}) is set
1657          * <em>and</em> the trailing array argument is not the only argument.
1658          * (If the trailing array argument is the only argument,
1659          * the given receiver value will be bound to it.)
1660          * <p>
1661          * This is almost equivalent to the following code, with some differences noted below:
1662          * <blockquote><pre>{@code
1663 import static java.lang.invoke.MethodHandles.*;
1664 import static java.lang.invoke.MethodType.*;
1665 ...
1666 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1667 MethodHandle mh1 = mh0.bindTo(receiver);
1668 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
1669 return mh1;
1670          * }</pre></blockquote>
1671          * where {@code defc} is either {@code receiver.getClass()} or a super
1672          * type of that class, in which the requested method is accessible
1673          * to the lookup class.
1674          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
1675          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
1676          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
1677          * the receiver is restricted by {@code findVirtual} to the lookup class.)
1678          * @param receiver the object from which the method is accessed
1679          * @param name the name of the method
1680          * @param type the type of the method, with the receiver argument omitted
1681          * @return the desired method handle
1682          * @throws NoSuchMethodException if the method does not exist
1683          * @throws IllegalAccessException if access checking fails
1684          *                                or if the method's variable arity modifier bit
1685          *                                is set and {@code asVarargsCollector} fails
1686          * @exception SecurityException if a security manager is present and it
1687          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1688          * @throws NullPointerException if any argument is null
1689          * @see MethodHandle#bindTo
1690          * @see #findVirtual
1691          */
1692         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1693             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1694             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1695             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerClass(method));
1696             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
1697                 throw new IllegalAccessException("The restricted defining class " +
1698                                                  mh.type().leadingReferenceParameter().getName() +
1699                                                  " is not assignable from receiver class " +
1700                                                  receiver.getClass().getName());
1701             }
1702             return mh.bindArgumentL(0, receiver).setVarargs(method);
1703         }
1704 
1705         /**
1706          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1707          * to <i>m</i>, if the lookup class has permission.
1708          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1709          * If <i>m</i> is virtual, overriding is respected on every call.
1710          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1711          * The type of the method handle will be that of the method,
1712          * with the receiver type prepended (but only if it is non-static).
1713          * If the method's {@code accessible} flag is not set,
1714          * access checking is performed immediately on behalf of the lookup class.
1715          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1716          * <p>
1717          * The returned method handle will have
1718          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1719          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1720          * <p>
1721          * If <i>m</i> is static, and
1722          * if the returned method handle is invoked, the method's class will
1723          * be initialized, if it has not already been initialized.
1724          * @param m the reflected method
1725          * @return a method handle which can invoke the reflected method
1726          * @throws IllegalAccessException if access checking fails
1727          *                                or if the method's variable arity modifier bit
1728          *                                is set and {@code asVarargsCollector} fails
1729          * @throws NullPointerException if the argument is null
1730          */
1731         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1732             if (m.getDeclaringClass() == MethodHandle.class) {
1733                 MethodHandle mh = unreflectForMH(m);
1734                 if (mh != null)  return mh;
1735             }
1736             if (m.getDeclaringClass() == VarHandle.class) {
1737                 MethodHandle mh = unreflectForVH(m);
1738                 if (mh != null)  return mh;
1739             }
1740             MemberName method = new MemberName(m);
1741             byte refKind = method.getReferenceKind();
1742             if (refKind == REF_invokeSpecial)
1743                 refKind = REF_invokeVirtual;
1744             assert(method.isMethod());
1745             @SuppressWarnings("deprecation")
1746             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1747             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1748         }
1749         private MethodHandle unreflectForMH(Method m) {
1750             // these names require special lookups because they throw UnsupportedOperationException
1751             if (MemberName.isMethodHandleInvokeName(m.getName()))
1752                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1753             return null;
1754         }
1755         private MethodHandle unreflectForVH(Method m) {
1756             // these names require special lookups because they throw UnsupportedOperationException
1757             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
1758                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
1759             return null;
1760         }
1761 
1762         /**
1763          * Produces a method handle for a reflected method.
1764          * It will bypass checks for overriding methods on the receiver,
1765          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1766          * instruction from within the explicitly specified {@code specialCaller}.
1767          * The type of the method handle will be that of the method,
1768          * with a suitably restricted receiver type prepended.
1769          * (The receiver type will be {@code specialCaller} or a subtype.)
1770          * If the method's {@code accessible} flag is not set,
1771          * access checking is performed immediately on behalf of the lookup class,
1772          * as if {@code invokespecial} instruction were being linked.
1773          * <p>
1774          * Before method resolution,
1775          * if the explicitly specified caller class is not identical with the
1776          * lookup class, or if this lookup object does not have
1777          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1778          * privileges, the access fails.
1779          * <p>
1780          * The returned method handle will have
1781          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1782          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1783          * @param m the reflected method
1784          * @param specialCaller the class nominally calling the method
1785          * @return a method handle which can invoke the reflected method
1786          * @throws IllegalAccessException if access checking fails,
1787          *                                or if the method is {@code static},
1788          *                                or if the method's variable arity modifier bit
1789          *                                is set and {@code asVarargsCollector} fails
1790          * @throws NullPointerException if any argument is null
1791          */
1792         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1793             checkSpecialCaller(specialCaller, null);
1794             Lookup specialLookup = this.in(specialCaller);
1795             MemberName method = new MemberName(m, true);
1796             assert(method.isMethod());
1797             // ignore m.isAccessible:  this is a new kind of access
1798             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1799         }
1800 
1801         /**
1802          * Produces a method handle for a reflected constructor.
1803          * The type of the method handle will be that of the constructor,
1804          * with the return type changed to the declaring class.
1805          * The method handle will perform a {@code newInstance} operation,
1806          * creating a new instance of the constructor's class on the
1807          * arguments passed to the method handle.
1808          * <p>
1809          * If the constructor's {@code accessible} flag is not set,
1810          * access checking is performed immediately on behalf of the lookup class.
1811          * <p>
1812          * The returned method handle will have
1813          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1814          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1815          * <p>
1816          * If the returned method handle is invoked, the constructor's class will
1817          * be initialized, if it has not already been initialized.
1818          * @param c the reflected constructor
1819          * @return a method handle which can invoke the reflected constructor
1820          * @throws IllegalAccessException if access checking fails
1821          *                                or if the method's variable arity modifier bit
1822          *                                is set and {@code asVarargsCollector} fails
1823          * @throws NullPointerException if the argument is null
1824          */
1825         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1826             MemberName ctor = new MemberName(c);
1827             assert(ctor.isConstructor());
1828             @SuppressWarnings("deprecation")
1829             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1830             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1831         }
1832 
1833         /**
1834          * Produces a method handle giving read access to a reflected field.
1835          * The type of the method handle will have a return type of the field's
1836          * value type.
1837          * If the field is static, the method handle will take no arguments.
1838          * Otherwise, its single argument will be the instance containing
1839          * the field.
1840          * If the field's {@code accessible} flag is not set,
1841          * access checking is performed immediately on behalf of the lookup class.
1842          * <p>
1843          * If the field is static, and
1844          * if the returned method handle is invoked, the field's class will
1845          * be initialized, if it has not already been initialized.
1846          * @param f the reflected field
1847          * @return a method handle which can load values from the reflected field
1848          * @throws IllegalAccessException if access checking fails
1849          * @throws NullPointerException if the argument is null
1850          */
1851         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1852             return unreflectField(f, false);
1853         }
1854         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1855             MemberName field = new MemberName(f, isSetter);
1856             assert(isSetter
1857                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1858                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1859             @SuppressWarnings("deprecation")
1860             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1861             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1862         }
1863 
1864         /**
1865          * Produces a method handle giving write access to a reflected field.
1866          * The type of the method handle will have a void return type.
1867          * If the field is static, the method handle will take a single
1868          * argument, of the field's value type, the value to be stored.
1869          * Otherwise, the two arguments will be the instance containing
1870          * the field, and the value to be stored.
1871          * If the field's {@code accessible} flag is not set,
1872          * access checking is performed immediately on behalf of the lookup class.
1873          * <p>
1874          * If the field is static, and
1875          * if the returned method handle is invoked, the field's class will
1876          * be initialized, if it has not already been initialized.
1877          * @param f the reflected field
1878          * @return a method handle which can store values into the reflected field
1879          * @throws IllegalAccessException if access checking fails
1880          * @throws NullPointerException if the argument is null
1881          */
1882         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1883             return unreflectField(f, true);
1884         }
1885 
1886         /**
1887          * Produces a VarHandle giving access to a reflected field {@code f}
1888          * of type {@code T} declared in a class of type {@code R}.
1889          * The VarHandle's variable type is {@code T}.
1890          * If the field is non-static the VarHandle has one coordinate type,
1891          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
1892          * coordinate types.
1893          * <p>
1894          * Access checking is performed immediately on behalf of the lookup
1895          * class, regardless of the value of the field's {@code accessible}
1896          * flag.
1897          * <p>
1898          * If the field is static, and if the returned VarHandle is operated
1899          * on, the field's declaring class will be initialized, if it has not
1900          * already been initialized.
1901          * <p>
1902          * Certain access modes of the returned VarHandle are unsupported under
1903          * the following conditions:
1904          * <ul>
1905          * <li>if the field is declared {@code final}, then the write, atomic
1906          *     update, numeric atomic update, and bitwise atomic update access
1907          *     modes are unsupported.
1908          * <li>if the field type is anything other than {@code byte},
1909          *     {@code short}, {@code char}, {@code int}, {@code long},
1910          *     {@code float}, or {@code double} then numeric atomic update
1911          *     access modes are unsupported.
1912          * <li>if the field type is anything other than {@code boolean},
1913          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1914          *     {@code long} then bitwise atomic update access modes are
1915          *     unsupported.
1916          * </ul>
1917          * <p>
1918          * If the field is declared {@code volatile} then the returned VarHandle
1919          * will override access to the field (effectively ignore the
1920          * {@code volatile} declaration) in accordance to its specified
1921          * access modes.
1922          * <p>
1923          * If the field type is {@code float} or {@code double} then numeric
1924          * and atomic update access modes compare values using their bitwise
1925          * representation (see {@link Float#floatToRawIntBits} and
1926          * {@link Double#doubleToRawLongBits}, respectively).
1927          * @apiNote
1928          * Bitwise comparison of {@code float} values or {@code double} values,
1929          * as performed by the numeric and atomic update access modes, differ
1930          * from the primitive {@code ==} operator and the {@link Float#equals}
1931          * and {@link Double#equals} methods, specifically with respect to
1932          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1933          * Care should be taken when performing a compare and set or a compare
1934          * and exchange operation with such values since the operation may
1935          * unexpectedly fail.
1936          * There are many possible NaN values that are considered to be
1937          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1938          * provided by Java can distinguish between them.  Operation failure can
1939          * occur if the expected or witness value is a NaN value and it is
1940          * transformed (perhaps in a platform specific manner) into another NaN
1941          * value, and thus has a different bitwise representation (see
1942          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1943          * details).
1944          * The values {@code -0.0} and {@code +0.0} have different bitwise
1945          * representations but are considered equal when using the primitive
1946          * {@code ==} operator.  Operation failure can occur if, for example, a
1947          * numeric algorithm computes an expected value to be say {@code -0.0}
1948          * and previously computed the witness value to be say {@code +0.0}.
1949          * @param f the reflected field, with a field of type {@code T}, and
1950          * a declaring class of type {@code R}
1951          * @return a VarHandle giving access to non-static fields or a static
1952          * field
1953          * @throws IllegalAccessException if access checking fails
1954          * @throws NullPointerException if the argument is null
1955          * @since 9
1956          */
1957         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
1958             MemberName getField = new MemberName(f, false);
1959             MemberName putField = new MemberName(f, true);
1960             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
1961                                                       f.getDeclaringClass(), getField, putField);
1962         }
1963 
1964         /**
1965          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1966          * created by this lookup object or a similar one.
1967          * Security and access checks are performed to ensure that this lookup object
1968          * is capable of reproducing the target method handle.
1969          * This means that the cracking may fail if target is a direct method handle
1970          * but was created by an unrelated lookup object.
1971          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1972          * and was created by a lookup object for a different class.
1973          * @param target a direct method handle to crack into symbolic reference components
1974          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
1975          * @exception SecurityException if a security manager is present and it
1976          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1977          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
1978          * @exception NullPointerException if the target is {@code null}
1979          * @see MethodHandleInfo
1980          * @since 1.8
1981          */
1982         public MethodHandleInfo revealDirect(MethodHandle target) {
1983             MemberName member = target.internalMemberName();
1984             if (member == null || (!member.isResolved() &&
1985                                    !member.isMethodHandleInvoke() &&
1986                                    !member.isVarHandleMethodInvoke()))
1987                 throw newIllegalArgumentException("not a direct method handle");
1988             Class<?> defc = member.getDeclaringClass();
1989             byte refKind = member.getReferenceKind();
1990             assert(MethodHandleNatives.refKindIsValid(refKind));
1991             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
1992                 // Devirtualized method invocation is usually formally virtual.
1993                 // To avoid creating extra MemberName objects for this common case,
1994                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
1995                 refKind = REF_invokeVirtual;
1996             if (refKind == REF_invokeVirtual && defc.isInterface())
1997                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
1998                 refKind = REF_invokeInterface;
1999             // Check SM permissions and member access before cracking.
2000             try {
2001                 checkAccess(refKind, defc, member);
2002                 checkSecurityManager(defc, member);
2003             } catch (IllegalAccessException ex) {
2004                 throw new IllegalArgumentException(ex);
2005             }
2006             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
2007                 Class<?> callerClass = target.internalCallerClass();
2008                 if (!hasPrivateAccess() || callerClass != lookupClass())
2009                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
2010             }
2011             // Produce the handle to the results.
2012             return new InfoFromMemberName(this, member, refKind);
2013         }
2014 
2015         /// Helper methods, all package-private.
2016 
2017         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2018             checkSymbolicClass(refc);  // do this before attempting to resolve
2019             Objects.requireNonNull(name);
2020             Objects.requireNonNull(type);
2021             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2022                                             NoSuchFieldException.class);
2023         }
2024 
2025         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2026             checkSymbolicClass(refc);  // do this before attempting to resolve
2027             Objects.requireNonNull(name);
2028             Objects.requireNonNull(type);
2029             checkMethodName(refKind, name);  // NPE check on name
2030             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2031                                             NoSuchMethodException.class);
2032         }
2033 
2034         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
2035             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
2036             Objects.requireNonNull(member.getName());
2037             Objects.requireNonNull(member.getType());
2038             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
2039                                             ReflectiveOperationException.class);
2040         }
2041 
2042         MemberName resolveOrNull(byte refKind, MemberName member) {
2043             // do this before attempting to resolve
2044             if (!isClassAccessible(member.getDeclaringClass())) {
2045                 return null;
2046             }
2047             Objects.requireNonNull(member.getName());
2048             Objects.requireNonNull(member.getType());
2049             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull());
2050         }
2051 
2052         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
2053             if (!isClassAccessible(refc)) {
2054                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
2055             }
2056         }
2057 
2058         boolean isClassAccessible(Class<?> refc) {
2059             Objects.requireNonNull(refc);
2060             Class<?> caller = lookupClassOrNull();
2061             return caller == null || VerifyAccess.isClassAccessible(refc, caller, allowedModes);
2062         }
2063 
2064         /** Check name for an illegal leading "&lt;" character. */
2065         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
2066             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
2067                 throw new NoSuchMethodException("illegal method name: "+name);
2068         }
2069 
2070 
2071         /**
2072          * Find my trustable caller class if m is a caller sensitive method.
2073          * If this lookup object has private access, then the caller class is the lookupClass.
2074          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
2075          */
2076         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
2077             Class<?> callerClass = null;
2078             if (MethodHandleNatives.isCallerSensitive(m)) {
2079                 // Only lookups with private access are allowed to resolve caller-sensitive methods
2080                 if (hasPrivateAccess()) {
2081                     callerClass = lookupClass;
2082                 } else {
2083                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
2084                 }
2085             }
2086             return callerClass;
2087         }
2088 
2089         /**
2090          * Returns {@code true} if this lookup has {@code PRIVATE} access.
2091          * @return {@code true} if this lookup has {@code PRIVATE} access.
2092          * @since 9
2093          */
2094         public boolean hasPrivateAccess() {
2095             return (allowedModes & PRIVATE) != 0;
2096         }
2097 
2098         /**
2099          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
2100          * Determines a trustable caller class to compare with refc, the symbolic reference class.
2101          * If this lookup object has private access, then the caller class is the lookupClass.
2102          */
2103         void checkSecurityManager(Class<?> refc, MemberName m) {
2104             SecurityManager smgr = System.getSecurityManager();
2105             if (smgr == null)  return;
2106             if (allowedModes == TRUSTED)  return;
2107 
2108             // Step 1:
2109             boolean fullPowerLookup = hasPrivateAccess();
2110             if (!fullPowerLookup ||
2111                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
2112                 ReflectUtil.checkPackageAccess(refc);
2113             }
2114 
2115             if (m == null) {  // findClass or accessClass
2116                 // Step 2b:
2117                 if (!fullPowerLookup) {
2118                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
2119                 }
2120                 return;
2121             }
2122 
2123             // Step 2a:
2124             if (m.isPublic()) return;
2125             if (!fullPowerLookup) {
2126                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
2127             }
2128 
2129             // Step 3:
2130             Class<?> defc = m.getDeclaringClass();
2131             if (!fullPowerLookup && defc != refc) {
2132                 ReflectUtil.checkPackageAccess(defc);
2133             }
2134         }
2135 
2136         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2137             boolean wantStatic = (refKind == REF_invokeStatic);
2138             String message;
2139             if (m.isConstructor())
2140                 message = "expected a method, not a constructor";
2141             else if (!m.isMethod())
2142                 message = "expected a method";
2143             else if (wantStatic != m.isStatic())
2144                 message = wantStatic ? "expected a static method" : "expected a non-static method";
2145             else
2146                 { checkAccess(refKind, refc, m); return; }
2147             throw m.makeAccessException(message, this);
2148         }
2149 
2150         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2151             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
2152             String message;
2153             if (wantStatic != m.isStatic())
2154                 message = wantStatic ? "expected a static field" : "expected a non-static field";
2155             else
2156                 { checkAccess(refKind, refc, m); return; }
2157             throw m.makeAccessException(message, this);
2158         }
2159 
2160         /** Check public/protected/private bits on the symbolic reference class and its member. */
2161         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2162             assert(m.referenceKindIsConsistentWith(refKind) &&
2163                    MethodHandleNatives.refKindIsValid(refKind) &&
2164                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
2165             int allowedModes = this.allowedModes;
2166             if (allowedModes == TRUSTED)  return;
2167             int mods = m.getModifiers();
2168             if (Modifier.isProtected(mods) &&
2169                     refKind == REF_invokeVirtual &&
2170                     m.getDeclaringClass() == Object.class &&
2171                     m.getName().equals("clone") &&
2172                     refc.isArray()) {
2173                 // The JVM does this hack also.
2174                 // (See ClassVerifier::verify_invoke_instructions
2175                 // and LinkResolver::check_method_accessability.)
2176                 // Because the JVM does not allow separate methods on array types,
2177                 // there is no separate method for int[].clone.
2178                 // All arrays simply inherit Object.clone.
2179                 // But for access checking logic, we make Object.clone
2180                 // (normally protected) appear to be public.
2181                 // Later on, when the DirectMethodHandle is created,
2182                 // its leading argument will be restricted to the
2183                 // requested array type.
2184                 // N.B. The return type is not adjusted, because
2185                 // that is *not* the bytecode behavior.
2186                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
2187             }
2188             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
2189                 // cannot "new" a protected ctor in a different package
2190                 mods ^= Modifier.PROTECTED;
2191             }
2192             if (Modifier.isFinal(mods) &&
2193                     MethodHandleNatives.refKindIsSetter(refKind))
2194                 throw m.makeAccessException("unexpected set of a final field", this);
2195             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
2196             if ((requestedModes & allowedModes) != 0) {
2197                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
2198                                                     mods, lookupClass(), allowedModes))
2199                     return;
2200             } else {
2201                 // Protected members can also be checked as if they were package-private.
2202                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
2203                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
2204                     return;
2205             }
2206             throw m.makeAccessException(accessFailedMessage(refc, m), this);
2207         }
2208 
2209         String accessFailedMessage(Class<?> refc, MemberName m) {
2210             Class<?> defc = m.getDeclaringClass();
2211             int mods = m.getModifiers();
2212             // check the class first:
2213             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
2214                                (defc == refc ||
2215                                 Modifier.isPublic(refc.getModifiers())));
2216             if (!classOK && (allowedModes & PACKAGE) != 0) {
2217                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), FULL_POWER_MODES) &&
2218                            (defc == refc ||
2219                             VerifyAccess.isClassAccessible(refc, lookupClass(), FULL_POWER_MODES)));
2220             }
2221             if (!classOK)
2222                 return "class is not public";
2223             if (Modifier.isPublic(mods))
2224                 return "access to public member failed";  // (how?, module not readable?)
2225             if (Modifier.isPrivate(mods))
2226                 return "member is private";
2227             if (Modifier.isProtected(mods))
2228                 return "member is protected";
2229             return "member is private to package";
2230         }
2231 
2232         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
2233             int allowedModes = this.allowedModes;
2234             if (allowedModes == TRUSTED)  return;
2235             if (!hasPrivateAccess()
2236                 || (specialCaller != lookupClass()
2237                        // ensure non-abstract methods in superinterfaces can be special-invoked
2238                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
2239                 throw new MemberName(specialCaller).
2240                     makeAccessException("no private access for invokespecial", this);
2241         }
2242 
2243         private boolean restrictProtectedReceiver(MemberName method) {
2244             // The accessing class only has the right to use a protected member
2245             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
2246             if (!method.isProtected() || method.isStatic()
2247                 || allowedModes == TRUSTED
2248                 || method.getDeclaringClass() == lookupClass()
2249                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
2250                 return false;
2251             return true;
2252         }
2253         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
2254             assert(!method.isStatic());
2255             // receiver type of mh is too wide; narrow to caller
2256             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
2257                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
2258             }
2259             MethodType rawType = mh.type();
2260             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
2261             MethodType narrowType = rawType.changeParameterType(0, caller);
2262             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
2263             assert(mh.viewAsTypeChecks(narrowType, true));
2264             return mh.copyWith(narrowType, mh.form);
2265         }
2266 
2267         /** Check access and get the requested method. */
2268         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2269             final boolean doRestrict    = true;
2270             final boolean checkSecurity = true;
2271             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2272         }
2273         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
2274         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2275             final boolean doRestrict    = false;
2276             final boolean checkSecurity = true;
2277             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, boundCallerClass);
2278         }
2279         /** Check access and get the requested method, eliding security manager checks. */
2280         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2281             final boolean doRestrict    = true;
2282             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2283             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2284         }
2285         /** Common code for all methods; do not call directly except from immediately above. */
2286         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
2287                                                    boolean checkSecurity,
2288                                                    boolean doRestrict, Class<?> boundCallerClass) throws IllegalAccessException {
2289 
2290             checkMethod(refKind, refc, method);
2291             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2292             if (checkSecurity)
2293                 checkSecurityManager(refc, method);
2294             assert(!method.isMethodHandleInvoke());
2295 
2296             if (refKind == REF_invokeSpecial &&
2297                 refc != lookupClass() &&
2298                 !refc.isInterface() &&
2299                 refc != lookupClass().getSuperclass() &&
2300                 refc.isAssignableFrom(lookupClass())) {
2301                 assert(!method.getName().equals("<init>"));  // not this code path
2302 
2303                 // Per JVMS 6.5, desc. of invokespecial instruction:
2304                 // If the method is in a superclass of the LC,
2305                 // and if our original search was above LC.super,
2306                 // repeat the search (symbolic lookup) from LC.super
2307                 // and continue with the direct superclass of that class,
2308                 // and so forth, until a match is found or no further superclasses exist.
2309                 // FIXME: MemberName.resolve should handle this instead.
2310                 Class<?> refcAsSuper = lookupClass();
2311                 MemberName m2;
2312                 do {
2313                     refcAsSuper = refcAsSuper.getSuperclass();
2314                     m2 = new MemberName(refcAsSuper,
2315                                         method.getName(),
2316                                         method.getMethodType(),
2317                                         REF_invokeSpecial);
2318                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
2319                 } while (m2 == null &&         // no method is found yet
2320                          refc != refcAsSuper); // search up to refc
2321                 if (m2 == null)  throw new InternalError(method.toString());
2322                 method = m2;
2323                 refc = refcAsSuper;
2324                 // redo basic checks
2325                 checkMethod(refKind, refc, method);
2326             }
2327 
2328             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
2329             MethodHandle mh = dmh;
2330             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
2331             if ((doRestrict && refKind == REF_invokeSpecial) ||
2332                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
2333                 mh = restrictReceiver(method, dmh, lookupClass());
2334             }
2335             mh = maybeBindCaller(method, mh, boundCallerClass);
2336             mh = mh.setVarargs(method);
2337             return mh;
2338         }
2339         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
2340                                              Class<?> boundCallerClass)
2341                                              throws IllegalAccessException {
2342             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
2343                 return mh;
2344             Class<?> hostClass = lookupClass;
2345             if (!hasPrivateAccess())  // caller must have private access
2346                 hostClass = boundCallerClass;  // boundCallerClass came from a security manager style stack walk
2347             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
2348             // Note: caller will apply varargs after this step happens.
2349             return cbmh;
2350         }
2351         /** Check access and get the requested field. */
2352         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2353             final boolean checkSecurity = true;
2354             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2355         }
2356         /** Check access and get the requested field, eliding security manager checks. */
2357         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2358             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2359             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2360         }
2361         /** Common code for all fields; do not call directly except from immediately above. */
2362         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
2363                                                   boolean checkSecurity) throws IllegalAccessException {
2364             checkField(refKind, refc, field);
2365             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2366             if (checkSecurity)
2367                 checkSecurityManager(refc, field);
2368             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
2369             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
2370                                     restrictProtectedReceiver(field));
2371             if (doRestrict)
2372                 return restrictReceiver(field, dmh, lookupClass());
2373             return dmh;
2374         }
2375         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
2376                                             Class<?> refc, MemberName getField, MemberName putField)
2377                 throws IllegalAccessException {
2378             final boolean checkSecurity = true;
2379             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2380         }
2381         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
2382                                                              Class<?> refc, MemberName getField, MemberName putField)
2383                 throws IllegalAccessException {
2384             final boolean checkSecurity = false;
2385             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2386         }
2387         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
2388                                                   Class<?> refc, MemberName getField, MemberName putField,
2389                                                   boolean checkSecurity) throws IllegalAccessException {
2390             assert getField.isStatic() == putField.isStatic();
2391             assert getField.isGetter() && putField.isSetter();
2392             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
2393             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
2394 
2395             checkField(getRefKind, refc, getField);
2396             if (checkSecurity)
2397                 checkSecurityManager(refc, getField);
2398 
2399             if (!putField.isFinal()) {
2400                 // A VarHandle does not support updates to final fields, any
2401                 // such VarHandle to a final field will be read-only and
2402                 // therefore the following write-based accessibility checks are
2403                 // only required for non-final fields
2404                 checkField(putRefKind, refc, putField);
2405                 if (checkSecurity)
2406                     checkSecurityManager(refc, putField);
2407             }
2408 
2409             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
2410                                   restrictProtectedReceiver(getField));
2411             if (doRestrict) {
2412                 assert !getField.isStatic();
2413                 // receiver type of VarHandle is too wide; narrow to caller
2414                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
2415                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
2416                 }
2417                 refc = lookupClass();
2418             }
2419             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED);
2420         }
2421         /** Check access and get the requested constructor. */
2422         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2423             final boolean checkSecurity = true;
2424             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2425         }
2426         /** Check access and get the requested constructor, eliding security manager checks. */
2427         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2428             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2429             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2430         }
2431         /** Common code for all constructors; do not call directly except from immediately above. */
2432         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
2433                                                   boolean checkSecurity) throws IllegalAccessException {
2434             assert(ctor.isConstructor());
2435             checkAccess(REF_newInvokeSpecial, refc, ctor);
2436             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2437             if (checkSecurity)
2438                 checkSecurityManager(refc, ctor);
2439             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
2440             return DirectMethodHandle.make(ctor).setVarargs(ctor);
2441         }
2442 
2443         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
2444          */
2445         /*non-public*/
2446         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
2447             if (!(type instanceof Class || type instanceof MethodType))
2448                 throw new InternalError("unresolved MemberName");
2449             MemberName member = new MemberName(refKind, defc, name, type);
2450             MethodHandle mh = LOOKASIDE_TABLE.get(member);
2451             if (mh != null) {
2452                 checkSymbolicClass(defc);
2453                 return mh;
2454             }
2455             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
2456                 // Treat MethodHandle.invoke and invokeExact specially.
2457                 mh = findVirtualForMH(member.getName(), member.getMethodType());
2458                 if (mh != null) {
2459                     return mh;
2460                 }
2461             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
2462                 // Treat signature-polymorphic methods on VarHandle specially.
2463                 mh = findVirtualForVH(member.getName(), member.getMethodType());
2464                 if (mh != null) {
2465                     return mh;
2466                 }
2467             }
2468             MemberName resolved = resolveOrFail(refKind, member);
2469             mh = getDirectMethodForConstant(refKind, defc, resolved);
2470             if (mh instanceof DirectMethodHandle
2471                     && canBeCached(refKind, defc, resolved)) {
2472                 MemberName key = mh.internalMemberName();
2473                 if (key != null) {
2474                     key = key.asNormalOriginal();
2475                 }
2476                 if (member.equals(key)) {  // better safe than sorry
2477                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
2478                 }
2479             }
2480             return mh;
2481         }
2482         private
2483         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
2484             if (refKind == REF_invokeSpecial) {
2485                 return false;
2486             }
2487             if (!Modifier.isPublic(defc.getModifiers()) ||
2488                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
2489                     !member.isPublic() ||
2490                     member.isCallerSensitive()) {
2491                 return false;
2492             }
2493             ClassLoader loader = defc.getClassLoader();
2494             if (loader != null) {
2495                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
2496                 boolean found = false;
2497                 while (sysl != null) {
2498                     if (loader == sysl) { found = true; break; }
2499                     sysl = sysl.getParent();
2500                 }
2501                 if (!found) {
2502                     return false;
2503                 }
2504             }
2505             try {
2506                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
2507                     new MemberName(refKind, defc, member.getName(), member.getType()));
2508                 if (resolved2 == null) {
2509                     return false;
2510                 }
2511                 checkSecurityManager(defc, resolved2);
2512             } catch (SecurityException ex) {
2513                 return false;
2514             }
2515             return true;
2516         }
2517         private
2518         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
2519                 throws ReflectiveOperationException {
2520             if (MethodHandleNatives.refKindIsField(refKind)) {
2521                 return getDirectFieldNoSecurityManager(refKind, defc, member);
2522             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
2523                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
2524             } else if (refKind == REF_newInvokeSpecial) {
2525                 return getDirectConstructorNoSecurityManager(defc, member);
2526             }
2527             // oops
2528             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
2529         }
2530 
2531         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
2532     }
2533 
2534     /**
2535      * Produces a method handle constructing arrays of a desired type,
2536      * as if by the {@code anewarray} bytecode.
2537      * The return type of the method handle will be the array type.
2538      * The type of its sole argument will be {@code int}, which specifies the size of the array.
2539      *
2540      * <p> If the returned method handle is invoked with a negative
2541      * array size, a {@code NegativeArraySizeException} will be thrown.
2542      *
2543      * @param arrayClass an array type
2544      * @return a method handle which can create arrays of the given type
2545      * @throws NullPointerException if the argument is {@code null}
2546      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
2547      * @see java.lang.reflect.Array#newInstance(Class, int)
2548      * @jvms 6.5 {@code anewarray} Instruction
2549      * @since 9
2550      */
2551     public static
2552     MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
2553         if (!arrayClass.isArray()) {
2554             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
2555         }
2556         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
2557                 bindTo(arrayClass.getComponentType());
2558         return ani.asType(ani.type().changeReturnType(arrayClass));
2559     }
2560 
2561     /**
2562      * Produces a method handle returning the length of an array,
2563      * as if by the {@code arraylength} bytecode.
2564      * The type of the method handle will have {@code int} as return type,
2565      * and its sole argument will be the array type.
2566      *
2567      * <p> If the returned method handle is invoked with a {@code null}
2568      * array reference, a {@code NullPointerException} will be thrown.
2569      *
2570      * @param arrayClass an array type
2571      * @return a method handle which can retrieve the length of an array of the given array type
2572      * @throws NullPointerException if the argument is {@code null}
2573      * @throws IllegalArgumentException if arrayClass is not an array type
2574      * @jvms 6.5 {@code arraylength} Instruction
2575      * @since 9
2576      */
2577     public static
2578     MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
2579         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
2580     }
2581 
2582     /**
2583      * Produces a method handle giving read access to elements of an array,
2584      * as if by the {@code aaload} bytecode.
2585      * The type of the method handle will have a return type of the array's
2586      * element type.  Its first argument will be the array type,
2587      * and the second will be {@code int}.
2588      *
2589      * <p> When the returned method handle is invoked,
2590      * the array reference and array index are checked.
2591      * A {@code NullPointerException} will be thrown if the array reference
2592      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2593      * thrown if the index is negative or if it is greater than or equal to
2594      * the length of the array.
2595      *
2596      * @param arrayClass an array type
2597      * @return a method handle which can load values from the given array type
2598      * @throws NullPointerException if the argument is null
2599      * @throws  IllegalArgumentException if arrayClass is not an array type
2600      * @jvms 6.5 {@code aaload} Instruction
2601      */
2602     public static
2603     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
2604         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
2605     }
2606 
2607     /**
2608      * Produces a method handle giving write access to elements of an array,
2609      * as if by the {@code astore} bytecode.
2610      * The type of the method handle will have a void return type.
2611      * Its last argument will be the array's element type.
2612      * The first and second arguments will be the array type and int.
2613      *
2614      * <p> When the returned method handle is invoked,
2615      * the array reference and array index are checked.
2616      * A {@code NullPointerException} will be thrown if the array reference
2617      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2618      * thrown if the index is negative or if it is greater than or equal to
2619      * the length of the array.
2620      *
2621      * @param arrayClass the class of an array
2622      * @return a method handle which can store values into the array type
2623      * @throws NullPointerException if the argument is null
2624      * @throws IllegalArgumentException if arrayClass is not an array type
2625      * @jvms 6.5 {@code aastore} Instruction
2626      */
2627     public static
2628     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
2629         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
2630     }
2631 
2632     /**
2633      * Produces a VarHandle giving access to elements of an array of type
2634      * {@code arrayClass}.  The VarHandle's variable type is the component type
2635      * of {@code arrayClass} and the list of coordinate types is
2636      * {@code (arrayClass, int)}, where the {@code int} coordinate type
2637      * corresponds to an argument that is an index into an array.
2638      * <p>
2639      * Certain access modes of the returned VarHandle are unsupported under
2640      * the following conditions:
2641      * <ul>
2642      * <li>if the component type is anything other than {@code byte},
2643      *     {@code short}, {@code char}, {@code int}, {@code long},
2644      *     {@code float}, or {@code double} then numeric atomic update access
2645      *     modes are unsupported.
2646      * <li>if the field type is anything other than {@code boolean},
2647      *     {@code byte}, {@code short}, {@code char}, {@code int} or
2648      *     {@code long} then bitwise atomic update access modes are
2649      *     unsupported.
2650      * </ul>
2651      * <p>
2652      * If the component type is {@code float} or {@code double} then numeric
2653      * and atomic update access modes compare values using their bitwise
2654      * representation (see {@link Float#floatToRawIntBits} and
2655      * {@link Double#doubleToRawLongBits}, respectively).
2656      *
2657      * <p> When the returned {@code VarHandle} is invoked,
2658      * the array reference and array index are checked.
2659      * A {@code NullPointerException} will be thrown if the array reference
2660      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2661      * thrown if the index is negative or if it is greater than or equal to
2662      * the length of the array.
2663      *
2664      * @apiNote
2665      * Bitwise comparison of {@code float} values or {@code double} values,
2666      * as performed by the numeric and atomic update access modes, differ
2667      * from the primitive {@code ==} operator and the {@link Float#equals}
2668      * and {@link Double#equals} methods, specifically with respect to
2669      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2670      * Care should be taken when performing a compare and set or a compare
2671      * and exchange operation with such values since the operation may
2672      * unexpectedly fail.
2673      * There are many possible NaN values that are considered to be
2674      * {@code NaN} in Java, although no IEEE 754 floating-point operation
2675      * provided by Java can distinguish between them.  Operation failure can
2676      * occur if the expected or witness value is a NaN value and it is
2677      * transformed (perhaps in a platform specific manner) into another NaN
2678      * value, and thus has a different bitwise representation (see
2679      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2680      * details).
2681      * The values {@code -0.0} and {@code +0.0} have different bitwise
2682      * representations but are considered equal when using the primitive
2683      * {@code ==} operator.  Operation failure can occur if, for example, a
2684      * numeric algorithm computes an expected value to be say {@code -0.0}
2685      * and previously computed the witness value to be say {@code +0.0}.
2686      * @param arrayClass the class of an array, of type {@code T[]}
2687      * @return a VarHandle giving access to elements of an array
2688      * @throws NullPointerException if the arrayClass is null
2689      * @throws IllegalArgumentException if arrayClass is not an array type
2690      * @since 9
2691      */
2692     public static
2693     VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
2694         return VarHandles.makeArrayElementHandle(arrayClass);
2695     }
2696 
2697     /**
2698      * Produces a VarHandle giving access to elements of a {@code byte[]} array
2699      * viewed as if it were a different primitive array type, such as
2700      * {@code int[]} or {@code long[]}.
2701      * The VarHandle's variable type is the component type of
2702      * {@code viewArrayClass} and the list of coordinate types is
2703      * {@code (byte[], int)}, where the {@code int} coordinate type
2704      * corresponds to an argument that is an index into a {@code byte[]} array.
2705      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
2706      * array, composing bytes to or from a value of the component type of
2707      * {@code viewArrayClass} according to the given endianness.
2708      * <p>
2709      * The supported component types (variables types) are {@code short},
2710      * {@code char}, {@code int}, {@code long}, {@code float} and
2711      * {@code double}.
2712      * <p>
2713      * Access of bytes at a given index will result in an
2714      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2715      * or greater than the {@code byte[]} array length minus the size (in bytes)
2716      * of {@code T}.
2717      * <p>
2718      * Access of bytes at an index may be aligned or misaligned for {@code T},
2719      * with respect to the underlying memory address, {@code A} say, associated
2720      * with the array and index.
2721      * If access is misaligned then access for anything other than the
2722      * {@code get} and {@code set} access modes will result in an
2723      * {@code IllegalStateException}.  In such cases atomic access is only
2724      * guaranteed with respect to the largest power of two that divides the GCD
2725      * of {@code A} and the size (in bytes) of {@code T}.
2726      * If access is aligned then following access modes are supported and are
2727      * guaranteed to support atomic access:
2728      * <ul>
2729      * <li>read write access modes for all {@code T}, with the exception of
2730      *     access modes {@code get} and {@code set} for {@code long} and
2731      *     {@code double} on 32-bit platforms.
2732      * <li>atomic update access modes for {@code int}, {@code long},
2733      *     {@code float} or {@code double}.
2734      *     (Future major platform releases of the JDK may support additional
2735      *     types for certain currently unsupported access modes.)
2736      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2737      *     (Future major platform releases of the JDK may support additional
2738      *     numeric types for certain currently unsupported access modes.)
2739      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2740      *     (Future major platform releases of the JDK may support additional
2741      *     numeric types for certain currently unsupported access modes.)
2742      * </ul>
2743      * <p>
2744      * Misaligned access, and therefore atomicity guarantees, may be determined
2745      * for {@code byte[]} arrays without operating on a specific array.  Given
2746      * an {@code index}, {@code T} and it's corresponding boxed type,
2747      * {@code T_BOX}, misalignment may be determined as follows:
2748      * <pre>{@code
2749      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2750      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
2751      *     alignmentOffset(0, sizeOfT);
2752      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
2753      * boolean isMisaligned = misalignedAtIndex != 0;
2754      * }</pre>
2755      * <p>
2756      * If the variable type is {@code float} or {@code double} then atomic
2757      * update access modes compare values using their bitwise representation
2758      * (see {@link Float#floatToRawIntBits} and
2759      * {@link Double#doubleToRawLongBits}, respectively).
2760      * @param viewArrayClass the view array class, with a component type of
2761      * type {@code T}
2762      * @param byteOrder the endianness of the view array elements, as
2763      * stored in the underlying {@code byte} array
2764      * @return a VarHandle giving access to elements of a {@code byte[]} array
2765      * viewed as if elements corresponding to the components type of the view
2766      * array class
2767      * @throws NullPointerException if viewArrayClass or byteOrder is null
2768      * @throws IllegalArgumentException if viewArrayClass is not an array type
2769      * @throws UnsupportedOperationException if the component type of
2770      * viewArrayClass is not supported as a variable type
2771      * @since 9
2772      */
2773     public static
2774     VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
2775                                      ByteOrder byteOrder) throws IllegalArgumentException {
2776         Objects.requireNonNull(byteOrder);
2777         return VarHandles.byteArrayViewHandle(viewArrayClass,
2778                                               byteOrder == ByteOrder.BIG_ENDIAN);
2779     }
2780 
2781     /**
2782      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
2783      * viewed as if it were an array of elements of a different primitive
2784      * component type to that of {@code byte}, such as {@code int[]} or
2785      * {@code long[]}.
2786      * The VarHandle's variable type is the component type of
2787      * {@code viewArrayClass} and the list of coordinate types is
2788      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
2789      * corresponds to an argument that is an index into a {@code byte[]} array.
2790      * The returned VarHandle accesses bytes at an index in a
2791      * {@code ByteBuffer}, composing bytes to or from a value of the component
2792      * type of {@code viewArrayClass} according to the given endianness.
2793      * <p>
2794      * The supported component types (variables types) are {@code short},
2795      * {@code char}, {@code int}, {@code long}, {@code float} and
2796      * {@code double}.
2797      * <p>
2798      * Access will result in a {@code ReadOnlyBufferException} for anything
2799      * other than the read access modes if the {@code ByteBuffer} is read-only.
2800      * <p>
2801      * Access of bytes at a given index will result in an
2802      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2803      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
2804      * {@code T}.
2805      * <p>
2806      * Access of bytes at an index may be aligned or misaligned for {@code T},
2807      * with respect to the underlying memory address, {@code A} say, associated
2808      * with the {@code ByteBuffer} and index.
2809      * If access is misaligned then access for anything other than the
2810      * {@code get} and {@code set} access modes will result in an
2811      * {@code IllegalStateException}.  In such cases atomic access is only
2812      * guaranteed with respect to the largest power of two that divides the GCD
2813      * of {@code A} and the size (in bytes) of {@code T}.
2814      * If access is aligned then following access modes are supported and are
2815      * guaranteed to support atomic access:
2816      * <ul>
2817      * <li>read write access modes for all {@code T}, with the exception of
2818      *     access modes {@code get} and {@code set} for {@code long} and
2819      *     {@code double} on 32-bit platforms.
2820      * <li>atomic update access modes for {@code int}, {@code long},
2821      *     {@code float} or {@code double}.
2822      *     (Future major platform releases of the JDK may support additional
2823      *     types for certain currently unsupported access modes.)
2824      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2825      *     (Future major platform releases of the JDK may support additional
2826      *     numeric types for certain currently unsupported access modes.)
2827      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2828      *     (Future major platform releases of the JDK may support additional
2829      *     numeric types for certain currently unsupported access modes.)
2830      * </ul>
2831      * <p>
2832      * Misaligned access, and therefore atomicity guarantees, may be determined
2833      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
2834      * {@code index}, {@code T} and it's corresponding boxed type,
2835      * {@code T_BOX}, as follows:
2836      * <pre>{@code
2837      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2838      * ByteBuffer bb = ...
2839      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
2840      * boolean isMisaligned = misalignedAtIndex != 0;
2841      * }</pre>
2842      * <p>
2843      * If the variable type is {@code float} or {@code double} then atomic
2844      * update access modes compare values using their bitwise representation
2845      * (see {@link Float#floatToRawIntBits} and
2846      * {@link Double#doubleToRawLongBits}, respectively).
2847      * @param viewArrayClass the view array class, with a component type of
2848      * type {@code T}
2849      * @param byteOrder the endianness of the view array elements, as
2850      * stored in the underlying {@code ByteBuffer} (Note this overrides the
2851      * endianness of a {@code ByteBuffer})
2852      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
2853      * viewed as if elements corresponding to the components type of the view
2854      * array class
2855      * @throws NullPointerException if viewArrayClass or byteOrder is null
2856      * @throws IllegalArgumentException if viewArrayClass is not an array type
2857      * @throws UnsupportedOperationException if the component type of
2858      * viewArrayClass is not supported as a variable type
2859      * @since 9
2860      */
2861     public static
2862     VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
2863                                       ByteOrder byteOrder) throws IllegalArgumentException {
2864         Objects.requireNonNull(byteOrder);
2865         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
2866                                                    byteOrder == ByteOrder.BIG_ENDIAN);
2867     }
2868 
2869 
2870     /// method handle invocation (reflective style)
2871 
2872     /**
2873      * Produces a method handle which will invoke any method handle of the
2874      * given {@code type}, with a given number of trailing arguments replaced by
2875      * a single trailing {@code Object[]} array.
2876      * The resulting invoker will be a method handle with the following
2877      * arguments:
2878      * <ul>
2879      * <li>a single {@code MethodHandle} target
2880      * <li>zero or more leading values (counted by {@code leadingArgCount})
2881      * <li>an {@code Object[]} array containing trailing arguments
2882      * </ul>
2883      * <p>
2884      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
2885      * the indicated {@code type}.
2886      * That is, if the target is exactly of the given {@code type}, it will behave
2887      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
2888      * is used to convert the target to the required {@code type}.
2889      * <p>
2890      * The type of the returned invoker will not be the given {@code type}, but rather
2891      * will have all parameters except the first {@code leadingArgCount}
2892      * replaced by a single array of type {@code Object[]}, which will be
2893      * the final parameter.
2894      * <p>
2895      * Before invoking its target, the invoker will spread the final array, apply
2896      * reference casts as necessary, and unbox and widen primitive arguments.
2897      * If, when the invoker is called, the supplied array argument does
2898      * not have the correct number of elements, the invoker will throw
2899      * an {@link IllegalArgumentException} instead of invoking the target.
2900      * <p>
2901      * This method is equivalent to the following code (though it may be more efficient):
2902      * <blockquote><pre>{@code
2903 MethodHandle invoker = MethodHandles.invoker(type);
2904 int spreadArgCount = type.parameterCount() - leadingArgCount;
2905 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
2906 return invoker;
2907      * }</pre></blockquote>
2908      * This method throws no reflective or security exceptions.
2909      * @param type the desired target type
2910      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
2911      * @return a method handle suitable for invoking any method handle of the given type
2912      * @throws NullPointerException if {@code type} is null
2913      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
2914      *                  the range from 0 to {@code type.parameterCount()} inclusive,
2915      *                  or if the resulting method handle's type would have
2916      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2917      */
2918     public static
2919     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
2920         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
2921             throw newIllegalArgumentException("bad argument count", leadingArgCount);
2922         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
2923         return type.invokers().spreadInvoker(leadingArgCount);
2924     }
2925 
2926     /**
2927      * Produces a special <em>invoker method handle</em> which can be used to
2928      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
2929      * The resulting invoker will have a type which is
2930      * exactly equal to the desired type, except that it will accept
2931      * an additional leading argument of type {@code MethodHandle}.
2932      * <p>
2933      * This method is equivalent to the following code (though it may be more efficient):
2934      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
2935      *
2936      * <p style="font-size:smaller;">
2937      * <em>Discussion:</em>
2938      * Invoker method handles can be useful when working with variable method handles
2939      * of unknown types.
2940      * For example, to emulate an {@code invokeExact} call to a variable method
2941      * handle {@code M}, extract its type {@code T},
2942      * look up the invoker method {@code X} for {@code T},
2943      * and call the invoker method, as {@code X.invoke(T, A...)}.
2944      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
2945      * is unknown.)
2946      * If spreading, collecting, or other argument transformations are required,
2947      * they can be applied once to the invoker {@code X} and reused on many {@code M}
2948      * method handle values, as long as they are compatible with the type of {@code X}.
2949      * <p style="font-size:smaller;">
2950      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2951      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2952      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2953      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2954      * <p>
2955      * This method throws no reflective or security exceptions.
2956      * @param type the desired target type
2957      * @return a method handle suitable for invoking any method handle of the given type
2958      * @throws IllegalArgumentException if the resulting method handle's type would have
2959      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2960      */
2961     public static
2962     MethodHandle exactInvoker(MethodType type) {
2963         return type.invokers().exactInvoker();
2964     }
2965 
2966     /**
2967      * Produces a special <em>invoker method handle</em> which can be used to
2968      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
2969      * The resulting invoker will have a type which is
2970      * exactly equal to the desired type, except that it will accept
2971      * an additional leading argument of type {@code MethodHandle}.
2972      * <p>
2973      * Before invoking its target, if the target differs from the expected type,
2974      * the invoker will apply reference casts as
2975      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
2976      * Similarly, the return value will be converted as necessary.
2977      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
2978      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
2979      * <p>
2980      * This method is equivalent to the following code (though it may be more efficient):
2981      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
2982      * <p style="font-size:smaller;">
2983      * <em>Discussion:</em>
2984      * A {@linkplain MethodType#genericMethodType general method type} is one which
2985      * mentions only {@code Object} arguments and return values.
2986      * An invoker for such a type is capable of calling any method handle
2987      * of the same arity as the general type.
2988      * <p style="font-size:smaller;">
2989      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2990      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2991      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2992      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2993      * <p>
2994      * This method throws no reflective or security exceptions.
2995      * @param type the desired target type
2996      * @return a method handle suitable for invoking any method handle convertible to the given type
2997      * @throws IllegalArgumentException if the resulting method handle's type would have
2998      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2999      */
3000     public static
3001     MethodHandle invoker(MethodType type) {
3002         return type.invokers().genericInvoker();
3003     }
3004 
3005     /**
3006      * Produces a special <em>invoker method handle</em> which can be used to
3007      * invoke a signature-polymorphic access mode method on any VarHandle whose
3008      * associated access mode type is compatible with the given type.
3009      * The resulting invoker will have a type which is exactly equal to the
3010      * desired given type, except that it will accept an additional leading
3011      * argument of type {@code VarHandle}.
3012      *
3013      * @param accessMode the VarHandle access mode
3014      * @param type the desired target type
3015      * @return a method handle suitable for invoking an access mode method of
3016      *         any VarHandle whose access mode type is of the given type.
3017      * @since 9
3018      */
3019     static public
3020     MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3021         return type.invokers().varHandleMethodExactInvoker(accessMode);
3022     }
3023 
3024     /**
3025      * Produces a special <em>invoker method handle</em> which can be used to
3026      * invoke a signature-polymorphic access mode method on any VarHandle whose
3027      * associated access mode type is compatible with the given type.
3028      * The resulting invoker will have a type which is exactly equal to the
3029      * desired given type, except that it will accept an additional leading
3030      * argument of type {@code VarHandle}.
3031      * <p>
3032      * Before invoking its target, if the access mode type differs from the
3033      * desired given type, the invoker will apply reference casts as necessary
3034      * and box, unbox, or widen primitive values, as if by
3035      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
3036      * converted as necessary.
3037      * <p>
3038      * This method is equivalent to the following code (though it may be more
3039      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
3040      *
3041      * @param accessMode the VarHandle access mode
3042      * @param type the desired target type
3043      * @return a method handle suitable for invoking an access mode method of
3044      *         any VarHandle whose access mode type is convertible to the given
3045      *         type.
3046      * @since 9
3047      */
3048     static public
3049     MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3050         return type.invokers().varHandleMethodInvoker(accessMode);
3051     }
3052 
3053     static /*non-public*/
3054     MethodHandle basicInvoker(MethodType type) {
3055         return type.invokers().basicInvoker();
3056     }
3057 
3058      /// method handle modification (creation from other method handles)
3059 
3060     /**
3061      * Produces a method handle which adapts the type of the
3062      * given method handle to a new type by pairwise argument and return type conversion.
3063      * The original type and new type must have the same number of arguments.
3064      * The resulting method handle is guaranteed to report a type
3065      * which is equal to the desired new type.
3066      * <p>
3067      * If the original type and new type are equal, returns target.
3068      * <p>
3069      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
3070      * and some additional conversions are also applied if those conversions fail.
3071      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
3072      * if possible, before or instead of any conversions done by {@code asType}:
3073      * <ul>
3074      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
3075      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
3076      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
3077      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
3078      *     the boolean is converted to a byte value, 1 for true, 0 for false.
3079      *     (This treatment follows the usage of the bytecode verifier.)
3080      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
3081      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
3082      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
3083      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
3084      *     then a Java casting conversion (JLS 5.5) is applied.
3085      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
3086      *     widening and/or narrowing.)
3087      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
3088      *     conversion will be applied at runtime, possibly followed
3089      *     by a Java casting conversion (JLS 5.5) on the primitive value,
3090      *     possibly followed by a conversion from byte to boolean by testing
3091      *     the low-order bit.
3092      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
3093      *     and if the reference is null at runtime, a zero value is introduced.
3094      * </ul>
3095      * @param target the method handle to invoke after arguments are retyped
3096      * @param newType the expected type of the new method handle
3097      * @return a method handle which delegates to the target after performing
3098      *           any necessary argument conversions, and arranges for any
3099      *           necessary return value conversions
3100      * @throws NullPointerException if either argument is null
3101      * @throws WrongMethodTypeException if the conversion cannot be made
3102      * @see MethodHandle#asType
3103      */
3104     public static
3105     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
3106         explicitCastArgumentsChecks(target, newType);
3107         // use the asTypeCache when possible:
3108         MethodType oldType = target.type();
3109         if (oldType == newType)  return target;
3110         if (oldType.explicitCastEquivalentToAsType(newType)) {
3111             return target.asFixedArity().asType(newType);
3112         }
3113         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
3114     }
3115 
3116     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
3117         if (target.type().parameterCount() != newType.parameterCount()) {
3118             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
3119         }
3120     }
3121 
3122     /**
3123      * Produces a method handle which adapts the calling sequence of the
3124      * given method handle to a new type, by reordering the arguments.
3125      * The resulting method handle is guaranteed to report a type
3126      * which is equal to the desired new type.
3127      * <p>
3128      * The given array controls the reordering.
3129      * Call {@code #I} the number of incoming parameters (the value
3130      * {@code newType.parameterCount()}, and call {@code #O} the number
3131      * of outgoing parameters (the value {@code target.type().parameterCount()}).
3132      * Then the length of the reordering array must be {@code #O},
3133      * and each element must be a non-negative number less than {@code #I}.
3134      * For every {@code N} less than {@code #O}, the {@code N}-th
3135      * outgoing argument will be taken from the {@code I}-th incoming
3136      * argument, where {@code I} is {@code reorder[N]}.
3137      * <p>
3138      * No argument or return value conversions are applied.
3139      * The type of each incoming argument, as determined by {@code newType},
3140      * must be identical to the type of the corresponding outgoing parameter
3141      * or parameters in the target method handle.
3142      * The return type of {@code newType} must be identical to the return
3143      * type of the original target.
3144      * <p>
3145      * The reordering array need not specify an actual permutation.
3146      * An incoming argument will be duplicated if its index appears
3147      * more than once in the array, and an incoming argument will be dropped
3148      * if its index does not appear in the array.
3149      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
3150      * incoming arguments which are not mentioned in the reordering array
3151      * may be of any type, as determined only by {@code newType}.
3152      * <blockquote><pre>{@code
3153 import static java.lang.invoke.MethodHandles.*;
3154 import static java.lang.invoke.MethodType.*;
3155 ...
3156 MethodType intfn1 = methodType(int.class, int.class);
3157 MethodType intfn2 = methodType(int.class, int.class, int.class);
3158 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
3159 assert(sub.type().equals(intfn2));
3160 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
3161 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
3162 assert((int)rsub.invokeExact(1, 100) == 99);
3163 MethodHandle add = ... (int x, int y) -> (x+y) ...;
3164 assert(add.type().equals(intfn2));
3165 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
3166 assert(twice.type().equals(intfn1));
3167 assert((int)twice.invokeExact(21) == 42);
3168      * }</pre></blockquote>
3169      * <p>
3170      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3171      * variable-arity method handle}, even if the original target method handle was.
3172      * @param target the method handle to invoke after arguments are reordered
3173      * @param newType the expected type of the new method handle
3174      * @param reorder an index array which controls the reordering
3175      * @return a method handle which delegates to the target after it
3176      *           drops unused arguments and moves and/or duplicates the other arguments
3177      * @throws NullPointerException if any argument is null
3178      * @throws IllegalArgumentException if the index array length is not equal to
3179      *                  the arity of the target, or if any index array element
3180      *                  not a valid index for a parameter of {@code newType},
3181      *                  or if two corresponding parameter types in
3182      *                  {@code target.type()} and {@code newType} are not identical,
3183      */
3184     public static
3185     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
3186         reorder = reorder.clone();  // get a private copy
3187         MethodType oldType = target.type();
3188         permuteArgumentChecks(reorder, newType, oldType);
3189         // first detect dropped arguments and handle them separately
3190         int[] originalReorder = reorder;
3191         BoundMethodHandle result = target.rebind();
3192         LambdaForm form = result.form;
3193         int newArity = newType.parameterCount();
3194         // Normalize the reordering into a real permutation,
3195         // by removing duplicates and adding dropped elements.
3196         // This somewhat improves lambda form caching, as well
3197         // as simplifying the transform by breaking it up into steps.
3198         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
3199             if (ddIdx > 0) {
3200                 // We found a duplicated entry at reorder[ddIdx].
3201                 // Example:  (x,y,z)->asList(x,y,z)
3202                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
3203                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
3204                 // The starred element corresponds to the argument
3205                 // deleted by the dupArgumentForm transform.
3206                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
3207                 boolean killFirst = false;
3208                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
3209                     // Set killFirst if the dup is larger than an intervening position.
3210                     // This will remove at least one inversion from the permutation.
3211                     if (dupVal > val) killFirst = true;
3212                 }
3213                 if (!killFirst) {
3214                     srcPos = dstPos;
3215                     dstPos = ddIdx;
3216                 }
3217                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
3218                 assert (reorder[srcPos] == reorder[dstPos]);
3219                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
3220                 // contract the reordering by removing the element at dstPos
3221                 int tailPos = dstPos + 1;
3222                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
3223                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
3224             } else {
3225                 int dropVal = ~ddIdx, insPos = 0;
3226                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
3227                     // Find first element of reorder larger than dropVal.
3228                     // This is where we will insert the dropVal.
3229                     insPos += 1;
3230                 }
3231                 Class<?> ptype = newType.parameterType(dropVal);
3232                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
3233                 oldType = oldType.insertParameterTypes(insPos, ptype);
3234                 // expand the reordering by inserting an element at insPos
3235                 int tailPos = insPos + 1;
3236                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
3237                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
3238                 reorder[insPos] = dropVal;
3239             }
3240             assert (permuteArgumentChecks(reorder, newType, oldType));
3241         }
3242         assert (reorder.length == newArity);  // a perfect permutation
3243         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
3244         form = form.editor().permuteArgumentsForm(1, reorder);
3245         if (newType == result.type() && form == result.internalForm())
3246             return result;
3247         return result.copyWith(newType, form);
3248     }
3249 
3250     /**
3251      * Return an indication of any duplicate or omission in reorder.
3252      * If the reorder contains a duplicate entry, return the index of the second occurrence.
3253      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
3254      * Otherwise, return zero.
3255      * If an element not in [0..newArity-1] is encountered, return reorder.length.
3256      */
3257     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
3258         final int BIT_LIMIT = 63;  // max number of bits in bit mask
3259         if (newArity < BIT_LIMIT) {
3260             long mask = 0;
3261             for (int i = 0; i < reorder.length; i++) {
3262                 int arg = reorder[i];
3263                 if (arg >= newArity) {
3264                     return reorder.length;
3265                 }
3266                 long bit = 1L << arg;
3267                 if ((mask & bit) != 0) {
3268                     return i;  // >0 indicates a dup
3269                 }
3270                 mask |= bit;
3271             }
3272             if (mask == (1L << newArity) - 1) {
3273                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
3274                 return 0;
3275             }
3276             // find first zero
3277             long zeroBit = Long.lowestOneBit(~mask);
3278             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
3279             assert(zeroPos <= newArity);
3280             if (zeroPos == newArity) {
3281                 return 0;
3282             }
3283             return ~zeroPos;
3284         } else {
3285             // same algorithm, different bit set
3286             BitSet mask = new BitSet(newArity);
3287             for (int i = 0; i < reorder.length; i++) {
3288                 int arg = reorder[i];
3289                 if (arg >= newArity) {
3290                     return reorder.length;
3291                 }
3292                 if (mask.get(arg)) {
3293                     return i;  // >0 indicates a dup
3294                 }
3295                 mask.set(arg);
3296             }
3297             int zeroPos = mask.nextClearBit(0);
3298             assert(zeroPos <= newArity);
3299             if (zeroPos == newArity) {
3300                 return 0;
3301             }
3302             return ~zeroPos;
3303         }
3304     }
3305 
3306     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
3307         if (newType.returnType() != oldType.returnType())
3308             throw newIllegalArgumentException("return types do not match",
3309                     oldType, newType);
3310         if (reorder.length == oldType.parameterCount()) {
3311             int limit = newType.parameterCount();
3312             boolean bad = false;
3313             for (int j = 0; j < reorder.length; j++) {
3314                 int i = reorder[j];
3315                 if (i < 0 || i >= limit) {
3316                     bad = true; break;
3317                 }
3318                 Class<?> src = newType.parameterType(i);
3319                 Class<?> dst = oldType.parameterType(j);
3320                 if (src != dst)
3321                     throw newIllegalArgumentException("parameter types do not match after reorder",
3322                             oldType, newType);
3323             }
3324             if (!bad)  return true;
3325         }
3326         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
3327     }
3328 
3329     /**
3330      * Produces a method handle of the requested return type which returns the given
3331      * constant value every time it is invoked.
3332      * <p>
3333      * Before the method handle is returned, the passed-in value is converted to the requested type.
3334      * If the requested type is primitive, widening primitive conversions are attempted,
3335      * else reference conversions are attempted.
3336      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
3337      * @param type the return type of the desired method handle
3338      * @param value the value to return
3339      * @return a method handle of the given return type and no arguments, which always returns the given value
3340      * @throws NullPointerException if the {@code type} argument is null
3341      * @throws ClassCastException if the value cannot be converted to the required return type
3342      * @throws IllegalArgumentException if the given type is {@code void.class}
3343      */
3344     public static
3345     MethodHandle constant(Class<?> type, Object value) {
3346         if (type.isPrimitive()) {
3347             if (type == void.class)
3348                 throw newIllegalArgumentException("void type");
3349             Wrapper w = Wrapper.forPrimitiveType(type);
3350             value = w.convert(value, type);
3351             if (w.zero().equals(value))
3352                 return zero(w, type);
3353             return insertArguments(identity(type), 0, value);
3354         } else {
3355             if (value == null)
3356                 return zero(Wrapper.OBJECT, type);
3357             return identity(type).bindTo(value);
3358         }
3359     }
3360 
3361     /**
3362      * Produces a method handle which returns its sole argument when invoked.
3363      * @param type the type of the sole parameter and return value of the desired method handle
3364      * @return a unary method handle which accepts and returns the given type
3365      * @throws NullPointerException if the argument is null
3366      * @throws IllegalArgumentException if the given type is {@code void.class}
3367      */
3368     public static
3369     MethodHandle identity(Class<?> type) {
3370         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
3371         int pos = btw.ordinal();
3372         MethodHandle ident = IDENTITY_MHS[pos];
3373         if (ident == null) {
3374             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
3375         }
3376         if (ident.type().returnType() == type)
3377             return ident;
3378         // something like identity(Foo.class); do not bother to intern these
3379         assert (btw == Wrapper.OBJECT);
3380         return makeIdentity(type);
3381     }
3382 
3383     /**
3384      * Produces a constant method handle of the requested return type which
3385      * returns the default value for that type every time it is invoked.
3386      * The resulting constant method handle will have no side effects.
3387      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
3388      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
3389      * since {@code explicitCastArguments} converts {@code null} to default values.
3390      * @param type the expected return type of the desired method handle
3391      * @return a constant method handle that takes no arguments
3392      *         and returns the default value of the given type (or void, if the type is void)
3393      * @throws NullPointerException if the argument is null
3394      * @see MethodHandles#constant
3395      * @see MethodHandles#empty
3396      * @see MethodHandles#explicitCastArguments
3397      * @since 9
3398      */
3399     public static MethodHandle zero(Class<?> type) {
3400         Objects.requireNonNull(type);
3401         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
3402     }
3403 
3404     private static MethodHandle identityOrVoid(Class<?> type) {
3405         return type == void.class ? zero(type) : identity(type);
3406     }
3407 
3408     /**
3409      * Produces a method handle of the requested type which ignores any arguments, does nothing,
3410      * and returns a suitable default depending on the return type.
3411      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
3412      * <p>The returned method handle is equivalent to
3413      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
3414      *
3415      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
3416      * {@code guardWithTest(pred, target, empty(target.type())}.
3417      * @param type the type of the desired method handle
3418      * @return a constant method handle of the given type, which returns a default value of the given return type
3419      * @throws NullPointerException if the argument is null
3420      * @see MethodHandles#zero
3421      * @see MethodHandles#constant
3422      * @since 9
3423      */
3424     public static  MethodHandle empty(MethodType type) {
3425         Objects.requireNonNull(type);
3426         return dropArguments(zero(type.returnType()), 0, type.parameterList());
3427     }
3428 
3429     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
3430     private static MethodHandle makeIdentity(Class<?> ptype) {
3431         MethodType mtype = methodType(ptype, ptype);
3432         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
3433         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
3434     }
3435 
3436     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
3437         int pos = btw.ordinal();
3438         MethodHandle zero = ZERO_MHS[pos];
3439         if (zero == null) {
3440             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
3441         }
3442         if (zero.type().returnType() == rtype)
3443             return zero;
3444         assert(btw == Wrapper.OBJECT);
3445         return makeZero(rtype);
3446     }
3447     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
3448     private static MethodHandle makeZero(Class<?> rtype) {
3449         MethodType mtype = methodType(rtype);
3450         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
3451         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
3452     }
3453 
3454     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
3455         // Simulate a CAS, to avoid racy duplication of results.
3456         MethodHandle prev = cache[pos];
3457         if (prev != null) return prev;
3458         return cache[pos] = value;
3459     }
3460 
3461     /**
3462      * Provides a target method handle with one or more <em>bound arguments</em>
3463      * in advance of the method handle's invocation.
3464      * The formal parameters to the target corresponding to the bound
3465      * arguments are called <em>bound parameters</em>.
3466      * Returns a new method handle which saves away the bound arguments.
3467      * When it is invoked, it receives arguments for any non-bound parameters,
3468      * binds the saved arguments to their corresponding parameters,
3469      * and calls the original target.
3470      * <p>
3471      * The type of the new method handle will drop the types for the bound
3472      * parameters from the original target type, since the new method handle
3473      * will no longer require those arguments to be supplied by its callers.
3474      * <p>
3475      * Each given argument object must match the corresponding bound parameter type.
3476      * If a bound parameter type is a primitive, the argument object
3477      * must be a wrapper, and will be unboxed to produce the primitive value.
3478      * <p>
3479      * The {@code pos} argument selects which parameters are to be bound.
3480      * It may range between zero and <i>N-L</i> (inclusively),
3481      * where <i>N</i> is the arity of the target method handle
3482      * and <i>L</i> is the length of the values array.
3483      * <p>
3484      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3485      * variable-arity method handle}, even if the original target method handle was.
3486      * @param target the method handle to invoke after the argument is inserted
3487      * @param pos where to insert the argument (zero for the first)
3488      * @param values the series of arguments to insert
3489      * @return a method handle which inserts an additional argument,
3490      *         before calling the original method handle
3491      * @throws NullPointerException if the target or the {@code values} array is null
3492      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
3493      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
3494      *         is the length of the values array.
3495      * @throws ClassCastException if an argument does not match the corresponding bound parameter
3496      *         type.
3497      * @see MethodHandle#bindTo
3498      */
3499     public static
3500     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
3501         int insCount = values.length;
3502         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
3503         if (insCount == 0)  return target;
3504         BoundMethodHandle result = target.rebind();
3505         for (int i = 0; i < insCount; i++) {
3506             Object value = values[i];
3507             Class<?> ptype = ptypes[pos+i];
3508             if (ptype.isPrimitive()) {
3509                 result = insertArgumentPrimitive(result, pos, ptype, value);
3510             } else {
3511                 value = ptype.cast(value);  // throw CCE if needed
3512                 result = result.bindArgumentL(pos, value);
3513             }
3514         }
3515         return result;
3516     }
3517 
3518     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
3519                                                              Class<?> ptype, Object value) {
3520         Wrapper w = Wrapper.forPrimitiveType(ptype);
3521         // perform unboxing and/or primitive conversion
3522         value = w.convert(value, ptype);
3523         switch (w) {
3524         case INT:     return result.bindArgumentI(pos, (int)value);
3525         case LONG:    return result.bindArgumentJ(pos, (long)value);
3526         case FLOAT:   return result.bindArgumentF(pos, (float)value);
3527         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
3528         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
3529         }
3530     }
3531 
3532     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
3533         MethodType oldType = target.type();
3534         int outargs = oldType.parameterCount();
3535         int inargs  = outargs - insCount;
3536         if (inargs < 0)
3537             throw newIllegalArgumentException("too many values to insert");
3538         if (pos < 0 || pos > inargs)
3539             throw newIllegalArgumentException("no argument type to append");
3540         return oldType.ptypes();
3541     }
3542 
3543     /**
3544      * Produces a method handle which will discard some dummy arguments
3545      * before calling some other specified <i>target</i> method handle.
3546      * The type of the new method handle will be the same as the target's type,
3547      * except it will also include the dummy argument types,
3548      * at some given position.
3549      * <p>
3550      * The {@code pos} argument may range between zero and <i>N</i>,
3551      * where <i>N</i> is the arity of the target.
3552      * If {@code pos} is zero, the dummy arguments will precede
3553      * the target's real arguments; if {@code pos} is <i>N</i>
3554      * they will come after.
3555      * <p>
3556      * <b>Example:</b>
3557      * <blockquote><pre>{@code
3558 import static java.lang.invoke.MethodHandles.*;
3559 import static java.lang.invoke.MethodType.*;
3560 ...
3561 MethodHandle cat = lookup().findVirtual(String.class,
3562   "concat", methodType(String.class, String.class));
3563 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3564 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
3565 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
3566 assertEquals(bigType, d0.type());
3567 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
3568      * }</pre></blockquote>
3569      * <p>
3570      * This method is also equivalent to the following code:
3571      * <blockquote><pre>
3572      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
3573      * </pre></blockquote>
3574      * @param target the method handle to invoke after the arguments are dropped
3575      * @param valueTypes the type(s) of the argument(s) to drop
3576      * @param pos position of first argument to drop (zero for the leftmost)
3577      * @return a method handle which drops arguments of the given types,
3578      *         before calling the original method handle
3579      * @throws NullPointerException if the target is null,
3580      *                              or if the {@code valueTypes} list or any of its elements is null
3581      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3582      *                  or if {@code pos} is negative or greater than the arity of the target,
3583      *                  or if the new method handle's type would have too many parameters
3584      */
3585     public static
3586     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3587         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
3588     }
3589 
3590     private static List<Class<?>> copyTypes(Object[] array) {
3591         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
3592     }
3593 
3594     private static
3595     MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3596         MethodType oldType = target.type();  // get NPE
3597         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
3598         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
3599         if (dropped == 0)  return target;
3600         BoundMethodHandle result = target.rebind();
3601         LambdaForm lform = result.form;
3602         int insertFormArg = 1 + pos;
3603         for (Class<?> ptype : valueTypes) {
3604             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
3605         }
3606         result = result.copyWith(newType, lform);
3607         return result;
3608     }
3609 
3610     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
3611         int dropped = valueTypes.size();
3612         MethodType.checkSlotCount(dropped);
3613         int outargs = oldType.parameterCount();
3614         int inargs  = outargs + dropped;
3615         if (pos < 0 || pos > outargs)
3616             throw newIllegalArgumentException("no argument type to remove"
3617                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
3618                     );
3619         return dropped;
3620     }
3621 
3622     /**
3623      * Produces a method handle which will discard some dummy arguments
3624      * before calling some other specified <i>target</i> method handle.
3625      * The type of the new method handle will be the same as the target's type,
3626      * except it will also include the dummy argument types,
3627      * at some given position.
3628      * <p>
3629      * The {@code pos} argument may range between zero and <i>N</i>,
3630      * where <i>N</i> is the arity of the target.
3631      * If {@code pos} is zero, the dummy arguments will precede
3632      * the target's real arguments; if {@code pos} is <i>N</i>
3633      * they will come after.
3634      * @apiNote
3635      * <blockquote><pre>{@code
3636 import static java.lang.invoke.MethodHandles.*;
3637 import static java.lang.invoke.MethodType.*;
3638 ...
3639 MethodHandle cat = lookup().findVirtual(String.class,
3640   "concat", methodType(String.class, String.class));
3641 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3642 MethodHandle d0 = dropArguments(cat, 0, String.class);
3643 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
3644 MethodHandle d1 = dropArguments(cat, 1, String.class);
3645 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
3646 MethodHandle d2 = dropArguments(cat, 2, String.class);
3647 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
3648 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
3649 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
3650      * }</pre></blockquote>
3651      * <p>
3652      * This method is also equivalent to the following code:
3653      * <blockquote><pre>
3654      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
3655      * </pre></blockquote>
3656      * @param target the method handle to invoke after the arguments are dropped
3657      * @param valueTypes the type(s) of the argument(s) to drop
3658      * @param pos position of first argument to drop (zero for the leftmost)
3659      * @return a method handle which drops arguments of the given types,
3660      *         before calling the original method handle
3661      * @throws NullPointerException if the target is null,
3662      *                              or if the {@code valueTypes} array or any of its elements is null
3663      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3664      *                  or if {@code pos} is negative or greater than the arity of the target,
3665      *                  or if the new method handle's type would have
3666      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
3667      */
3668     public static
3669     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
3670         return dropArguments0(target, pos, copyTypes(valueTypes));
3671     }
3672 
3673     // private version which allows caller some freedom with error handling
3674     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
3675                                       boolean nullOnFailure) {
3676         newTypes = copyTypes(newTypes.toArray());
3677         List<Class<?>> oldTypes = target.type().parameterList();
3678         int match = oldTypes.size();
3679         if (skip != 0) {
3680             if (skip < 0 || skip > match) {
3681                 throw newIllegalArgumentException("illegal skip", skip, target);
3682             }
3683             oldTypes = oldTypes.subList(skip, match);
3684             match -= skip;
3685         }
3686         List<Class<?>> addTypes = newTypes;
3687         int add = addTypes.size();
3688         if (pos != 0) {
3689             if (pos < 0 || pos > add) {
3690                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
3691             }
3692             addTypes = addTypes.subList(pos, add);
3693             add -= pos;
3694             assert(addTypes.size() == add);
3695         }
3696         // Do not add types which already match the existing arguments.
3697         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
3698             if (nullOnFailure) {
3699                 return null;
3700             }
3701             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
3702         }
3703         addTypes = addTypes.subList(match, add);
3704         add -= match;
3705         assert(addTypes.size() == add);
3706         // newTypes:     (   P*[pos], M*[match], A*[add] )
3707         // target: ( S*[skip],        M*[match]  )
3708         MethodHandle adapter = target;
3709         if (add > 0) {
3710             adapter = dropArguments0(adapter, skip+ match, addTypes);
3711         }
3712         // adapter: (S*[skip],        M*[match], A*[add] )
3713         if (pos > 0) {
3714             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
3715         }
3716         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
3717         return adapter;
3718     }
3719 
3720     /**
3721      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
3722      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
3723      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
3724      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
3725      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
3726      * {@link #dropArguments(MethodHandle, int, Class[])}.
3727      * <p>
3728      * The resulting handle will have the same return type as the target handle.
3729      * <p>
3730      * In more formal terms, assume these two type lists:<ul>
3731      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
3732      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
3733      * {@code newTypes}.
3734      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
3735      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
3736      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
3737      * sub-list.
3738      * </ul>
3739      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
3740      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
3741      * {@link #dropArguments(MethodHandle, int, Class[])}.
3742      *
3743      * @apiNote
3744      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
3745      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
3746      * <blockquote><pre>{@code
3747 import static java.lang.invoke.MethodHandles.*;
3748 import static java.lang.invoke.MethodType.*;
3749 ...
3750 ...
3751 MethodHandle h0 = constant(boolean.class, true);
3752 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
3753 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
3754 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
3755 if (h1.type().parameterCount() < h2.type().parameterCount())
3756     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
3757 else
3758     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
3759 MethodHandle h3 = guardWithTest(h0, h1, h2);
3760 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
3761      * }</pre></blockquote>
3762      * @param target the method handle to adapt
3763      * @param skip number of targets parameters to disregard (they will be unchanged)
3764      * @param newTypes the list of types to match {@code target}'s parameter type list to
3765      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
3766      * @return a possibly adapted method handle
3767      * @throws NullPointerException if either argument is null
3768      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
3769      *         or if {@code skip} is negative or greater than the arity of the target,
3770      *         or if {@code pos} is negative or greater than the newTypes list size,
3771      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
3772      *         {@code pos}.
3773      * @since 9
3774      */
3775     public static
3776     MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
3777         Objects.requireNonNull(target);
3778         Objects.requireNonNull(newTypes);
3779         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
3780     }
3781 
3782     /**
3783      * Adapts a target method handle by pre-processing
3784      * one or more of its arguments, each with its own unary filter function,
3785      * and then calling the target with each pre-processed argument
3786      * replaced by the result of its corresponding filter function.
3787      * <p>
3788      * The pre-processing is performed by one or more method handles,
3789      * specified in the elements of the {@code filters} array.
3790      * The first element of the filter array corresponds to the {@code pos}
3791      * argument of the target, and so on in sequence.
3792      * The filter functions are invoked in left to right order.
3793      * <p>
3794      * Null arguments in the array are treated as identity functions,
3795      * and the corresponding arguments left unchanged.
3796      * (If there are no non-null elements in the array, the original target is returned.)
3797      * Each filter is applied to the corresponding argument of the adapter.
3798      * <p>
3799      * If a filter {@code F} applies to the {@code N}th argument of
3800      * the target, then {@code F} must be a method handle which
3801      * takes exactly one argument.  The type of {@code F}'s sole argument
3802      * replaces the corresponding argument type of the target
3803      * in the resulting adapted method handle.
3804      * The return type of {@code F} must be identical to the corresponding
3805      * parameter type of the target.
3806      * <p>
3807      * It is an error if there are elements of {@code filters}
3808      * (null or not)
3809      * which do not correspond to argument positions in the target.
3810      * <p><b>Example:</b>
3811      * <blockquote><pre>{@code
3812 import static java.lang.invoke.MethodHandles.*;
3813 import static java.lang.invoke.MethodType.*;
3814 ...
3815 MethodHandle cat = lookup().findVirtual(String.class,
3816   "concat", methodType(String.class, String.class));
3817 MethodHandle upcase = lookup().findVirtual(String.class,
3818   "toUpperCase", methodType(String.class));
3819 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3820 MethodHandle f0 = filterArguments(cat, 0, upcase);
3821 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
3822 MethodHandle f1 = filterArguments(cat, 1, upcase);
3823 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
3824 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
3825 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
3826      * }</pre></blockquote>
3827      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3828      * denotes the return type of both the {@code target} and resulting adapter.
3829      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
3830      * of the parameters and arguments that precede and follow the filter position
3831      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
3832      * values of the filtered parameters and arguments; they also represent the
3833      * return types of the {@code filter[i]} handles. The latter accept arguments
3834      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
3835      * the resulting adapter.
3836      * <blockquote><pre>{@code
3837      * T target(P... p, A[i]... a[i], B... b);
3838      * A[i] filter[i](V[i]);
3839      * T adapter(P... p, V[i]... v[i], B... b) {
3840      *   return target(p..., filter[i](v[i])..., b...);
3841      * }
3842      * }</pre></blockquote>
3843      * <p>
3844      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3845      * variable-arity method handle}, even if the original target method handle was.
3846      *
3847      * @param target the method handle to invoke after arguments are filtered
3848      * @param pos the position of the first argument to filter
3849      * @param filters method handles to call initially on filtered arguments
3850      * @return method handle which incorporates the specified argument filtering logic
3851      * @throws NullPointerException if the target is null
3852      *                              or if the {@code filters} array is null
3853      * @throws IllegalArgumentException if a non-null element of {@code filters}
3854      *          does not match a corresponding argument type of target as described above,
3855      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
3856      *          or if the resulting method handle's type would have
3857      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3858      */
3859     public static
3860     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
3861         filterArgumentsCheckArity(target, pos, filters);
3862         MethodHandle adapter = target;
3863         // process filters in reverse order so that the invocation of
3864         // the resulting adapter will invoke the filters in left-to-right order
3865         for (int i = filters.length - 1; i >= 0; --i) {
3866             MethodHandle filter = filters[i];
3867             if (filter == null)  continue;  // ignore null elements of filters
3868             adapter = filterArgument(adapter, pos + i, filter);
3869         }
3870         return adapter;
3871     }
3872 
3873     /*non-public*/ static
3874     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
3875         filterArgumentChecks(target, pos, filter);
3876         MethodType targetType = target.type();
3877         MethodType filterType = filter.type();
3878         BoundMethodHandle result = target.rebind();
3879         Class<?> newParamType = filterType.parameterType(0);
3880         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
3881         MethodType newType = targetType.changeParameterType(pos, newParamType);
3882         result = result.copyWithExtendL(newType, lform, filter);
3883         return result;
3884     }
3885 
3886     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
3887         MethodType targetType = target.type();
3888         int maxPos = targetType.parameterCount();
3889         if (pos + filters.length > maxPos)
3890             throw newIllegalArgumentException("too many filters");
3891     }
3892 
3893     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
3894         MethodType targetType = target.type();
3895         MethodType filterType = filter.type();
3896         if (filterType.parameterCount() != 1
3897             || filterType.returnType() != targetType.parameterType(pos))
3898             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3899     }
3900 
3901     /**
3902      * Adapts a target method handle by pre-processing
3903      * a sub-sequence of its arguments with a filter (another method handle).
3904      * The pre-processed arguments are replaced by the result (if any) of the
3905      * filter function.
3906      * The target is then called on the modified (usually shortened) argument list.
3907      * <p>
3908      * If the filter returns a value, the target must accept that value as
3909      * its argument in position {@code pos}, preceded and/or followed by
3910      * any arguments not passed to the filter.
3911      * If the filter returns void, the target must accept all arguments
3912      * not passed to the filter.
3913      * No arguments are reordered, and a result returned from the filter
3914      * replaces (in order) the whole subsequence of arguments originally
3915      * passed to the adapter.
3916      * <p>
3917      * The argument types (if any) of the filter
3918      * replace zero or one argument types of the target, at position {@code pos},
3919      * in the resulting adapted method handle.
3920      * The return type of the filter (if any) must be identical to the
3921      * argument type of the target at position {@code pos}, and that target argument
3922      * is supplied by the return value of the filter.
3923      * <p>
3924      * In all cases, {@code pos} must be greater than or equal to zero, and
3925      * {@code pos} must also be less than or equal to the target's arity.
3926      * <p><b>Example:</b>
3927      * <blockquote><pre>{@code
3928 import static java.lang.invoke.MethodHandles.*;
3929 import static java.lang.invoke.MethodType.*;
3930 ...
3931 MethodHandle deepToString = publicLookup()
3932   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
3933 
3934 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
3935 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
3936 
3937 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
3938 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
3939 
3940 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
3941 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
3942 assertEquals("[top, [up, down], strange]",
3943              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
3944 
3945 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
3946 assertEquals("[top, [up, down], [strange]]",
3947              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
3948 
3949 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
3950 assertEquals("[top, [[up, down, strange], charm], bottom]",
3951              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
3952      * }</pre></blockquote>
3953      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3954      * represents the return type of the {@code target} and resulting adapter.
3955      * {@code V}/{@code v} stand for the return type and value of the
3956      * {@code filter}, which are also found in the signature and arguments of
3957      * the {@code target}, respectively, unless {@code V} is {@code void}.
3958      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
3959      * and values preceding and following the collection position, {@code pos},
3960      * in the {@code target}'s signature. They also turn up in the resulting
3961      * adapter's signature and arguments, where they surround
3962      * {@code B}/{@code b}, which represent the parameter types and arguments
3963      * to the {@code filter} (if any).
3964      * <blockquote><pre>{@code
3965      * T target(A...,V,C...);
3966      * V filter(B...);
3967      * T adapter(A... a,B... b,C... c) {
3968      *   V v = filter(b...);
3969      *   return target(a...,v,c...);
3970      * }
3971      * // and if the filter has no arguments:
3972      * T target2(A...,V,C...);
3973      * V filter2();
3974      * T adapter2(A... a,C... c) {
3975      *   V v = filter2();
3976      *   return target2(a...,v,c...);
3977      * }
3978      * // and if the filter has a void return:
3979      * T target3(A...,C...);
3980      * void filter3(B...);
3981      * T adapter3(A... a,B... b,C... c) {
3982      *   filter3(b...);
3983      *   return target3(a...,c...);
3984      * }
3985      * }</pre></blockquote>
3986      * <p>
3987      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
3988      * one which first "folds" the affected arguments, and then drops them, in separate
3989      * steps as follows:
3990      * <blockquote><pre>{@code
3991      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
3992      * mh = MethodHandles.foldArguments(mh, coll); //step 1
3993      * }</pre></blockquote>
3994      * If the target method handle consumes no arguments besides than the result
3995      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
3996      * is equivalent to {@code filterReturnValue(coll, mh)}.
3997      * If the filter method handle {@code coll} consumes one argument and produces
3998      * a non-void result, then {@code collectArguments(mh, N, coll)}
3999      * is equivalent to {@code filterArguments(mh, N, coll)}.
4000      * Other equivalences are possible but would require argument permutation.
4001      * <p>
4002      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4003      * variable-arity method handle}, even if the original target method handle was.
4004      *
4005      * @param target the method handle to invoke after filtering the subsequence of arguments
4006      * @param pos the position of the first adapter argument to pass to the filter,
4007      *            and/or the target argument which receives the result of the filter
4008      * @param filter method handle to call on the subsequence of arguments
4009      * @return method handle which incorporates the specified argument subsequence filtering logic
4010      * @throws NullPointerException if either argument is null
4011      * @throws IllegalArgumentException if the return type of {@code filter}
4012      *          is non-void and is not the same as the {@code pos} argument of the target,
4013      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
4014      *          or if the resulting method handle's type would have
4015      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4016      * @see MethodHandles#foldArguments
4017      * @see MethodHandles#filterArguments
4018      * @see MethodHandles#filterReturnValue
4019      */
4020     public static
4021     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
4022         MethodType newType = collectArgumentsChecks(target, pos, filter);
4023         MethodType collectorType = filter.type();
4024         BoundMethodHandle result = target.rebind();
4025         LambdaForm lform;
4026         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
4027             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
4028             if (lform != null) {
4029                 return result.copyWith(newType, lform);
4030             }
4031         }
4032         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
4033         return result.copyWithExtendL(newType, lform, filter);
4034     }
4035 
4036     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
4037         MethodType targetType = target.type();
4038         MethodType filterType = filter.type();
4039         Class<?> rtype = filterType.returnType();
4040         List<Class<?>> filterArgs = filterType.parameterList();
4041         if (rtype == void.class) {
4042             return targetType.insertParameterTypes(pos, filterArgs);
4043         }
4044         if (rtype != targetType.parameterType(pos)) {
4045             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4046         }
4047         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
4048     }
4049 
4050     /**
4051      * Adapts a target method handle by post-processing
4052      * its return value (if any) with a filter (another method handle).
4053      * The result of the filter is returned from the adapter.
4054      * <p>
4055      * If the target returns a value, the filter must accept that value as
4056      * its only argument.
4057      * If the target returns void, the filter must accept no arguments.
4058      * <p>
4059      * The return type of the filter
4060      * replaces the return type of the target
4061      * in the resulting adapted method handle.
4062      * The argument type of the filter (if any) must be identical to the
4063      * return type of the target.
4064      * <p><b>Example:</b>
4065      * <blockquote><pre>{@code
4066 import static java.lang.invoke.MethodHandles.*;
4067 import static java.lang.invoke.MethodType.*;
4068 ...
4069 MethodHandle cat = lookup().findVirtual(String.class,
4070   "concat", methodType(String.class, String.class));
4071 MethodHandle length = lookup().findVirtual(String.class,
4072   "length", methodType(int.class));
4073 System.out.println((String) cat.invokeExact("x", "y")); // xy
4074 MethodHandle f0 = filterReturnValue(cat, length);
4075 System.out.println((int) f0.invokeExact("x", "y")); // 2
4076      * }</pre></blockquote>
4077      * <p>Here is pseudocode for the resulting adapter. In the code,
4078      * {@code T}/{@code t} represent the result type and value of the
4079      * {@code target}; {@code V}, the result type of the {@code filter}; and
4080      * {@code A}/{@code a}, the types and values of the parameters and arguments
4081      * of the {@code target} as well as the resulting adapter.
4082      * <blockquote><pre>{@code
4083      * T target(A...);
4084      * V filter(T);
4085      * V adapter(A... a) {
4086      *   T t = target(a...);
4087      *   return filter(t);
4088      * }
4089      * // and if the target has a void return:
4090      * void target2(A...);
4091      * V filter2();
4092      * V adapter2(A... a) {
4093      *   target2(a...);
4094      *   return filter2();
4095      * }
4096      * // and if the filter has a void return:
4097      * T target3(A...);
4098      * void filter3(V);
4099      * void adapter3(A... a) {
4100      *   T t = target3(a...);
4101      *   filter3(t);
4102      * }
4103      * }</pre></blockquote>
4104      * <p>
4105      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4106      * variable-arity method handle}, even if the original target method handle was.
4107      * @param target the method handle to invoke before filtering the return value
4108      * @param filter method handle to call on the return value
4109      * @return method handle which incorporates the specified return value filtering logic
4110      * @throws NullPointerException if either argument is null
4111      * @throws IllegalArgumentException if the argument list of {@code filter}
4112      *          does not match the return type of target as described above
4113      */
4114     public static
4115     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
4116         MethodType targetType = target.type();
4117         MethodType filterType = filter.type();
4118         filterReturnValueChecks(targetType, filterType);
4119         BoundMethodHandle result = target.rebind();
4120         BasicType rtype = BasicType.basicType(filterType.returnType());
4121         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
4122         MethodType newType = targetType.changeReturnType(filterType.returnType());
4123         result = result.copyWithExtendL(newType, lform, filter);
4124         return result;
4125     }
4126 
4127     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
4128         Class<?> rtype = targetType.returnType();
4129         int filterValues = filterType.parameterCount();
4130         if (filterValues == 0
4131                 ? (rtype != void.class)
4132                 : (rtype != filterType.parameterType(0) || filterValues != 1))
4133             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4134     }
4135 
4136     /**
4137      * Adapts a target method handle by pre-processing
4138      * some of its arguments, and then calling the target with
4139      * the result of the pre-processing, inserted into the original
4140      * sequence of arguments.
4141      * <p>
4142      * The pre-processing is performed by {@code combiner}, a second method handle.
4143      * Of the arguments passed to the adapter, the first {@code N} arguments
4144      * are copied to the combiner, which is then called.
4145      * (Here, {@code N} is defined as the parameter count of the combiner.)
4146      * After this, control passes to the target, with any result
4147      * from the combiner inserted before the original {@code N} incoming
4148      * arguments.
4149      * <p>
4150      * If the combiner returns a value, the first parameter type of the target
4151      * must be identical with the return type of the combiner, and the next
4152      * {@code N} parameter types of the target must exactly match the parameters
4153      * of the combiner.
4154      * <p>
4155      * If the combiner has a void return, no result will be inserted,
4156      * and the first {@code N} parameter types of the target
4157      * must exactly match the parameters of the combiner.
4158      * <p>
4159      * The resulting adapter is the same type as the target, except that the
4160      * first parameter type is dropped,
4161      * if it corresponds to the result of the combiner.
4162      * <p>
4163      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
4164      * that either the combiner or the target does not wish to receive.
4165      * If some of the incoming arguments are destined only for the combiner,
4166      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
4167      * arguments will not need to be live on the stack on entry to the
4168      * target.)
4169      * <p><b>Example:</b>
4170      * <blockquote><pre>{@code
4171 import static java.lang.invoke.MethodHandles.*;
4172 import static java.lang.invoke.MethodType.*;
4173 ...
4174 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4175   "println", methodType(void.class, String.class))
4176     .bindTo(System.out);
4177 MethodHandle cat = lookup().findVirtual(String.class,
4178   "concat", methodType(String.class, String.class));
4179 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4180 MethodHandle catTrace = foldArguments(cat, trace);
4181 // also prints "boo":
4182 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4183      * }</pre></blockquote>
4184      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4185      * represents the result type of the {@code target} and resulting adapter.
4186      * {@code V}/{@code v} represent the type and value of the parameter and argument
4187      * of {@code target} that precedes the folding position; {@code V} also is
4188      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4189      * types and values of the {@code N} parameters and arguments at the folding
4190      * position. {@code B}/{@code b} represent the types and values of the
4191      * {@code target} parameters and arguments that follow the folded parameters
4192      * and arguments.
4193      * <blockquote><pre>{@code
4194      * // there are N arguments in A...
4195      * T target(V, A[N]..., B...);
4196      * V combiner(A...);
4197      * T adapter(A... a, B... b) {
4198      *   V v = combiner(a...);
4199      *   return target(v, a..., b...);
4200      * }
4201      * // and if the combiner has a void return:
4202      * T target2(A[N]..., B...);
4203      * void combiner2(A...);
4204      * T adapter2(A... a, B... b) {
4205      *   combiner2(a...);
4206      *   return target2(a..., b...);
4207      * }
4208      * }</pre></blockquote>
4209      * <p>
4210      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4211      * variable-arity method handle}, even if the original target method handle was.
4212      * @param target the method handle to invoke after arguments are combined
4213      * @param combiner method handle to call initially on the incoming arguments
4214      * @return method handle which incorporates the specified argument folding logic
4215      * @throws NullPointerException if either argument is null
4216      * @throws IllegalArgumentException if {@code combiner}'s return type
4217      *          is non-void and not the same as the first argument type of
4218      *          the target, or if the initial {@code N} argument types
4219      *          of the target
4220      *          (skipping one matching the {@code combiner}'s return type)
4221      *          are not identical with the argument types of {@code combiner}
4222      */
4223     public static
4224     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
4225         return foldArguments(target, 0, combiner);
4226     }
4227 
4228     /**
4229      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
4230      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
4231      * before the folded arguments.
4232      * <p>
4233      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
4234      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
4235      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
4236      * 0.
4237      *
4238      * @apiNote Example:
4239      * <blockquote><pre>{@code
4240     import static java.lang.invoke.MethodHandles.*;
4241     import static java.lang.invoke.MethodType.*;
4242     ...
4243     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4244     "println", methodType(void.class, String.class))
4245     .bindTo(System.out);
4246     MethodHandle cat = lookup().findVirtual(String.class,
4247     "concat", methodType(String.class, String.class));
4248     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4249     MethodHandle catTrace = foldArguments(cat, 1, trace);
4250     // also prints "jum":
4251     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4252      * }</pre></blockquote>
4253      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4254      * represents the result type of the {@code target} and resulting adapter.
4255      * {@code V}/{@code v} represent the type and value of the parameter and argument
4256      * of {@code target} that precedes the folding position; {@code V} also is
4257      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4258      * types and values of the {@code N} parameters and arguments at the folding
4259      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
4260      * and values of the {@code target} parameters and arguments that precede and
4261      * follow the folded parameters and arguments starting at {@code pos},
4262      * respectively.
4263      * <blockquote><pre>{@code
4264      * // there are N arguments in A...
4265      * T target(Z..., V, A[N]..., B...);
4266      * V combiner(A...);
4267      * T adapter(Z... z, A... a, B... b) {
4268      *   V v = combiner(a...);
4269      *   return target(z..., v, a..., b...);
4270      * }
4271      * // and if the combiner has a void return:
4272      * T target2(Z..., A[N]..., B...);
4273      * void combiner2(A...);
4274      * T adapter2(Z... z, A... a, B... b) {
4275      *   combiner2(a...);
4276      *   return target2(z..., a..., b...);
4277      * }
4278      * }</pre></blockquote>
4279      * <p>
4280      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4281      * variable-arity method handle}, even if the original target method handle was.
4282      *
4283      * @param target the method handle to invoke after arguments are combined
4284      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
4285      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4286      * @param combiner method handle to call initially on the incoming arguments
4287      * @return method handle which incorporates the specified argument folding logic
4288      * @throws NullPointerException if either argument is null
4289      * @throws IllegalArgumentException if either of the following two conditions holds:
4290      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
4291      *              {@code pos} of the target signature;
4292      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
4293      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
4294      *
4295      * @see #foldArguments(MethodHandle, MethodHandle)
4296      * @since 9
4297      */
4298     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
4299         MethodType targetType = target.type();
4300         MethodType combinerType = combiner.type();
4301         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
4302         BoundMethodHandle result = target.rebind();
4303         boolean dropResult = rtype == void.class;
4304         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
4305         MethodType newType = targetType;
4306         if (!dropResult) {
4307             newType = newType.dropParameterTypes(pos, pos + 1);
4308         }
4309         result = result.copyWithExtendL(newType, lform, combiner);
4310         return result;
4311     }
4312 
4313     /**
4314      * As {@see foldArguments(MethodHandle, int, MethodHandle)}, but with the
4315      * added capability of selecting the arguments from the targets parameters
4316      * to call the combiner with. This allows us to avoid some simple cases of
4317      * permutations and padding the combiner with dropArguments to select the
4318      * right argument, which may ultimately produce fewer intermediaries.
4319      */
4320     static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner, int ... argPositions) {
4321         MethodType targetType = target.type();
4322         MethodType combinerType = combiner.type();
4323         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType, argPositions);
4324         BoundMethodHandle result = target.rebind();
4325         boolean dropResult = rtype == void.class;
4326         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType(), argPositions);
4327         MethodType newType = targetType;
4328         if (!dropResult) {
4329             newType = newType.dropParameterTypes(pos, pos + 1);
4330         }
4331         result = result.copyWithExtendL(newType, lform, combiner);
4332         return result;
4333     }
4334 
4335     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
4336         int foldArgs   = combinerType.parameterCount();
4337         Class<?> rtype = combinerType.returnType();
4338         int foldVals = rtype == void.class ? 0 : 1;
4339         int afterInsertPos = foldPos + foldVals;
4340         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
4341         if (ok) {
4342             for (int i = 0; i < foldArgs; i++) {
4343                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
4344                     ok = false;
4345                     break;
4346                 }
4347             }
4348         }
4349         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
4350             ok = false;
4351         if (!ok)
4352             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4353         return rtype;
4354     }
4355 
4356     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType, int ... argPos) {
4357         int foldArgs = combinerType.parameterCount();
4358         if (argPos.length != foldArgs) {
4359             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
4360         }
4361         Class<?> rtype = combinerType.returnType();
4362         int foldVals = rtype == void.class ? 0 : 1;
4363         boolean ok = true;
4364         for (int i = 0; i < foldArgs; i++) {
4365             int arg = argPos[i];
4366             if (arg < 0 || arg > targetType.parameterCount()) {
4367                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
4368             }
4369             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
4370                 throw newIllegalArgumentException("target argument type at position " + arg
4371                         + " must match combiner argument type at index " + i + ": " + targetType
4372                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
4373             }
4374         }
4375         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos)) {
4376             ok = false;
4377         }
4378         if (!ok)
4379             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4380         return rtype;
4381     }
4382 
4383     /**
4384      * Makes a method handle which adapts a target method handle,
4385      * by guarding it with a test, a boolean-valued method handle.
4386      * If the guard fails, a fallback handle is called instead.
4387      * All three method handles must have the same corresponding
4388      * argument and return types, except that the return type
4389      * of the test must be boolean, and the test is allowed
4390      * to have fewer arguments than the other two method handles.
4391      * <p>
4392      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4393      * represents the uniform result type of the three involved handles;
4394      * {@code A}/{@code a}, the types and values of the {@code target}
4395      * parameters and arguments that are consumed by the {@code test}; and
4396      * {@code B}/{@code b}, those types and values of the {@code target}
4397      * parameters and arguments that are not consumed by the {@code test}.
4398      * <blockquote><pre>{@code
4399      * boolean test(A...);
4400      * T target(A...,B...);
4401      * T fallback(A...,B...);
4402      * T adapter(A... a,B... b) {
4403      *   if (test(a...))
4404      *     return target(a..., b...);
4405      *   else
4406      *     return fallback(a..., b...);
4407      * }
4408      * }</pre></blockquote>
4409      * Note that the test arguments ({@code a...} in the pseudocode) cannot
4410      * be modified by execution of the test, and so are passed unchanged
4411      * from the caller to the target or fallback as appropriate.
4412      * @param test method handle used for test, must return boolean
4413      * @param target method handle to call if test passes
4414      * @param fallback method handle to call if test fails
4415      * @return method handle which incorporates the specified if/then/else logic
4416      * @throws NullPointerException if any argument is null
4417      * @throws IllegalArgumentException if {@code test} does not return boolean,
4418      *          or if all three method types do not match (with the return
4419      *          type of {@code test} changed to match that of the target).
4420      */
4421     public static
4422     MethodHandle guardWithTest(MethodHandle test,
4423                                MethodHandle target,
4424                                MethodHandle fallback) {
4425         MethodType gtype = test.type();
4426         MethodType ttype = target.type();
4427         MethodType ftype = fallback.type();
4428         if (!ttype.equals(ftype))
4429             throw misMatchedTypes("target and fallback types", ttype, ftype);
4430         if (gtype.returnType() != boolean.class)
4431             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
4432         List<Class<?>> targs = ttype.parameterList();
4433         test = dropArgumentsToMatch(test, 0, targs, 0, true);
4434         if (test == null) {
4435             throw misMatchedTypes("target and test types", ttype, gtype);
4436         }
4437         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
4438     }
4439 
4440     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
4441         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
4442     }
4443 
4444     /**
4445      * Makes a method handle which adapts a target method handle,
4446      * by running it inside an exception handler.
4447      * If the target returns normally, the adapter returns that value.
4448      * If an exception matching the specified type is thrown, the fallback
4449      * handle is called instead on the exception, plus the original arguments.
4450      * <p>
4451      * The target and handler must have the same corresponding
4452      * argument and return types, except that handler may omit trailing arguments
4453      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
4454      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
4455      * <p>
4456      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4457      * represents the return type of the {@code target} and {@code handler},
4458      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
4459      * the types and values of arguments to the resulting handle consumed by
4460      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
4461      * resulting handle discarded by {@code handler}.
4462      * <blockquote><pre>{@code
4463      * T target(A..., B...);
4464      * T handler(ExType, A...);
4465      * T adapter(A... a, B... b) {
4466      *   try {
4467      *     return target(a..., b...);
4468      *   } catch (ExType ex) {
4469      *     return handler(ex, a...);
4470      *   }
4471      * }
4472      * }</pre></blockquote>
4473      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
4474      * be modified by execution of the target, and so are passed unchanged
4475      * from the caller to the handler, if the handler is invoked.
4476      * <p>
4477      * The target and handler must return the same type, even if the handler
4478      * always throws.  (This might happen, for instance, because the handler
4479      * is simulating a {@code finally} clause).
4480      * To create such a throwing handler, compose the handler creation logic
4481      * with {@link #throwException throwException},
4482      * in order to create a method handle of the correct return type.
4483      * @param target method handle to call
4484      * @param exType the type of exception which the handler will catch
4485      * @param handler method handle to call if a matching exception is thrown
4486      * @return method handle which incorporates the specified try/catch logic
4487      * @throws NullPointerException if any argument is null
4488      * @throws IllegalArgumentException if {@code handler} does not accept
4489      *          the given exception type, or if the method handle types do
4490      *          not match in their return types and their
4491      *          corresponding parameters
4492      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
4493      */
4494     public static
4495     MethodHandle catchException(MethodHandle target,
4496                                 Class<? extends Throwable> exType,
4497                                 MethodHandle handler) {
4498         MethodType ttype = target.type();
4499         MethodType htype = handler.type();
4500         if (!Throwable.class.isAssignableFrom(exType))
4501             throw new ClassCastException(exType.getName());
4502         if (htype.parameterCount() < 1 ||
4503             !htype.parameterType(0).isAssignableFrom(exType))
4504             throw newIllegalArgumentException("handler does not accept exception type "+exType);
4505         if (htype.returnType() != ttype.returnType())
4506             throw misMatchedTypes("target and handler return types", ttype, htype);
4507         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
4508         if (handler == null) {
4509             throw misMatchedTypes("target and handler types", ttype, htype);
4510         }
4511         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
4512     }
4513 
4514     /**
4515      * Produces a method handle which will throw exceptions of the given {@code exType}.
4516      * The method handle will accept a single argument of {@code exType},
4517      * and immediately throw it as an exception.
4518      * The method type will nominally specify a return of {@code returnType}.
4519      * The return type may be anything convenient:  It doesn't matter to the
4520      * method handle's behavior, since it will never return normally.
4521      * @param returnType the return type of the desired method handle
4522      * @param exType the parameter type of the desired method handle
4523      * @return method handle which can throw the given exceptions
4524      * @throws NullPointerException if either argument is null
4525      */
4526     public static
4527     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
4528         if (!Throwable.class.isAssignableFrom(exType))
4529             throw new ClassCastException(exType.getName());
4530         return MethodHandleImpl.throwException(methodType(returnType, exType));
4531     }
4532 
4533     /**
4534      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
4535      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
4536      * delivers the loop's result, which is the return value of the resulting handle.
4537      * <p>
4538      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
4539      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
4540      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
4541      * terms of method handles, each clause will specify up to four independent actions:<ul>
4542      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
4543      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
4544      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
4545      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
4546      * </ul>
4547      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
4548      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
4549      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
4550      * <p>
4551      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
4552      * this case. See below for a detailed description.
4553      * <p>
4554      * <em>Parameters optional everywhere:</em>
4555      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
4556      * As an exception, the init functions cannot take any {@code v} parameters,
4557      * because those values are not yet computed when the init functions are executed.
4558      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
4559      * In fact, any clause function may take no arguments at all.
4560      * <p>
4561      * <em>Loop parameters:</em>
4562      * A clause function may take all the iteration variable values it is entitled to, in which case
4563      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
4564      * with their types and values notated as {@code (A...)} and {@code (a...)}.
4565      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
4566      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
4567      * init function is automatically a loop parameter {@code a}.)
4568      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
4569      * These loop parameters act as loop-invariant values visible across the whole loop.
4570      * <p>
4571      * <em>Parameters visible everywhere:</em>
4572      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
4573      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
4574      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
4575      * Most clause functions will not need all of this information, but they will be formally connected to it
4576      * as if by {@link #dropArguments}.
4577      * <a id="astar"></a>
4578      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
4579      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
4580      * In that notation, the general form of an init function parameter list
4581      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
4582      * <p>
4583      * <em>Checking clause structure:</em>
4584      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
4585      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
4586      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
4587      * met by the inputs to the loop combinator.
4588      * <p>
4589      * <em>Effectively identical sequences:</em>
4590      * <a id="effid"></a>
4591      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
4592      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
4593      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
4594      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
4595      * that longest list.
4596      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
4597      * and the same is true if more sequences of the form {@code (V... A*)} are added.
4598      * <p>
4599      * <em>Step 0: Determine clause structure.</em><ol type="a">
4600      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
4601      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
4602      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
4603      * four. Padding takes place by appending elements to the array.
4604      * <li>Clauses with all {@code null}s are disregarded.
4605      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
4606      * </ol>
4607      * <p>
4608      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
4609      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
4610      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
4611      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
4612      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
4613      * iteration variable type.
4614      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
4615      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
4616      * </ol>
4617      * <p>
4618      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
4619      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
4620      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
4621      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
4622      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
4623      * (These types will checked in step 2, along with all the clause function types.)
4624      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
4625      * <li>All of the collected parameter lists must be effectively identical.
4626      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
4627      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
4628      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
4629      * the "internal parameter list".
4630      * </ul>
4631      * <p>
4632      * <em>Step 1C: Determine loop return type.</em><ol type="a">
4633      * <li>Examine fini function return types, disregarding omitted fini functions.
4634      * <li>If there are no fini functions, the loop return type is {@code void}.
4635      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
4636      * type.
4637      * </ol>
4638      * <p>
4639      * <em>Step 1D: Check other types.</em><ol type="a">
4640      * <li>There must be at least one non-omitted pred function.
4641      * <li>Every non-omitted pred function must have a {@code boolean} return type.
4642      * </ol>
4643      * <p>
4644      * <em>Step 2: Determine parameter lists.</em><ol type="a">
4645      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
4646      * <li>The parameter list for init functions will be adjusted to the external parameter list.
4647      * (Note that their parameter lists are already effectively identical to this list.)
4648      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
4649      * effectively identical to the internal parameter list {@code (V... A...)}.
4650      * </ol>
4651      * <p>
4652      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
4653      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
4654      * type.
4655      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
4656      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
4657      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
4658      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
4659      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
4660      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
4661      * loop return type.
4662      * </ol>
4663      * <p>
4664      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
4665      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
4666      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
4667      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
4668      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
4669      * pad out the end of the list.
4670      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
4671      * </ol>
4672      * <p>
4673      * <em>Final observations.</em><ol type="a">
4674      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
4675      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
4676      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
4677      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
4678      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
4679      * <li>Each pair of init and step functions agrees in their return type {@code V}.
4680      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
4681      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
4682      * </ol>
4683      * <p>
4684      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
4685      * <ul>
4686      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
4687      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
4688      * (Only one {@code Pn} has to be non-{@code null}.)
4689      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
4690      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
4691      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
4692      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
4693      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
4694      * the resulting loop handle's parameter types {@code (A...)}.
4695      * </ul>
4696      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
4697      * which is natural if most of the loop computation happens in the steps.  For some loops,
4698      * the burden of computation might be heaviest in the pred functions, and so the pred functions
4699      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
4700      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
4701      * where the init functions will need the extra parameters.  For such reasons, the rules for
4702      * determining these parameters are as symmetric as possible, across all clause parts.
4703      * In general, the loop parameters function as common invariant values across the whole
4704      * loop, while the iteration variables function as common variant values, or (if there is
4705      * no step function) as internal loop invariant temporaries.
4706      * <p>
4707      * <em>Loop execution.</em><ol type="a">
4708      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
4709      * every clause function. These locals are loop invariant.
4710      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
4711      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
4712      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
4713      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
4714      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
4715      * (in argument order).
4716      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
4717      * returns {@code false}.
4718      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
4719      * sequence {@code (v...)} of loop variables.
4720      * The updated value is immediately visible to all subsequent function calls.
4721      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
4722      * (of type {@code R}) is returned from the loop as a whole.
4723      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
4724      * except by throwing an exception.
4725      * </ol>
4726      * <p>
4727      * <em>Usage tips.</em>
4728      * <ul>
4729      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
4730      * sometimes a step function only needs to observe the current value of its own variable.
4731      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
4732      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
4733      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
4734      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
4735      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
4736      * <li>If some of the clause functions are virtual methods on an instance, the instance
4737      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
4738      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
4739      * will be the first iteration variable value, and it will be easy to use virtual
4740      * methods as clause parts, since all of them will take a leading instance reference matching that value.
4741      * </ul>
4742      * <p>
4743      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
4744      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
4745      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
4746      * <blockquote><pre>{@code
4747      * V... init...(A...);
4748      * boolean pred...(V..., A...);
4749      * V... step...(V..., A...);
4750      * R fini...(V..., A...);
4751      * R loop(A... a) {
4752      *   V... v... = init...(a...);
4753      *   for (;;) {
4754      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
4755      *       v = s(v..., a...);
4756      *       if (!p(v..., a...)) {
4757      *         return f(v..., a...);
4758      *       }
4759      *     }
4760      *   }
4761      * }
4762      * }</pre></blockquote>
4763      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
4764      * to their full length, even though individual clause functions may neglect to take them all.
4765      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
4766      *
4767      * @apiNote Example:
4768      * <blockquote><pre>{@code
4769      * // iterative implementation of the factorial function as a loop handle
4770      * static int one(int k) { return 1; }
4771      * static int inc(int i, int acc, int k) { return i + 1; }
4772      * static int mult(int i, int acc, int k) { return i * acc; }
4773      * static boolean pred(int i, int acc, int k) { return i < k; }
4774      * static int fin(int i, int acc, int k) { return acc; }
4775      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4776      * // null initializer for counter, should initialize to 0
4777      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4778      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4779      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4780      * assertEquals(120, loop.invoke(5));
4781      * }</pre></blockquote>
4782      * The same example, dropping arguments and using combinators:
4783      * <blockquote><pre>{@code
4784      * // simplified implementation of the factorial function as a loop handle
4785      * static int inc(int i) { return i + 1; } // drop acc, k
4786      * static int mult(int i, int acc) { return i * acc; } //drop k
4787      * static boolean cmp(int i, int k) { return i < k; }
4788      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
4789      * // null initializer for counter, should initialize to 0
4790      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4791      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
4792      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
4793      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4794      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4795      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4796      * assertEquals(720, loop.invoke(6));
4797      * }</pre></blockquote>
4798      * A similar example, using a helper object to hold a loop parameter:
4799      * <blockquote><pre>{@code
4800      * // instance-based implementation of the factorial function as a loop handle
4801      * static class FacLoop {
4802      *   final int k;
4803      *   FacLoop(int k) { this.k = k; }
4804      *   int inc(int i) { return i + 1; }
4805      *   int mult(int i, int acc) { return i * acc; }
4806      *   boolean pred(int i) { return i < k; }
4807      *   int fin(int i, int acc) { return acc; }
4808      * }
4809      * // assume MH_FacLoop is a handle to the constructor
4810      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4811      * // null initializer for counter, should initialize to 0
4812      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4813      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
4814      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4815      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4816      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
4817      * assertEquals(5040, loop.invoke(7));
4818      * }</pre></blockquote>
4819      *
4820      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
4821      *
4822      * @return a method handle embodying the looping behavior as defined by the arguments.
4823      *
4824      * @throws IllegalArgumentException in case any of the constraints described above is violated.
4825      *
4826      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
4827      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
4828      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
4829      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
4830      * @since 9
4831      */
4832     public static MethodHandle loop(MethodHandle[]... clauses) {
4833         // Step 0: determine clause structure.
4834         loopChecks0(clauses);
4835 
4836         List<MethodHandle> init = new ArrayList<>();
4837         List<MethodHandle> step = new ArrayList<>();
4838         List<MethodHandle> pred = new ArrayList<>();
4839         List<MethodHandle> fini = new ArrayList<>();
4840 
4841         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
4842             init.add(clause[0]); // all clauses have at least length 1
4843             step.add(clause.length <= 1 ? null : clause[1]);
4844             pred.add(clause.length <= 2 ? null : clause[2]);
4845             fini.add(clause.length <= 3 ? null : clause[3]);
4846         });
4847 
4848         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
4849         final int nclauses = init.size();
4850 
4851         // Step 1A: determine iteration variables (V...).
4852         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
4853         for (int i = 0; i < nclauses; ++i) {
4854             MethodHandle in = init.get(i);
4855             MethodHandle st = step.get(i);
4856             if (in == null && st == null) {
4857                 iterationVariableTypes.add(void.class);
4858             } else if (in != null && st != null) {
4859                 loopChecks1a(i, in, st);
4860                 iterationVariableTypes.add(in.type().returnType());
4861             } else {
4862                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
4863             }
4864         }
4865         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
4866                 collect(Collectors.toList());
4867 
4868         // Step 1B: determine loop parameters (A...).
4869         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
4870         loopChecks1b(init, commonSuffix);
4871 
4872         // Step 1C: determine loop return type.
4873         // Step 1D: check other types.
4874         // local variable required here; see JDK-8223553
4875         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
4876                 .map(MethodType::returnType);
4877         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
4878         loopChecks1cd(pred, fini, loopReturnType);
4879 
4880         // Step 2: determine parameter lists.
4881         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
4882         commonParameterSequence.addAll(commonSuffix);
4883         loopChecks2(step, pred, fini, commonParameterSequence);
4884 
4885         // Step 3: fill in omitted functions.
4886         for (int i = 0; i < nclauses; ++i) {
4887             Class<?> t = iterationVariableTypes.get(i);
4888             if (init.get(i) == null) {
4889                 init.set(i, empty(methodType(t, commonSuffix)));
4890             }
4891             if (step.get(i) == null) {
4892                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
4893             }
4894             if (pred.get(i) == null) {
4895                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
4896             }
4897             if (fini.get(i) == null) {
4898                 fini.set(i, empty(methodType(t, commonParameterSequence)));
4899             }
4900         }
4901 
4902         // Step 4: fill in missing parameter types.
4903         // Also convert all handles to fixed-arity handles.
4904         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
4905         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
4906         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
4907         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
4908 
4909         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
4910                 allMatch(pl -> pl.equals(commonSuffix));
4911         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
4912                 allMatch(pl -> pl.equals(commonParameterSequence));
4913 
4914         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
4915     }
4916 
4917     private static void loopChecks0(MethodHandle[][] clauses) {
4918         if (clauses == null || clauses.length == 0) {
4919             throw newIllegalArgumentException("null or no clauses passed");
4920         }
4921         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
4922             throw newIllegalArgumentException("null clauses are not allowed");
4923         }
4924         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
4925             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
4926         }
4927     }
4928 
4929     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
4930         if (in.type().returnType() != st.type().returnType()) {
4931             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
4932                     st.type().returnType());
4933         }
4934     }
4935 
4936     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
4937         final List<Class<?>> empty = List.of();
4938         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
4939                 // take only those that can contribute to a common suffix because they are longer than the prefix
4940                         map(MethodHandle::type).
4941                         filter(t -> t.parameterCount() > skipSize).
4942                         map(MethodType::parameterList).
4943                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
4944         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
4945     }
4946 
4947     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
4948         final List<Class<?>> empty = List.of();
4949         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
4950     }
4951 
4952     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
4953         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
4954         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
4955         return longestParameterList(Arrays.asList(longest1, longest2));
4956     }
4957 
4958     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
4959         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
4960                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
4961             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
4962                     " (common suffix: " + commonSuffix + ")");
4963         }
4964     }
4965 
4966     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
4967         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4968                 anyMatch(t -> t != loopReturnType)) {
4969             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
4970                     loopReturnType + ")");
4971         }
4972 
4973         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
4974             throw newIllegalArgumentException("no predicate found", pred);
4975         }
4976         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4977                 anyMatch(t -> t != boolean.class)) {
4978             throw newIllegalArgumentException("predicates must have boolean return type", pred);
4979         }
4980     }
4981 
4982     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
4983         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
4984                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
4985             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
4986                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
4987         }
4988     }
4989 
4990     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
4991         return hs.stream().map(h -> {
4992             int pc = h.type().parameterCount();
4993             int tpsize = targetParams.size();
4994             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
4995         }).collect(Collectors.toList());
4996     }
4997 
4998     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
4999         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
5000     }
5001 
5002     /**
5003      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
5004      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5005      * <p>
5006      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5007      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
5008      * evaluates to {@code true}).
5009      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
5010      * <p>
5011      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5012      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5013      * and updated with the value returned from its invocation. The result of loop execution will be
5014      * the final value of the additional loop-local variable (if present).
5015      * <p>
5016      * The following rules hold for these argument handles:<ul>
5017      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5018      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5019      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5020      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5021      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5022      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5023      * It will constrain the parameter lists of the other loop parts.
5024      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5025      * list {@code (A...)} is called the <em>external parameter list</em>.
5026      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5027      * additional state variable of the loop.
5028      * The body must both accept and return a value of this type {@code V}.
5029      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5030      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5031      * <a href="MethodHandles.html#effid">effectively identical</a>
5032      * to the external parameter list {@code (A...)}.
5033      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5034      * {@linkplain #empty default value}.
5035      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5036      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5037      * effectively identical to the internal parameter list.
5038      * </ul>
5039      * <p>
5040      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5041      * <li>The loop handle's result type is the result type {@code V} of the body.
5042      * <li>The loop handle's parameter types are the types {@code (A...)},
5043      * from the external parameter list.
5044      * </ul>
5045      * <p>
5046      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5047      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5048      * passed to the loop.
5049      * <blockquote><pre>{@code
5050      * V init(A...);
5051      * boolean pred(V, A...);
5052      * V body(V, A...);
5053      * V whileLoop(A... a...) {
5054      *   V v = init(a...);
5055      *   while (pred(v, a...)) {
5056      *     v = body(v, a...);
5057      *   }
5058      *   return v;
5059      * }
5060      * }</pre></blockquote>
5061      *
5062      * @apiNote Example:
5063      * <blockquote><pre>{@code
5064      * // implement the zip function for lists as a loop handle
5065      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
5066      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
5067      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
5068      *   zip.add(a.next());
5069      *   zip.add(b.next());
5070      *   return zip;
5071      * }
5072      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
5073      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
5074      * List<String> a = Arrays.asList("a", "b", "c", "d");
5075      * List<String> b = Arrays.asList("e", "f", "g", "h");
5076      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
5077      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
5078      * }</pre></blockquote>
5079      *
5080      *
5081      * @apiNote The implementation of this method can be expressed as follows:
5082      * <blockquote><pre>{@code
5083      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5084      *     MethodHandle fini = (body.type().returnType() == void.class
5085      *                         ? null : identity(body.type().returnType()));
5086      *     MethodHandle[]
5087      *         checkExit = { null, null, pred, fini },
5088      *         varBody   = { init, body };
5089      *     return loop(checkExit, varBody);
5090      * }
5091      * }</pre></blockquote>
5092      *
5093      * @param init optional initializer, providing the initial value of the loop variable.
5094      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5095      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5096      *             above for other constraints.
5097      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5098      *             See above for other constraints.
5099      *
5100      * @return a method handle implementing the {@code while} loop as described by the arguments.
5101      * @throws IllegalArgumentException if the rules for the arguments are violated.
5102      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5103      *
5104      * @see #loop(MethodHandle[][])
5105      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
5106      * @since 9
5107      */
5108     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5109         whileLoopChecks(init, pred, body);
5110         MethodHandle fini = identityOrVoid(body.type().returnType());
5111         MethodHandle[] checkExit = { null, null, pred, fini };
5112         MethodHandle[] varBody = { init, body };
5113         return loop(checkExit, varBody);
5114     }
5115 
5116     /**
5117      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
5118      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5119      * <p>
5120      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5121      * method will, in each iteration, first execute its body and then evaluate the predicate.
5122      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
5123      * <p>
5124      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5125      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5126      * and updated with the value returned from its invocation. The result of loop execution will be
5127      * the final value of the additional loop-local variable (if present).
5128      * <p>
5129      * The following rules hold for these argument handles:<ul>
5130      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5131      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5132      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5133      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5134      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5135      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5136      * It will constrain the parameter lists of the other loop parts.
5137      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5138      * list {@code (A...)} is called the <em>external parameter list</em>.
5139      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5140      * additional state variable of the loop.
5141      * The body must both accept and return a value of this type {@code V}.
5142      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5143      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5144      * <a href="MethodHandles.html#effid">effectively identical</a>
5145      * to the external parameter list {@code (A...)}.
5146      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5147      * {@linkplain #empty default value}.
5148      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5149      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5150      * effectively identical to the internal parameter list.
5151      * </ul>
5152      * <p>
5153      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5154      * <li>The loop handle's result type is the result type {@code V} of the body.
5155      * <li>The loop handle's parameter types are the types {@code (A...)},
5156      * from the external parameter list.
5157      * </ul>
5158      * <p>
5159      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5160      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5161      * passed to the loop.
5162      * <blockquote><pre>{@code
5163      * V init(A...);
5164      * boolean pred(V, A...);
5165      * V body(V, A...);
5166      * V doWhileLoop(A... a...) {
5167      *   V v = init(a...);
5168      *   do {
5169      *     v = body(v, a...);
5170      *   } while (pred(v, a...));
5171      *   return v;
5172      * }
5173      * }</pre></blockquote>
5174      *
5175      * @apiNote Example:
5176      * <blockquote><pre>{@code
5177      * // int i = 0; while (i < limit) { ++i; } return i; => limit
5178      * static int zero(int limit) { return 0; }
5179      * static int step(int i, int limit) { return i + 1; }
5180      * static boolean pred(int i, int limit) { return i < limit; }
5181      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
5182      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
5183      * assertEquals(23, loop.invoke(23));
5184      * }</pre></blockquote>
5185      *
5186      *
5187      * @apiNote The implementation of this method can be expressed as follows:
5188      * <blockquote><pre>{@code
5189      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5190      *     MethodHandle fini = (body.type().returnType() == void.class
5191      *                         ? null : identity(body.type().returnType()));
5192      *     MethodHandle[] clause = { init, body, pred, fini };
5193      *     return loop(clause);
5194      * }
5195      * }</pre></blockquote>
5196      *
5197      * @param init optional initializer, providing the initial value of the loop variable.
5198      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5199      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5200      *             See above for other constraints.
5201      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5202      *             above for other constraints.
5203      *
5204      * @return a method handle implementing the {@code while} loop as described by the arguments.
5205      * @throws IllegalArgumentException if the rules for the arguments are violated.
5206      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5207      *
5208      * @see #loop(MethodHandle[][])
5209      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
5210      * @since 9
5211      */
5212     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5213         whileLoopChecks(init, pred, body);
5214         MethodHandle fini = identityOrVoid(body.type().returnType());
5215         MethodHandle[] clause = {init, body, pred, fini };
5216         return loop(clause);
5217     }
5218 
5219     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
5220         Objects.requireNonNull(pred);
5221         Objects.requireNonNull(body);
5222         MethodType bodyType = body.type();
5223         Class<?> returnType = bodyType.returnType();
5224         List<Class<?>> innerList = bodyType.parameterList();
5225         List<Class<?>> outerList = innerList;
5226         if (returnType == void.class) {
5227             // OK
5228         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
5229             // leading V argument missing => error
5230             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5231             throw misMatchedTypes("body function", bodyType, expected);
5232         } else {
5233             outerList = innerList.subList(1, innerList.size());
5234         }
5235         MethodType predType = pred.type();
5236         if (predType.returnType() != boolean.class ||
5237                 !predType.effectivelyIdenticalParameters(0, innerList)) {
5238             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
5239         }
5240         if (init != null) {
5241             MethodType initType = init.type();
5242             if (initType.returnType() != returnType ||
5243                     !initType.effectivelyIdenticalParameters(0, outerList)) {
5244                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5245             }
5246         }
5247     }
5248 
5249     /**
5250      * Constructs a loop that runs a given number of iterations.
5251      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5252      * <p>
5253      * The number of iterations is determined by the {@code iterations} handle evaluation result.
5254      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
5255      * It will be initialized to 0 and incremented by 1 in each iteration.
5256      * <p>
5257      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5258      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5259      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5260      * <p>
5261      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5262      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5263      * iteration variable.
5264      * The result of the loop handle execution will be the final {@code V} value of that variable
5265      * (or {@code void} if there is no {@code V} variable).
5266      * <p>
5267      * The following rules hold for the argument handles:<ul>
5268      * <li>The {@code iterations} handle must not be {@code null}, and must return
5269      * the type {@code int}, referred to here as {@code I} in parameter type lists.
5270      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5271      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5272      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5273      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5274      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5275      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5276      * of types called the <em>internal parameter list</em>.
5277      * It will constrain the parameter lists of the other loop parts.
5278      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5279      * with no additional {@code A} types, then the internal parameter list is extended by
5280      * the argument types {@code A...} of the {@code iterations} handle.
5281      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5282      * list {@code (A...)} is called the <em>external parameter list</em>.
5283      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5284      * additional state variable of the loop.
5285      * The body must both accept a leading parameter and return a value of this type {@code V}.
5286      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5287      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5288      * <a href="MethodHandles.html#effid">effectively identical</a>
5289      * to the external parameter list {@code (A...)}.
5290      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5291      * {@linkplain #empty default value}.
5292      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
5293      * effectively identical to the external parameter list {@code (A...)}.
5294      * </ul>
5295      * <p>
5296      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5297      * <li>The loop handle's result type is the result type {@code V} of the body.
5298      * <li>The loop handle's parameter types are the types {@code (A...)},
5299      * from the external parameter list.
5300      * </ul>
5301      * <p>
5302      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5303      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5304      * arguments passed to the loop.
5305      * <blockquote><pre>{@code
5306      * int iterations(A...);
5307      * V init(A...);
5308      * V body(V, int, A...);
5309      * V countedLoop(A... a...) {
5310      *   int end = iterations(a...);
5311      *   V v = init(a...);
5312      *   for (int i = 0; i < end; ++i) {
5313      *     v = body(v, i, a...);
5314      *   }
5315      *   return v;
5316      * }
5317      * }</pre></blockquote>
5318      *
5319      * @apiNote Example with a fully conformant body method:
5320      * <blockquote><pre>{@code
5321      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5322      * // => a variation on a well known theme
5323      * static String step(String v, int counter, String init) { return "na " + v; }
5324      * // assume MH_step is a handle to the method above
5325      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
5326      * MethodHandle start = MethodHandles.identity(String.class);
5327      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
5328      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
5329      * }</pre></blockquote>
5330      *
5331      * @apiNote Example with the simplest possible body method type,
5332      * and passing the number of iterations to the loop invocation:
5333      * <blockquote><pre>{@code
5334      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5335      * // => a variation on a well known theme
5336      * static String step(String v, int counter ) { return "na " + v; }
5337      * // assume MH_step is a handle to the method above
5338      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
5339      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
5340      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
5341      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
5342      * }</pre></blockquote>
5343      *
5344      * @apiNote Example that treats the number of iterations, string to append to, and string to append
5345      * as loop parameters:
5346      * <blockquote><pre>{@code
5347      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5348      * // => a variation on a well known theme
5349      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
5350      * // assume MH_step is a handle to the method above
5351      * MethodHandle count = MethodHandles.identity(int.class);
5352      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
5353      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
5354      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
5355      * }</pre></blockquote>
5356      *
5357      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
5358      * to enforce a loop type:
5359      * <blockquote><pre>{@code
5360      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5361      * // => a variation on a well known theme
5362      * static String step(String v, int counter, String pre) { return pre + " " + v; }
5363      * // assume MH_step is a handle to the method above
5364      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
5365      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
5366      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
5367      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
5368      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
5369      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
5370      * }</pre></blockquote>
5371      *
5372      * @apiNote The implementation of this method can be expressed as follows:
5373      * <blockquote><pre>{@code
5374      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5375      *     return countedLoop(empty(iterations.type()), iterations, init, body);
5376      * }
5377      * }</pre></blockquote>
5378      *
5379      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
5380      *                   result type must be {@code int}. See above for other constraints.
5381      * @param init optional initializer, providing the initial value of the loop variable.
5382      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5383      * @param body body of the loop, which may not be {@code null}.
5384      *             It controls the loop parameters and result type in the standard case (see above for details).
5385      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5386      *             and may accept any number of additional types.
5387      *             See above for other constraints.
5388      *
5389      * @return a method handle representing the loop.
5390      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
5391      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5392      *
5393      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
5394      * @since 9
5395      */
5396     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5397         return countedLoop(empty(iterations.type()), iterations, init, body);
5398     }
5399 
5400     /**
5401      * Constructs a loop that counts over a range of numbers.
5402      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5403      * <p>
5404      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
5405      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
5406      * values of the loop counter.
5407      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
5408      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
5409      * <p>
5410      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5411      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5412      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5413      * <p>
5414      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5415      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5416      * iteration variable.
5417      * The result of the loop handle execution will be the final {@code V} value of that variable
5418      * (or {@code void} if there is no {@code V} variable).
5419      * <p>
5420      * The following rules hold for the argument handles:<ul>
5421      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
5422      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
5423      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5424      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5425      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5426      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5427      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5428      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5429      * of types called the <em>internal parameter list</em>.
5430      * It will constrain the parameter lists of the other loop parts.
5431      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5432      * with no additional {@code A} types, then the internal parameter list is extended by
5433      * the argument types {@code A...} of the {@code end} handle.
5434      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5435      * list {@code (A...)} is called the <em>external parameter list</em>.
5436      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5437      * additional state variable of the loop.
5438      * The body must both accept a leading parameter and return a value of this type {@code V}.
5439      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5440      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5441      * <a href="MethodHandles.html#effid">effectively identical</a>
5442      * to the external parameter list {@code (A...)}.
5443      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5444      * {@linkplain #empty default value}.
5445      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
5446      * effectively identical to the external parameter list {@code (A...)}.
5447      * <li>Likewise, the parameter list of {@code end} must be effectively identical
5448      * to the external parameter list.
5449      * </ul>
5450      * <p>
5451      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5452      * <li>The loop handle's result type is the result type {@code V} of the body.
5453      * <li>The loop handle's parameter types are the types {@code (A...)},
5454      * from the external parameter list.
5455      * </ul>
5456      * <p>
5457      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5458      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5459      * arguments passed to the loop.
5460      * <blockquote><pre>{@code
5461      * int start(A...);
5462      * int end(A...);
5463      * V init(A...);
5464      * V body(V, int, A...);
5465      * V countedLoop(A... a...) {
5466      *   int e = end(a...);
5467      *   int s = start(a...);
5468      *   V v = init(a...);
5469      *   for (int i = s; i < e; ++i) {
5470      *     v = body(v, i, a...);
5471      *   }
5472      *   return v;
5473      * }
5474      * }</pre></blockquote>
5475      *
5476      * @apiNote The implementation of this method can be expressed as follows:
5477      * <blockquote><pre>{@code
5478      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5479      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
5480      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
5481      *     // the following semantics:
5482      *     // MH_increment: (int limit, int counter) -> counter + 1
5483      *     // MH_predicate: (int limit, int counter) -> counter < limit
5484      *     Class<?> counterType = start.type().returnType();  // int
5485      *     Class<?> returnType = body.type().returnType();
5486      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
5487      *     if (returnType != void.class) {  // ignore the V variable
5488      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5489      *         pred = dropArguments(pred, 1, returnType);  // ditto
5490      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
5491      *     }
5492      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
5493      *     MethodHandle[]
5494      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5495      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
5496      *         indexVar   = { start, incr };           // i = start(); i = i + 1
5497      *     return loop(loopLimit, bodyClause, indexVar);
5498      * }
5499      * }</pre></blockquote>
5500      *
5501      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
5502      *              See above for other constraints.
5503      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
5504      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
5505      * @param init optional initializer, providing the initial value of the loop variable.
5506      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5507      * @param body body of the loop, which may not be {@code null}.
5508      *             It controls the loop parameters and result type in the standard case (see above for details).
5509      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5510      *             and may accept any number of additional types.
5511      *             See above for other constraints.
5512      *
5513      * @return a method handle representing the loop.
5514      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
5515      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5516      *
5517      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
5518      * @since 9
5519      */
5520     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5521         countedLoopChecks(start, end, init, body);
5522         Class<?> counterType = start.type().returnType();  // int, but who's counting?
5523         Class<?> limitType   = end.type().returnType();    // yes, int again
5524         Class<?> returnType  = body.type().returnType();
5525         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
5526         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
5527         MethodHandle retv = null;
5528         if (returnType != void.class) {
5529             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5530             pred = dropArguments(pred, 1, returnType);  // ditto
5531             retv = dropArguments(identity(returnType), 0, counterType);
5532         }
5533         body = dropArguments(body, 0, counterType);  // ignore the limit variable
5534         MethodHandle[]
5535             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5536             bodyClause = { init, body },            // v = init(); v = body(v, i)
5537             indexVar   = { start, incr };           // i = start(); i = i + 1
5538         return loop(loopLimit, bodyClause, indexVar);
5539     }
5540 
5541     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5542         Objects.requireNonNull(start);
5543         Objects.requireNonNull(end);
5544         Objects.requireNonNull(body);
5545         Class<?> counterType = start.type().returnType();
5546         if (counterType != int.class) {
5547             MethodType expected = start.type().changeReturnType(int.class);
5548             throw misMatchedTypes("start function", start.type(), expected);
5549         } else if (end.type().returnType() != counterType) {
5550             MethodType expected = end.type().changeReturnType(counterType);
5551             throw misMatchedTypes("end function", end.type(), expected);
5552         }
5553         MethodType bodyType = body.type();
5554         Class<?> returnType = bodyType.returnType();
5555         List<Class<?>> innerList = bodyType.parameterList();
5556         // strip leading V value if present
5557         int vsize = (returnType == void.class ? 0 : 1);
5558         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
5559             // argument list has no "V" => error
5560             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5561             throw misMatchedTypes("body function", bodyType, expected);
5562         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
5563             // missing I type => error
5564             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
5565             throw misMatchedTypes("body function", bodyType, expected);
5566         }
5567         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
5568         if (outerList.isEmpty()) {
5569             // special case; take lists from end handle
5570             outerList = end.type().parameterList();
5571             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
5572         }
5573         MethodType expected = methodType(counterType, outerList);
5574         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
5575             throw misMatchedTypes("start parameter types", start.type(), expected);
5576         }
5577         if (end.type() != start.type() &&
5578             !end.type().effectivelyIdenticalParameters(0, outerList)) {
5579             throw misMatchedTypes("end parameter types", end.type(), expected);
5580         }
5581         if (init != null) {
5582             MethodType initType = init.type();
5583             if (initType.returnType() != returnType ||
5584                 !initType.effectivelyIdenticalParameters(0, outerList)) {
5585                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5586             }
5587         }
5588     }
5589 
5590     /**
5591      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
5592      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5593      * <p>
5594      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
5595      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
5596      * <p>
5597      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5598      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5599      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5600      * <p>
5601      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5602      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5603      * iteration variable.
5604      * The result of the loop handle execution will be the final {@code V} value of that variable
5605      * (or {@code void} if there is no {@code V} variable).
5606      * <p>
5607      * The following rules hold for the argument handles:<ul>
5608      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5609      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
5610      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5611      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
5612      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
5613      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
5614      * of types called the <em>internal parameter list</em>.
5615      * It will constrain the parameter lists of the other loop parts.
5616      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
5617      * with no additional {@code A} types, then the internal parameter list is extended by
5618      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
5619      * single type {@code Iterable} is added and constitutes the {@code A...} list.
5620      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
5621      * list {@code (A...)} is called the <em>external parameter list</em>.
5622      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5623      * additional state variable of the loop.
5624      * The body must both accept a leading parameter and return a value of this type {@code V}.
5625      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5626      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5627      * <a href="MethodHandles.html#effid">effectively identical</a>
5628      * to the external parameter list {@code (A...)}.
5629      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5630      * {@linkplain #empty default value}.
5631      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
5632      * type {@code java.util.Iterator} or a subtype thereof.
5633      * The iterator it produces when the loop is executed will be assumed
5634      * to yield values which can be converted to type {@code T}.
5635      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
5636      * effectively identical to the external parameter list {@code (A...)}.
5637      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
5638      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
5639      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
5640      * handle parameter is adjusted to accept the leading {@code A} type, as if by
5641      * the {@link MethodHandle#asType asType} conversion method.
5642      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
5643      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
5644      * </ul>
5645      * <p>
5646      * The type {@code T} may be either a primitive or reference.
5647      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
5648      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
5649      * as if by the {@link MethodHandle#asType asType} conversion method.
5650      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
5651      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
5652      * <p>
5653      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5654      * <li>The loop handle's result type is the result type {@code V} of the body.
5655      * <li>The loop handle's parameter types are the types {@code (A...)},
5656      * from the external parameter list.
5657      * </ul>
5658      * <p>
5659      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5660      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
5661      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
5662      * <blockquote><pre>{@code
5663      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
5664      * V init(A...);
5665      * V body(V,T,A...);
5666      * V iteratedLoop(A... a...) {
5667      *   Iterator<T> it = iterator(a...);
5668      *   V v = init(a...);
5669      *   while (it.hasNext()) {
5670      *     T t = it.next();
5671      *     v = body(v, t, a...);
5672      *   }
5673      *   return v;
5674      * }
5675      * }</pre></blockquote>
5676      *
5677      * @apiNote Example:
5678      * <blockquote><pre>{@code
5679      * // get an iterator from a list
5680      * static List<String> reverseStep(List<String> r, String e) {
5681      *   r.add(0, e);
5682      *   return r;
5683      * }
5684      * static List<String> newArrayList() { return new ArrayList<>(); }
5685      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
5686      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
5687      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
5688      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
5689      * assertEquals(reversedList, (List<String>) loop.invoke(list));
5690      * }</pre></blockquote>
5691      *
5692      * @apiNote The implementation of this method can be expressed approximately as follows:
5693      * <blockquote><pre>{@code
5694      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5695      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
5696      *     Class<?> returnType = body.type().returnType();
5697      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5698      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
5699      *     MethodHandle retv = null, step = body, startIter = iterator;
5700      *     if (returnType != void.class) {
5701      *         // the simple thing first:  in (I V A...), drop the I to get V
5702      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
5703      *         // body type signature (V T A...), internal loop types (I V A...)
5704      *         step = swapArguments(body, 0, 1);  // swap V <-> T
5705      *     }
5706      *     if (startIter == null)  startIter = MH_getIter;
5707      *     MethodHandle[]
5708      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
5709      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
5710      *     return loop(iterVar, bodyClause);
5711      * }
5712      * }</pre></blockquote>
5713      *
5714      * @param iterator an optional handle to return the iterator to start the loop.
5715      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
5716      *                 See above for other constraints.
5717      * @param init optional initializer, providing the initial value of the loop variable.
5718      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5719      * @param body body of the loop, which may not be {@code null}.
5720      *             It controls the loop parameters and result type in the standard case (see above for details).
5721      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
5722      *             and may accept any number of additional types.
5723      *             See above for other constraints.
5724      *
5725      * @return a method handle embodying the iteration loop functionality.
5726      * @throws NullPointerException if the {@code body} handle is {@code null}.
5727      * @throws IllegalArgumentException if any argument violates the above requirements.
5728      *
5729      * @since 9
5730      */
5731     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5732         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
5733         Class<?> returnType = body.type().returnType();
5734         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
5735         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
5736         MethodHandle startIter;
5737         MethodHandle nextVal;
5738         {
5739             MethodType iteratorType;
5740             if (iterator == null) {
5741                 // derive argument type from body, if available, else use Iterable
5742                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
5743                 iteratorType = startIter.type().changeParameterType(0, iterableType);
5744             } else {
5745                 // force return type to the internal iterator class
5746                 iteratorType = iterator.type().changeReturnType(Iterator.class);
5747                 startIter = iterator;
5748             }
5749             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5750             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
5751 
5752             // perform the asType transforms under an exception transformer, as per spec.:
5753             try {
5754                 startIter = startIter.asType(iteratorType);
5755                 nextVal = nextRaw.asType(nextValType);
5756             } catch (WrongMethodTypeException ex) {
5757                 throw new IllegalArgumentException(ex);
5758             }
5759         }
5760 
5761         MethodHandle retv = null, step = body;
5762         if (returnType != void.class) {
5763             // the simple thing first:  in (I V A...), drop the I to get V
5764             retv = dropArguments(identity(returnType), 0, Iterator.class);
5765             // body type signature (V T A...), internal loop types (I V A...)
5766             step = swapArguments(body, 0, 1);  // swap V <-> T
5767         }
5768 
5769         MethodHandle[]
5770             iterVar    = { startIter, null, hasNext, retv },
5771             bodyClause = { init, filterArgument(step, 0, nextVal) };
5772         return loop(iterVar, bodyClause);
5773     }
5774 
5775     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5776         Objects.requireNonNull(body);
5777         MethodType bodyType = body.type();
5778         Class<?> returnType = bodyType.returnType();
5779         List<Class<?>> internalParamList = bodyType.parameterList();
5780         // strip leading V value if present
5781         int vsize = (returnType == void.class ? 0 : 1);
5782         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
5783             // argument list has no "V" => error
5784             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5785             throw misMatchedTypes("body function", bodyType, expected);
5786         } else if (internalParamList.size() <= vsize) {
5787             // missing T type => error
5788             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
5789             throw misMatchedTypes("body function", bodyType, expected);
5790         }
5791         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
5792         Class<?> iterableType = null;
5793         if (iterator != null) {
5794             // special case; if the body handle only declares V and T then
5795             // the external parameter list is obtained from iterator handle
5796             if (externalParamList.isEmpty()) {
5797                 externalParamList = iterator.type().parameterList();
5798             }
5799             MethodType itype = iterator.type();
5800             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
5801                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
5802             }
5803             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
5804                 MethodType expected = methodType(itype.returnType(), externalParamList);
5805                 throw misMatchedTypes("iterator parameters", itype, expected);
5806             }
5807         } else {
5808             if (externalParamList.isEmpty()) {
5809                 // special case; if the iterator handle is null and the body handle
5810                 // only declares V and T then the external parameter list consists
5811                 // of Iterable
5812                 externalParamList = Arrays.asList(Iterable.class);
5813                 iterableType = Iterable.class;
5814             } else {
5815                 // special case; if the iterator handle is null and the external
5816                 // parameter list is not empty then the first parameter must be
5817                 // assignable to Iterable
5818                 iterableType = externalParamList.get(0);
5819                 if (!Iterable.class.isAssignableFrom(iterableType)) {
5820                     throw newIllegalArgumentException(
5821                             "inferred first loop argument must inherit from Iterable: " + iterableType);
5822                 }
5823             }
5824         }
5825         if (init != null) {
5826             MethodType initType = init.type();
5827             if (initType.returnType() != returnType ||
5828                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
5829                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
5830             }
5831         }
5832         return iterableType;  // help the caller a bit
5833     }
5834 
5835     /*non-public*/ static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
5836         // there should be a better way to uncross my wires
5837         int arity = mh.type().parameterCount();
5838         int[] order = new int[arity];
5839         for (int k = 0; k < arity; k++)  order[k] = k;
5840         order[i] = j; order[j] = i;
5841         Class<?>[] types = mh.type().parameterArray();
5842         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
5843         MethodType swapType = methodType(mh.type().returnType(), types);
5844         return permuteArguments(mh, swapType, order);
5845     }
5846 
5847     /**
5848      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
5849      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
5850      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
5851      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
5852      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
5853      * {@code try-finally} handle.
5854      * <p>
5855      * The {@code cleanup} handle will be passed one or two additional leading arguments.
5856      * The first is the exception thrown during the
5857      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
5858      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
5859      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
5860      * The second argument is not present if the {@code target} handle has a {@code void} return type.
5861      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
5862      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
5863      * <p>
5864      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
5865      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
5866      * two extra leading parameters:<ul>
5867      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
5868      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
5869      * the result from the execution of the {@code target} handle.
5870      * This parameter is not present if the {@code target} returns {@code void}.
5871      * </ul>
5872      * <p>
5873      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
5874      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
5875      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
5876      * the cleanup.
5877      * <blockquote><pre>{@code
5878      * V target(A..., B...);
5879      * V cleanup(Throwable, V, A...);
5880      * V adapter(A... a, B... b) {
5881      *   V result = (zero value for V);
5882      *   Throwable throwable = null;
5883      *   try {
5884      *     result = target(a..., b...);
5885      *   } catch (Throwable t) {
5886      *     throwable = t;
5887      *     throw t;
5888      *   } finally {
5889      *     result = cleanup(throwable, result, a...);
5890      *   }
5891      *   return result;
5892      * }
5893      * }</pre></blockquote>
5894      * <p>
5895      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
5896      * be modified by execution of the target, and so are passed unchanged
5897      * from the caller to the cleanup, if it is invoked.
5898      * <p>
5899      * The target and cleanup must return the same type, even if the cleanup
5900      * always throws.
5901      * To create such a throwing cleanup, compose the cleanup logic
5902      * with {@link #throwException throwException},
5903      * in order to create a method handle of the correct return type.
5904      * <p>
5905      * Note that {@code tryFinally} never converts exceptions into normal returns.
5906      * In rare cases where exceptions must be converted in that way, first wrap
5907      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
5908      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
5909      * <p>
5910      * It is recommended that the first parameter type of {@code cleanup} be
5911      * declared {@code Throwable} rather than a narrower subtype.  This ensures
5912      * {@code cleanup} will always be invoked with whatever exception that
5913      * {@code target} throws.  Declaring a narrower type may result in a
5914      * {@code ClassCastException} being thrown by the {@code try-finally}
5915      * handle if the type of the exception thrown by {@code target} is not
5916      * assignable to the first parameter type of {@code cleanup}.  Note that
5917      * various exception types of {@code VirtualMachineError},
5918      * {@code LinkageError}, and {@code RuntimeException} can in principle be
5919      * thrown by almost any kind of Java code, and a finally clause that
5920      * catches (say) only {@code IOException} would mask any of the others
5921      * behind a {@code ClassCastException}.
5922      *
5923      * @param target the handle whose execution is to be wrapped in a {@code try} block.
5924      * @param cleanup the handle that is invoked in the finally block.
5925      *
5926      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
5927      * @throws NullPointerException if any argument is null
5928      * @throws IllegalArgumentException if {@code cleanup} does not accept
5929      *          the required leading arguments, or if the method handle types do
5930      *          not match in their return types and their
5931      *          corresponding trailing parameters
5932      *
5933      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
5934      * @since 9
5935      */
5936     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
5937         List<Class<?>> targetParamTypes = target.type().parameterList();
5938         Class<?> rtype = target.type().returnType();
5939 
5940         tryFinallyChecks(target, cleanup);
5941 
5942         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
5943         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
5944         // target parameter list.
5945         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
5946 
5947         // Ensure that the intrinsic type checks the instance thrown by the
5948         // target against the first parameter of cleanup
5949         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
5950 
5951         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
5952         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
5953     }
5954 
5955     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
5956         Class<?> rtype = target.type().returnType();
5957         if (rtype != cleanup.type().returnType()) {
5958             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
5959         }
5960         MethodType cleanupType = cleanup.type();
5961         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
5962             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
5963         }
5964         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
5965             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
5966         }
5967         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
5968         // target parameter list.
5969         int cleanupArgIndex = rtype == void.class ? 1 : 2;
5970         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
5971             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
5972                     cleanup.type(), target.type());
5973         }
5974     }
5975 
5976 }