1 /*
   2  * Copyright (c) 2008, 2016, 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 java.lang.reflect.*;
  29 import java.util.ArrayList;
  30 import java.util.BitSet;
  31 import java.util.Iterator;
  32 import java.util.List;
  33 import java.util.Arrays;
  34 import java.util.Objects;
  35 import java.security.AccessController;
  36 import java.security.PrivilegedAction;
  37 
  38 import sun.invoke.util.ValueConversions;
  39 import sun.invoke.util.VerifyAccess;
  40 import sun.invoke.util.Wrapper;
  41 import jdk.internal.reflect.CallerSensitive;
  42 import jdk.internal.reflect.Reflection;
  43 import sun.reflect.misc.ReflectUtil;
  44 import sun.security.util.SecurityConstants;
  45 import java.lang.invoke.LambdaForm.BasicType;
  46 
  47 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  48 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  49 import java.util.concurrent.ConcurrentHashMap;
  50 import java.util.stream.Collectors;
  51 import java.util.stream.Stream;
  52 
  53 import jdk.internal.org.objectweb.asm.ClassWriter;
  54 import jdk.internal.org.objectweb.asm.Opcodes;
  55 
  56 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  57 import static java.lang.invoke.MethodType.methodType;
  58 
  59 /**
  60  * This class consists exclusively of static methods that operate on or return
  61  * method handles. They fall into several categories:
  62  * <ul>
  63  * <li>Lookup methods which help create method handles for methods and fields.
  64  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  65  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  66  * </ul>
  67  *
  68  * @author John Rose, JSR 292 EG
  69  * @since 1.7
  70  */
  71 public class MethodHandles {
  72 
  73     private MethodHandles() { }  // do not instantiate
  74 
  75     private static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  76 
  77     // See IMPL_LOOKUP below.
  78 
  79     //// Method handle creation from ordinary methods.
  80 
  81     /**
  82      * Returns a {@link Lookup lookup object} with
  83      * full capabilities to emulate all supported bytecode behaviors of the caller.
  84      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  85      * Factory methods on the lookup object can create
  86      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  87      * for any member that the caller has access to via bytecodes,
  88      * including protected and private fields and methods.
  89      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
  90      * Do not store it in place where untrusted code can access it.
  91      * <p>
  92      * This method is caller sensitive, which means that it may return different
  93      * values to different callers.
  94      * <p>
  95      * For any given caller class {@code C}, the lookup object returned by this call
  96      * has equivalent capabilities to any lookup object
  97      * supplied by the JVM to the bootstrap method of an
  98      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
  99      * executing in the same caller class {@code C}.
 100      * @return a lookup object for the caller of this method, with private access
 101      */
 102     @CallerSensitive
 103     public static Lookup lookup() {
 104         return new Lookup(Reflection.getCallerClass());
 105     }
 106 
 107     /**
 108      * Returns a {@link Lookup lookup object} which is trusted minimally.
 109      * It can only be used to create method handles to public members in
 110      * public classes in packages that are exported unconditionally.
 111      * <p>
 112      * For now, the {@linkplain Lookup#lookupClass lookup class} of this lookup
 113      * object is in an unnamed module.
 114      * Consequently, the lookup context of this lookup object will be the bootstrap
 115      * class loader, which means it cannot find user classes.
 116      *
 117      * <p style="font-size:smaller;">
 118      * <em>Discussion:</em>
 119      * The lookup class can be changed to any other class {@code C} using an expression of the form
 120      * {@link Lookup#in publicLookup().in(C.class)}.
 121      * but may change the lookup context by virtue of changing the class loader.
 122      * A public lookup object is always subject to
 123      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 124      * Also, it cannot access
 125      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 126      * @return a lookup object which is trusted minimally
 127      */
 128     public static Lookup publicLookup() {
 129         // During VM startup then only classes in the java.base module can be
 130         // loaded and linked. This is because java.base exports aren't setup until
 131         // the module system is initialized, hence types in the unnamed module
 132         // (or any named module) can't link to java/lang/Object.
 133         if (!jdk.internal.misc.VM.isModuleSystemInited()) {
 134             return new Lookup(Object.class, Lookup.PUBLIC);
 135         } else {
 136             return LookupHelper.PUBLIC_LOOKUP;
 137         }
 138     }
 139 
 140     /**
 141      * Performs an unchecked "crack" of a
 142      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 143      * The result is as if the user had obtained a lookup object capable enough
 144      * to crack the target method handle, called
 145      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 146      * on the target to obtain its symbolic reference, and then called
 147      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 148      * to resolve the symbolic reference to a member.
 149      * <p>
 150      * If there is a security manager, its {@code checkPermission} method
 151      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 152      * @param <T> the desired type of the result, either {@link Member} or a subtype
 153      * @param target a direct method handle to crack into symbolic reference components
 154      * @param expected a class object representing the desired result type {@code T}
 155      * @return a reference to the method, constructor, or field object
 156      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 157      * @exception NullPointerException if either argument is {@code null}
 158      * @exception IllegalArgumentException if the target is not a direct method handle
 159      * @exception ClassCastException if the member is not of the expected type
 160      * @since 1.8
 161      */
 162     public static <T extends Member> T
 163     reflectAs(Class<T> expected, MethodHandle target) {
 164         SecurityManager smgr = System.getSecurityManager();
 165         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 166         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 167         return lookup.revealDirect(target).reflectAs(expected, lookup);
 168     }
 169     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 170     private static final java.security.Permission ACCESS_PERMISSION =
 171         new ReflectPermission("suppressAccessChecks");
 172 
 173     /**
 174      * A <em>lookup object</em> is a factory for creating method handles,
 175      * when the creation requires access checking.
 176      * Method handles do not perform
 177      * access checks when they are called, but rather when they are created.
 178      * Therefore, method handle access
 179      * restrictions must be enforced when a method handle is created.
 180      * The caller class against which those restrictions are enforced
 181      * is known as the {@linkplain #lookupClass lookup class}.
 182      * <p>
 183      * A lookup class which needs to create method handles will call
 184      * {@link MethodHandles#lookup MethodHandles.lookup} to create a factory for itself.
 185      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 186      * determined, and securely stored in the {@code Lookup} object.
 187      * The lookup class (or its delegates) may then use factory methods
 188      * on the {@code Lookup} object to create method handles for access-checked members.
 189      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 190      * even private ones.
 191      *
 192      * <h1><a name="lookups"></a>Lookup Factory Methods</h1>
 193      * The factory methods on a {@code Lookup} object correspond to all major
 194      * use cases for methods, constructors, and fields.
 195      * Each method handle created by a factory method is the functional
 196      * equivalent of a particular <em>bytecode behavior</em>.
 197      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 198      * Here is a summary of the correspondence between these factory methods and
 199      * the behavior of the resulting method handles:
 200      * <table border=1 cellpadding=5 summary="lookup method behaviors">
 201      * <tr>
 202      *     <th><a name="equiv"></a>lookup expression</th>
 203      *     <th>member</th>
 204      *     <th>bytecode behavior</th>
 205      * </tr>
 206      * <tr>
 207      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</td>
 208      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 209      * </tr>
 210      * <tr>
 211      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</td>
 212      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 213      * </tr>
 214      * <tr>
 215      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</td>
 216      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 217      * </tr>
 218      * <tr>
 219      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</td>
 220      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 221      * </tr>
 222      * <tr>
 223      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</td>
 224      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 225      * </tr>
 226      * <tr>
 227      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</td>
 228      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 229      * </tr>
 230      * <tr>
 231      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</td>
 232      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 233      * </tr>
 234      * <tr>
 235      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</td>
 236      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 237      * </tr>
 238      * <tr>
 239      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</td>
 240      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 241      * </tr>
 242      * <tr>
 243      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</td>
 244      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 245      * </tr>
 246      * <tr>
 247      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 248      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 249      * </tr>
 250      * <tr>
 251      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</td>
 252      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 253      * </tr>
 254      * <tr>
 255      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 256      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 257      * </tr>
 258      * <tr>
 259      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</td>
 260      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 261      * </tr>
 262      * </table>
 263      *
 264      * Here, the type {@code C} is the class or interface being searched for a member,
 265      * documented as a parameter named {@code refc} in the lookup methods.
 266      * The method type {@code MT} is composed from the return type {@code T}
 267      * and the sequence of argument types {@code A*}.
 268      * The constructor also has a sequence of argument types {@code A*} and
 269      * is deemed to return the newly-created object of type {@code C}.
 270      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 271      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 272      * if it is present, it is always the leading argument to the method handle invocation.
 273      * (In the case of some {@code protected} members, {@code this} may be
 274      * restricted in type to the lookup class; see below.)
 275      * The name {@code arg} stands for all the other method handle arguments.
 276      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 277      * stands for a null reference if the accessed method or field is static,
 278      * and {@code this} otherwise.
 279      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 280      * for reflective objects corresponding to the given members.
 281      * <p>
 282      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 283      * as if by {@code ldc CONSTANT_Class}.
 284      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 285      * <p>
 286      * In cases where the given member is of variable arity (i.e., a method or constructor)
 287      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 288      * In all other cases, the returned method handle will be of fixed arity.
 289      * <p style="font-size:smaller;">
 290      * <em>Discussion:</em>
 291      * The equivalence between looked-up method handles and underlying
 292      * class members and bytecode behaviors
 293      * can break down in a few ways:
 294      * <ul style="font-size:smaller;">
 295      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 296      * the lookup can still succeed, even when there is no equivalent
 297      * Java expression or bytecoded constant.
 298      * <li>Likewise, if {@code T} or {@code MT}
 299      * is not symbolically accessible from the lookup class's loader,
 300      * the lookup can still succeed.
 301      * For example, lookups for {@code MethodHandle.invokeExact} and
 302      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 303      * <li>If there is a security manager installed, it can forbid the lookup
 304      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 305      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 306      * constant is not subject to security manager checks.
 307      * <li>If the looked-up method has a
 308      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 309      * the method handle creation may fail, due to the method handle
 310      * type having too many parameters.
 311      * </ul>
 312      *
 313      * <h1><a name="access"></a>Access checking</h1>
 314      * Access checks are applied in the factory methods of {@code Lookup},
 315      * when a method handle is created.
 316      * This is a key difference from the Core Reflection API, since
 317      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 318      * performs access checking against every caller, on every call.
 319      * <p>
 320      * All access checks start from a {@code Lookup} object, which
 321      * compares its recorded lookup class against all requests to
 322      * create method handles.
 323      * A single {@code Lookup} object can be used to create any number
 324      * of access-checked method handles, all checked against a single
 325      * lookup class.
 326      * <p>
 327      * A {@code Lookup} object can be shared with other trusted code,
 328      * such as a metaobject protocol.
 329      * A shared {@code Lookup} object delegates the capability
 330      * to create method handles on private members of the lookup class.
 331      * Even if privileged code uses the {@code Lookup} object,
 332      * the access checking is confined to the privileges of the
 333      * original lookup class.
 334      * <p>
 335      * A lookup can fail, because
 336      * the containing class is not accessible to the lookup class, or
 337      * because the desired class member is missing, or because the
 338      * desired class member is not accessible to the lookup class, or
 339      * because the lookup object is not trusted enough to access the member.
 340      * In any of these cases, a {@code ReflectiveOperationException} will be
 341      * thrown from the attempted lookup.  The exact class will be one of
 342      * the following:
 343      * <ul>
 344      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 345      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 346      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 347      * </ul>
 348      * <p>
 349      * In general, the conditions under which a method handle may be
 350      * looked up for a method {@code M} are no more restrictive than the conditions
 351      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 352      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 353      * a method handle lookup will generally raise a corresponding
 354      * checked exception, such as {@code NoSuchMethodException}.
 355      * And the effect of invoking the method handle resulting from the lookup
 356      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 357      * to executing the compiled, verified, and resolved call to {@code M}.
 358      * The same point is true of fields and constructors.
 359      * <p style="font-size:smaller;">
 360      * <em>Discussion:</em>
 361      * Access checks only apply to named and reflected methods,
 362      * constructors, and fields.
 363      * Other method handle creation methods, such as
 364      * {@link MethodHandle#asType MethodHandle.asType},
 365      * do not require any access checks, and are used
 366      * independently of any {@code Lookup} object.
 367      * <p>
 368      * If the desired member is {@code protected}, the usual JVM rules apply,
 369      * including the requirement that the lookup class must be either be in the
 370      * same package as the desired member, or must inherit that member.
 371      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 372      * In addition, if the desired member is a non-static field or method
 373      * in a different package, the resulting method handle may only be applied
 374      * to objects of the lookup class or one of its subclasses.
 375      * This requirement is enforced by narrowing the type of the leading
 376      * {@code this} parameter from {@code C}
 377      * (which will necessarily be a superclass of the lookup class)
 378      * to the lookup class itself.
 379      * <p>
 380      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 381      * that the receiver argument must match both the resolved method <em>and</em>
 382      * the current class.  Again, this requirement is enforced by narrowing the
 383      * type of the leading parameter to the resulting method handle.
 384      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 385      * <p>
 386      * The JVM represents constructors and static initializer blocks as internal methods
 387      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 388      * The internal syntax of invocation instructions allows them to refer to such internal
 389      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 390      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 391      * <p>
 392      * In some cases, access between nested classes is obtained by the Java compiler by creating
 393      * an wrapper method to access a private method of another class
 394      * in the same top-level declaration.
 395      * For example, a nested class {@code C.D}
 396      * can access private members within other related classes such as
 397      * {@code C}, {@code C.D.E}, or {@code C.B},
 398      * but the Java compiler may need to generate wrapper methods in
 399      * those related classes.  In such cases, a {@code Lookup} object on
 400      * {@code C.E} would be unable to those private members.
 401      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 402      * which can transform a lookup on {@code C.E} into one on any of those other
 403      * classes, without special elevation of privilege.
 404      * <p>
 405      * The accesses permitted to a given lookup object may be limited,
 406      * according to its set of {@link #lookupModes lookupModes},
 407      * to a subset of members normally accessible to the lookup class.
 408      * For example, the {@link MethodHandles#publicLookup publicLookup}
 409      * method produces a lookup object which is only allowed to access
 410      * public members in public classes of exported packages.
 411      * The caller sensitive method {@link MethodHandles#lookup lookup}
 412      * produces a lookup object with full capabilities relative to
 413      * its caller class, to emulate all supported bytecode behaviors.
 414      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 415      * with fewer access modes than the original lookup object.
 416      *
 417      * <p style="font-size:smaller;">
 418      * <a name="privacc"></a>
 419      * <em>Discussion of private access:</em>
 420      * We say that a lookup has <em>private access</em>
 421      * if its {@linkplain #lookupModes lookup modes}
 422      * include the possibility of accessing {@code private} members.
 423      * As documented in the relevant methods elsewhere,
 424      * only lookups with private access possess the following capabilities:
 425      * <ul style="font-size:smaller;">
 426      * <li>access private fields, methods, and constructors of the lookup class
 427      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 428      *     such as {@code Class.forName}
 429      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 430      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 431      *     for classes accessible to the lookup class
 432      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 433      *     within the same package member
 434      * </ul>
 435      * <p style="font-size:smaller;">
 436      * Each of these permissions is a consequence of the fact that a lookup object
 437      * with private access can be securely traced back to an originating class,
 438      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 439      * can be reliably determined and emulated by method handles.
 440      *
 441      * <h1><a name="secmgr"></a>Security manager interactions</h1>
 442      * Although bytecode instructions can only refer to classes in
 443      * a related class loader, this API can search for methods in any
 444      * class, as long as a reference to its {@code Class} object is
 445      * available.  Such cross-loader references are also possible with the
 446      * Core Reflection API, and are impossible to bytecode instructions
 447      * such as {@code invokestatic} or {@code getfield}.
 448      * There is a {@linkplain java.lang.SecurityManager security manager API}
 449      * to allow applications to check such cross-loader references.
 450      * These checks apply to both the {@code MethodHandles.Lookup} API
 451      * and the Core Reflection API
 452      * (as found on {@link java.lang.Class Class}).
 453      * <p>
 454      * If a security manager is present, member and class lookups are subject to
 455      * additional checks.
 456      * From one to three calls are made to the security manager.
 457      * Any of these calls can refuse access by throwing a
 458      * {@link java.lang.SecurityException SecurityException}.
 459      * Define {@code smgr} as the security manager,
 460      * {@code lookc} as the lookup class of the current lookup object,
 461      * {@code refc} as the containing class in which the member
 462      * is being sought, and {@code defc} as the class in which the
 463      * member is actually defined.
 464      * (If a class or other type is being accessed,
 465      * the {@code refc} and {@code defc} values are the class itself.)
 466      * The value {@code lookc} is defined as <em>not present</em>
 467      * if the current lookup object does not have
 468      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 469      * The calls are made according to the following rules:
 470      * <ul>
 471      * <li><b>Step 1:</b>
 472      *     If {@code lookc} is not present, or if its class loader is not
 473      *     the same as or an ancestor of the class loader of {@code refc},
 474      *     then {@link SecurityManager#checkPackageAccess
 475      *     smgr.checkPackageAccess(refcPkg)} is called,
 476      *     where {@code refcPkg} is the package of {@code refc}.
 477      * <li><b>Step 2a:</b>
 478      *     If the retrieved member is not public and
 479      *     {@code lookc} is not present, then
 480      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 481      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 482      * <li><b>Step 2b:</b>
 483      *     If the retrieved class has a {@code null} class loader,
 484      *     and {@code lookc} is not present, then
 485      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 486      *     with {@code RuntimePermission("getClassLoader")} is called.
 487      * <li><b>Step 3:</b>
 488      *     If the retrieved member is not public,
 489      *     and if {@code lookc} is not present,
 490      *     and if {@code defc} and {@code refc} are different,
 491      *     then {@link SecurityManager#checkPackageAccess
 492      *     smgr.checkPackageAccess(defcPkg)} is called,
 493      *     where {@code defcPkg} is the package of {@code defc}.
 494      * </ul>
 495      * Security checks are performed after other access checks have passed.
 496      * Therefore, the above rules presuppose a member or class that is public,
 497      * or else that is being accessed from a lookup class that has
 498      * rights to access the member or class.
 499      *
 500      * <h1><a name="callsens"></a>Caller sensitive methods</h1>
 501      * A small number of Java methods have a special property called caller sensitivity.
 502      * A <em>caller-sensitive</em> method can behave differently depending on the
 503      * identity of its immediate caller.
 504      * <p>
 505      * If a method handle for a caller-sensitive method is requested,
 506      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 507      * but they take account of the lookup class in a special way.
 508      * The resulting method handle behaves as if it were called
 509      * from an instruction contained in the lookup class,
 510      * so that the caller-sensitive method detects the lookup class.
 511      * (By contrast, the invoker of the method handle is disregarded.)
 512      * Thus, in the case of caller-sensitive methods,
 513      * different lookup classes may give rise to
 514      * differently behaving method handles.
 515      * <p>
 516      * In cases where the lookup object is
 517      * {@link MethodHandles#publicLookup() publicLookup()},
 518      * or some other lookup object without
 519      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 520      * the lookup class is disregarded.
 521      * In such cases, no caller-sensitive method handle can be created,
 522      * access is forbidden, and the lookup fails with an
 523      * {@code IllegalAccessException}.
 524      * <p style="font-size:smaller;">
 525      * <em>Discussion:</em>
 526      * For example, the caller-sensitive method
 527      * {@link java.lang.Class#forName(String) Class.forName(x)}
 528      * can return varying classes or throw varying exceptions,
 529      * depending on the class loader of the class that calls it.
 530      * A public lookup of {@code Class.forName} will fail, because
 531      * there is no reasonable way to determine its bytecode behavior.
 532      * <p style="font-size:smaller;">
 533      * If an application caches method handles for broad sharing,
 534      * it should use {@code publicLookup()} to create them.
 535      * If there is a lookup of {@code Class.forName}, it will fail,
 536      * and the application must take appropriate action in that case.
 537      * It may be that a later lookup, perhaps during the invocation of a
 538      * bootstrap method, can incorporate the specific identity
 539      * of the caller, making the method accessible.
 540      * <p style="font-size:smaller;">
 541      * The function {@code MethodHandles.lookup} is caller sensitive
 542      * so that there can be a secure foundation for lookups.
 543      * Nearly all other methods in the JSR 292 API rely on lookup
 544      * objects to check access requests.
 545      */
 546     public static final
 547     class Lookup {
 548         /** The class on behalf of whom the lookup is being performed. */
 549         private final Class<?> lookupClass;
 550 
 551         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 552         private final int allowedModes;
 553 
 554         /** A single-bit mask representing {@code public} access,
 555          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 556          *  The value, {@code 0x01}, happens to be the same as the value of the
 557          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 558          */
 559         public static final int PUBLIC = Modifier.PUBLIC;
 560 
 561         /** A single-bit mask representing {@code private} access,
 562          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 563          *  The value, {@code 0x02}, happens to be the same as the value of the
 564          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 565          */
 566         public static final int PRIVATE = Modifier.PRIVATE;
 567 
 568         /** A single-bit mask representing {@code protected} access,
 569          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 570          *  The value, {@code 0x04}, happens to be the same as the value of the
 571          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 572          */
 573         public static final int PROTECTED = Modifier.PROTECTED;
 574 
 575         /** A single-bit mask representing {@code package} access (default access),
 576          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 577          *  The value is {@code 0x08}, which does not correspond meaningfully to
 578          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 579          */
 580         public static final int PACKAGE = Modifier.STATIC;
 581 
 582         /** A single-bit mask representing {@code module} access (default access),
 583          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 584          *  The value is {@code 0x10}, which does not correspond meaningfully to
 585          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 586          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 587          *  with this lookup mode can access all public types in the module of the
 588          *  lookup class and public types in packages exported by other modules
 589          *  to the module of the lookup class.
 590          *  @since 9
 591          */
 592         public static final int MODULE = PACKAGE << 1;
 593 
 594         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE);
 595         private static final int TRUSTED   = -1;
 596 
 597         private static int fixmods(int mods) {
 598             mods &= (ALL_MODES - PACKAGE - MODULE);
 599             return (mods != 0) ? mods : (PACKAGE | MODULE);
 600         }
 601 
 602         /** Tells which class is performing the lookup.  It is this class against
 603          *  which checks are performed for visibility and access permissions.
 604          *  <p>
 605          *  The class implies a maximum level of access permission,
 606          *  but the permissions may be additionally limited by the bitmask
 607          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 608          *  can be accessed.
 609          *  @return the lookup class, on behalf of which this lookup object finds members
 610          */
 611         public Class<?> lookupClass() {
 612             return lookupClass;
 613         }
 614 
 615         // This is just for calling out to MethodHandleImpl.
 616         private Class<?> lookupClassOrNull() {
 617             return (allowedModes == TRUSTED) ? null : lookupClass;
 618         }
 619 
 620         /** Tells which access-protection classes of members this lookup object can produce.
 621          *  The result is a bit-mask of the bits
 622          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 623          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 624          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 625          *  {@linkplain #PACKAGE PACKAGE (0x08)},
 626          *  and {@linkplain #MODULE MODULE (0x10)}.
 627          *  <p>
 628          *  A freshly-created lookup object
 629          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class}
 630          *  has all possible bits set, since the caller class can access all its own members,
 631          *  all public types in the caller's module, and all public types in packages exported
 632          *  by other modules to the caller's module.
 633          *  A lookup object on a new lookup class
 634          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 635          *  may have some mode bits set to zero.
 636          *  The purpose of this is to restrict access via the new lookup object,
 637          *  so that it can access only names which can be reached by the original
 638          *  lookup object, and also by the new lookup class.
 639          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 640          */
 641         public int lookupModes() {
 642             return allowedModes & ALL_MODES;
 643         }
 644 
 645         /** Embody the current class (the lookupClass) as a lookup class
 646          * for method handle creation.
 647          * Must be called by from a method in this package,
 648          * which in turn is called by a method not in this package.
 649          */
 650         Lookup(Class<?> lookupClass) {
 651             this(lookupClass, ALL_MODES);
 652             // make sure we haven't accidentally picked up a privileged class:
 653             checkUnprivilegedlookupClass(lookupClass, ALL_MODES);
 654         }
 655 
 656         private Lookup(Class<?> lookupClass, int allowedModes) {
 657             this.lookupClass = lookupClass;
 658             this.allowedModes = allowedModes;
 659         }
 660 
 661         /**
 662          * Creates a lookup on the specified new lookup class.
 663          * The resulting object will report the specified
 664          * class as its own {@link #lookupClass lookupClass}.
 665          * <p>
 666          * However, the resulting {@code Lookup} object is guaranteed
 667          * to have no more access capabilities than the original.
 668          * In particular, access capabilities can be lost as follows:<ul>
 669          * <li>If the lookup class for this {@code Lookup} is not in a named module,
 670          * and the new lookup class is in a named module {@code M}, then no members in
 671          * {@code M}'s non-exported packages will be accessible.
 672          * <li>If the lookup for this {@code Lookup} is in a named module, and the
 673          * new lookup class is in a different module {@code M}, then no members, not even
 674          * public members in {@code M}'s exported packages, will be accessible.
 675          * <li>If the new lookup class differs from the old one,
 676          * protected members will not be accessible by virtue of inheritance.
 677          * (Protected members may continue to be accessible because of package sharing.)
 678          * <li>If the new lookup class is in a different package
 679          * than the old one, protected and default (package) members will not be accessible.
 680          * <li>If the new lookup class is not within the same package member
 681          * as the old one, private members will not be accessible.
 682          * <li>If the new lookup class is not accessible to the old lookup class,
 683          * then no members, not even public members, will be accessible.
 684          * (In all other cases, public members will continue to be accessible.)
 685          * </ul>
 686          * <p>
 687          * The resulting lookup's capabilities for loading classes
 688          * (used during {@link #findClass} invocations)
 689          * are determined by the lookup class' loader,
 690          * which may change due to this operation.
 691          *
 692          * @param requestedLookupClass the desired lookup class for the new lookup object
 693          * @return a lookup object which reports the desired lookup class
 694          * @throws NullPointerException if the argument is null
 695          */
 696         public Lookup in(Class<?> requestedLookupClass) {
 697             Objects.requireNonNull(requestedLookupClass);
 698             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 699                 return new Lookup(requestedLookupClass, ALL_MODES);
 700             if (requestedLookupClass == this.lookupClass)
 701                 return this;  // keep same capabilities
 702 
 703             int newModes = (allowedModes & (ALL_MODES & ~PROTECTED));
 704             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 705                 // Allowed to teleport from an unnamed to a named module but resulting
 706                 // Lookup has no access to module private members
 707                 if (this.lookupClass.getModule().isNamed()) {
 708                     newModes = 0;
 709                 } else {
 710                     newModes &= ~MODULE;
 711                 }
 712             }
 713             if ((newModes & PACKAGE) != 0
 714                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 715                 newModes &= ~(PACKAGE|PRIVATE);
 716             }
 717             // Allow nestmate lookups to be created without special privilege:
 718             if ((newModes & PRIVATE) != 0
 719                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 720                 newModes &= ~PRIVATE;
 721             }
 722             if ((newModes & PUBLIC) != 0
 723                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 724                 // The requested class it not accessible from the lookup class.
 725                 // No permissions.
 726                 newModes = 0;
 727             }
 728 
 729             checkUnprivilegedlookupClass(requestedLookupClass, newModes);
 730             return new Lookup(requestedLookupClass, newModes);
 731         }
 732 
 733         // Make sure outer class is initialized first.
 734         static { IMPL_NAMES.getClass(); }
 735 
 736         /** Package-private version of lookup which is trusted. */
 737         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 738 
 739         private static void checkUnprivilegedlookupClass(Class<?> lookupClass, int allowedModes) {
 740             String name = lookupClass.getName();
 741             if (name.startsWith("java.lang.invoke."))
 742                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
 743 
 744             // For caller-sensitive MethodHandles.lookup()
 745             // disallow lookup more restricted packages
 746             if (allowedModes == ALL_MODES && lookupClass.getClassLoader() == null) {
 747                 if (name.startsWith("java.") ||
 748                         (name.startsWith("sun.") && !name.startsWith("sun.invoke."))) {
 749                     throw newIllegalArgumentException("illegal lookupClass: " + lookupClass);
 750                 }
 751             }
 752         }
 753 
 754         /**
 755          * Displays the name of the class from which lookups are to be made.
 756          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
 757          * If there are restrictions on the access permitted to this lookup,
 758          * this is indicated by adding a suffix to the class name, consisting
 759          * of a slash and a keyword.  The keyword represents the strongest
 760          * allowed access, and is chosen as follows:
 761          * <ul>
 762          * <li>If no access is allowed, the suffix is "/noaccess".
 763          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
 764          * <li>If only public and module access are allowed, the suffix is "/module".
 765          * <li>If only public, module and package access are allowed, the suffix is "/package".
 766          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
 767          * </ul>
 768          * If none of the above cases apply, it is the case that full
 769          * access (public, module, package, private, and protected) is allowed.
 770          * In this case, no suffix is added.
 771          * This is true only of an object obtained originally from
 772          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
 773          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
 774          * always have restricted access, and will display a suffix.
 775          * <p>
 776          * (It may seem strange that protected access should be
 777          * stronger than private access.  Viewed independently from
 778          * package access, protected access is the first to be lost,
 779          * because it requires a direct subclass relationship between
 780          * caller and callee.)
 781          * @see #in
 782          */
 783         @Override
 784         public String toString() {
 785             String cname = lookupClass.getName();
 786             switch (allowedModes) {
 787             case 0:  // no privileges
 788                 return cname + "/noaccess";
 789             case PUBLIC:
 790                 return cname + "/public";
 791             case PUBLIC|MODULE:
 792                 return cname + "/module";
 793             case PUBLIC|MODULE|PACKAGE:
 794                 return cname + "/package";
 795             case ALL_MODES & ~PROTECTED:
 796                 return cname + "/private";
 797             case ALL_MODES:
 798                 return cname;
 799             case TRUSTED:
 800                 return "/trusted";  // internal only; not exported
 801             default:  // Should not happen, but it's a bitfield...
 802                 cname = cname + "/" + Integer.toHexString(allowedModes);
 803                 assert(false) : cname;
 804                 return cname;
 805             }
 806         }
 807 
 808         /**
 809          * Produces a method handle for a static method.
 810          * The type of the method handle will be that of the method.
 811          * (Since static methods do not take receivers, there is no
 812          * additional receiver argument inserted into the method handle type,
 813          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
 814          * The method and all its argument types must be accessible to the lookup object.
 815          * <p>
 816          * The returned method handle will have
 817          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 818          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 819          * <p>
 820          * If the returned method handle is invoked, the method's class will
 821          * be initialized, if it has not already been initialized.
 822          * <p><b>Example:</b>
 823          * <blockquote><pre>{@code
 824 import static java.lang.invoke.MethodHandles.*;
 825 import static java.lang.invoke.MethodType.*;
 826 ...
 827 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
 828   "asList", methodType(List.class, Object[].class));
 829 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
 830          * }</pre></blockquote>
 831          * @param refc the class from which the method is accessed
 832          * @param name the name of the method
 833          * @param type the type of the method
 834          * @return the desired method handle
 835          * @throws NoSuchMethodException if the method does not exist
 836          * @throws IllegalAccessException if access checking fails,
 837          *                                or if the method is not {@code static},
 838          *                                or if the method's variable arity modifier bit
 839          *                                is set and {@code asVarargsCollector} fails
 840          * @exception SecurityException if a security manager is present and it
 841          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 842          * @throws NullPointerException if any argument is null
 843          */
 844         public
 845         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 846             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
 847             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
 848         }
 849 
 850         /**
 851          * Produces a method handle for a virtual method.
 852          * The type of the method handle will be that of the method,
 853          * with the receiver type (usually {@code refc}) prepended.
 854          * The method and all its argument types must be accessible to the lookup object.
 855          * <p>
 856          * When called, the handle will treat the first argument as a receiver
 857          * and dispatch on the receiver's type to determine which method
 858          * implementation to enter.
 859          * (The dispatching action is identical with that performed by an
 860          * {@code invokevirtual} or {@code invokeinterface} instruction.)
 861          * <p>
 862          * The first argument will be of type {@code refc} if the lookup
 863          * class has full privileges to access the member.  Otherwise
 864          * the member must be {@code protected} and the first argument
 865          * will be restricted in type to the lookup class.
 866          * <p>
 867          * The returned method handle will have
 868          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 869          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 870          * <p>
 871          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
 872          * instructions and method handles produced by {@code findVirtual},
 873          * if the class is {@code MethodHandle} and the name string is
 874          * {@code invokeExact} or {@code invoke}, the resulting
 875          * method handle is equivalent to one produced by
 876          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
 877          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
 878          * with the same {@code type} argument.
 879          * <p>
 880          * If the class is {@code VarHandle} and the name string corresponds to
 881          * the name of a signature-polymorphic access mode method, the resulting
 882          * method handle is equivalent to one produced by
 883          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
 884          * the access mode corresponding to the name string and with the same
 885          * {@code type} arguments.
 886          * <p>
 887          * <b>Example:</b>
 888          * <blockquote><pre>{@code
 889 import static java.lang.invoke.MethodHandles.*;
 890 import static java.lang.invoke.MethodType.*;
 891 ...
 892 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
 893   "concat", methodType(String.class, String.class));
 894 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
 895   "hashCode", methodType(int.class));
 896 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
 897   "hashCode", methodType(int.class));
 898 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
 899 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
 900 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
 901 // interface method:
 902 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
 903   "subSequence", methodType(CharSequence.class, int.class, int.class));
 904 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
 905 // constructor "internal method" must be accessed differently:
 906 MethodType MT_newString = methodType(void.class); //()V for new String()
 907 try { assertEquals("impossible", lookup()
 908         .findVirtual(String.class, "<init>", MT_newString));
 909  } catch (NoSuchMethodException ex) { } // OK
 910 MethodHandle MH_newString = publicLookup()
 911   .findConstructor(String.class, MT_newString);
 912 assertEquals("", (String) MH_newString.invokeExact());
 913          * }</pre></blockquote>
 914          *
 915          * @param refc the class or interface from which the method is accessed
 916          * @param name the name of the method
 917          * @param type the type of the method, with the receiver argument omitted
 918          * @return the desired method handle
 919          * @throws NoSuchMethodException if the method does not exist
 920          * @throws IllegalAccessException if access checking fails,
 921          *                                or if the method is {@code static},
 922          *                                or if the method is {@code private} method of interface,
 923          *                                or if the method's variable arity modifier bit
 924          *                                is set and {@code asVarargsCollector} fails
 925          * @exception SecurityException if a security manager is present and it
 926          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 927          * @throws NullPointerException if any argument is null
 928          */
 929         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 930             if (refc == MethodHandle.class) {
 931                 MethodHandle mh = findVirtualForMH(name, type);
 932                 if (mh != null)  return mh;
 933             } else if (refc == VarHandle.class) {
 934                 MethodHandle mh = findVirtualForVH(name, type);
 935                 if (mh != null)  return mh;
 936             }
 937             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
 938             MemberName method = resolveOrFail(refKind, refc, name, type);
 939             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
 940         }
 941         private MethodHandle findVirtualForMH(String name, MethodType type) {
 942             // these names require special lookups because of the implicit MethodType argument
 943             if ("invoke".equals(name))
 944                 return invoker(type);
 945             if ("invokeExact".equals(name))
 946                 return exactInvoker(type);
 947             if ("invokeBasic".equals(name))
 948                 return basicInvoker(type);
 949             assert(!MemberName.isMethodHandleInvokeName(name));
 950             return null;
 951         }
 952         private MethodHandle findVirtualForVH(String name, MethodType type) {
 953             try {
 954                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
 955             } catch (IllegalArgumentException e) {
 956                 return null;
 957             }
 958         }
 959 
 960         /**
 961          * Produces a method handle which creates an object and initializes it, using
 962          * the constructor of the specified type.
 963          * The parameter types of the method handle will be those of the constructor,
 964          * while the return type will be a reference to the constructor's class.
 965          * The constructor and all its argument types must be accessible to the lookup object.
 966          * <p>
 967          * The requested type must have a return type of {@code void}.
 968          * (This is consistent with the JVM's treatment of constructor type descriptors.)
 969          * <p>
 970          * The returned method handle will have
 971          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 972          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
 973          * <p>
 974          * If the returned method handle is invoked, the constructor's class will
 975          * be initialized, if it has not already been initialized.
 976          * <p><b>Example:</b>
 977          * <blockquote><pre>{@code
 978 import static java.lang.invoke.MethodHandles.*;
 979 import static java.lang.invoke.MethodType.*;
 980 ...
 981 MethodHandle MH_newArrayList = publicLookup().findConstructor(
 982   ArrayList.class, methodType(void.class, Collection.class));
 983 Collection orig = Arrays.asList("x", "y");
 984 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
 985 assert(orig != copy);
 986 assertEquals(orig, copy);
 987 // a variable-arity constructor:
 988 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
 989   ProcessBuilder.class, methodType(void.class, String[].class));
 990 ProcessBuilder pb = (ProcessBuilder)
 991   MH_newProcessBuilder.invoke("x", "y", "z");
 992 assertEquals("[x, y, z]", pb.command().toString());
 993          * }</pre></blockquote>
 994          * @param refc the class or interface from which the method is accessed
 995          * @param type the type of the method, with the receiver argument omitted, and a void return type
 996          * @return the desired method handle
 997          * @throws NoSuchMethodException if the constructor does not exist
 998          * @throws IllegalAccessException if access checking fails
 999          *                                or if the method's variable arity modifier bit
1000          *                                is set and {@code asVarargsCollector} fails
1001          * @exception SecurityException if a security manager is present and it
1002          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1003          * @throws NullPointerException if any argument is null
1004          */
1005         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1006             String name = "<init>";
1007             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1008             return getDirectConstructor(refc, ctor);
1009         }
1010 
1011         /**
1012          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1013          * initializer of the class is not run.
1014          * <p>
1015          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1016          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1017          * load the requested class, and then determines whether the class is accessible to this lookup object.
1018          *
1019          * @param targetName the fully qualified name of the class to be looked up.
1020          * @return the requested class.
1021          * @exception SecurityException if a security manager is present and it
1022          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1023          * @throws LinkageError if the linkage fails
1024          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
1025          * @throws IllegalAccessException if the class is not accessible, using the allowed access
1026          * modes.
1027          * @exception SecurityException if a security manager is present and it
1028          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1029          * @since 9
1030          */
1031         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1032             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1033             return accessClass(targetClass);
1034         }
1035 
1036         /**
1037          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1038          * static initializer of the class is not run.
1039          * <p>
1040          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1041          * {@linkplain #lookupModes() lookup modes}.
1042          *
1043          * @param targetClass the class to be access-checked
1044          *
1045          * @return the class that has been access-checked
1046          *
1047          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1048          * modes.
1049          * @exception SecurityException if a security manager is present and it
1050          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1051          * @since 9
1052          */
1053         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1054             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1055                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1056             }
1057             checkSecurityManager(targetClass, null);
1058             return targetClass;
1059         }
1060 
1061         /**
1062          * Produces an early-bound method handle for a virtual method.
1063          * It will bypass checks for overriding methods on the receiver,
1064          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1065          * instruction from within the explicitly specified {@code specialCaller}.
1066          * The type of the method handle will be that of the method,
1067          * with a suitably restricted receiver type prepended.
1068          * (The receiver type will be {@code specialCaller} or a subtype.)
1069          * The method and all its argument types must be accessible
1070          * to the lookup object.
1071          * <p>
1072          * Before method resolution,
1073          * if the explicitly specified caller class is not identical with the
1074          * lookup class, or if this lookup object does not have
1075          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1076          * privileges, the access fails.
1077          * <p>
1078          * The returned method handle will have
1079          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1080          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1081          * <p style="font-size:smaller;">
1082          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1083          * even though the {@code invokespecial} instruction can refer to them
1084          * in special circumstances.  Use {@link #findConstructor findConstructor}
1085          * to access instance initialization methods in a safe manner.)</em>
1086          * <p><b>Example:</b>
1087          * <blockquote><pre>{@code
1088 import static java.lang.invoke.MethodHandles.*;
1089 import static java.lang.invoke.MethodType.*;
1090 ...
1091 static class Listie extends ArrayList {
1092   public String toString() { return "[wee Listie]"; }
1093   static Lookup lookup() { return MethodHandles.lookup(); }
1094 }
1095 ...
1096 // no access to constructor via invokeSpecial:
1097 MethodHandle MH_newListie = Listie.lookup()
1098   .findConstructor(Listie.class, methodType(void.class));
1099 Listie l = (Listie) MH_newListie.invokeExact();
1100 try { assertEquals("impossible", Listie.lookup().findSpecial(
1101         Listie.class, "<init>", methodType(void.class), Listie.class));
1102  } catch (NoSuchMethodException ex) { } // OK
1103 // access to super and self methods via invokeSpecial:
1104 MethodHandle MH_super = Listie.lookup().findSpecial(
1105   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1106 MethodHandle MH_this = Listie.lookup().findSpecial(
1107   Listie.class, "toString" , methodType(String.class), Listie.class);
1108 MethodHandle MH_duper = Listie.lookup().findSpecial(
1109   Object.class, "toString" , methodType(String.class), Listie.class);
1110 assertEquals("[]", (String) MH_super.invokeExact(l));
1111 assertEquals(""+l, (String) MH_this.invokeExact(l));
1112 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1113 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1114         String.class, "toString", methodType(String.class), Listie.class));
1115  } catch (IllegalAccessException ex) { } // OK
1116 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1117 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1118          * }</pre></blockquote>
1119          *
1120          * @param refc the class or interface from which the method is accessed
1121          * @param name the name of the method (which must not be "&lt;init&gt;")
1122          * @param type the type of the method, with the receiver argument omitted
1123          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1124          * @return the desired method handle
1125          * @throws NoSuchMethodException if the method does not exist
1126          * @throws IllegalAccessException if access checking fails,
1127          *                                or if the method is {@code static},
1128          *                                or if the method's variable arity modifier bit
1129          *                                is set and {@code asVarargsCollector} fails
1130          * @exception SecurityException if a security manager is present and it
1131          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1132          * @throws NullPointerException if any argument is null
1133          */
1134         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1135                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1136             checkSpecialCaller(specialCaller, refc);
1137             Lookup specialLookup = this.in(specialCaller);
1138             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1139             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1140         }
1141 
1142         /**
1143          * Produces a method handle giving read access to a non-static field.
1144          * The type of the method handle will have a return type of the field's
1145          * value type.
1146          * The method handle's single argument will be the instance containing
1147          * the field.
1148          * Access checking is performed immediately on behalf of the lookup class.
1149          * @param refc the class or interface from which the method is accessed
1150          * @param name the field's name
1151          * @param type the field's type
1152          * @return a method handle which can load values from the field
1153          * @throws NoSuchFieldException if the field does not exist
1154          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1155          * @exception SecurityException if a security manager is present and it
1156          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1157          * @throws NullPointerException if any argument is null
1158          * @see #findVarHandle(Class, String, Class)
1159          */
1160         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1161             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1162             return getDirectField(REF_getField, refc, field);
1163         }
1164 
1165         /**
1166          * Produces a method handle giving write access to a non-static field.
1167          * The type of the method handle will have a void return type.
1168          * The method handle will take two arguments, the instance containing
1169          * the field, and the value to be stored.
1170          * The second argument will be of the field's value type.
1171          * Access checking is performed immediately on behalf of the lookup class.
1172          * @param refc the class or interface from which the method is accessed
1173          * @param name the field's name
1174          * @param type the field's type
1175          * @return a method handle which can store values into the field
1176          * @throws NoSuchFieldException if the field does not exist
1177          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1178          * @exception SecurityException if a security manager is present and it
1179          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1180          * @throws NullPointerException if any argument is null
1181          * @see #findVarHandle(Class, String, Class)
1182          */
1183         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1184             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1185             return getDirectField(REF_putField, refc, field);
1186         }
1187 
1188         /**
1189          * Produces a VarHandle giving access to non-static fields of type
1190          * {@code T} declared by a receiver class of type {@code R}, supporting
1191          * shape {@code (R : T)}.
1192          * <p>
1193          * Access checking is performed immediately on behalf of the lookup
1194          * class.
1195          * <p>
1196          * Certain access modes of the returned VarHandle are unsupported under
1197          * the following conditions:
1198          * <ul>
1199          * <li>if the field is declared {@code final}, then the write, atomic
1200          *     update, and numeric atomic update access modes are unsupported.
1201          * <li>if the field type is anything other than {@code int},
1202          *     {@code long} or a reference type, then atomic update access modes
1203          *     are unsupported.  (Future major platform releases of the JDK may
1204          *     support additional types for certain currently unsupported access
1205          *     modes.)
1206          * <li>if the field type is anything other than {@code int} or
1207          *     {@code long}, then numeric atomic update access modes are
1208          *     unsupported.  (Future major platform releases of the JDK may
1209          *     support additional numeric types for certain currently
1210          *     unsupported access modes.)
1211          * </ul>
1212          * <p>
1213          * If the field is declared {@code volatile} then the returned VarHandle
1214          * will override access to the field (effectively ignore the
1215          * {@code volatile} declaration) in accordance to it's specified
1216          * access modes.
1217          * @param recv the receiver class, of type {@code R}, that declares the
1218          * non-static field
1219          * @param name the field's name
1220          * @param type the field's type, of type {@code T}
1221          * @return a VarHandle giving access to non-static fields.
1222          * @throws NoSuchFieldException if the field does not exist
1223          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1224          * @exception SecurityException if a security manager is present and it
1225          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1226          * @throws NullPointerException if any argument is null
1227          * @since 9
1228          */
1229         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1230             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
1231             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
1232             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
1233         }
1234 
1235         /**
1236          * Produces a method handle giving read access to a static field.
1237          * The type of the method handle will have a return type of the field's
1238          * value type.
1239          * The method handle will take no arguments.
1240          * Access checking is performed immediately on behalf of the lookup class.
1241          * <p>
1242          * If the returned method handle is invoked, the field's class will
1243          * be initialized, if it has not already been initialized.
1244          * @param refc the class or interface from which the method is accessed
1245          * @param name the field's name
1246          * @param type the field's type
1247          * @return a method handle which can load values from the field
1248          * @throws NoSuchFieldException if the field does not exist
1249          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1250          * @exception SecurityException if a security manager is present and it
1251          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1252          * @throws NullPointerException if any argument is null
1253          */
1254         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1255             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1256             return getDirectField(REF_getStatic, refc, field);
1257         }
1258 
1259         /**
1260          * Produces a method handle giving write access to a static field.
1261          * The type of the method handle will have a void return type.
1262          * The method handle will take a single
1263          * argument, of the field's value type, the value to be stored.
1264          * Access checking is performed immediately on behalf of the lookup class.
1265          * <p>
1266          * If the returned method handle is invoked, the field's class will
1267          * be initialized, if it has not already been initialized.
1268          * @param refc the class or interface from which the method is accessed
1269          * @param name the field's name
1270          * @param type the field's type
1271          * @return a method handle which can store values into the field
1272          * @throws NoSuchFieldException if the field does not exist
1273          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1274          * @exception SecurityException if a security manager is present and it
1275          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1276          * @throws NullPointerException if any argument is null
1277          */
1278         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1279             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1280             return getDirectField(REF_putStatic, refc, field);
1281         }
1282 
1283         /**
1284          * Produces a VarHandle giving access to a static field of type
1285          * {@code T} declared by a given declaring class, supporting shape
1286          * {@code ((empty) : T)}.
1287          * <p>
1288          * Access checking is performed immediately on behalf of the lookup
1289          * class.
1290          * <p>
1291          * If the returned VarHandle is operated on, the declaring class will be
1292          * initialized, if it has not already been initialized.
1293          * <p>
1294          * Certain access modes of the returned VarHandle are unsupported under
1295          * the following conditions:
1296          * <ul>
1297          * <li>if the field is declared {@code final}, then the write, atomic
1298          *     update, and numeric atomic update access modes are unsupported.
1299          * <li>if the field type is anything other than {@code int},
1300          *     {@code long} or a reference type, then atomic update access modes
1301          *     are unsupported.  (Future major platform releases of the JDK may
1302          *     support additional types for certain currently unsupported access
1303          *     modes.)
1304          * <li>if the field type is anything other than {@code int} or
1305          *     {@code long}, then numeric atomic update access modes are
1306          *     unsupported.  (Future major platform releases of the JDK may
1307          *     support additional numeric types for certain currently
1308          *     unsupported access modes.)
1309          * </ul>
1310          * <p>
1311          * If the field is declared {@code volatile} then the returned VarHandle
1312          * will override access to the field (effectively ignore the
1313          * {@code volatile} declaration) in accordance to it's specified
1314          * access modes.
1315          * @param decl the class that declares the static field
1316          * @param name the field's name
1317          * @param type the field's type, of type {@code T}
1318          * @return a VarHandle giving access to a static field
1319          * @throws NoSuchFieldException if the field does not exist
1320          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1321          * @exception SecurityException if a security manager is present and it
1322          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1323          * @throws NullPointerException if any argument is null
1324          * @since 9
1325          */
1326         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1327             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
1328             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
1329             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
1330         }
1331 
1332         /**
1333          * Produces an early-bound method handle for a non-static method.
1334          * The receiver must have a supertype {@code defc} in which a method
1335          * of the given name and type is accessible to the lookup class.
1336          * The method and all its argument types must be accessible to the lookup object.
1337          * The type of the method handle will be that of the method,
1338          * without any insertion of an additional receiver parameter.
1339          * The given receiver will be bound into the method handle,
1340          * so that every call to the method handle will invoke the
1341          * requested method on the given receiver.
1342          * <p>
1343          * The returned method handle will have
1344          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1345          * the method's variable arity modifier bit ({@code 0x0080}) is set
1346          * <em>and</em> the trailing array argument is not the only argument.
1347          * (If the trailing array argument is the only argument,
1348          * the given receiver value will be bound to it.)
1349          * <p>
1350          * This is equivalent to the following code:
1351          * <blockquote><pre>{@code
1352 import static java.lang.invoke.MethodHandles.*;
1353 import static java.lang.invoke.MethodType.*;
1354 ...
1355 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1356 MethodHandle mh1 = mh0.bindTo(receiver);
1357 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
1358 return mh1;
1359          * }</pre></blockquote>
1360          * where {@code defc} is either {@code receiver.getClass()} or a super
1361          * type of that class, in which the requested method is accessible
1362          * to the lookup class.
1363          * (Note that {@code bindTo} does not preserve variable arity.)
1364          * @param receiver the object from which the method is accessed
1365          * @param name the name of the method
1366          * @param type the type of the method, with the receiver argument omitted
1367          * @return the desired method handle
1368          * @throws NoSuchMethodException if the method does not exist
1369          * @throws IllegalAccessException if access checking fails
1370          *                                or if the method's variable arity modifier bit
1371          *                                is set and {@code asVarargsCollector} fails
1372          * @exception SecurityException if a security manager is present and it
1373          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1374          * @throws NullPointerException if any argument is null
1375          * @see MethodHandle#bindTo
1376          * @see #findVirtual
1377          */
1378         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1379             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1380             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1381             MethodHandle mh = getDirectMethodNoRestrict(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1382             return mh.bindArgumentL(0, receiver).setVarargs(method);
1383         }
1384 
1385         /**
1386          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1387          * to <i>m</i>, if the lookup class has permission.
1388          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1389          * If <i>m</i> is virtual, overriding is respected on every call.
1390          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1391          * The type of the method handle will be that of the method,
1392          * with the receiver type prepended (but only if it is non-static).
1393          * If the method's {@code accessible} flag is not set,
1394          * access checking is performed immediately on behalf of the lookup class.
1395          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1396          * <p>
1397          * The returned method handle will have
1398          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1399          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1400          * <p>
1401          * If <i>m</i> is static, and
1402          * if the returned method handle is invoked, the method's class will
1403          * be initialized, if it has not already been initialized.
1404          * @param m the reflected method
1405          * @return a method handle which can invoke the reflected method
1406          * @throws IllegalAccessException if access checking fails
1407          *                                or if the method's variable arity modifier bit
1408          *                                is set and {@code asVarargsCollector} fails
1409          * @throws NullPointerException if the argument is null
1410          */
1411         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1412             if (m.getDeclaringClass() == MethodHandle.class) {
1413                 MethodHandle mh = unreflectForMH(m);
1414                 if (mh != null)  return mh;
1415             }
1416             if (m.getDeclaringClass() == VarHandle.class) {
1417                 MethodHandle mh = unreflectForVH(m);
1418                 if (mh != null)  return mh;
1419             }
1420             MemberName method = new MemberName(m);
1421             byte refKind = method.getReferenceKind();
1422             if (refKind == REF_invokeSpecial)
1423                 refKind = REF_invokeVirtual;
1424             assert(method.isMethod());
1425             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1426             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1427         }
1428         private MethodHandle unreflectForMH(Method m) {
1429             // these names require special lookups because they throw UnsupportedOperationException
1430             if (MemberName.isMethodHandleInvokeName(m.getName()))
1431                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1432             return null;
1433         }
1434         private MethodHandle unreflectForVH(Method m) {
1435             // these names require special lookups because they throw UnsupportedOperationException
1436             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
1437                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
1438             return null;
1439         }
1440 
1441         /**
1442          * Produces a method handle for a reflected method.
1443          * It will bypass checks for overriding methods on the receiver,
1444          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1445          * instruction from within the explicitly specified {@code specialCaller}.
1446          * The type of the method handle will be that of the method,
1447          * with a suitably restricted receiver type prepended.
1448          * (The receiver type will be {@code specialCaller} or a subtype.)
1449          * If the method's {@code accessible} flag is not set,
1450          * access checking is performed immediately on behalf of the lookup class,
1451          * as if {@code invokespecial} instruction were being linked.
1452          * <p>
1453          * Before method resolution,
1454          * if the explicitly specified caller class is not identical with the
1455          * lookup class, or if this lookup object does not have
1456          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1457          * privileges, the access fails.
1458          * <p>
1459          * The returned method handle will have
1460          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1461          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1462          * @param m the reflected method
1463          * @param specialCaller the class nominally calling the method
1464          * @return a method handle which can invoke the reflected method
1465          * @throws IllegalAccessException if access checking fails,
1466          *                                or if the method is {@code static},
1467          *                                or if the method's variable arity modifier bit
1468          *                                is set and {@code asVarargsCollector} fails
1469          * @throws NullPointerException if any argument is null
1470          */
1471         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1472             checkSpecialCaller(specialCaller, null);
1473             Lookup specialLookup = this.in(specialCaller);
1474             MemberName method = new MemberName(m, true);
1475             assert(method.isMethod());
1476             // ignore m.isAccessible:  this is a new kind of access
1477             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1478         }
1479 
1480         /**
1481          * Produces a method handle for a reflected constructor.
1482          * The type of the method handle will be that of the constructor,
1483          * with the return type changed to the declaring class.
1484          * The method handle will perform a {@code newInstance} operation,
1485          * creating a new instance of the constructor's class on the
1486          * arguments passed to the method handle.
1487          * <p>
1488          * If the constructor's {@code accessible} flag is not set,
1489          * access checking is performed immediately on behalf of the lookup class.
1490          * <p>
1491          * The returned method handle will have
1492          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1493          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1494          * <p>
1495          * If the returned method handle is invoked, the constructor's class will
1496          * be initialized, if it has not already been initialized.
1497          * @param c the reflected constructor
1498          * @return a method handle which can invoke the reflected constructor
1499          * @throws IllegalAccessException if access checking fails
1500          *                                or if the method's variable arity modifier bit
1501          *                                is set and {@code asVarargsCollector} fails
1502          * @throws NullPointerException if the argument is null
1503          */
1504         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1505             MemberName ctor = new MemberName(c);
1506             assert(ctor.isConstructor());
1507             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1508             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1509         }
1510 
1511         /**
1512          * Produces a method handle giving read access to a reflected field.
1513          * The type of the method handle will have a return type of the field's
1514          * value type.
1515          * If the field is static, the method handle will take no arguments.
1516          * Otherwise, its single argument will be the instance containing
1517          * the field.
1518          * If the field's {@code accessible} flag is not set,
1519          * access checking is performed immediately on behalf of the lookup class.
1520          * <p>
1521          * If the field is static, and
1522          * if the returned method handle is invoked, the field's class will
1523          * be initialized, if it has not already been initialized.
1524          * @param f the reflected field
1525          * @return a method handle which can load values from the reflected field
1526          * @throws IllegalAccessException if access checking fails
1527          * @throws NullPointerException if the argument is null
1528          */
1529         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1530             return unreflectField(f, false);
1531         }
1532         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1533             MemberName field = new MemberName(f, isSetter);
1534             assert(isSetter
1535                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1536                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1537             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1538             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1539         }
1540 
1541         /**
1542          * Produces a method handle giving write access to a reflected field.
1543          * The type of the method handle will have a void return type.
1544          * If the field is static, the method handle will take a single
1545          * argument, of the field's value type, the value to be stored.
1546          * Otherwise, the two arguments will be the instance containing
1547          * the field, and the value to be stored.
1548          * If the field's {@code accessible} flag is not set,
1549          * access checking is performed immediately on behalf of the lookup class.
1550          * <p>
1551          * If the field is static, and
1552          * if the returned method handle is invoked, the field's class will
1553          * be initialized, if it has not already been initialized.
1554          * @param f the reflected field
1555          * @return a method handle which can store values into the reflected field
1556          * @throws IllegalAccessException if access checking fails
1557          * @throws NullPointerException if the argument is null
1558          */
1559         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1560             return unreflectField(f, true);
1561         }
1562 
1563         /**
1564          * Produces a VarHandle that accesses fields of type {@code T} declared
1565          * by a class of type {@code R}, as described by the given reflected
1566          * field.
1567          * If the field is non-static the VarHandle supports a shape of
1568          * {@code (R : T)}, otherwise supports a shape of {@code ((empty) : T)}.
1569          * <p>
1570          * Access checking is performed immediately on behalf of the lookup
1571          * class, regardless of the value of the field's {@code accessible}
1572          * flag.
1573          * <p>
1574          * If the field is static, and if the returned VarHandle is operated
1575          * on, the field's declaring class will be initialized, if it has not
1576          * already been initialized.
1577          * <p>
1578          * Certain access modes of the returned VarHandle are unsupported under
1579          * the following conditions:
1580          * <ul>
1581          * <li>if the field is declared {@code final}, then the write, atomic
1582          *     update, and numeric atomic update access modes are unsupported.
1583          * <li>if the field type is anything other than {@code int},
1584          *     {@code long} or a reference type, then atomic update access modes
1585          *     are unsupported.  (Future major platform releases of the JDK may
1586          *     support additional types for certain currently unsupported access
1587          *     modes.)
1588          * <li>if the field type is anything other than {@code int} or
1589          *     {@code long}, then numeric atomic update access modes are
1590          *     unsupported.  (Future major platform releases of the JDK may
1591          *     support additional numeric types for certain currently
1592          *     unsupported access modes.)
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 it's specified
1598          * access modes.
1599          * @param f the reflected field, with a field of type {@code T}, and
1600          * a declaring class of type {@code R}
1601          * @return a VarHandle giving access to non-static fields or a static
1602          * field
1603          * @throws IllegalAccessException if access checking fails
1604          * @throws NullPointerException if the argument is null
1605          * @since 9
1606          */
1607         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
1608             MemberName getField = new MemberName(f, false);
1609             MemberName putField = new MemberName(f, true);
1610             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
1611                                                       f.getDeclaringClass(), getField, putField);
1612         }
1613 
1614         /**
1615          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1616          * created by this lookup object or a similar one.
1617          * Security and access checks are performed to ensure that this lookup object
1618          * is capable of reproducing the target method handle.
1619          * This means that the cracking may fail if target is a direct method handle
1620          * but was created by an unrelated lookup object.
1621          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1622          * and was created by a lookup object for a different class.
1623          * @param target a direct method handle to crack into symbolic reference components
1624          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
1625          * @exception SecurityException if a security manager is present and it
1626          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1627          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
1628          * @exception NullPointerException if the target is {@code null}
1629          * @see MethodHandleInfo
1630          * @since 1.8
1631          */
1632         public MethodHandleInfo revealDirect(MethodHandle target) {
1633             MemberName member = target.internalMemberName();
1634             if (member == null || (!member.isResolved() &&
1635                                    !member.isMethodHandleInvoke() &&
1636                                    !member.isVarHandleMethodInvoke()))
1637                 throw newIllegalArgumentException("not a direct method handle");
1638             Class<?> defc = member.getDeclaringClass();
1639             byte refKind = member.getReferenceKind();
1640             assert(MethodHandleNatives.refKindIsValid(refKind));
1641             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
1642                 // Devirtualized method invocation is usually formally virtual.
1643                 // To avoid creating extra MemberName objects for this common case,
1644                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
1645                 refKind = REF_invokeVirtual;
1646             if (refKind == REF_invokeVirtual && defc.isInterface())
1647                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
1648                 refKind = REF_invokeInterface;
1649             // Check SM permissions and member access before cracking.
1650             try {
1651                 checkAccess(refKind, defc, member);
1652                 checkSecurityManager(defc, member);
1653             } catch (IllegalAccessException ex) {
1654                 throw new IllegalArgumentException(ex);
1655             }
1656             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
1657                 Class<?> callerClass = target.internalCallerClass();
1658                 if (!hasPrivateAccess() || callerClass != lookupClass())
1659                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
1660             }
1661             // Produce the handle to the results.
1662             return new InfoFromMemberName(this, member, refKind);
1663         }
1664 
1665         /// Helper methods, all package-private.
1666 
1667         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1668             checkSymbolicClass(refc);  // do this before attempting to resolve
1669             Objects.requireNonNull(name);
1670             Objects.requireNonNull(type);
1671             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1672                                             NoSuchFieldException.class);
1673         }
1674 
1675         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1676             checkSymbolicClass(refc);  // do this before attempting to resolve
1677             Objects.requireNonNull(name);
1678             Objects.requireNonNull(type);
1679             checkMethodName(refKind, name);  // NPE check on name
1680             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1681                                             NoSuchMethodException.class);
1682         }
1683 
1684         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
1685             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
1686             Objects.requireNonNull(member.getName());
1687             Objects.requireNonNull(member.getType());
1688             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
1689                                             ReflectiveOperationException.class);
1690         }
1691 
1692         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
1693             Objects.requireNonNull(refc);
1694             Class<?> caller = lookupClassOrNull();
1695             if (caller != null && !VerifyAccess.isClassAccessible(refc, caller, allowedModes))
1696                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
1697         }
1698 
1699         /** Check name for an illegal leading "&lt;" character. */
1700         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
1701             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
1702                 throw new NoSuchMethodException("illegal method name: "+name);
1703         }
1704 
1705 
1706         /**
1707          * Find my trustable caller class if m is a caller sensitive method.
1708          * If this lookup object has private access, then the caller class is the lookupClass.
1709          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
1710          */
1711         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
1712             Class<?> callerClass = null;
1713             if (MethodHandleNatives.isCallerSensitive(m)) {
1714                 // Only lookups with private access are allowed to resolve caller-sensitive methods
1715                 if (hasPrivateAccess()) {
1716                     callerClass = lookupClass;
1717                 } else {
1718                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
1719                 }
1720             }
1721             return callerClass;
1722         }
1723 
1724         private boolean hasPrivateAccess() {
1725             return (allowedModes & PRIVATE) != 0;
1726         }
1727 
1728         /**
1729          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
1730          * Determines a trustable caller class to compare with refc, the symbolic reference class.
1731          * If this lookup object has private access, then the caller class is the lookupClass.
1732          */
1733         void checkSecurityManager(Class<?> refc, MemberName m) {
1734             SecurityManager smgr = System.getSecurityManager();
1735             if (smgr == null)  return;
1736             if (allowedModes == TRUSTED)  return;
1737 
1738             // Step 1:
1739             boolean fullPowerLookup = hasPrivateAccess();
1740             if (!fullPowerLookup ||
1741                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
1742                 ReflectUtil.checkPackageAccess(refc);
1743             }
1744 
1745             if (m == null) {  // findClass or accessClass
1746                 // Step 2b:
1747                 if (!fullPowerLookup) {
1748                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
1749                 }
1750                 return;
1751             }
1752 
1753             // Step 2a:
1754             if (m.isPublic()) return;
1755             if (!fullPowerLookup) {
1756                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
1757             }
1758 
1759             // Step 3:
1760             Class<?> defc = m.getDeclaringClass();
1761             if (!fullPowerLookup && defc != refc) {
1762                 ReflectUtil.checkPackageAccess(defc);
1763             }
1764         }
1765 
1766         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1767             boolean wantStatic = (refKind == REF_invokeStatic);
1768             String message;
1769             if (m.isConstructor())
1770                 message = "expected a method, not a constructor";
1771             else if (!m.isMethod())
1772                 message = "expected a method";
1773             else if (wantStatic != m.isStatic())
1774                 message = wantStatic ? "expected a static method" : "expected a non-static method";
1775             else
1776                 { checkAccess(refKind, refc, m); return; }
1777             throw m.makeAccessException(message, this);
1778         }
1779 
1780         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1781             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
1782             String message;
1783             if (wantStatic != m.isStatic())
1784                 message = wantStatic ? "expected a static field" : "expected a non-static field";
1785             else
1786                 { checkAccess(refKind, refc, m); return; }
1787             throw m.makeAccessException(message, this);
1788         }
1789 
1790         /** Check public/protected/private bits on the symbolic reference class and its member. */
1791         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1792             assert(m.referenceKindIsConsistentWith(refKind) &&
1793                    MethodHandleNatives.refKindIsValid(refKind) &&
1794                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
1795             int allowedModes = this.allowedModes;
1796             if (allowedModes == TRUSTED)  return;
1797             int mods = m.getModifiers();
1798             if (Modifier.isProtected(mods) &&
1799                     refKind == REF_invokeVirtual &&
1800                     m.getDeclaringClass() == Object.class &&
1801                     m.getName().equals("clone") &&
1802                     refc.isArray()) {
1803                 // The JVM does this hack also.
1804                 // (See ClassVerifier::verify_invoke_instructions
1805                 // and LinkResolver::check_method_accessability.)
1806                 // Because the JVM does not allow separate methods on array types,
1807                 // there is no separate method for int[].clone.
1808                 // All arrays simply inherit Object.clone.
1809                 // But for access checking logic, we make Object.clone
1810                 // (normally protected) appear to be public.
1811                 // Later on, when the DirectMethodHandle is created,
1812                 // its leading argument will be restricted to the
1813                 // requested array type.
1814                 // N.B. The return type is not adjusted, because
1815                 // that is *not* the bytecode behavior.
1816                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
1817             }
1818             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
1819                 // cannot "new" a protected ctor in a different package
1820                 mods ^= Modifier.PROTECTED;
1821             }
1822             if (Modifier.isFinal(mods) &&
1823                     MethodHandleNatives.refKindIsSetter(refKind))
1824                 throw m.makeAccessException("unexpected set of a final field", this);
1825             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
1826             if ((requestedModes & allowedModes) != 0) {
1827                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
1828                                                     mods, lookupClass(), allowedModes))
1829                     return;
1830             } else {
1831                 // Protected members can also be checked as if they were package-private.
1832                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
1833                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
1834                     return;
1835             }
1836             throw m.makeAccessException(accessFailedMessage(refc, m), this);
1837         }
1838 
1839         String accessFailedMessage(Class<?> refc, MemberName m) {
1840             Class<?> defc = m.getDeclaringClass();
1841             int mods = m.getModifiers();
1842             // check the class first:
1843             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
1844                                (defc == refc ||
1845                                 Modifier.isPublic(refc.getModifiers())));
1846             if (!classOK && (allowedModes & PACKAGE) != 0) {
1847                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), ALL_MODES) &&
1848                            (defc == refc ||
1849                             VerifyAccess.isClassAccessible(refc, lookupClass(), ALL_MODES)));
1850             }
1851             if (!classOK)
1852                 return "class is not public";
1853             if (Modifier.isPublic(mods))
1854                 return "access to public member failed";  // (how?, module not readable?)
1855             if (Modifier.isPrivate(mods))
1856                 return "member is private";
1857             if (Modifier.isProtected(mods))
1858                 return "member is protected";
1859             return "member is private to package";
1860         }
1861 
1862         private static final boolean ALLOW_NESTMATE_ACCESS = false;
1863 
1864         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
1865             int allowedModes = this.allowedModes;
1866             if (allowedModes == TRUSTED)  return;
1867             if (!hasPrivateAccess()
1868                 || (specialCaller != lookupClass()
1869                        // ensure non-abstract methods in superinterfaces can be special-invoked
1870                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))
1871                     && !(ALLOW_NESTMATE_ACCESS &&
1872                          VerifyAccess.isSamePackageMember(specialCaller, lookupClass()))))
1873                 throw new MemberName(specialCaller).
1874                     makeAccessException("no private access for invokespecial", this);
1875         }
1876 
1877         private boolean restrictProtectedReceiver(MemberName method) {
1878             // The accessing class only has the right to use a protected member
1879             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
1880             if (!method.isProtected() || method.isStatic()
1881                 || allowedModes == TRUSTED
1882                 || method.getDeclaringClass() == lookupClass()
1883                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())
1884                 || (ALLOW_NESTMATE_ACCESS &&
1885                     VerifyAccess.isSamePackageMember(method.getDeclaringClass(), lookupClass())))
1886                 return false;
1887             return true;
1888         }
1889         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
1890             assert(!method.isStatic());
1891             // receiver type of mh is too wide; narrow to caller
1892             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
1893                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
1894             }
1895             MethodType rawType = mh.type();
1896             if (rawType.parameterType(0) == caller)  return mh;
1897             MethodType narrowType = rawType.changeParameterType(0, caller);
1898             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
1899             assert(mh.viewAsTypeChecks(narrowType, true));
1900             return mh.copyWith(narrowType, mh.form);
1901         }
1902 
1903         /** Check access and get the requested method. */
1904         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1905             final boolean doRestrict    = true;
1906             final boolean checkSecurity = true;
1907             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1908         }
1909         /** Check access and get the requested method, eliding receiver narrowing rules. */
1910         private MethodHandle getDirectMethodNoRestrict(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1911             final boolean doRestrict    = false;
1912             final boolean checkSecurity = true;
1913             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1914         }
1915         /** Check access and get the requested method, eliding security manager checks. */
1916         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1917             final boolean doRestrict    = true;
1918             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1919             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1920         }
1921         /** Common code for all methods; do not call directly except from immediately above. */
1922         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
1923                                                    boolean checkSecurity,
1924                                                    boolean doRestrict, Class<?> callerClass) throws IllegalAccessException {
1925             checkMethod(refKind, refc, method);
1926             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1927             if (checkSecurity)
1928                 checkSecurityManager(refc, method);
1929             assert(!method.isMethodHandleInvoke());
1930 
1931             if (refKind == REF_invokeSpecial &&
1932                 refc != lookupClass() &&
1933                 !refc.isInterface() &&
1934                 refc != lookupClass().getSuperclass() &&
1935                 refc.isAssignableFrom(lookupClass())) {
1936                 assert(!method.getName().equals("<init>"));  // not this code path
1937                 // Per JVMS 6.5, desc. of invokespecial instruction:
1938                 // If the method is in a superclass of the LC,
1939                 // and if our original search was above LC.super,
1940                 // repeat the search (symbolic lookup) from LC.super
1941                 // and continue with the direct superclass of that class,
1942                 // and so forth, until a match is found or no further superclasses exist.
1943                 // FIXME: MemberName.resolve should handle this instead.
1944                 Class<?> refcAsSuper = lookupClass();
1945                 MemberName m2;
1946                 do {
1947                     refcAsSuper = refcAsSuper.getSuperclass();
1948                     m2 = new MemberName(refcAsSuper,
1949                                         method.getName(),
1950                                         method.getMethodType(),
1951                                         REF_invokeSpecial);
1952                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
1953                 } while (m2 == null &&         // no method is found yet
1954                          refc != refcAsSuper); // search up to refc
1955                 if (m2 == null)  throw new InternalError(method.toString());
1956                 method = m2;
1957                 refc = refcAsSuper;
1958                 // redo basic checks
1959                 checkMethod(refKind, refc, method);
1960             }
1961 
1962             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method);
1963             MethodHandle mh = dmh;
1964             // Optionally narrow the receiver argument to refc using restrictReceiver.
1965             if (doRestrict &&
1966                    (refKind == REF_invokeSpecial ||
1967                        (MethodHandleNatives.refKindHasReceiver(refKind) &&
1968                            restrictProtectedReceiver(method)))) {
1969                 mh = restrictReceiver(method, dmh, lookupClass());
1970             }
1971             mh = maybeBindCaller(method, mh, callerClass);
1972             mh = mh.setVarargs(method);
1973             return mh;
1974         }
1975         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
1976                                              Class<?> callerClass)
1977                                              throws IllegalAccessException {
1978             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
1979                 return mh;
1980             Class<?> hostClass = lookupClass;
1981             if (!hasPrivateAccess())  // caller must have private access
1982                 hostClass = callerClass;  // callerClass came from a security manager style stack walk
1983             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
1984             // Note: caller will apply varargs after this step happens.
1985             return cbmh;
1986         }
1987         /** Check access and get the requested field. */
1988         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1989             final boolean checkSecurity = true;
1990             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1991         }
1992         /** Check access and get the requested field, eliding security manager checks. */
1993         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1994             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1995             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1996         }
1997         /** Common code for all fields; do not call directly except from immediately above. */
1998         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
1999                                                   boolean checkSecurity) throws IllegalAccessException {
2000             checkField(refKind, refc, field);
2001             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2002             if (checkSecurity)
2003                 checkSecurityManager(refc, field);
2004             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
2005             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
2006                                     restrictProtectedReceiver(field));
2007             if (doRestrict)
2008                 return restrictReceiver(field, dmh, lookupClass());
2009             return dmh;
2010         }
2011         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
2012                                             Class<?> refc, MemberName getField, MemberName putField)
2013                 throws IllegalAccessException {
2014             final boolean checkSecurity = true;
2015             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2016         }
2017         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
2018                                                              Class<?> refc, MemberName getField, MemberName putField)
2019                 throws IllegalAccessException {
2020             final boolean checkSecurity = false;
2021             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2022         }
2023         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
2024                                                   Class<?> refc, MemberName getField, MemberName putField,
2025                                                   boolean checkSecurity) throws IllegalAccessException {
2026             assert getField.isStatic() == putField.isStatic();
2027             assert getField.isGetter() && putField.isSetter();
2028             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
2029             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
2030 
2031             checkField(getRefKind, refc, getField);
2032             if (checkSecurity)
2033                 checkSecurityManager(refc, getField);
2034 
2035             if (!putField.isFinal()) {
2036                 // A VarHandle does not support updates to final fields, any
2037                 // such VarHandle to a final field will be read-only and
2038                 // therefore the following write-based accessibility checks are
2039                 // only required for non-final fields
2040                 checkField(putRefKind, refc, putField);
2041                 if (checkSecurity)
2042                     checkSecurityManager(refc, putField);
2043             }
2044 
2045             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
2046                                   restrictProtectedReceiver(getField));
2047             if (doRestrict) {
2048                 assert !getField.isStatic();
2049                 // receiver type of VarHandle is too wide; narrow to caller
2050                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
2051                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
2052                 }
2053                 refc = lookupClass();
2054             }
2055             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED);
2056         }
2057         /** Check access and get the requested constructor. */
2058         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2059             final boolean checkSecurity = true;
2060             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2061         }
2062         /** Check access and get the requested constructor, eliding security manager checks. */
2063         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2064             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2065             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2066         }
2067         /** Common code for all constructors; do not call directly except from immediately above. */
2068         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
2069                                                   boolean checkSecurity) throws IllegalAccessException {
2070             assert(ctor.isConstructor());
2071             checkAccess(REF_newInvokeSpecial, refc, ctor);
2072             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2073             if (checkSecurity)
2074                 checkSecurityManager(refc, ctor);
2075             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
2076             return DirectMethodHandle.make(ctor).setVarargs(ctor);
2077         }
2078 
2079         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
2080          */
2081         /*non-public*/
2082         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
2083             if (!(type instanceof Class || type instanceof MethodType))
2084                 throw new InternalError("unresolved MemberName");
2085             MemberName member = new MemberName(refKind, defc, name, type);
2086             MethodHandle mh = LOOKASIDE_TABLE.get(member);
2087             if (mh != null) {
2088                 checkSymbolicClass(defc);
2089                 return mh;
2090             }
2091             // Treat MethodHandle.invoke and invokeExact specially.
2092             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
2093                 mh = findVirtualForMH(member.getName(), member.getMethodType());
2094                 if (mh != null) {
2095                     return mh;
2096                 }
2097             }
2098             MemberName resolved = resolveOrFail(refKind, member);
2099             mh = getDirectMethodForConstant(refKind, defc, resolved);
2100             if (mh instanceof DirectMethodHandle
2101                     && canBeCached(refKind, defc, resolved)) {
2102                 MemberName key = mh.internalMemberName();
2103                 if (key != null) {
2104                     key = key.asNormalOriginal();
2105                 }
2106                 if (member.equals(key)) {  // better safe than sorry
2107                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
2108                 }
2109             }
2110             return mh;
2111         }
2112         private
2113         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
2114             if (refKind == REF_invokeSpecial) {
2115                 return false;
2116             }
2117             if (!Modifier.isPublic(defc.getModifiers()) ||
2118                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
2119                     !member.isPublic() ||
2120                     member.isCallerSensitive()) {
2121                 return false;
2122             }
2123             ClassLoader loader = defc.getClassLoader();
2124             if (!jdk.internal.misc.VM.isSystemDomainLoader(loader)) {
2125                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
2126                 boolean found = false;
2127                 while (sysl != null) {
2128                     if (loader == sysl) { found = true; break; }
2129                     sysl = sysl.getParent();
2130                 }
2131                 if (!found) {
2132                     return false;
2133                 }
2134             }
2135             try {
2136                 MemberName resolved2 = publicLookup().resolveOrFail(refKind,
2137                     new MemberName(refKind, defc, member.getName(), member.getType()));
2138                 checkSecurityManager(defc, resolved2);
2139             } catch (ReflectiveOperationException | SecurityException ex) {
2140                 return false;
2141             }
2142             return true;
2143         }
2144         private
2145         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
2146                 throws ReflectiveOperationException {
2147             if (MethodHandleNatives.refKindIsField(refKind)) {
2148                 return getDirectFieldNoSecurityManager(refKind, defc, member);
2149             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
2150                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
2151             } else if (refKind == REF_newInvokeSpecial) {
2152                 return getDirectConstructorNoSecurityManager(defc, member);
2153             }
2154             // oops
2155             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
2156         }
2157 
2158         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
2159     }
2160 
2161     /**
2162      * Helper class used to lazily create PUBLIC_LOOKUP with a lookup class
2163      * in an <em>unnamed module</em>.
2164      *
2165      * @see Lookup#publicLookup
2166      */
2167     private static class LookupHelper {
2168         private static final String UNNAMED = "Unnamed";
2169         private static final String OBJECT  = "java/lang/Object";
2170 
2171         private static Class<?> createClass() {
2172             try {
2173                 ClassWriter cw = new ClassWriter(0);
2174                 cw.visit(Opcodes.V1_8,
2175                          Opcodes.ACC_FINAL + Opcodes.ACC_SUPER,
2176                          UNNAMED,
2177                          null,
2178                          OBJECT,
2179                          null);
2180                 cw.visitSource(UNNAMED, null);
2181                 cw.visitEnd();
2182                 byte[] bytes = cw.toByteArray();
2183                 ClassLoader loader = new ClassLoader(null) {
2184                     @Override
2185                     protected Class<?> findClass(String cn) throws ClassNotFoundException {
2186                         if (cn.equals(UNNAMED))
2187                             return super.defineClass(UNNAMED, bytes, 0, bytes.length);
2188                         throw new ClassNotFoundException(cn);
2189                     }
2190                 };
2191                 return loader.loadClass(UNNAMED);
2192             } catch (Exception e) {
2193                 throw new InternalError(e);
2194             }
2195         }
2196 
2197         private static final Class<?> PUBLIC_LOOKUP_CLASS;
2198         static {
2199             PrivilegedAction<Class<?>> pa = new PrivilegedAction<Class<?>>() {
2200                 public Class<?> run() {
2201                     return createClass();
2202                 }
2203             };
2204             PUBLIC_LOOKUP_CLASS = AccessController.doPrivileged(pa);
2205         }
2206 
2207         /**
2208          * Lookup that is trusted minimally. It can only be used to create
2209          * method handles to publicly accessible members in exported packages.
2210          *
2211          * @see MethodHandles#publicLookup
2212          */
2213         static final Lookup PUBLIC_LOOKUP = new Lookup(PUBLIC_LOOKUP_CLASS, Lookup.PUBLIC);
2214     }
2215 
2216     /**
2217      * Produces a method handle giving read access to elements of an array.
2218      * The type of the method handle will have a return type of the array's
2219      * element type.  Its first argument will be the array type,
2220      * and the second will be {@code int}.
2221      * @param arrayClass an array type
2222      * @return a method handle which can load values from the given array type
2223      * @throws NullPointerException if the argument is null
2224      * @throws  IllegalArgumentException if arrayClass is not an array type
2225      */
2226     public static
2227     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
2228         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, false);
2229     }
2230 
2231     /**
2232      * Produces a method handle giving write access to elements of an array.
2233      * The type of the method handle will have a void return type.
2234      * Its last argument will be the array's element type.
2235      * The first and second arguments will be the array type and int.
2236      * @param arrayClass the class of an array
2237      * @return a method handle which can store values into the array type
2238      * @throws NullPointerException if the argument is null
2239      * @throws IllegalArgumentException if arrayClass is not an array type
2240      */
2241     public static
2242     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
2243         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, true);
2244     }
2245 
2246     /**
2247      *
2248      * Produces a VarHandle giving access to elements of an array type
2249      * {@code T[]}, supporting shape {@code (T[], int : T)}.
2250      * <p>
2251      * Certain access modes of the returned VarHandle are unsupported under
2252      * the following conditions:
2253      * <ul>
2254      * <li>if the component type is anything other than {@code int},
2255      *     {@code long} or a reference type, then atomic update access modes
2256      *     are unsupported.  (Future major platform releases of the JDK may
2257      *     support additional types for certain currently unsupported access
2258      *     modes.)
2259      * <li>if the component type is anything other than {@code int} or
2260      *     {@code long}, then numeric atomic update access modes are
2261      *     unsupported.  (Future major platform releases of the JDK may
2262      *     support additional numeric types for certain currently
2263      *     unsupported access modes.)
2264      * </ul>
2265      * @param arrayClass the class of an array, of type {@code T[]}
2266      * @return a VarHandle giving access to elements of an array
2267      * @throws NullPointerException if the arrayClass is null
2268      * @throws IllegalArgumentException if arrayClass is not an array type
2269      * @since 9
2270      */
2271     public static
2272     VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
2273         return VarHandles.makeArrayElementHandle(arrayClass);
2274     }
2275 
2276     /**
2277      * Produces a VarHandle giving access to elements of a {@code byte[]} array
2278      * viewed as if it were a different primitive array type, such as
2279      * {@code int[]} or {@code long[]}.  The shape of the resulting VarHandle is
2280      * {@code (byte[], int : T)}, where the {@code int} coordinate type
2281      * corresponds to an argument that is an index in a {@code byte[]} array,
2282      * and {@code T} is the component type of the given view array class.  The
2283      * returned VarHandle accesses bytes at an index in a {@code byte[]} array,
2284      * composing bytes to or from a value of {@code T} according to the given
2285      * endianness.
2286      * <p>
2287      * The supported component types (variables types) are {@code short},
2288      * {@code char}, {@code int}, {@code long}, {@code float} and
2289      * {@code double}.
2290      * <p>
2291      * Access of bytes at a given index will result in an
2292      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2293      * or greater than the {@code byte[]} array length minus the size (in bytes)
2294      * of {@code T}.
2295      * <p>
2296      * Access of bytes at an index may be aligned or misaligned for {@code T},
2297      * with respect to the underlying memory address, {@code A} say, associated
2298      * with the array and index.
2299      * If access is misaligned then access for anything other than the
2300      * {@code get} and {@code set} access modes will result in an
2301      * {@code IllegalStateException}.  In such cases atomic access is only
2302      * guaranteed with respect to the largest power of two that divides the GCD
2303      * of {@code A} and the size (in bytes) of {@code T}.
2304      * If access is aligned then following access modes are supported and are
2305      * guaranteed to support atomic access:
2306      * <ul>
2307      * <li>read write access modes for all {@code T};
2308      * <li>atomic update access modes for {@code int}, {@code long},
2309      *     {@code float} or {@code double}.
2310      *     (Future major platform releases of the JDK may support additional
2311      *     types for certain currently unsupported access modes.)
2312      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2313      *     (Future major platform releases of the JDK may support additional
2314      *     numeric types for certain currently unsupported access modes.)
2315      * </ul>
2316      * <p>
2317      * Misaligned access, and therefore atomicity guarantees, may be determined
2318      * for {@code byte[]} arrays without operating on a specific array.  Given
2319      * an {@code index}, {@code T} and it's corresponding boxed type,
2320      * {@code T_BOX}, misalignment may be determined as follows:
2321      * <pre>{@code
2322      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2323      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
2324      *     alignmentOffset(0, sizeOfT);
2325      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
2326      * boolean isMisaligned = misalignedAtIndex != 0;
2327      * }</pre>
2328      *
2329      * @implNote
2330      * The variable types {@code float} and {@code double} are supported as if
2331      * by transformation to and access with the variable types {@code int} and
2332      * {@code long} respectively.  For example, the transformation of a
2333      * {@code double} value to a long value is performed as if using
2334      * {@link Double#doubleToRawLongBits(double)}, and the reverse
2335      * transformation is performed as if using
2336      * {@link Double#longBitsToDouble(long)}.
2337      *
2338      * @param viewArrayClass the view array class, with a component type of
2339      * type {@code T}
2340      * @param bigEndian true if the endianness of the view array elements, as
2341      * stored in the underlying {@code byte} array, is big endian, otherwise
2342      * little endian
2343      * @return a VarHandle giving access to elements of a {@code byte[]} array
2344      * viewed as if elements corresponding to the components type of the view
2345      * array class
2346      * @throws NullPointerException if viewArrayClass is null
2347      * @throws IllegalArgumentException if viewArrayClass is not an array type
2348      * @throws UnsupportedOperationException if the component type of
2349      * viewArrayClass is not supported as a variable type
2350      * @since 9
2351      */
2352     public static
2353     VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
2354                                      boolean bigEndian) throws IllegalArgumentException {
2355         return VarHandles.byteArrayViewHandle(viewArrayClass, bigEndian);
2356     }
2357 
2358     /**
2359      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
2360      * viewed as if it were an array of elements of a different primitive
2361      * component type to that of {@code byte}, such as {@code int[]} or
2362      * {@code long[]}.  The shape of the resulting VarHandle is
2363      * {@code (ByteBuffer, int : T)}, where the {@code int} coordinate type
2364      * corresponds to an argument that is an index in a {@code ByteBuffer}, and
2365      * {@code T} is the component type of the given view array class.  The
2366      * returned VarHandle accesses bytes at an index in a {@code ByteBuffer},
2367      * composing bytes to or from a value of {@code T} according to the given
2368      * endianness.
2369      * <p>
2370      * The supported component types (variables types) are {@code short},
2371      * {@code char}, {@code int}, {@code long}, {@code float} and
2372      * {@code double}.
2373      * <p>
2374      * Access will result in a {@code ReadOnlyBufferException} for anything
2375      * other than the read access modes if the {@code ByteBuffer} is read-only.
2376      * <p>
2377      * Access of bytes at a given index will result in an
2378      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2379      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
2380      * {@code T}.
2381      * <p>
2382      * Access of bytes at an index may be aligned or misaligned for {@code T},
2383      * with respect to the underlying memory address, {@code A} say, associated
2384      * with the {@code ByteBuffer} and index.
2385      * If access is misaligned then access for anything other than the
2386      * {@code get} and {@code set} access modes will result in an
2387      * {@code IllegalStateException}.  In such cases atomic access is only
2388      * guaranteed with respect to the largest power of two that divides the GCD
2389      * of {@code A} and the size (in bytes) of {@code T}.
2390      * If access is aligned then following access modes are supported and are
2391      * guaranteed to support atomic access:
2392      * <ul>
2393      * <li>read write access modes for all {@code T};
2394      * <li>atomic update access modes for {@code int}, {@code long},
2395      *     {@code float} or {@code double}.
2396      *     (Future major platform releases of the JDK may support additional
2397      *     types for certain currently unsupported access modes.)
2398      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2399      *     (Future major platform releases of the JDK may support additional
2400      *     numeric types for certain currently unsupported access modes.)
2401      * </ul>
2402      * <p>
2403      * Misaligned access, and therefore atomicity guarantees, may be determined
2404      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
2405      * {@code index}, {@code T} and it's corresponding boxed type,
2406      * {@code T_BOX}, as follows:
2407      * <pre>{@code
2408      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2409      * ByteBuffer bb = ...
2410      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
2411      * boolean isMisaligned = misalignedAtIndex != 0;
2412      * }</pre>
2413      *
2414      * @implNote
2415      * The variable types {@code float} and {@code double} are supported as if
2416      * by transformation to and access with the variable types {@code int} and
2417      * {@code long} respectively.  For example, the transformation of a
2418      * {@code double} value to a long value is performed as if using
2419      * {@link Double#doubleToRawLongBits(double)}, and the reverse
2420      * transformation is performed as if using
2421      * {@link Double#longBitsToDouble(long)}.
2422      *
2423      * @param viewArrayClass the view array class, with a component type of
2424      * type {@code T}
2425      * @param bigEndian true if the endianness of the view array elements, as
2426      * stored in the underlying {@code ByteBuffer}, is big endian, otherwise
2427      * little endian (Note this overrides the endianness of a
2428      * {@code ByteBuffer})
2429      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
2430      * viewed as if elements corresponding to the components type of the view
2431      * array class
2432      * @throws NullPointerException if viewArrayClass is null
2433      * @throws IllegalArgumentException if viewArrayClass is not an array type
2434      * @throws UnsupportedOperationException if the component type of
2435      * viewArrayClass is not supported as a variable type
2436      * @since 9
2437      */
2438     public static
2439     VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
2440                                       boolean bigEndian) throws IllegalArgumentException {
2441         return VarHandles.makeByteBufferViewHandle(viewArrayClass, bigEndian);
2442     }
2443 
2444 
2445     /// method handle invocation (reflective style)
2446 
2447     /**
2448      * Produces a method handle which will invoke any method handle of the
2449      * given {@code type}, with a given number of trailing arguments replaced by
2450      * a single trailing {@code Object[]} array.
2451      * The resulting invoker will be a method handle with the following
2452      * arguments:
2453      * <ul>
2454      * <li>a single {@code MethodHandle} target
2455      * <li>zero or more leading values (counted by {@code leadingArgCount})
2456      * <li>an {@code Object[]} array containing trailing arguments
2457      * </ul>
2458      * <p>
2459      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
2460      * the indicated {@code type}.
2461      * That is, if the target is exactly of the given {@code type}, it will behave
2462      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
2463      * is used to convert the target to the required {@code type}.
2464      * <p>
2465      * The type of the returned invoker will not be the given {@code type}, but rather
2466      * will have all parameters except the first {@code leadingArgCount}
2467      * replaced by a single array of type {@code Object[]}, which will be
2468      * the final parameter.
2469      * <p>
2470      * Before invoking its target, the invoker will spread the final array, apply
2471      * reference casts as necessary, and unbox and widen primitive arguments.
2472      * If, when the invoker is called, the supplied array argument does
2473      * not have the correct number of elements, the invoker will throw
2474      * an {@link IllegalArgumentException} instead of invoking the target.
2475      * <p>
2476      * This method is equivalent to the following code (though it may be more efficient):
2477      * <blockquote><pre>{@code
2478 MethodHandle invoker = MethodHandles.invoker(type);
2479 int spreadArgCount = type.parameterCount() - leadingArgCount;
2480 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
2481 return invoker;
2482      * }</pre></blockquote>
2483      * This method throws no reflective or security exceptions.
2484      * @param type the desired target type
2485      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
2486      * @return a method handle suitable for invoking any method handle of the given type
2487      * @throws NullPointerException if {@code type} is null
2488      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
2489      *                  the range from 0 to {@code type.parameterCount()} inclusive,
2490      *                  or if the resulting method handle's type would have
2491      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2492      */
2493     public static
2494     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
2495         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
2496             throw newIllegalArgumentException("bad argument count", leadingArgCount);
2497         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
2498         return type.invokers().spreadInvoker(leadingArgCount);
2499     }
2500 
2501     /**
2502      * Produces a special <em>invoker method handle</em> which can be used to
2503      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
2504      * The resulting invoker will have a type which is
2505      * exactly equal to the desired type, except that it will accept
2506      * an additional leading argument of type {@code MethodHandle}.
2507      * <p>
2508      * This method is equivalent to the following code (though it may be more efficient):
2509      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
2510      *
2511      * <p style="font-size:smaller;">
2512      * <em>Discussion:</em>
2513      * Invoker method handles can be useful when working with variable method handles
2514      * of unknown types.
2515      * For example, to emulate an {@code invokeExact} call to a variable method
2516      * handle {@code M}, extract its type {@code T},
2517      * look up the invoker method {@code X} for {@code T},
2518      * and call the invoker method, as {@code X.invoke(T, A...)}.
2519      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
2520      * is unknown.)
2521      * If spreading, collecting, or other argument transformations are required,
2522      * they can be applied once to the invoker {@code X} and reused on many {@code M}
2523      * method handle values, as long as they are compatible with the type of {@code X}.
2524      * <p style="font-size:smaller;">
2525      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2526      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2527      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2528      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2529      * <p>
2530      * This method throws no reflective or security exceptions.
2531      * @param type the desired target type
2532      * @return a method handle suitable for invoking any method handle of the given type
2533      * @throws IllegalArgumentException if the resulting method handle's type would have
2534      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2535      */
2536     public static
2537     MethodHandle exactInvoker(MethodType type) {
2538         return type.invokers().exactInvoker();
2539     }
2540 
2541     /**
2542      * Produces a special <em>invoker method handle</em> which can be used to
2543      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
2544      * The resulting invoker will have a type which is
2545      * exactly equal to the desired type, except that it will accept
2546      * an additional leading argument of type {@code MethodHandle}.
2547      * <p>
2548      * Before invoking its target, if the target differs from the expected type,
2549      * the invoker will apply reference casts as
2550      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
2551      * Similarly, the return value will be converted as necessary.
2552      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
2553      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
2554      * <p>
2555      * This method is equivalent to the following code (though it may be more efficient):
2556      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
2557      * <p style="font-size:smaller;">
2558      * <em>Discussion:</em>
2559      * A {@linkplain MethodType#genericMethodType general method type} is one which
2560      * mentions only {@code Object} arguments and return values.
2561      * An invoker for such a type is capable of calling any method handle
2562      * of the same arity as the general type.
2563      * <p style="font-size:smaller;">
2564      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2565      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2566      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2567      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2568      * <p>
2569      * This method throws no reflective or security exceptions.
2570      * @param type the desired target type
2571      * @return a method handle suitable for invoking any method handle convertible to the given type
2572      * @throws IllegalArgumentException if the resulting method handle's type would have
2573      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2574      */
2575     public static
2576     MethodHandle invoker(MethodType type) {
2577         return type.invokers().genericInvoker();
2578     }
2579 
2580     /**
2581      * Produces a special <em>invoker method handle</em> which can be used to
2582      * invoke a signature-polymorphic access mode method on any VarHandle whose
2583      * associated access mode type is compatible with the given type.
2584      * The resulting invoker will have a type which is exactly equal to the
2585      * desired given type, except that it will accept an additional leading
2586      * argument of type {@code VarHandle}.
2587      *
2588      * @param accessMode the VarHandle access mode
2589      * @param type the desired target type
2590      * @return a method handle suitable for invoking an access mode method of
2591      *         any VarHandle whose access mode type is of the given type.
2592      * @since 9
2593      */
2594     static public
2595     MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
2596         return type.invokers().varHandleMethodExactInvoker(accessMode);
2597     }
2598 
2599     /**
2600      * Produces a special <em>invoker method handle</em> which can be used to
2601      * invoke a signature-polymorphic access mode method on any VarHandle whose
2602      * associated access mode type is compatible with the given type.
2603      * The resulting invoker will have a type which is exactly equal to the
2604      * desired given type, except that it will accept an additional leading
2605      * argument of type {@code VarHandle}.
2606      * <p>
2607      * Before invoking its target, if the access mode type differs from the
2608      * desired given type, the invoker will apply reference casts as necessary
2609      * and box, unbox, or widen primitive values, as if by
2610      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
2611      * converted as necessary.
2612      * <p>
2613      * This method is equivalent to the following code (though it may be more
2614      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
2615      *
2616      * @param accessMode the VarHandle access mode
2617      * @param type the desired target type
2618      * @return a method handle suitable for invoking an access mode method of
2619      *         any VarHandle whose access mode type is convertible to the given
2620      *         type.
2621      * @since 9
2622      */
2623     static public
2624     MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
2625         return type.invokers().varHandleMethodInvoker(accessMode);
2626     }
2627 
2628     static /*non-public*/
2629     MethodHandle basicInvoker(MethodType type) {
2630         return type.invokers().basicInvoker();
2631     }
2632 
2633      /// method handle modification (creation from other method handles)
2634 
2635     /**
2636      * Produces a method handle which adapts the type of the
2637      * given method handle to a new type by pairwise argument and return type conversion.
2638      * The original type and new type must have the same number of arguments.
2639      * The resulting method handle is guaranteed to report a type
2640      * which is equal to the desired new type.
2641      * <p>
2642      * If the original type and new type are equal, returns target.
2643      * <p>
2644      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
2645      * and some additional conversions are also applied if those conversions fail.
2646      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
2647      * if possible, before or instead of any conversions done by {@code asType}:
2648      * <ul>
2649      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
2650      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
2651      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
2652      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
2653      *     the boolean is converted to a byte value, 1 for true, 0 for false.
2654      *     (This treatment follows the usage of the bytecode verifier.)
2655      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
2656      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
2657      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
2658      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
2659      *     then a Java casting conversion (JLS 5.5) is applied.
2660      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
2661      *     widening and/or narrowing.)
2662      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
2663      *     conversion will be applied at runtime, possibly followed
2664      *     by a Java casting conversion (JLS 5.5) on the primitive value,
2665      *     possibly followed by a conversion from byte to boolean by testing
2666      *     the low-order bit.
2667      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
2668      *     and if the reference is null at runtime, a zero value is introduced.
2669      * </ul>
2670      * @param target the method handle to invoke after arguments are retyped
2671      * @param newType the expected type of the new method handle
2672      * @return a method handle which delegates to the target after performing
2673      *           any necessary argument conversions, and arranges for any
2674      *           necessary return value conversions
2675      * @throws NullPointerException if either argument is null
2676      * @throws WrongMethodTypeException if the conversion cannot be made
2677      * @see MethodHandle#asType
2678      */
2679     public static
2680     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
2681         explicitCastArgumentsChecks(target, newType);
2682         // use the asTypeCache when possible:
2683         MethodType oldType = target.type();
2684         if (oldType == newType)  return target;
2685         if (oldType.explicitCastEquivalentToAsType(newType)) {
2686             return target.asFixedArity().asType(newType);
2687         }
2688         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
2689     }
2690 
2691     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
2692         if (target.type().parameterCount() != newType.parameterCount()) {
2693             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
2694         }
2695     }
2696 
2697     /**
2698      * Produces a method handle which adapts the calling sequence of the
2699      * given method handle to a new type, by reordering the arguments.
2700      * The resulting method handle is guaranteed to report a type
2701      * which is equal to the desired new type.
2702      * <p>
2703      * The given array controls the reordering.
2704      * Call {@code #I} the number of incoming parameters (the value
2705      * {@code newType.parameterCount()}, and call {@code #O} the number
2706      * of outgoing parameters (the value {@code target.type().parameterCount()}).
2707      * Then the length of the reordering array must be {@code #O},
2708      * and each element must be a non-negative number less than {@code #I}.
2709      * For every {@code N} less than {@code #O}, the {@code N}-th
2710      * outgoing argument will be taken from the {@code I}-th incoming
2711      * argument, where {@code I} is {@code reorder[N]}.
2712      * <p>
2713      * No argument or return value conversions are applied.
2714      * The type of each incoming argument, as determined by {@code newType},
2715      * must be identical to the type of the corresponding outgoing parameter
2716      * or parameters in the target method handle.
2717      * The return type of {@code newType} must be identical to the return
2718      * type of the original target.
2719      * <p>
2720      * The reordering array need not specify an actual permutation.
2721      * An incoming argument will be duplicated if its index appears
2722      * more than once in the array, and an incoming argument will be dropped
2723      * if its index does not appear in the array.
2724      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
2725      * incoming arguments which are not mentioned in the reordering array
2726      * may be of any type, as determined only by {@code newType}.
2727      * <blockquote><pre>{@code
2728 import static java.lang.invoke.MethodHandles.*;
2729 import static java.lang.invoke.MethodType.*;
2730 ...
2731 MethodType intfn1 = methodType(int.class, int.class);
2732 MethodType intfn2 = methodType(int.class, int.class, int.class);
2733 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
2734 assert(sub.type().equals(intfn2));
2735 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
2736 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
2737 assert((int)rsub.invokeExact(1, 100) == 99);
2738 MethodHandle add = ... (int x, int y) -> (x+y) ...;
2739 assert(add.type().equals(intfn2));
2740 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
2741 assert(twice.type().equals(intfn1));
2742 assert((int)twice.invokeExact(21) == 42);
2743      * }</pre></blockquote>
2744      * <p>
2745      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
2746      * variable-arity method handle}, even if the original target method handle was.
2747      * @param target the method handle to invoke after arguments are reordered
2748      * @param newType the expected type of the new method handle
2749      * @param reorder an index array which controls the reordering
2750      * @return a method handle which delegates to the target after it
2751      *           drops unused arguments and moves and/or duplicates the other arguments
2752      * @throws NullPointerException if any argument is null
2753      * @throws IllegalArgumentException if the index array length is not equal to
2754      *                  the arity of the target, or if any index array element
2755      *                  not a valid index for a parameter of {@code newType},
2756      *                  or if two corresponding parameter types in
2757      *                  {@code target.type()} and {@code newType} are not identical,
2758      */
2759     public static
2760     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
2761         reorder = reorder.clone();  // get a private copy
2762         MethodType oldType = target.type();
2763         permuteArgumentChecks(reorder, newType, oldType);
2764         // first detect dropped arguments and handle them separately
2765         int[] originalReorder = reorder;
2766         BoundMethodHandle result = target.rebind();
2767         LambdaForm form = result.form;
2768         int newArity = newType.parameterCount();
2769         // Normalize the reordering into a real permutation,
2770         // by removing duplicates and adding dropped elements.
2771         // This somewhat improves lambda form caching, as well
2772         // as simplifying the transform by breaking it up into steps.
2773         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
2774             if (ddIdx > 0) {
2775                 // We found a duplicated entry at reorder[ddIdx].
2776                 // Example:  (x,y,z)->asList(x,y,z)
2777                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
2778                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
2779                 // The starred element corresponds to the argument
2780                 // deleted by the dupArgumentForm transform.
2781                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
2782                 boolean killFirst = false;
2783                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
2784                     // Set killFirst if the dup is larger than an intervening position.
2785                     // This will remove at least one inversion from the permutation.
2786                     if (dupVal > val) killFirst = true;
2787                 }
2788                 if (!killFirst) {
2789                     srcPos = dstPos;
2790                     dstPos = ddIdx;
2791                 }
2792                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
2793                 assert (reorder[srcPos] == reorder[dstPos]);
2794                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
2795                 // contract the reordering by removing the element at dstPos
2796                 int tailPos = dstPos + 1;
2797                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
2798                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
2799             } else {
2800                 int dropVal = ~ddIdx, insPos = 0;
2801                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
2802                     // Find first element of reorder larger than dropVal.
2803                     // This is where we will insert the dropVal.
2804                     insPos += 1;
2805                 }
2806                 Class<?> ptype = newType.parameterType(dropVal);
2807                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
2808                 oldType = oldType.insertParameterTypes(insPos, ptype);
2809                 // expand the reordering by inserting an element at insPos
2810                 int tailPos = insPos + 1;
2811                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
2812                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
2813                 reorder[insPos] = dropVal;
2814             }
2815             assert (permuteArgumentChecks(reorder, newType, oldType));
2816         }
2817         assert (reorder.length == newArity);  // a perfect permutation
2818         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
2819         form = form.editor().permuteArgumentsForm(1, reorder);
2820         if (newType == result.type() && form == result.internalForm())
2821             return result;
2822         return result.copyWith(newType, form);
2823     }
2824 
2825     /**
2826      * Return an indication of any duplicate or omission in reorder.
2827      * If the reorder contains a duplicate entry, return the index of the second occurrence.
2828      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
2829      * Otherwise, return zero.
2830      * If an element not in [0..newArity-1] is encountered, return reorder.length.
2831      */
2832     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
2833         final int BIT_LIMIT = 63;  // max number of bits in bit mask
2834         if (newArity < BIT_LIMIT) {
2835             long mask = 0;
2836             for (int i = 0; i < reorder.length; i++) {
2837                 int arg = reorder[i];
2838                 if (arg >= newArity) {
2839                     return reorder.length;
2840                 }
2841                 long bit = 1L << arg;
2842                 if ((mask & bit) != 0) {
2843                     return i;  // >0 indicates a dup
2844                 }
2845                 mask |= bit;
2846             }
2847             if (mask == (1L << newArity) - 1) {
2848                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
2849                 return 0;
2850             }
2851             // find first zero
2852             long zeroBit = Long.lowestOneBit(~mask);
2853             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
2854             assert(zeroPos <= newArity);
2855             if (zeroPos == newArity) {
2856                 return 0;
2857             }
2858             return ~zeroPos;
2859         } else {
2860             // same algorithm, different bit set
2861             BitSet mask = new BitSet(newArity);
2862             for (int i = 0; i < reorder.length; i++) {
2863                 int arg = reorder[i];
2864                 if (arg >= newArity) {
2865                     return reorder.length;
2866                 }
2867                 if (mask.get(arg)) {
2868                     return i;  // >0 indicates a dup
2869                 }
2870                 mask.set(arg);
2871             }
2872             int zeroPos = mask.nextClearBit(0);
2873             assert(zeroPos <= newArity);
2874             if (zeroPos == newArity) {
2875                 return 0;
2876             }
2877             return ~zeroPos;
2878         }
2879     }
2880 
2881     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
2882         if (newType.returnType() != oldType.returnType())
2883             throw newIllegalArgumentException("return types do not match",
2884                     oldType, newType);
2885         if (reorder.length == oldType.parameterCount()) {
2886             int limit = newType.parameterCount();
2887             boolean bad = false;
2888             for (int j = 0; j < reorder.length; j++) {
2889                 int i = reorder[j];
2890                 if (i < 0 || i >= limit) {
2891                     bad = true; break;
2892                 }
2893                 Class<?> src = newType.parameterType(i);
2894                 Class<?> dst = oldType.parameterType(j);
2895                 if (src != dst)
2896                     throw newIllegalArgumentException("parameter types do not match after reorder",
2897                             oldType, newType);
2898             }
2899             if (!bad)  return true;
2900         }
2901         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
2902     }
2903 
2904     /**
2905      * Produces a method handle of the requested return type which returns the given
2906      * constant value every time it is invoked.
2907      * <p>
2908      * Before the method handle is returned, the passed-in value is converted to the requested type.
2909      * If the requested type is primitive, widening primitive conversions are attempted,
2910      * else reference conversions are attempted.
2911      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
2912      * @param type the return type of the desired method handle
2913      * @param value the value to return
2914      * @return a method handle of the given return type and no arguments, which always returns the given value
2915      * @throws NullPointerException if the {@code type} argument is null
2916      * @throws ClassCastException if the value cannot be converted to the required return type
2917      * @throws IllegalArgumentException if the given type is {@code void.class}
2918      */
2919     public static
2920     MethodHandle constant(Class<?> type, Object value) {
2921         if (type.isPrimitive()) {
2922             if (type == void.class)
2923                 throw newIllegalArgumentException("void type");
2924             Wrapper w = Wrapper.forPrimitiveType(type);
2925             value = w.convert(value, type);
2926             if (w.zero().equals(value))
2927                 return zero(w, type);
2928             return insertArguments(identity(type), 0, value);
2929         } else {
2930             if (value == null)
2931                 return zero(Wrapper.OBJECT, type);
2932             return identity(type).bindTo(value);
2933         }
2934     }
2935 
2936     /**
2937      * Produces a method handle which returns its sole argument when invoked.
2938      * @param type the type of the sole parameter and return value of the desired method handle
2939      * @return a unary method handle which accepts and returns the given type
2940      * @throws NullPointerException if the argument is null
2941      * @throws IllegalArgumentException if the given type is {@code void.class}
2942      */
2943     public static
2944     MethodHandle identity(Class<?> type) {
2945         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
2946         int pos = btw.ordinal();
2947         MethodHandle ident = IDENTITY_MHS[pos];
2948         if (ident == null) {
2949             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
2950         }
2951         if (ident.type().returnType() == type)
2952             return ident;
2953         // something like identity(Foo.class); do not bother to intern these
2954         assert (btw == Wrapper.OBJECT);
2955         return makeIdentity(type);
2956     }
2957 
2958     /**
2959      * Produces a constant method handle of the requested return type which
2960      * returns the default value for that type every time it is invoked.
2961      * The resulting constant method handle will have no side effects.
2962      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
2963      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
2964      * since {@code explicitCastArguments} converts {@code null} to default values.
2965      * @param type the expected return type of the desired method handle
2966      * @return a constant method handle that takes no arguments
2967      *         and returns the default value of the given type (or void, if the type is void)
2968      * @throws NullPointerException if the argument is null
2969      * @see MethodHandles#constant
2970      * @see MethodHandles#empty
2971      * @since 9
2972      */
2973     public static  MethodHandle zero(Class<?> type) {
2974         Objects.requireNonNull(type);
2975         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
2976     }
2977 
2978     private static MethodHandle identityOrVoid(Class<?> type) {
2979         return type == void.class ? zero(type) : identity(type);
2980     }
2981 
2982     /**
2983      * Produces a method handle of the requested type which ignores any arguments, does nothing,
2984      * and returns a suitable default depending on the return type.
2985      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
2986      * <p>The returned method handle is equivalent to
2987      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
2988      * <p>
2989      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
2990      * {@code guardWithTest(pred, target, empty(target.type())}.
2991      * @param type the type of the desired method handle
2992      * @return a constant method handle of the given type, which returns a default value of the given return type
2993      * @throws NullPointerException if the argument is null
2994      * @see MethodHandles#zero
2995      * @see MethodHandles#constant
2996      * @since 9
2997      */
2998     public static  MethodHandle empty(MethodType type) {
2999         Objects.requireNonNull(type);
3000         return dropArguments(zero(type.returnType()), 0, type.parameterList());
3001     }
3002 
3003     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.values().length];
3004     private static MethodHandle makeIdentity(Class<?> ptype) {
3005         MethodType mtype = methodType(ptype, ptype);
3006         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
3007         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
3008     }
3009 
3010     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
3011         int pos = btw.ordinal();
3012         MethodHandle zero = ZERO_MHS[pos];
3013         if (zero == null) {
3014             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
3015         }
3016         if (zero.type().returnType() == rtype)
3017             return zero;
3018         assert(btw == Wrapper.OBJECT);
3019         return makeZero(rtype);
3020     }
3021     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.values().length];
3022     private static MethodHandle makeZero(Class<?> rtype) {
3023         MethodType mtype = methodType(rtype);
3024         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
3025         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
3026     }
3027 
3028     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
3029         // Simulate a CAS, to avoid racy duplication of results.
3030         MethodHandle prev = cache[pos];
3031         if (prev != null) return prev;
3032         return cache[pos] = value;
3033     }
3034 
3035     /**
3036      * Provides a target method handle with one or more <em>bound arguments</em>
3037      * in advance of the method handle's invocation.
3038      * The formal parameters to the target corresponding to the bound
3039      * arguments are called <em>bound parameters</em>.
3040      * Returns a new method handle which saves away the bound arguments.
3041      * When it is invoked, it receives arguments for any non-bound parameters,
3042      * binds the saved arguments to their corresponding parameters,
3043      * and calls the original target.
3044      * <p>
3045      * The type of the new method handle will drop the types for the bound
3046      * parameters from the original target type, since the new method handle
3047      * will no longer require those arguments to be supplied by its callers.
3048      * <p>
3049      * Each given argument object must match the corresponding bound parameter type.
3050      * If a bound parameter type is a primitive, the argument object
3051      * must be a wrapper, and will be unboxed to produce the primitive value.
3052      * <p>
3053      * The {@code pos} argument selects which parameters are to be bound.
3054      * It may range between zero and <i>N-L</i> (inclusively),
3055      * where <i>N</i> is the arity of the target method handle
3056      * and <i>L</i> is the length of the values array.
3057      * <p>
3058      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3059      * variable-arity method handle}, even if the original target method handle was.
3060      * @param target the method handle to invoke after the argument is inserted
3061      * @param pos where to insert the argument (zero for the first)
3062      * @param values the series of arguments to insert
3063      * @return a method handle which inserts an additional argument,
3064      *         before calling the original method handle
3065      * @throws NullPointerException if the target or the {@code values} array is null
3066      * @see MethodHandle#bindTo
3067      */
3068     public static
3069     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
3070         int insCount = values.length;
3071         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
3072         if (insCount == 0)  return target;
3073         BoundMethodHandle result = target.rebind();
3074         for (int i = 0; i < insCount; i++) {
3075             Object value = values[i];
3076             Class<?> ptype = ptypes[pos+i];
3077             if (ptype.isPrimitive()) {
3078                 result = insertArgumentPrimitive(result, pos, ptype, value);
3079             } else {
3080                 value = ptype.cast(value);  // throw CCE if needed
3081                 result = result.bindArgumentL(pos, value);
3082             }
3083         }
3084         return result;
3085     }
3086 
3087     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
3088                                                              Class<?> ptype, Object value) {
3089         Wrapper w = Wrapper.forPrimitiveType(ptype);
3090         // perform unboxing and/or primitive conversion
3091         value = w.convert(value, ptype);
3092         switch (w) {
3093         case INT:     return result.bindArgumentI(pos, (int)value);
3094         case LONG:    return result.bindArgumentJ(pos, (long)value);
3095         case FLOAT:   return result.bindArgumentF(pos, (float)value);
3096         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
3097         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
3098         }
3099     }
3100 
3101     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
3102         MethodType oldType = target.type();
3103         int outargs = oldType.parameterCount();
3104         int inargs  = outargs - insCount;
3105         if (inargs < 0)
3106             throw newIllegalArgumentException("too many values to insert");
3107         if (pos < 0 || pos > inargs)
3108             throw newIllegalArgumentException("no argument type to append");
3109         return oldType.ptypes();
3110     }
3111 
3112     /**
3113      * Produces a method handle which will discard some dummy arguments
3114      * before calling some other specified <i>target</i> method handle.
3115      * The type of the new method handle will be the same as the target's type,
3116      * except it will also include the dummy argument types,
3117      * at some given position.
3118      * <p>
3119      * The {@code pos} argument may range between zero and <i>N</i>,
3120      * where <i>N</i> is the arity of the target.
3121      * If {@code pos} is zero, the dummy arguments will precede
3122      * the target's real arguments; if {@code pos} is <i>N</i>
3123      * they will come after.
3124      * <p>
3125      * <b>Example:</b>
3126      * <blockquote><pre>{@code
3127 import static java.lang.invoke.MethodHandles.*;
3128 import static java.lang.invoke.MethodType.*;
3129 ...
3130 MethodHandle cat = lookup().findVirtual(String.class,
3131   "concat", methodType(String.class, String.class));
3132 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3133 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
3134 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
3135 assertEquals(bigType, d0.type());
3136 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
3137      * }</pre></blockquote>
3138      * <p>
3139      * This method is also equivalent to the following code:
3140      * <blockquote><pre>
3141      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
3142      * </pre></blockquote>
3143      * @param target the method handle to invoke after the arguments are dropped
3144      * @param valueTypes the type(s) of the argument(s) to drop
3145      * @param pos position of first argument to drop (zero for the leftmost)
3146      * @return a method handle which drops arguments of the given types,
3147      *         before calling the original method handle
3148      * @throws NullPointerException if the target is null,
3149      *                              or if the {@code valueTypes} list or any of its elements is null
3150      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3151      *                  or if {@code pos} is negative or greater than the arity of the target,
3152      *                  or if the new method handle's type would have too many parameters
3153      */
3154     public static
3155     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3156         MethodType oldType = target.type();  // get NPE
3157         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
3158         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
3159         if (dropped == 0)  return target;
3160         BoundMethodHandle result = target.rebind();
3161         LambdaForm lform = result.form;
3162         int insertFormArg = 1 + pos;
3163         for (Class<?> ptype : valueTypes) {
3164             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
3165         }
3166         result = result.copyWith(newType, lform);
3167         return result;
3168     }
3169 
3170     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
3171         int dropped = valueTypes.size();
3172         MethodType.checkSlotCount(dropped);
3173         int outargs = oldType.parameterCount();
3174         int inargs  = outargs + dropped;
3175         if (pos < 0 || pos > outargs)
3176             throw newIllegalArgumentException("no argument type to remove"
3177                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
3178                     );
3179         return dropped;
3180     }
3181 
3182     /**
3183      * Produces a method handle which will discard some dummy arguments
3184      * before calling some other specified <i>target</i> method handle.
3185      * The type of the new method handle will be the same as the target's type,
3186      * except it will also include the dummy argument types,
3187      * at some given position.
3188      * <p>
3189      * The {@code pos} argument may range between zero and <i>N</i>,
3190      * where <i>N</i> is the arity of the target.
3191      * If {@code pos} is zero, the dummy arguments will precede
3192      * the target's real arguments; if {@code pos} is <i>N</i>
3193      * they will come after.
3194      * @apiNote
3195      * <blockquote><pre>{@code
3196 import static java.lang.invoke.MethodHandles.*;
3197 import static java.lang.invoke.MethodType.*;
3198 ...
3199 MethodHandle cat = lookup().findVirtual(String.class,
3200   "concat", methodType(String.class, String.class));
3201 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3202 MethodHandle d0 = dropArguments(cat, 0, String.class);
3203 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
3204 MethodHandle d1 = dropArguments(cat, 1, String.class);
3205 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
3206 MethodHandle d2 = dropArguments(cat, 2, String.class);
3207 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
3208 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
3209 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
3210      * }</pre></blockquote>
3211      * <p>
3212      * This method is also equivalent to the following code:
3213      * <blockquote><pre>
3214      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
3215      * </pre></blockquote>
3216      * @param target the method handle to invoke after the arguments are dropped
3217      * @param valueTypes the type(s) of the argument(s) to drop
3218      * @param pos position of first argument to drop (zero for the leftmost)
3219      * @return a method handle which drops arguments of the given types,
3220      *         before calling the original method handle
3221      * @throws NullPointerException if the target is null,
3222      *                              or if the {@code valueTypes} array or any of its elements is null
3223      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3224      *                  or if {@code pos} is negative or greater than the arity of the target,
3225      *                  or if the new method handle's type would have
3226      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
3227      */
3228     public static
3229     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
3230         return dropArguments(target, pos, Arrays.asList(valueTypes));
3231     }
3232 
3233     // private version which allows caller some freedom with error handling
3234     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
3235                                       boolean nullOnFailure) {
3236         List<Class<?>> oldTypes = target.type().parameterList();
3237         int match = oldTypes.size();
3238         if (skip != 0) {
3239             if (skip < 0 || skip > match) {
3240                 throw newIllegalArgumentException("illegal skip", skip, target);
3241             }
3242             oldTypes = oldTypes.subList(skip, match);
3243             match -= skip;
3244         }
3245         List<Class<?>> addTypes = newTypes;
3246         int add = addTypes.size();
3247         if (pos != 0) {
3248             if (pos < 0 || pos > add) {
3249                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
3250             }
3251             addTypes = addTypes.subList(pos, add);
3252             add -= pos; assert(addTypes.size() == add);
3253         }
3254         // Do not add types which already match the existing arguments.
3255         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
3256             if (nullOnFailure) {
3257                 return null;
3258             }
3259             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
3260         }
3261         addTypes = addTypes.subList(match, add);
3262         add -= match; assert(addTypes.size() == add);
3263         // newTypes:     (   P*[pos], M*[match], A*[add] )
3264         // target: ( S*[skip],        M*[match]  )
3265         MethodHandle adapter = target;
3266         if (add > 0) {
3267             adapter = dropArguments(adapter, skip+ match, addTypes);
3268         }
3269         // adapter: (S*[skip],        M*[match], A*[add] )
3270         if (pos > 0) {
3271             adapter = dropArguments(adapter, skip, newTypes.subList(0, pos));
3272        }
3273         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
3274         return adapter;
3275     }
3276 
3277     /**
3278      * Adapts a target method handle to match the given parameter type list, if necessary, by adding dummy arguments.
3279      * Some leading parameters are first skipped; they will be left unchanged and are otherwise ignored.
3280      * The remaining types in the target's parameter type list must be contained as a sub-list of the given type list,
3281      * at the given position.
3282      * Any non-matching parameter types (before or after the matching sub-list) are inserted in corresponding
3283      * positions of the target method handle's parameters, as if by {@link #dropArguments}.
3284      * (More precisely, elements in the new list before {@code pos} are inserted into the target list at {@code skip},
3285      * while elements in the new list after the match beginning at {@code pos} are inserted at the end of the
3286      * target list.)
3287      * The target's return type will be unchanged.
3288      * @apiNote
3289      * Two method handles whose argument lists are "effectively identical" (i.e., identical
3290      * in a common prefix) may be mutually converted to a common type
3291      * by two calls to {@code dropArgumentsToMatch}, as follows:
3292      * <blockquote><pre>{@code
3293 import static java.lang.invoke.MethodHandles.*;
3294 import static java.lang.invoke.MethodType.*;
3295 ...
3296 ...
3297 MethodHandle h0 = constant(boolean.class, true);
3298 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
3299 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
3300 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
3301 if (h1.type().parameterCount() < h2.type().parameterCount())
3302     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
3303 else
3304     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
3305 MethodHandle h3 = guardWithTest(h0, h1, h2);
3306 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
3307      * }</pre></blockquote>
3308      * @param target the method handle to adapt
3309      * @param skip number of targets parameters to disregard (they will be unchanged)
3310      * @param newTypes the desired argument list of the method handle
3311      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
3312      * @return a possibly adapted method handle
3313      * @throws NullPointerException if either argument is null
3314      * @throws IllegalArgumentException
3315      *         if either index is out of range in its corresponding list, or
3316      *         if the non-skipped target parameter types match the new types at {@code pos}
3317      * @since 9
3318      */
3319     public static
3320     MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
3321         Objects.requireNonNull(target);
3322         Objects.requireNonNull(newTypes);
3323         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
3324     }
3325 
3326     /**
3327      * Adapts a target method handle by pre-processing
3328      * one or more of its arguments, each with its own unary filter function,
3329      * and then calling the target with each pre-processed argument
3330      * replaced by the result of its corresponding filter function.
3331      * <p>
3332      * The pre-processing is performed by one or more method handles,
3333      * specified in the elements of the {@code filters} array.
3334      * The first element of the filter array corresponds to the {@code pos}
3335      * argument of the target, and so on in sequence.
3336      * <p>
3337      * Null arguments in the array are treated as identity functions,
3338      * and the corresponding arguments left unchanged.
3339      * (If there are no non-null elements in the array, the original target is returned.)
3340      * Each filter is applied to the corresponding argument of the adapter.
3341      * <p>
3342      * If a filter {@code F} applies to the {@code N}th argument of
3343      * the target, then {@code F} must be a method handle which
3344      * takes exactly one argument.  The type of {@code F}'s sole argument
3345      * replaces the corresponding argument type of the target
3346      * in the resulting adapted method handle.
3347      * The return type of {@code F} must be identical to the corresponding
3348      * parameter type of the target.
3349      * <p>
3350      * It is an error if there are elements of {@code filters}
3351      * (null or not)
3352      * which do not correspond to argument positions in the target.
3353      * <p><b>Example:</b>
3354      * <blockquote><pre>{@code
3355 import static java.lang.invoke.MethodHandles.*;
3356 import static java.lang.invoke.MethodType.*;
3357 ...
3358 MethodHandle cat = lookup().findVirtual(String.class,
3359   "concat", methodType(String.class, String.class));
3360 MethodHandle upcase = lookup().findVirtual(String.class,
3361   "toUpperCase", methodType(String.class));
3362 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3363 MethodHandle f0 = filterArguments(cat, 0, upcase);
3364 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
3365 MethodHandle f1 = filterArguments(cat, 1, upcase);
3366 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
3367 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
3368 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
3369      * }</pre></blockquote>
3370      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3371      * denotes the return type of both the {@code target} and resulting adapter.
3372      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
3373      * of the parameters and arguments that precede and follow the filter position
3374      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
3375      * values of the filtered parameters and arguments; they also represent the
3376      * return types of the {@code filter[i]} handles. The latter accept arguments
3377      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
3378      * the resulting adapter.
3379      * <blockquote><pre>{@code
3380      * T target(P... p, A[i]... a[i], B... b);
3381      * A[i] filter[i](V[i]);
3382      * T adapter(P... p, V[i]... v[i], B... b) {
3383      *   return target(p..., filter[i](v[i])..., b...);
3384      * }
3385      * }</pre></blockquote>
3386      * <p>
3387      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3388      * variable-arity method handle}, even if the original target method handle was.
3389      *
3390      * @param target the method handle to invoke after arguments are filtered
3391      * @param pos the position of the first argument to filter
3392      * @param filters method handles to call initially on filtered arguments
3393      * @return method handle which incorporates the specified argument filtering logic
3394      * @throws NullPointerException if the target is null
3395      *                              or if the {@code filters} array is null
3396      * @throws IllegalArgumentException if a non-null element of {@code filters}
3397      *          does not match a corresponding argument type of target as described above,
3398      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
3399      *          or if the resulting method handle's type would have
3400      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3401      */
3402     public static
3403     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
3404         filterArgumentsCheckArity(target, pos, filters);
3405         MethodHandle adapter = target;
3406         int curPos = pos-1;  // pre-incremented
3407         for (MethodHandle filter : filters) {
3408             curPos += 1;
3409             if (filter == null)  continue;  // ignore null elements of filters
3410             adapter = filterArgument(adapter, curPos, filter);
3411         }
3412         return adapter;
3413     }
3414 
3415     /*non-public*/ static
3416     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
3417         filterArgumentChecks(target, pos, filter);
3418         MethodType targetType = target.type();
3419         MethodType filterType = filter.type();
3420         BoundMethodHandle result = target.rebind();
3421         Class<?> newParamType = filterType.parameterType(0);
3422         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
3423         MethodType newType = targetType.changeParameterType(pos, newParamType);
3424         result = result.copyWithExtendL(newType, lform, filter);
3425         return result;
3426     }
3427 
3428     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
3429         MethodType targetType = target.type();
3430         int maxPos = targetType.parameterCount();
3431         if (pos + filters.length > maxPos)
3432             throw newIllegalArgumentException("too many filters");
3433     }
3434 
3435     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
3436         MethodType targetType = target.type();
3437         MethodType filterType = filter.type();
3438         if (filterType.parameterCount() != 1
3439             || filterType.returnType() != targetType.parameterType(pos))
3440             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3441     }
3442 
3443     /**
3444      * Adapts a target method handle by pre-processing
3445      * a sub-sequence of its arguments with a filter (another method handle).
3446      * The pre-processed arguments are replaced by the result (if any) of the
3447      * filter function.
3448      * The target is then called on the modified (usually shortened) argument list.
3449      * <p>
3450      * If the filter returns a value, the target must accept that value as
3451      * its argument in position {@code pos}, preceded and/or followed by
3452      * any arguments not passed to the filter.
3453      * If the filter returns void, the target must accept all arguments
3454      * not passed to the filter.
3455      * No arguments are reordered, and a result returned from the filter
3456      * replaces (in order) the whole subsequence of arguments originally
3457      * passed to the adapter.
3458      * <p>
3459      * The argument types (if any) of the filter
3460      * replace zero or one argument types of the target, at position {@code pos},
3461      * in the resulting adapted method handle.
3462      * The return type of the filter (if any) must be identical to the
3463      * argument type of the target at position {@code pos}, and that target argument
3464      * is supplied by the return value of the filter.
3465      * <p>
3466      * In all cases, {@code pos} must be greater than or equal to zero, and
3467      * {@code pos} must also be less than or equal to the target's arity.
3468      * <p><b>Example:</b>
3469      * <blockquote><pre>{@code
3470 import static java.lang.invoke.MethodHandles.*;
3471 import static java.lang.invoke.MethodType.*;
3472 ...
3473 MethodHandle deepToString = publicLookup()
3474   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
3475 
3476 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
3477 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
3478 
3479 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
3480 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
3481 
3482 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
3483 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
3484 assertEquals("[top, [up, down], strange]",
3485              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
3486 
3487 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
3488 assertEquals("[top, [up, down], [strange]]",
3489              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
3490 
3491 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
3492 assertEquals("[top, [[up, down, strange], charm], bottom]",
3493              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
3494      * }</pre></blockquote>
3495      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3496      * represents the return type of the {@code target} and resulting adapter.
3497      * {@code V}/{@code v} stand for the return type and value of the
3498      * {@code filter}, which are also found in the signature and arguments of
3499      * the {@code target}, respectively, unless {@code V} is {@code void}.
3500      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
3501      * and values preceding and following the collection position, {@code pos},
3502      * in the {@code target}'s signature. They also turn up in the resulting
3503      * adapter's signature and arguments, where they surround
3504      * {@code B}/{@code b}, which represent the parameter types and arguments
3505      * to the {@code filter} (if any).
3506      * <blockquote><pre>{@code
3507      * T target(A...,V,C...);
3508      * V filter(B...);
3509      * T adapter(A... a,B... b,C... c) {
3510      *   V v = filter(b...);
3511      *   return target(a...,v,c...);
3512      * }
3513      * // and if the filter has no arguments:
3514      * T target2(A...,V,C...);
3515      * V filter2();
3516      * T adapter2(A... a,C... c) {
3517      *   V v = filter2();
3518      *   return target2(a...,v,c...);
3519      * }
3520      * // and if the filter has a void return:
3521      * T target3(A...,C...);
3522      * void filter3(B...);
3523      * T adapter3(A... a,B... b,C... c) {
3524      *   filter3(b...);
3525      *   return target3(a...,c...);
3526      * }
3527      * }</pre></blockquote>
3528      * <p>
3529      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
3530      * one which first "folds" the affected arguments, and then drops them, in separate
3531      * steps as follows:
3532      * <blockquote><pre>{@code
3533      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
3534      * mh = MethodHandles.foldArguments(mh, coll); //step 1
3535      * }</pre></blockquote>
3536      * If the target method handle consumes no arguments besides than the result
3537      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
3538      * is equivalent to {@code filterReturnValue(coll, mh)}.
3539      * If the filter method handle {@code coll} consumes one argument and produces
3540      * a non-void result, then {@code collectArguments(mh, N, coll)}
3541      * is equivalent to {@code filterArguments(mh, N, coll)}.
3542      * Other equivalences are possible but would require argument permutation.
3543      * <p>
3544      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3545      * variable-arity method handle}, even if the original target method handle was.
3546      *
3547      * @param target the method handle to invoke after filtering the subsequence of arguments
3548      * @param pos the position of the first adapter argument to pass to the filter,
3549      *            and/or the target argument which receives the result of the filter
3550      * @param filter method handle to call on the subsequence of arguments
3551      * @return method handle which incorporates the specified argument subsequence filtering logic
3552      * @throws NullPointerException if either argument is null
3553      * @throws IllegalArgumentException if the return type of {@code filter}
3554      *          is non-void and is not the same as the {@code pos} argument of the target,
3555      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
3556      *          or if the resulting method handle's type would have
3557      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3558      * @see MethodHandles#foldArguments
3559      * @see MethodHandles#filterArguments
3560      * @see MethodHandles#filterReturnValue
3561      */
3562     public static
3563     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
3564         MethodType newType = collectArgumentsChecks(target, pos, filter);
3565         MethodType collectorType = filter.type();
3566         BoundMethodHandle result = target.rebind();
3567         LambdaForm lform;
3568         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
3569             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
3570             if (lform != null) {
3571                 return result.copyWith(newType, lform);
3572             }
3573         }
3574         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
3575         return result.copyWithExtendL(newType, lform, filter);
3576     }
3577 
3578     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
3579         MethodType targetType = target.type();
3580         MethodType filterType = filter.type();
3581         Class<?> rtype = filterType.returnType();
3582         List<Class<?>> filterArgs = filterType.parameterList();
3583         if (rtype == void.class) {
3584             return targetType.insertParameterTypes(pos, filterArgs);
3585         }
3586         if (rtype != targetType.parameterType(pos)) {
3587             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3588         }
3589         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
3590     }
3591 
3592     /**
3593      * Adapts a target method handle by post-processing
3594      * its return value (if any) with a filter (another method handle).
3595      * The result of the filter is returned from the adapter.
3596      * <p>
3597      * If the target returns a value, the filter must accept that value as
3598      * its only argument.
3599      * If the target returns void, the filter must accept no arguments.
3600      * <p>
3601      * The return type of the filter
3602      * replaces the return type of the target
3603      * in the resulting adapted method handle.
3604      * The argument type of the filter (if any) must be identical to the
3605      * return type of the target.
3606      * <p><b>Example:</b>
3607      * <blockquote><pre>{@code
3608 import static java.lang.invoke.MethodHandles.*;
3609 import static java.lang.invoke.MethodType.*;
3610 ...
3611 MethodHandle cat = lookup().findVirtual(String.class,
3612   "concat", methodType(String.class, String.class));
3613 MethodHandle length = lookup().findVirtual(String.class,
3614   "length", methodType(int.class));
3615 System.out.println((String) cat.invokeExact("x", "y")); // xy
3616 MethodHandle f0 = filterReturnValue(cat, length);
3617 System.out.println((int) f0.invokeExact("x", "y")); // 2
3618      * }</pre></blockquote>
3619      * <p>Here is pseudocode for the resulting adapter. In the code,
3620      * {@code T}/{@code t} represent the result type and value of the
3621      * {@code target}; {@code V}, the result type of the {@code filter}; and
3622      * {@code A}/{@code a}, the types and values of the parameters and arguments
3623      * of the {@code target} as well as the resulting adapter.
3624      * <blockquote><pre>{@code
3625      * T target(A...);
3626      * V filter(T);
3627      * V adapter(A... a) {
3628      *   T t = target(a...);
3629      *   return filter(t);
3630      * }
3631      * // and if the target has a void return:
3632      * void target2(A...);
3633      * V filter2();
3634      * V adapter2(A... a) {
3635      *   target2(a...);
3636      *   return filter2();
3637      * }
3638      * // and if the filter has a void return:
3639      * T target3(A...);
3640      * void filter3(V);
3641      * void adapter3(A... a) {
3642      *   T t = target3(a...);
3643      *   filter3(t);
3644      * }
3645      * }</pre></blockquote>
3646      * <p>
3647      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3648      * variable-arity method handle}, even if the original target method handle was.
3649      * @param target the method handle to invoke before filtering the return value
3650      * @param filter method handle to call on the return value
3651      * @return method handle which incorporates the specified return value filtering logic
3652      * @throws NullPointerException if either argument is null
3653      * @throws IllegalArgumentException if the argument list of {@code filter}
3654      *          does not match the return type of target as described above
3655      */
3656     public static
3657     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
3658         MethodType targetType = target.type();
3659         MethodType filterType = filter.type();
3660         filterReturnValueChecks(targetType, filterType);
3661         BoundMethodHandle result = target.rebind();
3662         BasicType rtype = BasicType.basicType(filterType.returnType());
3663         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
3664         MethodType newType = targetType.changeReturnType(filterType.returnType());
3665         result = result.copyWithExtendL(newType, lform, filter);
3666         return result;
3667     }
3668 
3669     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
3670         Class<?> rtype = targetType.returnType();
3671         int filterValues = filterType.parameterCount();
3672         if (filterValues == 0
3673                 ? (rtype != void.class)
3674                 : (rtype != filterType.parameterType(0)))
3675             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3676     }
3677 
3678     /**
3679      * Adapts a target method handle by pre-processing
3680      * some of its arguments, and then calling the target with
3681      * the result of the pre-processing, inserted into the original
3682      * sequence of arguments.
3683      * <p>
3684      * The pre-processing is performed by {@code combiner}, a second method handle.
3685      * Of the arguments passed to the adapter, the first {@code N} arguments
3686      * are copied to the combiner, which is then called.
3687      * (Here, {@code N} is defined as the parameter count of the combiner.)
3688      * After this, control passes to the target, with any result
3689      * from the combiner inserted before the original {@code N} incoming
3690      * arguments.
3691      * <p>
3692      * If the combiner returns a value, the first parameter type of the target
3693      * must be identical with the return type of the combiner, and the next
3694      * {@code N} parameter types of the target must exactly match the parameters
3695      * of the combiner.
3696      * <p>
3697      * If the combiner has a void return, no result will be inserted,
3698      * and the first {@code N} parameter types of the target
3699      * must exactly match the parameters of the combiner.
3700      * <p>
3701      * The resulting adapter is the same type as the target, except that the
3702      * first parameter type is dropped,
3703      * if it corresponds to the result of the combiner.
3704      * <p>
3705      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
3706      * that either the combiner or the target does not wish to receive.
3707      * If some of the incoming arguments are destined only for the combiner,
3708      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
3709      * arguments will not need to be live on the stack on entry to the
3710      * target.)
3711      * <p><b>Example:</b>
3712      * <blockquote><pre>{@code
3713 import static java.lang.invoke.MethodHandles.*;
3714 import static java.lang.invoke.MethodType.*;
3715 ...
3716 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
3717   "println", methodType(void.class, String.class))
3718     .bindTo(System.out);
3719 MethodHandle cat = lookup().findVirtual(String.class,
3720   "concat", methodType(String.class, String.class));
3721 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
3722 MethodHandle catTrace = foldArguments(cat, trace);
3723 // also prints "boo":
3724 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
3725      * }</pre></blockquote>
3726      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3727      * represents the result type of the {@code target} and resulting adapter.
3728      * {@code V}/{@code v} represent the type and value of the parameter and argument
3729      * of {@code target} that precedes the folding position; {@code V} also is
3730      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
3731      * types and values of the {@code N} parameters and arguments at the folding
3732      * position. {@code B}/{@code b} represent the types and values of the
3733      * {@code target} parameters and arguments that follow the folded parameters
3734      * and arguments.
3735      * <blockquote><pre>{@code
3736      * // there are N arguments in A...
3737      * T target(V, A[N]..., B...);
3738      * V combiner(A...);
3739      * T adapter(A... a, B... b) {
3740      *   V v = combiner(a...);
3741      *   return target(v, a..., b...);
3742      * }
3743      * // and if the combiner has a void return:
3744      * T target2(A[N]..., B...);
3745      * void combiner2(A...);
3746      * T adapter2(A... a, B... b) {
3747      *   combiner2(a...);
3748      *   return target2(a..., b...);
3749      * }
3750      * }</pre></blockquote>
3751      * <p>
3752      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3753      * variable-arity method handle}, even if the original target method handle was.
3754      * @param target the method handle to invoke after arguments are combined
3755      * @param combiner method handle to call initially on the incoming arguments
3756      * @return method handle which incorporates the specified argument folding logic
3757      * @throws NullPointerException if either argument is null
3758      * @throws IllegalArgumentException if {@code combiner}'s return type
3759      *          is non-void and not the same as the first argument type of
3760      *          the target, or if the initial {@code N} argument types
3761      *          of the target
3762      *          (skipping one matching the {@code combiner}'s return type)
3763      *          are not identical with the argument types of {@code combiner}
3764      */
3765     public static
3766     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
3767         return foldArguments(target, 0, combiner);
3768     }
3769 
3770     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
3771         int foldArgs   = combinerType.parameterCount();
3772         Class<?> rtype = combinerType.returnType();
3773         int foldVals = rtype == void.class ? 0 : 1;
3774         int afterInsertPos = foldPos + foldVals;
3775         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
3776         if (ok && !(combinerType.parameterList()
3777                     .equals(targetType.parameterList().subList(afterInsertPos,
3778                                                                afterInsertPos + foldArgs))))
3779             ok = false;
3780         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
3781             ok = false;
3782         if (!ok)
3783             throw misMatchedTypes("target and combiner types", targetType, combinerType);
3784         return rtype;
3785     }
3786 
3787     /**
3788      * Makes a method handle which adapts a target method handle,
3789      * by guarding it with a test, a boolean-valued method handle.
3790      * If the guard fails, a fallback handle is called instead.
3791      * All three method handles must have the same corresponding
3792      * argument and return types, except that the return type
3793      * of the test must be boolean, and the test is allowed
3794      * to have fewer arguments than the other two method handles.
3795      * <p>
3796      * Here is pseudocode for the resulting adapter. In the code, {@code T}
3797      * represents the uniform result type of the three involved handles;
3798      * {@code A}/{@code a}, the types and values of the {@code target}
3799      * parameters and arguments that are consumed by the {@code test}; and
3800      * {@code B}/{@code b}, those types and values of the {@code target}
3801      * parameters and arguments that are not consumed by the {@code test}.
3802      * <blockquote><pre>{@code
3803      * boolean test(A...);
3804      * T target(A...,B...);
3805      * T fallback(A...,B...);
3806      * T adapter(A... a,B... b) {
3807      *   if (test(a...))
3808      *     return target(a..., b...);
3809      *   else
3810      *     return fallback(a..., b...);
3811      * }
3812      * }</pre></blockquote>
3813      * Note that the test arguments ({@code a...} in the pseudocode) cannot
3814      * be modified by execution of the test, and so are passed unchanged
3815      * from the caller to the target or fallback as appropriate.
3816      * @param test method handle used for test, must return boolean
3817      * @param target method handle to call if test passes
3818      * @param fallback method handle to call if test fails
3819      * @return method handle which incorporates the specified if/then/else logic
3820      * @throws NullPointerException if any argument is null
3821      * @throws IllegalArgumentException if {@code test} does not return boolean,
3822      *          or if all three method types do not match (with the return
3823      *          type of {@code test} changed to match that of the target).
3824      */
3825     public static
3826     MethodHandle guardWithTest(MethodHandle test,
3827                                MethodHandle target,
3828                                MethodHandle fallback) {
3829         MethodType gtype = test.type();
3830         MethodType ttype = target.type();
3831         MethodType ftype = fallback.type();
3832         if (!ttype.equals(ftype))
3833             throw misMatchedTypes("target and fallback types", ttype, ftype);
3834         if (gtype.returnType() != boolean.class)
3835             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
3836         List<Class<?>> targs = ttype.parameterList();
3837         test = dropArgumentsToMatch(test, 0, targs, 0, true);
3838         if (test == null) {
3839             throw misMatchedTypes("target and test types", ttype, gtype);
3840         }
3841         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
3842     }
3843 
3844     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
3845         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
3846     }
3847 
3848     /**
3849      * Makes a method handle which adapts a target method handle,
3850      * by running it inside an exception handler.
3851      * If the target returns normally, the adapter returns that value.
3852      * If an exception matching the specified type is thrown, the fallback
3853      * handle is called instead on the exception, plus the original arguments.
3854      * <p>
3855      * The target and handler must have the same corresponding
3856      * argument and return types, except that handler may omit trailing arguments
3857      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
3858      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
3859      * <p>
3860      * Here is pseudocode for the resulting adapter. In the code, {@code T}
3861      * represents the return type of the {@code target} and {@code handler},
3862      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
3863      * the types and values of arguments to the resulting handle consumed by
3864      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
3865      * resulting handle discarded by {@code handler}.
3866      * <blockquote><pre>{@code
3867      * T target(A..., B...);
3868      * T handler(ExType, A...);
3869      * T adapter(A... a, B... b) {
3870      *   try {
3871      *     return target(a..., b...);
3872      *   } catch (ExType ex) {
3873      *     return handler(ex, a...);
3874      *   }
3875      * }
3876      * }</pre></blockquote>
3877      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
3878      * be modified by execution of the target, and so are passed unchanged
3879      * from the caller to the handler, if the handler is invoked.
3880      * <p>
3881      * The target and handler must return the same type, even if the handler
3882      * always throws.  (This might happen, for instance, because the handler
3883      * is simulating a {@code finally} clause).
3884      * To create such a throwing handler, compose the handler creation logic
3885      * with {@link #throwException throwException},
3886      * in order to create a method handle of the correct return type.
3887      * @param target method handle to call
3888      * @param exType the type of exception which the handler will catch
3889      * @param handler method handle to call if a matching exception is thrown
3890      * @return method handle which incorporates the specified try/catch logic
3891      * @throws NullPointerException if any argument is null
3892      * @throws IllegalArgumentException if {@code handler} does not accept
3893      *          the given exception type, or if the method handle types do
3894      *          not match in their return types and their
3895      *          corresponding parameters
3896      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
3897      */
3898     public static
3899     MethodHandle catchException(MethodHandle target,
3900                                 Class<? extends Throwable> exType,
3901                                 MethodHandle handler) {
3902         MethodType ttype = target.type();
3903         MethodType htype = handler.type();
3904         if (!Throwable.class.isAssignableFrom(exType))
3905             throw new ClassCastException(exType.getName());
3906         if (htype.parameterCount() < 1 ||
3907             !htype.parameterType(0).isAssignableFrom(exType))
3908             throw newIllegalArgumentException("handler does not accept exception type "+exType);
3909         if (htype.returnType() != ttype.returnType())
3910             throw misMatchedTypes("target and handler return types", ttype, htype);
3911         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
3912         if (handler == null) {
3913             throw misMatchedTypes("target and handler types", ttype, htype);
3914         }
3915         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
3916     }
3917 
3918     /**
3919      * Produces a method handle which will throw exceptions of the given {@code exType}.
3920      * The method handle will accept a single argument of {@code exType},
3921      * and immediately throw it as an exception.
3922      * The method type will nominally specify a return of {@code returnType}.
3923      * The return type may be anything convenient:  It doesn't matter to the
3924      * method handle's behavior, since it will never return normally.
3925      * @param returnType the return type of the desired method handle
3926      * @param exType the parameter type of the desired method handle
3927      * @return method handle which can throw the given exceptions
3928      * @throws NullPointerException if either argument is null
3929      */
3930     public static
3931     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
3932         if (!Throwable.class.isAssignableFrom(exType))
3933             throw new ClassCastException(exType.getName());
3934         return MethodHandleImpl.throwException(methodType(returnType, exType));
3935     }
3936 
3937     /**
3938      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
3939      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
3940      * delivers the loop's result, which is the return value of the resulting handle.
3941      * <p>
3942      * Intuitively, every loop is formed by one or more "clauses", each specifying a local iteration value and/or a loop
3943      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
3944      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
3945      * terms of method handles, each clause will determine four actions:<ul>
3946      * <li>Before the loop executes, the initialization of an iteration variable or loop invariant local.
3947      * <li>When a clause executes, an update step for the iteration variable.
3948      * <li>When a clause executes, a predicate execution to test for loop exit.
3949      * <li>If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
3950      * </ul>
3951      * <p>
3952      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
3953      * this case. See below for a detailed description.
3954      * <p>
3955      * Each clause function, with the exception of clause initializers, is able to observe the entire loop state,
3956      * because it will be passed <em>all</em> current iteration variable values, as well as all incoming loop
3957      * parameters. Most clause functions will not need all of this information, but they will be formally connected as
3958      * if by {@link #dropArguments}.
3959      * <p>
3960      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
3961      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
3962      * corresponds to a place where {@link IllegalArgumentException} may be thrown if the required constraint is not met
3963      * by the inputs to the loop combinator. The term "effectively identical", applied to parameter type lists, means
3964      * that they must be identical, or else one list must be a proper prefix of the other.
3965      * <p>
3966      * <em>Step 0: Determine clause structure.</em><ol type="a">
3967      * <li>The clause array (of type {@code MethodHandle[][]} must be non-{@code null} and contain at least one element.
3968      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
3969      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
3970      * four. Padding takes place by appending elements to the array.
3971      * <li>Clauses with all {@code null}s are disregarded.
3972      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
3973      * </ol>
3974      * <p>
3975      * <em>Step 1A: Determine iteration variables.</em><ol type="a">
3976      * <li>Examine init and step function return types, pairwise, to determine each clause's iteration variable type.
3977      * <li>If both functions are omitted, use {@code void}; else if one is omitted, use the other's return type; else
3978      * use the common return type (they must be identical).
3979      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
3980      * <li>This list of types is called the "common prefix".
3981      * </ol>
3982      * <p>
3983      * <em>Step 1B: Determine loop parameters.</em><ul>
3984      * <li><b>If at least one init function is given,</b><ol type="a">
3985      *   <li>Examine init function parameter lists.
3986      *   <li>Omitted init functions are deemed to have {@code null} parameter lists.
3987      *   <li>All init function parameter lists must be effectively identical.
3988      *   <li>The longest parameter list (which is necessarily unique) is called the "common suffix".
3989      * </ol>
3990      * <li><b>If no init function is given,</b><ol type="a">
3991      *   <li>Examine the suffixes of the step, pred, and fini parameter lists, after removing the "common prefix".
3992      *   <li>The longest of these suffixes is taken as the "common suffix".
3993      * </ol></ul>
3994      * <p>
3995      * <em>Step 1C: Determine loop return type.</em><ol type="a">
3996      * <li>Examine fini function return types, disregarding omitted fini functions.
3997      * <li>If there are no fini functions, use {@code void} as the loop return type.
3998      * <li>Otherwise, use the common return type of the fini functions; they must all be identical.
3999      * </ol>
4000      * <p>
4001      * <em>Step 1D: Check other types.</em><ol type="a">
4002      * <li>There must be at least one non-omitted pred function.
4003      * <li>Every non-omitted pred function must have a {@code boolean} return type.
4004      * </ol>
4005      * <p>
4006      * <em>Step 2: Determine parameter lists.</em><ol type="a">
4007      * <li>The parameter list for the resulting loop handle will be the "common suffix".
4008      * <li>The parameter list for init functions will be adjusted to the "common suffix". (Note that their parameter
4009      * lists are already effectively identical to the common suffix.)
4010      * <li>The parameter list for non-init (step, pred, and fini) functions will be adjusted to the common prefix
4011      * followed by the common suffix, called the "common parameter sequence".
4012      * <li>Every non-init, non-omitted function parameter list must be effectively identical to the common parameter
4013      * sequence.
4014      * </ol>
4015      * <p>
4016      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
4017      * <li>If an init function is omitted, use a {@linkplain #constant constant function} of the appropriate
4018      * {@code null}/zero/{@code false}/{@code void} type. (For this purpose, a constant {@code void} is simply a
4019      * function which does nothing and returns {@code void}; it can be obtained from another constant function by
4020      * {@linkplain MethodHandle#asType type conversion}.)
4021      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
4022      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
4023      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
4024      * <li>If a pred function is omitted, the corresponding fini function must also be omitted.
4025      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
4026      * as this clause is concerned.)
4027      * <li>If a fini function is omitted, use a constant {@code null}/zero/{@code false}/{@code void} function of the
4028      * loop return type.
4029      * </ol>
4030      * <p>
4031      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
4032      * <li>At this point, every init function parameter list is effectively identical to the common suffix, but some
4033      * lists may be shorter. For every init function with a short parameter list, pad out the end of the list by
4034      * {@linkplain #dropArguments dropping arguments}.
4035      * <li>At this point, every non-init function parameter list is effectively identical to the common parameter
4036      * sequence, but some lists may be shorter. For every non-init function with a short parameter list, pad out the end
4037      * of the list by {@linkplain #dropArguments dropping arguments}.
4038      * </ol>
4039      * <p>
4040      * <em>Final observations.</em><ol type="a">
4041      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
4042      * <li>All init functions have a common parameter type list, which the final loop handle will also have.
4043      * <li>All fini functions have a common return type, which the final loop handle will also have.
4044      * <li>All non-init functions have a common parameter type list, which is the common parameter sequence, of
4045      * (non-{@code void}) iteration variables followed by loop parameters.
4046      * <li>Each pair of init and step functions agrees in their return types.
4047      * <li>Each non-init function will be able to observe the current values of all iteration variables, by means of the
4048      * common prefix.
4049      * </ol>
4050      * <p>
4051      * <em>Loop execution.</em><ol type="a">
4052      * <li>When the loop is called, the loop input values are saved in locals, to be passed (as the common suffix) to
4053      * every clause function. These locals are loop invariant.
4054      * <li>Each init function is executed in clause order (passing the common suffix) and the non-{@code void} values
4055      * are saved (as the common prefix) into locals. These locals are loop varying (unless their steps are identity
4056      * functions, as noted above).
4057      * <li>All function executions (except init functions) will be passed the common parameter sequence, consisting of
4058      * the non-{@code void} iteration values (in clause order) and then the loop inputs (in argument order).
4059      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
4060      * returns {@code false}.
4061      * <li>The non-{@code void} result from a step function call is used to update the corresponding loop variable. The
4062      * updated value is immediately visible to all subsequent function calls.
4063      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
4064      * is returned from the loop as a whole.
4065      * </ol>
4066      * <p>
4067      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the types / values
4068      * of loop variables; {@code A}/{@code a}, those of arguments passed to the resulting loop; and {@code R}, the
4069      * result types of finalizers as well as of the resulting loop.
4070      * <blockquote><pre>{@code
4071      * V... init...(A...);
4072      * boolean pred...(V..., A...);
4073      * V... step...(V..., A...);
4074      * R fini...(V..., A...);
4075      * R loop(A... a) {
4076      *   V... v... = init...(a...);
4077      *   for (;;) {
4078      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
4079      *       v = s(v..., a...);
4080      *       if (!p(v..., a...)) {
4081      *         return f(v..., a...);
4082      *       }
4083      *     }
4084      *   }
4085      * }
4086      * }</pre></blockquote>
4087      * <p>
4088      * @apiNote Example:
4089      * <blockquote><pre>{@code
4090      * // iterative implementation of the factorial function as a loop handle
4091      * static int one(int k) { return 1; }
4092      * static int inc(int i, int acc, int k) { return i + 1; }
4093      * static int mult(int i, int acc, int k) { return i * acc; }
4094      * static boolean pred(int i, int acc, int k) { return i < k; }
4095      * static int fin(int i, int acc, int k) { return acc; }
4096      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4097      * // null initializer for counter, should initialize to 0
4098      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4099      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4100      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4101      * assertEquals(120, loop.invoke(5));
4102      * }</pre></blockquote>
4103      *
4104      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
4105      *
4106      * @return a method handle embodying the looping behavior as defined by the arguments.
4107      *
4108      * @throws IllegalArgumentException in case any of the constraints described above is violated.
4109      *
4110      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
4111      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
4112      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
4113      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
4114      * @since 9
4115      */
4116     public static MethodHandle loop(MethodHandle[]... clauses) {
4117         // Step 0: determine clause structure.
4118         checkLoop0(clauses);
4119 
4120         List<MethodHandle> init = new ArrayList<>();
4121         List<MethodHandle> step = new ArrayList<>();
4122         List<MethodHandle> pred = new ArrayList<>();
4123         List<MethodHandle> fini = new ArrayList<>();
4124 
4125         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
4126             init.add(clause[0]); // all clauses have at least length 1
4127             step.add(clause.length <= 1 ? null : clause[1]);
4128             pred.add(clause.length <= 2 ? null : clause[2]);
4129             fini.add(clause.length <= 3 ? null : clause[3]);
4130         });
4131 
4132         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
4133         final int nclauses = init.size();
4134 
4135         // Step 1A: determine iteration variables.
4136         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
4137         for (int i = 0; i < nclauses; ++i) {
4138             MethodHandle in = init.get(i);
4139             MethodHandle st = step.get(i);
4140             if (in == null && st == null) {
4141                 iterationVariableTypes.add(void.class);
4142             } else if (in != null && st != null) {
4143                 checkLoop1a(i, in, st);
4144                 iterationVariableTypes.add(in.type().returnType());
4145             } else {
4146                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
4147             }
4148         }
4149         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
4150                 collect(Collectors.toList());
4151 
4152         // Step 1B: determine loop parameters.
4153         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
4154         checkLoop1b(init, commonSuffix);
4155 
4156         // Step 1C: determine loop return type.
4157         // Step 1D: check other types.
4158         final Class<?> loopReturnType = fini.stream().filter(Objects::nonNull).map(MethodHandle::type).
4159                 map(MethodType::returnType).findFirst().orElse(void.class);
4160         checkLoop1cd(pred, fini, loopReturnType);
4161 
4162         // Step 2: determine parameter lists.
4163         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
4164         commonParameterSequence.addAll(commonSuffix);
4165         checkLoop2(step, pred, fini, commonParameterSequence);
4166 
4167         // Step 3: fill in omitted functions.
4168         for (int i = 0; i < nclauses; ++i) {
4169             Class<?> t = iterationVariableTypes.get(i);
4170             if (init.get(i) == null) {
4171                 init.set(i, empty(methodType(t, commonSuffix)));
4172             }
4173             if (step.get(i) == null) {
4174                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
4175             }
4176             if (pred.get(i) == null) {
4177                 pred.set(i, dropArguments(constant(boolean.class, true), 0, commonParameterSequence));
4178             }
4179             if (fini.get(i) == null) {
4180                 fini.set(i, empty(methodType(t, commonParameterSequence)));
4181             }
4182         }
4183 
4184         // Step 4: fill in missing parameter types.
4185         List<MethodHandle> finit = fillParameterTypes(init, commonSuffix);
4186         List<MethodHandle> fstep = fillParameterTypes(step, commonParameterSequence);
4187         List<MethodHandle> fpred = fillParameterTypes(pred, commonParameterSequence);
4188         List<MethodHandle> ffini = fillParameterTypes(fini, commonParameterSequence);
4189 
4190         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
4191                 allMatch(pl -> pl.equals(commonSuffix));
4192         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
4193                 allMatch(pl -> pl.equals(commonParameterSequence));
4194 
4195         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, commonPrefix, finit, fstep, fpred, ffini);
4196     }
4197 
4198     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
4199         return hs.stream().map(h -> {
4200             int pc = h.type().parameterCount();
4201             int tpsize = targetParams.size();
4202             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
4203         }).collect(Collectors.toList());
4204     }
4205 
4206     /**
4207      * Constructs a {@code while} loop from an initializer, a body, and a predicate. This is a convenience wrapper for
4208      * the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
4209      * <p>
4210      * The loop handle's result type is the same as the sole loop variable's, i.e., the result type of {@code init}.
4211      * The parameter type list of {@code init} also determines that of the resulting handle. The {@code pred} handle
4212      * must have an additional leading parameter of the same type as {@code init}'s result, and so must the {@code
4213      * body}. These constraints follow directly from those described for the {@linkplain MethodHandles#loop(MethodHandle[][])
4214      * generic loop combinator}.
4215      * <p>
4216      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
4217      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
4218      * passed to the loop.
4219      * <blockquote><pre>{@code
4220      * V init(A);
4221      * boolean pred(V, A);
4222      * V body(V, A);
4223      * V whileLoop(A a) {
4224      *   V v = init(a);
4225      *   while (pred(v, a)) {
4226      *     v = body(v, a);
4227      *   }
4228      *   return v;
4229      * }
4230      * }</pre></blockquote>
4231      * <p>
4232      * @apiNote Example:
4233      * <blockquote><pre>{@code
4234      * // implement the zip function for lists as a loop handle
4235      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
4236      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
4237      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
4238      *   zip.add(a.next());
4239      *   zip.add(b.next());
4240      *   return zip;
4241      * }
4242      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
4243      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
4244      * List<String> a = Arrays.asList("a", "b", "c", "d");
4245      * List<String> b = Arrays.asList("e", "f", "g", "h");
4246      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
4247      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
4248      * }</pre></blockquote>
4249      *
4250      * <p>
4251      * @implSpec The implementation of this method is equivalent to:
4252      * <blockquote><pre>{@code
4253      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
4254      *     MethodHandle[]
4255      *         checkExit = {null, null, pred, identity(init.type().returnType())},
4256      *         varBody = {init, body};
4257      *     return loop(checkExit, varBody);
4258      * }
4259      * }</pre></blockquote>
4260      *
4261      * @param init initializer: it should provide the initial value of the loop variable. This controls the loop's
4262      *             result type. Passing {@code null} or a {@code void} init function will make the loop's result type
4263      *             {@code void}.
4264      * @param pred condition for the loop, which may not be {@code null}.
4265      * @param body body of the loop, which may not be {@code null}.
4266      *
4267      * @return the value of the loop variable as the loop terminates.
4268      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
4269      *
4270      * @see MethodHandles#loop(MethodHandle[][])
4271      * @since 9
4272      */
4273     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
4274         MethodHandle fin = init == null || init.type().returnType() == void.class ? zero(void.class) :
4275                 identity(init.type().returnType());
4276         MethodHandle[] checkExit = {null, null, pred, fin};
4277         MethodHandle[] varBody = {init, body};
4278         return loop(checkExit, varBody);
4279     }
4280 
4281     /**
4282      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate. This is a convenience wrapper
4283      * for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
4284      * <p>
4285      * The loop handle's result type is the same as the sole loop variable's, i.e., the result type of {@code init}.
4286      * The parameter type list of {@code init} also determines that of the resulting handle. The {@code pred} handle
4287      * must have an additional leading parameter of the same type as {@code init}'s result, and so must the {@code
4288      * body}. These constraints follow directly from those described for the {@linkplain MethodHandles#loop(MethodHandle[][])
4289      * generic loop combinator}.
4290      * <p>
4291      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
4292      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
4293      * passed to the loop.
4294      * <blockquote><pre>{@code
4295      * V init(A);
4296      * boolean pred(V, A);
4297      * V body(V, A);
4298      * V doWhileLoop(A a) {
4299      *   V v = init(a);
4300      *   do {
4301      *     v = body(v, a);
4302      *   } while (pred(v, a));
4303      *   return v;
4304      * }
4305      * }</pre></blockquote>
4306      * <p>
4307      * @apiNote Example:
4308      * <blockquote><pre>{@code
4309      * // int i = 0; while (i < limit) { ++i; } return i; => limit
4310      * static int zero(int limit) { return 0; }
4311      * static int step(int i, int limit) { return i + 1; }
4312      * static boolean pred(int i, int limit) { return i < limit; }
4313      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
4314      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
4315      * assertEquals(23, loop.invoke(23));
4316      * }</pre></blockquote>
4317      *
4318      * <p>
4319      * @implSpec The implementation of this method is equivalent to:
4320      * <blockquote><pre>{@code
4321      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
4322      *     MethodHandle[] clause = { init, body, pred, identity(init.type().returnType()) };
4323      *     return loop(clause);
4324      * }
4325      * }</pre></blockquote>
4326      *
4327      *
4328      * @param init initializer: it should provide the initial value of the loop variable. This controls the loop's
4329      *             result type. Passing {@code null} or a {@code void} init function will make the loop's result type
4330      *             {@code void}.
4331      * @param pred condition for the loop, which may not be {@code null}.
4332      * @param body body of the loop, which may not be {@code null}.
4333      *
4334      * @return the value of the loop variable as the loop terminates.
4335      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
4336      *
4337      * @see MethodHandles#loop(MethodHandle[][])
4338      * @since 9
4339      */
4340     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
4341         MethodHandle fin = init == null || init.type().returnType() == void.class ? zero(void.class) :
4342                 identity(init.type().returnType());
4343         MethodHandle[] clause = {init, body, pred, fin};
4344         return loop(clause);
4345     }
4346 
4347     /**
4348      * Constructs a loop that runs a given number of iterations. The loop counter is an {@code int} initialized from the
4349      * {@code iterations} handle evaluation result. The counter is passed to the {@code body} function, so that must
4350      * accept an initial {@code int} argument. The result of the loop execution is the final value of the additional
4351      * local state. This is a convenience wrapper for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop
4352      * combinator}.
4353      * <p>
4354      * The result type and parameter type list of {@code init} determine those of the resulting handle. The {@code
4355      * iterations} handle must accept the same parameter types as {@code init} but return an {@code int}. The {@code
4356      * body} handle must accept the same parameter types as well, preceded by an {@code int} parameter for the counter,
4357      * and a parameter of the same type as {@code init}'s result. These constraints follow directly from those described
4358      * for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
4359      * <p>
4360      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
4361      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
4362      * passed to the loop.
4363      * <blockquote><pre>{@code
4364      * int iterations(A);
4365      * V init(A);
4366      * V body(int, V, A);
4367      * V countedLoop(A a) {
4368      *   int end = iterations(a);
4369      *   V v = init(a);
4370      *   for (int i = 0; i < end; ++i) {
4371      *     v = body(i, v, a);
4372      *   }
4373      *   return v;
4374      * }
4375      * }</pre></blockquote>
4376      * <p>
4377      * @apiNote Example:
4378      * <blockquote><pre>{@code
4379      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
4380      * // => a variation on a well known theme
4381      * static String start(String arg) { return arg; }
4382      * static String step(int counter, String v, String arg) { return "na " + v; }
4383      * // assume MH_start and MH_step are handles to the two methods above
4384      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
4385      * MethodHandle loop = MethodHandles.countedLoop(fit13, MH_start, MH_step);
4386      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
4387      * }</pre></blockquote>
4388      *
4389      * <p>
4390      * @implSpec The implementation of this method is equivalent to:
4391      * <blockquote><pre>{@code
4392      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
4393      *     return countedLoop(null, iterations, init, body);  // null => constant zero
4394      * }
4395      * }</pre></blockquote>
4396      *
4397      * @param iterations a handle to return the number of iterations this loop should run.
4398      * @param init initializer for additional loop state. This determines the loop's result type.
4399      *             Passing {@code null} or a {@code void} init function will make the loop's result type
4400      *             {@code void}.
4401      * @param body the body of the loop, which must not be {@code null}.
4402      *             It must accept an initial {@code int} parameter (for the counter), and then any
4403      *             additional loop-local variable plus loop parameters.
4404      *
4405      * @return a method handle representing the loop.
4406      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
4407      *
4408      * @since 9
4409      */
4410     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
4411         return countedLoop(null, iterations, init, body);
4412     }
4413 
4414     /**
4415      * Constructs a loop that counts over a range of numbers. The loop counter is an {@code int} that will be
4416      * initialized to the {@code int} value returned from the evaluation of the {@code start} handle and run to the
4417      * value returned from {@code end} (exclusively) with a step width of 1. The counter value is passed to the {@code
4418      * body} function in each iteration; it has to accept an initial {@code int} parameter
4419      * for that. The result of the loop execution is the final value of the additional local state
4420      * obtained by running {@code init}.
4421      * This is a
4422      * convenience wrapper for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
4423      * <p>
4424      * The constraints for the {@code init} and {@code body} handles are the same as for {@link
4425      * #countedLoop(MethodHandle, MethodHandle, MethodHandle)}. Additionally, the {@code start} and {@code end} handles
4426      * must return an {@code int} and accept the same parameters as {@code init}.
4427      * <p>
4428      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
4429      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
4430      * passed to the loop.
4431      * <blockquote><pre>{@code
4432      * int start(A);
4433      * int end(A);
4434      * V init(A);
4435      * V body(int, V, A);
4436      * V countedLoop(A a) {
4437      *   int s = start(a);
4438      *   int e = end(a);
4439      *   V v = init(a);
4440      *   for (int i = s; i < e; ++i) {
4441      *     v = body(i, v, a);
4442      *   }
4443      *   return v;
4444      * }
4445      * }</pre></blockquote>
4446      *
4447      * <p>
4448      * @implSpec The implementation of this method is equivalent to:
4449      * <blockquote><pre>{@code
4450      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
4451      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
4452      *     // assume MH_increment and MH_lessThan are handles to x+1 and x<y of type int,
4453      *     // assume MH_decrement is a handle to x-1 of type int
4454      *     MethodHandle[]
4455      *         indexVar = {start, MH_increment}, // i = start; i = i+1
4456      *         loopLimit = {end, null, MH_lessThan, returnVar }, // i<end
4457      *         bodyClause = {init,
4458      *                       filterArgument(dropArguments(body, 1, int.class), 0, MH_decrement}; // v = body(i-1, v)
4459      *     return loop(indexVar, loopLimit, bodyClause);
4460      * }
4461      * }</pre></blockquote>
4462      *
4463      * @param start a handle to return the start value of the loop counter.
4464      *              If it is {@code null}, a constant zero is assumed.
4465      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to {@code end-1}).
4466      * @param init initializer for additional loop state. This determines the loop's result type.
4467      *             Passing {@code null} or a {@code void} init function will make the loop's result type
4468      *             {@code void}.
4469      * @param body the body of the loop, which must not be {@code null}.
4470      *             It must accept an initial {@code int} parameter (for the counter), and then any
4471      *             additional loop-local variable plus loop parameters.
4472      *
4473      * @return a method handle representing the loop.
4474      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
4475      *
4476      * @since 9
4477      */
4478     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
4479         MethodHandle defaultResultHandle = init == null || init.type().returnType() == void.class ?
4480                 zero(void.class) :
4481                 identity(init.type().returnType());
4482         MethodHandle adaptedBody = body == null ? dropArguments(defaultResultHandle, 0, int.class) : body;
4483         MethodHandle returnVar = dropArguments(defaultResultHandle, 0, int.class, int.class);
4484         MethodHandle[] indexVar = {start, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep)};
4485         MethodHandle[] loopLimit = {end, null, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred),
4486                 returnVar};
4487         MethodHandle[] bodyClause = {init,
4488                 filterArgument(dropArguments(adaptedBody, 1, int.class), 0,
4489                         MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_decrementCounter))};
4490         return loop(indexVar, loopLimit, bodyClause);
4491     }
4492 
4493     /**
4494      * Constructs a loop that ranges over the elements produced by an {@code Iterator<T>}.
4495      * The iterator will be produced by the evaluation of the {@code iterator} handle.
4496      * This handle must have {@link java.util.Iterator} as its return type.
4497      * If this handle is passed as {@code null} the method {@link Iterable#iterator} will be used instead,
4498      * and will be applied to a leading argument of the loop handle.
4499      * Each value produced by the iterator is passed to the {@code body}, which must accept an initial {@code T} parameter.
4500      * The result of the loop execution is the final value of the additional local state
4501      * obtained by running {@code init}.
4502      * <p>
4503      * This is a convenience wrapper for the
4504      * {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}, and the constraints imposed on the {@code body}
4505      * handle follow directly from those described for the latter.
4506      * <p>
4507      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
4508      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
4509      * structure the loop iterates over, and {@code A}/{@code a}, that of the argument passed to the loop.
4510      * <blockquote><pre>{@code
4511      * Iterator<T> iterator(A);  // defaults to Iterable::iterator
4512      * V init(A);
4513      * V body(T,V,A);
4514      * V iteratedLoop(A a) {
4515      *   Iterator<T> it = iterator(a);
4516      *   V v = init(a);
4517      *   for (T t : it) {
4518      *     v = body(t, v, a);
4519      *   }
4520      *   return v;
4521      * }
4522      * }</pre></blockquote>
4523      * <p>
4524      * The type {@code T} may be either a primitive or reference.
4525      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type
4526      * {@code Iterator}, the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body}
4527      * to {@code Object} as if by the {@link MethodHandle#asType asType} conversion method.
4528      * Therefore, if an iterator of the wrong type appears as the loop is executed,
4529      * runtime exceptions may occur as the result of dynamic conversions performed by {@code asType}.
4530      * <p>
4531      * @apiNote Example:
4532      * <blockquote><pre>{@code
4533      * // reverse a list
4534      * static List<String> reverseStep(String e, List<String> r, List<String> l) {
4535      *   r.add(0, e);
4536      *   return r;
4537      * }
4538      * static List<String> newArrayList(List<String> l) { return new ArrayList<>(); }
4539      * // assume MH_reverseStep, MH_newArrayList are handles to the above methods
4540      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
4541      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
4542      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
4543      * assertEquals(reversedList, (List<String>) loop.invoke(list));
4544      * }</pre></blockquote>
4545      * <p>
4546      * @implSpec The implementation of this method is equivalent to (excluding error handling):
4547      * <blockquote><pre>{@code
4548      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
4549      *     // assume MH_next and MH_hasNext are handles to methods of Iterator
4550      *     Class<?> itype = iterator.type().returnType();
4551      *     Class<?> ttype = body.type().parameterType(0);
4552      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, itype);
4553      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
4554      *     MethodHandle[]
4555      *         iterVar = {iterator, null, MH_hasNext, returnVar}, // it = iterator(); while (it.hasNext)
4556      *         bodyClause = {init, filterArgument(body, 0, nextVal)};  // v = body(t, v, a);
4557      *     return loop(iterVar, bodyClause);
4558      * }
4559      * }</pre></blockquote>
4560      *
4561      * @param iterator a handle to return the iterator to start the loop.
4562      *             The handle must have {@link java.util.Iterator} as its return type.
4563      *             Passing {@code null} will make the loop call {@link Iterable#iterator()} on the first
4564      *             incoming value.
4565      * @param init initializer for additional loop state. This determines the loop's result type.
4566      *             Passing {@code null} or a {@code void} init function will make the loop's result type
4567      *             {@code void}.
4568      * @param body the body of the loop, which must not be {@code null}.
4569      *             It must accept an initial {@code T} parameter (for the iterated values), and then any
4570      *             additional loop-local variable plus loop parameters.
4571      *
4572      * @return a method handle embodying the iteration loop functionality.
4573      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
4574      *
4575      * @since 9
4576      */
4577     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
4578         checkIteratedLoop(iterator, body);
4579         Class<?> resultType = init == null ?
4580                 body == null ? void.class : body.type().returnType() :
4581                 init.type().returnType();
4582         boolean voidResult = resultType == void.class;
4583 
4584         MethodHandle initIterator;
4585         if (iterator == null) {
4586             MethodHandle initit = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
4587             initIterator = initit.asType(initit.type().changeParameterType(0,
4588                     body.type().parameterType(voidResult ? 1 : 2)));
4589         } else {
4590             initIterator = iterator.asType(iterator.type().changeReturnType(Iterator.class));
4591         }
4592 
4593         Class<?> ttype = body.type().parameterType(0);
4594 
4595         MethodHandle returnVar =
4596                 dropArguments(voidResult ? zero(void.class) : identity(resultType), 0, Iterator.class);
4597         MethodHandle initnx = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
4598         MethodHandle nextVal = initnx.asType(initnx.type().changeReturnType(ttype));
4599 
4600         MethodHandle[] iterVar = {initIterator, null, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred),
4601                 returnVar};
4602         MethodHandle[] bodyClause = {init, filterArgument(body, 0, nextVal)};
4603 
4604         return loop(iterVar, bodyClause);
4605     }
4606 
4607     /**
4608      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
4609      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
4610      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
4611      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
4612      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
4613      * {@code try-finally} handle.
4614      * <p>
4615      * The {@code cleanup} handle will be passed one or two additional leading arguments.
4616      * The first is the exception thrown during the
4617      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
4618      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
4619      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
4620      * The second argument is not present if the {@code target} handle has a {@code void} return type.
4621      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
4622      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
4623      * <p>
4624      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
4625      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
4626      * two extra leading parameters:<ul>
4627      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
4628      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
4629      * the result from the execution of the {@code target} handle.
4630      * This parameter is not present if the {@code target} returns {@code void}.
4631      * </ul>
4632      * <p>
4633      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
4634      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
4635      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
4636      * the cleanup.
4637      * <blockquote><pre>{@code
4638      * V target(A..., B...);
4639      * V cleanup(Throwable, V, A...);
4640      * V adapter(A... a, B... b) {
4641      *   V result = (zero value for V);
4642      *   Throwable throwable = null;
4643      *   try {
4644      *     result = target(a..., b...);
4645      *   } catch (Throwable t) {
4646      *     throwable = t;
4647      *     throw t;
4648      *   } finally {
4649      *     result = cleanup(throwable, result, a...);
4650      *   }
4651      *   return result;
4652      * }
4653      * }</pre></blockquote>
4654      * <p>
4655      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
4656      * be modified by execution of the target, and so are passed unchanged
4657      * from the caller to the cleanup, if it is invoked.
4658      * <p>
4659      * The target and cleanup must return the same type, even if the cleanup
4660      * always throws.
4661      * To create such a throwing cleanup, compose the cleanup logic
4662      * with {@link #throwException throwException},
4663      * in order to create a method handle of the correct return type.
4664      * <p>
4665      * Note that {@code tryFinally} never converts exceptions into normal returns.
4666      * In rare cases where exceptions must be converted in that way, first wrap
4667      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
4668      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
4669      *
4670      * @param target the handle whose execution is to be wrapped in a {@code try} block.
4671      * @param cleanup the handle that is invoked in the finally block.
4672      *
4673      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
4674      * @throws NullPointerException if any argument is null
4675      * @throws IllegalArgumentException if {@code cleanup} does not accept
4676      *          the required leading arguments, or if the method handle types do
4677      *          not match in their return types and their
4678      *          corresponding trailing parameters
4679      *
4680      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
4681      * @since 9
4682      */
4683     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
4684         List<Class<?>> targetParamTypes = target.type().parameterList();
4685         List<Class<?>> cleanupParamTypes = cleanup.type().parameterList();
4686         Class<?> rtype = target.type().returnType();
4687 
4688         checkTryFinally(target, cleanup);
4689 
4690         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
4691         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
4692         // target parameter list.
4693         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
4694         MethodHandle aTarget = target.asSpreader(Object[].class, target.type().parameterCount());
4695         MethodHandle aCleanup = cleanup.asSpreader(Object[].class, targetParamTypes.size());
4696 
4697         return MethodHandleImpl.makeTryFinally(aTarget, aCleanup, rtype, targetParamTypes);
4698     }
4699 
4700     /**
4701      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
4702      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
4703      * before the folded arguments.
4704      * <p>
4705      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
4706      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
4707      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
4708      * 0.
4709      * <p>
4710      * @apiNote Example:
4711      * <blockquote><pre>{@code
4712     import static java.lang.invoke.MethodHandles.*;
4713     import static java.lang.invoke.MethodType.*;
4714     ...
4715     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4716     "println", methodType(void.class, String.class))
4717     .bindTo(System.out);
4718     MethodHandle cat = lookup().findVirtual(String.class,
4719     "concat", methodType(String.class, String.class));
4720     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4721     MethodHandle catTrace = foldArguments(cat, 1, trace);
4722     // also prints "jum":
4723     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4724      * }</pre></blockquote>
4725      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4726      * represents the result type of the {@code target} and resulting adapter.
4727      * {@code V}/{@code v} represent the type and value of the parameter and argument
4728      * of {@code target} that precedes the folding position; {@code V} also is
4729      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4730      * types and values of the {@code N} parameters and arguments at the folding
4731      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
4732      * and values of the {@code target} parameters and arguments that precede and
4733      * follow the folded parameters and arguments starting at {@code pos},
4734      * respectively.
4735      * <blockquote><pre>{@code
4736      * // there are N arguments in A...
4737      * T target(Z..., V, A[N]..., B...);
4738      * V combiner(A...);
4739      * T adapter(Z... z, A... a, B... b) {
4740      *   V v = combiner(a...);
4741      *   return target(z..., v, a..., b...);
4742      * }
4743      * // and if the combiner has a void return:
4744      * T target2(Z..., A[N]..., B...);
4745      * void combiner2(A...);
4746      * T adapter2(Z... z, A... a, B... b) {
4747      *   combiner2(a...);
4748      *   return target2(z..., a..., b...);
4749      * }
4750      * }</pre></blockquote>
4751      * <p>
4752      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4753      * variable-arity method handle}, even if the original target method handle was.
4754      *
4755      * @param target the method handle to invoke after arguments are combined
4756      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
4757      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4758      * @param combiner method handle to call initially on the incoming arguments
4759      * @return method handle which incorporates the specified argument folding logic
4760      * @throws NullPointerException if either argument is null
4761      * @throws IllegalArgumentException if {@code combiner}'s return type
4762      *          is non-void and not the same as the argument type at position {@code pos} of
4763      *          the target signature, or if the {@code N} argument types at position {@code pos}
4764      *          of the target signature
4765      *          (skipping one matching the {@code combiner}'s return type)
4766      *          are not identical with the argument types of {@code combiner}
4767      *
4768      * @see #foldArguments(MethodHandle, MethodHandle)
4769      * @since 9
4770      */
4771     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
4772         MethodType targetType = target.type();
4773         MethodType combinerType = combiner.type();
4774         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
4775         BoundMethodHandle result = target.rebind();
4776         boolean dropResult = rtype == void.class;
4777         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
4778         MethodType newType = targetType;
4779         if (!dropResult) {
4780             newType = newType.dropParameterTypes(pos, pos + 1);
4781         }
4782         result = result.copyWithExtendL(newType, lform, combiner);
4783         return result;
4784     }
4785 
4786 
4787     private static void checkLoop0(MethodHandle[][] clauses) {
4788         if (clauses == null || clauses.length == 0) {
4789             throw newIllegalArgumentException("null or no clauses passed");
4790         }
4791         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
4792             throw newIllegalArgumentException("null clauses are not allowed");
4793         }
4794         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
4795             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
4796         }
4797     }
4798 
4799     private static void checkLoop1a(int i, MethodHandle in, MethodHandle st) {
4800         if (in.type().returnType() != st.type().returnType()) {
4801             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
4802                     st.type().returnType());
4803         }
4804     }
4805 
4806     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
4807         final List<Class<?>> empty = List.of();
4808         final List<MethodHandle> nonNullInits = init.stream().filter(Objects::nonNull).collect(Collectors.toList());
4809         if (nonNullInits.isEmpty()) {
4810             final List<Class<?>> longest = Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).
4811                     // take only those that can contribute to a common suffix because they are longer than the prefix
4812                     map(MethodHandle::type).filter(t -> t.parameterCount() > cpSize).map(MethodType::parameterList).
4813                     reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
4814             return longest.size() == 0 ? empty : longest.subList(cpSize, longest.size());
4815         } else {
4816             return nonNullInits.stream().map(MethodHandle::type).map(MethodType::parameterList).
4817                     reduce((p, q) -> p.size() >= q.size() ? p : q).get();
4818         }
4819     }
4820 
4821     private static void checkLoop1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
4822         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::parameterList).
4823                 anyMatch(pl -> !pl.equals(commonSuffix.subList(0, pl.size())))) {
4824             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
4825                     " (common suffix: " + commonSuffix + ")");
4826         }
4827     }
4828 
4829     private static void checkLoop1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
4830         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4831                 anyMatch(t -> t != loopReturnType)) {
4832             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
4833                     loopReturnType + ")");
4834         }
4835 
4836         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
4837             throw newIllegalArgumentException("no predicate found", pred);
4838         }
4839         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4840                 anyMatch(t -> t != boolean.class)) {
4841             throw newIllegalArgumentException("predicates must have boolean return type", pred);
4842         }
4843     }
4844 
4845     private static void checkLoop2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
4846         final int cpSize = commonParameterSequence.size();
4847         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
4848                 map(MethodType::parameterList).
4849                 anyMatch(pl -> pl.size() > cpSize || !pl.equals(commonParameterSequence.subList(0, pl.size())))) {
4850             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
4851                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
4852         }
4853     }
4854 
4855     private static void checkIteratedLoop(MethodHandle iterator, MethodHandle body) {
4856         if (null != iterator && !Iterator.class.isAssignableFrom(iterator.type().returnType())) {
4857             throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
4858         }
4859         if (null == body) {
4860             throw newIllegalArgumentException("iterated loop body must not be null");
4861         }
4862     }
4863 
4864     private static void checkTryFinally(MethodHandle target, MethodHandle cleanup) {
4865         Class<?> rtype = target.type().returnType();
4866         if (rtype != cleanup.type().returnType()) {
4867             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
4868         }
4869         List<Class<?>> cleanupParamTypes = cleanup.type().parameterList();
4870         if (!Throwable.class.isAssignableFrom(cleanupParamTypes.get(0))) {
4871             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
4872         }
4873         if (rtype != void.class && cleanupParamTypes.get(1) != rtype) {
4874             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
4875         }
4876         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
4877         // target parameter list.
4878         int cleanupArgIndex = rtype == void.class ? 1 : 2;
4879         List<Class<?>> cleanupArgSuffix = cleanupParamTypes.subList(cleanupArgIndex, cleanupParamTypes.size());
4880         List<Class<?>> targetParamTypes = target.type().parameterList();
4881         if (targetParamTypes.size() < cleanupArgSuffix.size() ||
4882                 !cleanupArgSuffix.equals(targetParamTypes.subList(0, cleanupParamTypes.size() - cleanupArgIndex))) {
4883             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
4884                     cleanup.type(), target.type());
4885         }
4886     }
4887 
4888 }