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.*;
  30 
  31 import sun.invoke.util.ValueConversions;
  32 import sun.invoke.util.VerifyAccess;
  33 import sun.invoke.util.Wrapper;
  34 import sun.reflect.CallerSensitive;
  35 import sun.reflect.Reflection;
  36 import sun.reflect.misc.ReflectUtil;
  37 import sun.security.util.SecurityConstants;
  38 import java.lang.invoke.LambdaForm.BasicType;
  39 
  40 import static java.lang.invoke.MethodHandleStatics.*;
  41 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  42 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  43 import java.util.concurrent.ConcurrentHashMap;
  44 import java.util.stream.Collectors;
  45 import java.util.stream.Stream;
  46 
  47 /**
  48  * This class consists exclusively of static methods that operate on or return
  49  * method handles. They fall into several categories:
  50  * <ul>
  51  * <li>Lookup methods which help create method handles for methods and fields.
  52  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  53  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  54  * </ul>
  55  *
  56  * @author John Rose, JSR 292 EG
  57  * @since 1.7
  58  */
  59 public class MethodHandles {
  60 
  61     private MethodHandles() { }  // do not instantiate
  62 
  63     private static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  64 
  65     // See IMPL_LOOKUP below.
  66 
  67     //// Method handle creation from ordinary methods.
  68 
  69     /**
  70      * Returns a {@link Lookup lookup object} with
  71      * full capabilities to emulate all supported bytecode behaviors of the caller.
  72      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  73      * Factory methods on the lookup object can create
  74      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  75      * for any member that the caller has access to via bytecodes,
  76      * including protected and private fields and methods.
  77      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
  78      * Do not store it in place where untrusted code can access it.
  79      * <p>
  80      * This method is caller sensitive, which means that it may return different
  81      * values to different callers.
  82      * <p>
  83      * For any given caller class {@code C}, the lookup object returned by this call
  84      * has equivalent capabilities to any lookup object
  85      * supplied by the JVM to the bootstrap method of an
  86      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
  87      * executing in the same caller class {@code C}.
  88      * @return a lookup object for the caller of this method, with private access
  89      */
  90     @CallerSensitive
  91     public static Lookup lookup() {
  92         return new Lookup(Reflection.getCallerClass());
  93     }
  94 
  95     /**
  96      * Returns a {@link Lookup lookup object} which is trusted minimally.
  97      * It can only be used to create method handles to
  98      * publicly accessible fields and methods.
  99      * <p>
 100      * As a matter of pure convention, the {@linkplain Lookup#lookupClass lookup class}
 101      * of this lookup object will be {@link java.lang.Object}.
 102      *
 103      * <p style="font-size:smaller;">
 104      * <em>Discussion:</em>
 105      * The lookup class can be changed to any other class {@code C} using an expression of the form
 106      * {@link Lookup#in publicLookup().in(C.class)}.
 107      * Since all classes have equal access to public names,
 108      * such a change would confer no new access rights.
 109      * A public lookup object is always subject to
 110      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 111      * Also, it cannot access
 112      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 113      * @return a lookup object which is trusted minimally
 114      */
 115     public static Lookup publicLookup() {
 116         return Lookup.PUBLIC_LOOKUP;
 117     }
 118 
 119     /**
 120      * Performs an unchecked "crack" of a
 121      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 122      * The result is as if the user had obtained a lookup object capable enough
 123      * to crack the target method handle, called
 124      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 125      * on the target to obtain its symbolic reference, and then called
 126      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 127      * to resolve the symbolic reference to a member.
 128      * <p>
 129      * If there is a security manager, its {@code checkPermission} method
 130      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 131      * @param <T> the desired type of the result, either {@link Member} or a subtype
 132      * @param target a direct method handle to crack into symbolic reference components
 133      * @param expected a class object representing the desired result type {@code T}
 134      * @return a reference to the method, constructor, or field object
 135      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 136      * @exception NullPointerException if either argument is {@code null}
 137      * @exception IllegalArgumentException if the target is not a direct method handle
 138      * @exception ClassCastException if the member is not of the expected type
 139      * @since 1.8
 140      */
 141     public static <T extends Member> T
 142     reflectAs(Class<T> expected, MethodHandle target) {
 143         SecurityManager smgr = System.getSecurityManager();
 144         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 145         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 146         return lookup.revealDirect(target).reflectAs(expected, lookup);
 147     }
 148     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 149     private static final java.security.Permission ACCESS_PERMISSION =
 150         new ReflectPermission("suppressAccessChecks");
 151 
 152     /**
 153      * A <em>lookup object</em> is a factory for creating method handles,
 154      * when the creation requires access checking.
 155      * Method handles do not perform
 156      * access checks when they are called, but rather when they are created.
 157      * Therefore, method handle access
 158      * restrictions must be enforced when a method handle is created.
 159      * The caller class against which those restrictions are enforced
 160      * is known as the {@linkplain #lookupClass lookup class}.
 161      * <p>
 162      * A lookup class which needs to create method handles will call
 163      * {@link MethodHandles#lookup MethodHandles.lookup} to create a factory for itself.
 164      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 165      * determined, and securely stored in the {@code Lookup} object.
 166      * The lookup class (or its delegates) may then use factory methods
 167      * on the {@code Lookup} object to create method handles for access-checked members.
 168      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 169      * even private ones.
 170      *
 171      * <h1><a name="lookups"></a>Lookup Factory Methods</h1>
 172      * The factory methods on a {@code Lookup} object correspond to all major
 173      * use cases for methods, constructors, and fields.
 174      * Each method handle created by a factory method is the functional
 175      * equivalent of a particular <em>bytecode behavior</em>.
 176      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 177      * Here is a summary of the correspondence between these factory methods and
 178      * the behavior of the resulting method handles:
 179      * <table border=1 cellpadding=5 summary="lookup method behaviors">
 180      * <tr>
 181      *     <th><a name="equiv"></a>lookup expression</th>
 182      *     <th>member</th>
 183      *     <th>bytecode behavior</th>
 184      * </tr>
 185      * <tr>
 186      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</td>
 187      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 188      * </tr>
 189      * <tr>
 190      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</td>
 191      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 192      * </tr>
 193      * <tr>
 194      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</td>
 195      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 196      * </tr>
 197      * <tr>
 198      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</td>
 199      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 200      * </tr>
 201      * <tr>
 202      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</td>
 203      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 204      * </tr>
 205      * <tr>
 206      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</td>
 207      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 208      * </tr>
 209      * <tr>
 210      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</td>
 211      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 212      * </tr>
 213      * <tr>
 214      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</td>
 215      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 216      * </tr>
 217      * <tr>
 218      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</td>
 219      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 220      * </tr>
 221      * <tr>
 222      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</td>
 223      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 224      * </tr>
 225      * <tr>
 226      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 227      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 228      * </tr>
 229      * <tr>
 230      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</td>
 231      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 232      * </tr>
 233      * <tr>
 234      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 235      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 236      * </tr>
 237      * <tr>
 238      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</td>
 239      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 240      * </tr>
 241      * </table>
 242      *
 243      * Here, the type {@code C} is the class or interface being searched for a member,
 244      * documented as a parameter named {@code refc} in the lookup methods.
 245      * The method type {@code MT} is composed from the return type {@code T}
 246      * and the sequence of argument types {@code A*}.
 247      * The constructor also has a sequence of argument types {@code A*} and
 248      * is deemed to return the newly-created object of type {@code C}.
 249      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 250      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 251      * if it is present, it is always the leading argument to the method handle invocation.
 252      * (In the case of some {@code protected} members, {@code this} may be
 253      * restricted in type to the lookup class; see below.)
 254      * The name {@code arg} stands for all the other method handle arguments.
 255      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 256      * stands for a null reference if the accessed method or field is static,
 257      * and {@code this} otherwise.
 258      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 259      * for reflective objects corresponding to the given members.
 260      * <p>
 261      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 262      * as if by {@code ldc CONSTANT_Class}.
 263      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 264      * <p>
 265      * In cases where the given member is of variable arity (i.e., a method or constructor)
 266      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 267      * In all other cases, the returned method handle will be of fixed arity.
 268      * <p style="font-size:smaller;">
 269      * <em>Discussion:</em>
 270      * The equivalence between looked-up method handles and underlying
 271      * class members and bytecode behaviors
 272      * can break down in a few ways:
 273      * <ul style="font-size:smaller;">
 274      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 275      * the lookup can still succeed, even when there is no equivalent
 276      * Java expression or bytecoded constant.
 277      * <li>Likewise, if {@code T} or {@code MT}
 278      * is not symbolically accessible from the lookup class's loader,
 279      * the lookup can still succeed.
 280      * For example, lookups for {@code MethodHandle.invokeExact} and
 281      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 282      * <li>If there is a security manager installed, it can forbid the lookup
 283      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 284      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 285      * constant is not subject to security manager checks.
 286      * <li>If the looked-up method has a
 287      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 288      * the method handle creation may fail, due to the method handle
 289      * type having too many parameters.
 290      * </ul>
 291      *
 292      * <h1><a name="access"></a>Access checking</h1>
 293      * Access checks are applied in the factory methods of {@code Lookup},
 294      * when a method handle is created.
 295      * This is a key difference from the Core Reflection API, since
 296      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 297      * performs access checking against every caller, on every call.
 298      * <p>
 299      * All access checks start from a {@code Lookup} object, which
 300      * compares its recorded lookup class against all requests to
 301      * create method handles.
 302      * A single {@code Lookup} object can be used to create any number
 303      * of access-checked method handles, all checked against a single
 304      * lookup class.
 305      * <p>
 306      * A {@code Lookup} object can be shared with other trusted code,
 307      * such as a metaobject protocol.
 308      * A shared {@code Lookup} object delegates the capability
 309      * to create method handles on private members of the lookup class.
 310      * Even if privileged code uses the {@code Lookup} object,
 311      * the access checking is confined to the privileges of the
 312      * original lookup class.
 313      * <p>
 314      * A lookup can fail, because
 315      * the containing class is not accessible to the lookup class, or
 316      * because the desired class member is missing, or because the
 317      * desired class member is not accessible to the lookup class, or
 318      * because the lookup object is not trusted enough to access the member.
 319      * In any of these cases, a {@code ReflectiveOperationException} will be
 320      * thrown from the attempted lookup.  The exact class will be one of
 321      * the following:
 322      * <ul>
 323      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 324      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 325      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 326      * </ul>
 327      * <p>
 328      * In general, the conditions under which a method handle may be
 329      * looked up for a method {@code M} are no more restrictive than the conditions
 330      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 331      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 332      * a method handle lookup will generally raise a corresponding
 333      * checked exception, such as {@code NoSuchMethodException}.
 334      * And the effect of invoking the method handle resulting from the lookup
 335      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 336      * to executing the compiled, verified, and resolved call to {@code M}.
 337      * The same point is true of fields and constructors.
 338      * <p style="font-size:smaller;">
 339      * <em>Discussion:</em>
 340      * Access checks only apply to named and reflected methods,
 341      * constructors, and fields.
 342      * Other method handle creation methods, such as
 343      * {@link MethodHandle#asType MethodHandle.asType},
 344      * do not require any access checks, and are used
 345      * independently of any {@code Lookup} object.
 346      * <p>
 347      * If the desired member is {@code protected}, the usual JVM rules apply,
 348      * including the requirement that the lookup class must be either be in the
 349      * same package as the desired member, or must inherit that member.
 350      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 351      * In addition, if the desired member is a non-static field or method
 352      * in a different package, the resulting method handle may only be applied
 353      * to objects of the lookup class or one of its subclasses.
 354      * This requirement is enforced by narrowing the type of the leading
 355      * {@code this} parameter from {@code C}
 356      * (which will necessarily be a superclass of the lookup class)
 357      * to the lookup class itself.
 358      * <p>
 359      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 360      * that the receiver argument must match both the resolved method <em>and</em>
 361      * the current class.  Again, this requirement is enforced by narrowing the
 362      * type of the leading parameter to the resulting method handle.
 363      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 364      * <p>
 365      * The JVM represents constructors and static initializer blocks as internal methods
 366      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 367      * The internal syntax of invocation instructions allows them to refer to such internal
 368      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 369      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 370      * <p>
 371      * In some cases, access between nested classes is obtained by the Java compiler by creating
 372      * an wrapper method to access a private method of another class
 373      * in the same top-level declaration.
 374      * For example, a nested class {@code C.D}
 375      * can access private members within other related classes such as
 376      * {@code C}, {@code C.D.E}, or {@code C.B},
 377      * but the Java compiler may need to generate wrapper methods in
 378      * those related classes.  In such cases, a {@code Lookup} object on
 379      * {@code C.E} would be unable to those private members.
 380      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 381      * which can transform a lookup on {@code C.E} into one on any of those other
 382      * classes, without special elevation of privilege.
 383      * <p>
 384      * The accesses permitted to a given lookup object may be limited,
 385      * according to its set of {@link #lookupModes lookupModes},
 386      * to a subset of members normally accessible to the lookup class.
 387      * For example, the {@link MethodHandles#publicLookup publicLookup}
 388      * method produces a lookup object which is only allowed to access
 389      * public members in public classes.
 390      * The caller sensitive method {@link MethodHandles#lookup lookup}
 391      * produces a lookup object with full capabilities relative to
 392      * its caller class, to emulate all supported bytecode behaviors.
 393      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 394      * with fewer access modes than the original lookup object.
 395      *
 396      * <p style="font-size:smaller;">
 397      * <a name="privacc"></a>
 398      * <em>Discussion of private access:</em>
 399      * We say that a lookup has <em>private access</em>
 400      * if its {@linkplain #lookupModes lookup modes}
 401      * include the possibility of accessing {@code private} members.
 402      * As documented in the relevant methods elsewhere,
 403      * only lookups with private access possess the following capabilities:
 404      * <ul style="font-size:smaller;">
 405      * <li>access private fields, methods, and constructors of the lookup class
 406      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 407      *     such as {@code Class.forName}
 408      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 409      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 410      *     for classes accessible to the lookup class
 411      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 412      *     within the same package member
 413      * </ul>
 414      * <p style="font-size:smaller;">
 415      * Each of these permissions is a consequence of the fact that a lookup object
 416      * with private access can be securely traced back to an originating class,
 417      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 418      * can be reliably determined and emulated by method handles.
 419      *
 420      * <h1><a name="secmgr"></a>Security manager interactions</h1>
 421      * Although bytecode instructions can only refer to classes in
 422      * a related class loader, this API can search for methods in any
 423      * class, as long as a reference to its {@code Class} object is
 424      * available.  Such cross-loader references are also possible with the
 425      * Core Reflection API, and are impossible to bytecode instructions
 426      * such as {@code invokestatic} or {@code getfield}.
 427      * There is a {@linkplain java.lang.SecurityManager security manager API}
 428      * to allow applications to check such cross-loader references.
 429      * These checks apply to both the {@code MethodHandles.Lookup} API
 430      * and the Core Reflection API
 431      * (as found on {@link java.lang.Class Class}).
 432      * <p>
 433      * If a security manager is present, member and class lookups are subject to
 434      * additional checks.
 435      * From one to three calls are made to the security manager.
 436      * Any of these calls can refuse access by throwing a
 437      * {@link java.lang.SecurityException SecurityException}.
 438      * Define {@code smgr} as the security manager,
 439      * {@code lookc} as the lookup class of the current lookup object,
 440      * {@code refc} as the containing class in which the member
 441      * is being sought, and {@code defc} as the class in which the
 442      * member is actually defined.
 443      * (If a class or other type is being accessed,
 444      * the {@code refc} and {@code defc} values are the class itself.)
 445      * The value {@code lookc} is defined as <em>not present</em>
 446      * if the current lookup object does not have
 447      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 448      * The calls are made according to the following rules:
 449      * <ul>
 450      * <li><b>Step 1:</b>
 451      *     If {@code lookc} is not present, or if its class loader is not
 452      *     the same as or an ancestor of the class loader of {@code refc},
 453      *     then {@link SecurityManager#checkPackageAccess
 454      *     smgr.checkPackageAccess(refcPkg)} is called,
 455      *     where {@code refcPkg} is the package of {@code refc}.
 456      * <li><b>Step 2a:</b>
 457      *     If the retrieved member is not public and
 458      *     {@code lookc} is not present, then
 459      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 460      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 461      * <li><b>Step 2b:</b>
 462      *     If the retrieved class has a {@code null} class loader,
 463      *     and {@code lookc} is not present, then
 464      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 465      *     with {@code RuntimePermission("getClassLoader")} is called.
 466      * <li><b>Step 3:</b>
 467      *     If the retrieved member is not public,
 468      *     and if {@code lookc} is not present,
 469      *     and if {@code defc} and {@code refc} are different,
 470      *     then {@link SecurityManager#checkPackageAccess
 471      *     smgr.checkPackageAccess(defcPkg)} is called,
 472      *     where {@code defcPkg} is the package of {@code defc}.
 473      * </ul>
 474      * Security checks are performed after other access checks have passed.
 475      * Therefore, the above rules presuppose a member or class that is public,
 476      * or else that is being accessed from a lookup class that has
 477      * rights to access the member or class.
 478      *
 479      * <h1><a name="callsens"></a>Caller sensitive methods</h1>
 480      * A small number of Java methods have a special property called caller sensitivity.
 481      * A <em>caller-sensitive</em> method can behave differently depending on the
 482      * identity of its immediate caller.
 483      * <p>
 484      * If a method handle for a caller-sensitive method is requested,
 485      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 486      * but they take account of the lookup class in a special way.
 487      * The resulting method handle behaves as if it were called
 488      * from an instruction contained in the lookup class,
 489      * so that the caller-sensitive method detects the lookup class.
 490      * (By contrast, the invoker of the method handle is disregarded.)
 491      * Thus, in the case of caller-sensitive methods,
 492      * different lookup classes may give rise to
 493      * differently behaving method handles.
 494      * <p>
 495      * In cases where the lookup object is
 496      * {@link MethodHandles#publicLookup() publicLookup()},
 497      * or some other lookup object without
 498      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 499      * the lookup class is disregarded.
 500      * In such cases, no caller-sensitive method handle can be created,
 501      * access is forbidden, and the lookup fails with an
 502      * {@code IllegalAccessException}.
 503      * <p style="font-size:smaller;">
 504      * <em>Discussion:</em>
 505      * For example, the caller-sensitive method
 506      * {@link java.lang.Class#forName(String) Class.forName(x)}
 507      * can return varying classes or throw varying exceptions,
 508      * depending on the class loader of the class that calls it.
 509      * A public lookup of {@code Class.forName} will fail, because
 510      * there is no reasonable way to determine its bytecode behavior.
 511      * <p style="font-size:smaller;">
 512      * If an application caches method handles for broad sharing,
 513      * it should use {@code publicLookup()} to create them.
 514      * If there is a lookup of {@code Class.forName}, it will fail,
 515      * and the application must take appropriate action in that case.
 516      * It may be that a later lookup, perhaps during the invocation of a
 517      * bootstrap method, can incorporate the specific identity
 518      * of the caller, making the method accessible.
 519      * <p style="font-size:smaller;">
 520      * The function {@code MethodHandles.lookup} is caller sensitive
 521      * so that there can be a secure foundation for lookups.
 522      * Nearly all other methods in the JSR 292 API rely on lookup
 523      * objects to check access requests.
 524      */
 525     public static final
 526     class Lookup {
 527         /** The class on behalf of whom the lookup is being performed. */
 528         private final Class<?> lookupClass;
 529 
 530         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 531         private final int allowedModes;
 532 
 533         /** A single-bit mask representing {@code public} access,
 534          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 535          *  The value, {@code 0x01}, happens to be the same as the value of the
 536          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 537          */
 538         public static final int PUBLIC = Modifier.PUBLIC;
 539 
 540         /** A single-bit mask representing {@code private} access,
 541          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 542          *  The value, {@code 0x02}, happens to be the same as the value of the
 543          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 544          */
 545         public static final int PRIVATE = Modifier.PRIVATE;
 546 
 547         /** A single-bit mask representing {@code protected} access,
 548          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 549          *  The value, {@code 0x04}, happens to be the same as the value of the
 550          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 551          */
 552         public static final int PROTECTED = Modifier.PROTECTED;
 553 
 554         /** A single-bit mask representing {@code package} access (default access),
 555          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 556          *  The value is {@code 0x08}, which does not correspond meaningfully to
 557          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 558          */
 559         public static final int PACKAGE = Modifier.STATIC;
 560 
 561         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE);
 562         private static final int TRUSTED   = -1;
 563 
 564         private static int fixmods(int mods) {
 565             mods &= (ALL_MODES - PACKAGE);
 566             return (mods != 0) ? mods : PACKAGE;
 567         }
 568 
 569         /** Tells which class is performing the lookup.  It is this class against
 570          *  which checks are performed for visibility and access permissions.
 571          *  <p>
 572          *  The class implies a maximum level of access permission,
 573          *  but the permissions may be additionally limited by the bitmask
 574          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 575          *  can be accessed.
 576          *  @return the lookup class, on behalf of which this lookup object finds members
 577          */
 578         public Class<?> lookupClass() {
 579             return lookupClass;
 580         }
 581 
 582         // This is just for calling out to MethodHandleImpl.
 583         private Class<?> lookupClassOrNull() {
 584             return (allowedModes == TRUSTED) ? null : lookupClass;
 585         }
 586 
 587         /** Tells which access-protection classes of members this lookup object can produce.
 588          *  The result is a bit-mask of the bits
 589          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 590          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 591          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 592          *  and {@linkplain #PACKAGE PACKAGE (0x08)}.
 593          *  <p>
 594          *  A freshly-created lookup object
 595          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class}
 596          *  has all possible bits set, since the caller class can access all its own members.
 597          *  A lookup object on a new lookup class
 598          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 599          *  may have some mode bits set to zero.
 600          *  The purpose of this is to restrict access via the new lookup object,
 601          *  so that it can access only names which can be reached by the original
 602          *  lookup object, and also by the new lookup class.
 603          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 604          */
 605         public int lookupModes() {
 606             return allowedModes & ALL_MODES;
 607         }
 608 
 609         /** Embody the current class (the lookupClass) as a lookup class
 610          * for method handle creation.
 611          * Must be called by from a method in this package,
 612          * which in turn is called by a method not in this package.
 613          */
 614         Lookup(Class<?> lookupClass) {
 615             this(lookupClass, ALL_MODES);
 616             // make sure we haven't accidentally picked up a privileged class:
 617             checkUnprivilegedlookupClass(lookupClass, ALL_MODES);
 618         }
 619 
 620         private Lookup(Class<?> lookupClass, int allowedModes) {
 621             this.lookupClass = lookupClass;
 622             this.allowedModes = allowedModes;
 623         }
 624 
 625         /**
 626          * Creates a lookup on the specified new lookup class.
 627          * The resulting object will report the specified
 628          * class as its own {@link #lookupClass lookupClass}.
 629          * <p>
 630          * However, the resulting {@code Lookup} object is guaranteed
 631          * to have no more access capabilities than the original.
 632          * In particular, access capabilities can be lost as follows:<ul>
 633          * <li>If the new lookup class differs from the old one,
 634          * protected members will not be accessible by virtue of inheritance.
 635          * (Protected members may continue to be accessible because of package sharing.)
 636          * <li>If the new lookup class is in a different package
 637          * than the old one, protected and default (package) members will not be accessible.
 638          * <li>If the new lookup class is not within the same package member
 639          * as the old one, private members will not be accessible.
 640          * <li>If the new lookup class is not accessible to the old lookup class,
 641          * then no members, not even public members, will be accessible.
 642          * (In all other cases, public members will continue to be accessible.)
 643          * </ul>
 644          *
 645          * @param requestedLookupClass the desired lookup class for the new lookup object
 646          * @return a lookup object which reports the desired lookup class
 647          * @throws NullPointerException if the argument is null
 648          */
 649         public Lookup in(Class<?> requestedLookupClass) {
 650             Objects.requireNonNull(requestedLookupClass);
 651             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 652                 return new Lookup(requestedLookupClass, ALL_MODES);
 653             if (requestedLookupClass == this.lookupClass)
 654                 return this;  // keep same capabilities
 655             int newModes = (allowedModes & (ALL_MODES & ~PROTECTED));
 656             if ((newModes & PACKAGE) != 0
 657                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 658                 newModes &= ~(PACKAGE|PRIVATE);
 659             }
 660             // Allow nestmate lookups to be created without special privilege:
 661             if ((newModes & PRIVATE) != 0
 662                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 663                 newModes &= ~PRIVATE;
 664             }
 665             if ((newModes & PUBLIC) != 0
 666                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 667                 // The requested class it not accessible from the lookup class.
 668                 // No permissions.
 669                 newModes = 0;
 670             }
 671             checkUnprivilegedlookupClass(requestedLookupClass, newModes);
 672             return new Lookup(requestedLookupClass, newModes);
 673         }
 674 
 675         // Make sure outer class is initialized first.
 676         static { IMPL_NAMES.getClass(); }
 677 
 678         /** Version of lookup which is trusted minimally.
 679          *  It can only be used to create method handles to
 680          *  publicly accessible members.
 681          */
 682         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, PUBLIC);
 683 
 684         /** Package-private version of lookup which is trusted. */
 685         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 686 
 687         private static void checkUnprivilegedlookupClass(Class<?> lookupClass, int allowedModes) {
 688             String name = lookupClass.getName();
 689             if (name.startsWith("java.lang.invoke."))
 690                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
 691 
 692             // For caller-sensitive MethodHandles.lookup()
 693             // disallow lookup more restricted packages
 694             if (allowedModes == ALL_MODES && lookupClass.getClassLoader() == null) {
 695                 if (name.startsWith("java.") ||
 696                         (name.startsWith("sun.") && !name.startsWith("sun.invoke."))) {
 697                     throw newIllegalArgumentException("illegal lookupClass: " + lookupClass);
 698                 }
 699             }
 700         }
 701 
 702         /**
 703          * Displays the name of the class from which lookups are to be made.
 704          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
 705          * If there are restrictions on the access permitted to this lookup,
 706          * this is indicated by adding a suffix to the class name, consisting
 707          * of a slash and a keyword.  The keyword represents the strongest
 708          * allowed access, and is chosen as follows:
 709          * <ul>
 710          * <li>If no access is allowed, the suffix is "/noaccess".
 711          * <li>If only public access is allowed, the suffix is "/public".
 712          * <li>If only public and package access are allowed, the suffix is "/package".
 713          * <li>If only public, package, and private access are allowed, the suffix is "/private".
 714          * </ul>
 715          * If none of the above cases apply, it is the case that full
 716          * access (public, package, private, and protected) is allowed.
 717          * In this case, no suffix is added.
 718          * This is true only of an object obtained originally from
 719          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
 720          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
 721          * always have restricted access, and will display a suffix.
 722          * <p>
 723          * (It may seem strange that protected access should be
 724          * stronger than private access.  Viewed independently from
 725          * package access, protected access is the first to be lost,
 726          * because it requires a direct subclass relationship between
 727          * caller and callee.)
 728          * @see #in
 729          */
 730         @Override
 731         public String toString() {
 732             String cname = lookupClass.getName();
 733             switch (allowedModes) {
 734             case 0:  // no privileges
 735                 return cname + "/noaccess";
 736             case PUBLIC:
 737                 return cname + "/public";
 738             case PUBLIC|PACKAGE:
 739                 return cname + "/package";
 740             case ALL_MODES & ~PROTECTED:
 741                 return cname + "/private";
 742             case ALL_MODES:
 743                 return cname;
 744             case TRUSTED:
 745                 return "/trusted";  // internal only; not exported
 746             default:  // Should not happen, but it's a bitfield...
 747                 cname = cname + "/" + Integer.toHexString(allowedModes);
 748                 assert(false) : cname;
 749                 return cname;
 750             }
 751         }
 752 
 753         /**
 754          * Produces a method handle for a static method.
 755          * The type of the method handle will be that of the method.
 756          * (Since static methods do not take receivers, there is no
 757          * additional receiver argument inserted into the method handle type,
 758          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
 759          * The method and all its argument types must be accessible to the lookup object.
 760          * <p>
 761          * The returned method handle will have
 762          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 763          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 764          * <p>
 765          * If the returned method handle is invoked, the method's class will
 766          * be initialized, if it has not already been initialized.
 767          * <p><b>Example:</b>
 768          * <blockquote><pre>{@code
 769 import static java.lang.invoke.MethodHandles.*;
 770 import static java.lang.invoke.MethodType.*;
 771 ...
 772 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
 773   "asList", methodType(List.class, Object[].class));
 774 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
 775          * }</pre></blockquote>
 776          * @param refc the class from which the method is accessed
 777          * @param name the name of the method
 778          * @param type the type of the method
 779          * @return the desired method handle
 780          * @throws NoSuchMethodException if the method does not exist
 781          * @throws IllegalAccessException if access checking fails,
 782          *                                or if the method is not {@code static},
 783          *                                or if the method's variable arity modifier bit
 784          *                                is set and {@code asVarargsCollector} fails
 785          * @exception SecurityException if a security manager is present and it
 786          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 787          * @throws NullPointerException if any argument is null
 788          */
 789         public
 790         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 791             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
 792             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
 793         }
 794 
 795         /**
 796          * Produces a method handle for a virtual method.
 797          * The type of the method handle will be that of the method,
 798          * with the receiver type (usually {@code refc}) prepended.
 799          * The method and all its argument types must be accessible to the lookup object.
 800          * <p>
 801          * When called, the handle will treat the first argument as a receiver
 802          * and dispatch on the receiver's type to determine which method
 803          * implementation to enter.
 804          * (The dispatching action is identical with that performed by an
 805          * {@code invokevirtual} or {@code invokeinterface} instruction.)
 806          * <p>
 807          * The first argument will be of type {@code refc} if the lookup
 808          * class has full privileges to access the member.  Otherwise
 809          * the member must be {@code protected} and the first argument
 810          * will be restricted in type to the lookup class.
 811          * <p>
 812          * The returned method handle will have
 813          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 814          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 815          * <p>
 816          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
 817          * instructions and method handles produced by {@code findVirtual},
 818          * if the class is {@code MethodHandle} and the name string is
 819          * {@code invokeExact} or {@code invoke}, the resulting
 820          * method handle is equivalent to one produced by
 821          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
 822          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
 823          * with the same {@code type} argument.
 824          *
 825          * <b>Example:</b>
 826          * <blockquote><pre>{@code
 827 import static java.lang.invoke.MethodHandles.*;
 828 import static java.lang.invoke.MethodType.*;
 829 ...
 830 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
 831   "concat", methodType(String.class, String.class));
 832 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
 833   "hashCode", methodType(int.class));
 834 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
 835   "hashCode", methodType(int.class));
 836 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
 837 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
 838 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
 839 // interface method:
 840 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
 841   "subSequence", methodType(CharSequence.class, int.class, int.class));
 842 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
 843 // constructor "internal method" must be accessed differently:
 844 MethodType MT_newString = methodType(void.class); //()V for new String()
 845 try { assertEquals("impossible", lookup()
 846         .findVirtual(String.class, "<init>", MT_newString));
 847  } catch (NoSuchMethodException ex) { } // OK
 848 MethodHandle MH_newString = publicLookup()
 849   .findConstructor(String.class, MT_newString);
 850 assertEquals("", (String) MH_newString.invokeExact());
 851          * }</pre></blockquote>
 852          *
 853          * @param refc the class or interface from which the method is accessed
 854          * @param name the name of the method
 855          * @param type the type of the method, with the receiver argument omitted
 856          * @return the desired method handle
 857          * @throws NoSuchMethodException if the method does not exist
 858          * @throws IllegalAccessException if access checking fails,
 859          *                                or if the method is {@code static},
 860          *                                or if the method is {@code private} method of interface,
 861          *                                or if the method's variable arity modifier bit
 862          *                                is set and {@code asVarargsCollector} fails
 863          * @exception SecurityException if a security manager is present and it
 864          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 865          * @throws NullPointerException if any argument is null
 866          */
 867         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 868             if (refc == MethodHandle.class) {
 869                 MethodHandle mh = findVirtualForMH(name, type);
 870                 if (mh != null)  return mh;
 871             }
 872             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
 873             MemberName method = resolveOrFail(refKind, refc, name, type);
 874             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
 875         }
 876         private MethodHandle findVirtualForMH(String name, MethodType type) {
 877             // these names require special lookups because of the implicit MethodType argument
 878             if ("invoke".equals(name))
 879                 return invoker(type);
 880             if ("invokeExact".equals(name))
 881                 return exactInvoker(type);
 882             if ("invokeBasic".equals(name))
 883                 return basicInvoker(type);
 884             assert(!MemberName.isMethodHandleInvokeName(name));
 885             return null;
 886         }
 887 
 888         /**
 889          * Produces a method handle which creates an object and initializes it, using
 890          * the constructor of the specified type.
 891          * The parameter types of the method handle will be those of the constructor,
 892          * while the return type will be a reference to the constructor's class.
 893          * The constructor and all its argument types must be accessible to the lookup object.
 894          * <p>
 895          * The requested type must have a return type of {@code void}.
 896          * (This is consistent with the JVM's treatment of constructor type descriptors.)
 897          * <p>
 898          * The returned method handle will have
 899          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 900          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
 901          * <p>
 902          * If the returned method handle is invoked, the constructor's class will
 903          * be initialized, if it has not already been initialized.
 904          * <p><b>Example:</b>
 905          * <blockquote><pre>{@code
 906 import static java.lang.invoke.MethodHandles.*;
 907 import static java.lang.invoke.MethodType.*;
 908 ...
 909 MethodHandle MH_newArrayList = publicLookup().findConstructor(
 910   ArrayList.class, methodType(void.class, Collection.class));
 911 Collection orig = Arrays.asList("x", "y");
 912 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
 913 assert(orig != copy);
 914 assertEquals(orig, copy);
 915 // a variable-arity constructor:
 916 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
 917   ProcessBuilder.class, methodType(void.class, String[].class));
 918 ProcessBuilder pb = (ProcessBuilder)
 919   MH_newProcessBuilder.invoke("x", "y", "z");
 920 assertEquals("[x, y, z]", pb.command().toString());
 921          * }</pre></blockquote>
 922          * @param refc the class or interface from which the method is accessed
 923          * @param type the type of the method, with the receiver argument omitted, and a void return type
 924          * @return the desired method handle
 925          * @throws NoSuchMethodException if the constructor does not exist
 926          * @throws IllegalAccessException if access checking fails
 927          *                                or if the method's variable arity modifier bit
 928          *                                is set and {@code asVarargsCollector} fails
 929          * @exception SecurityException if a security manager is present and it
 930          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 931          * @throws NullPointerException if any argument is null
 932          */
 933         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 934             String name = "<init>";
 935             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
 936             return getDirectConstructor(refc, ctor);
 937         }
 938 
 939         /**
 940          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
 941          * initializer of the class is not run.
 942          *
 943          * @param targetName the fully qualified name of the class to be looked up.
 944          * @return the requested class.
 945          * @exception SecurityException if a security manager is present and it
 946          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 947          * @throws LinkageError if the linkage fails
 948          * @throws ClassNotFoundException if the class does not exist.
 949          * @throws IllegalAccessException if the class is not accessible, using the allowed access
 950          * modes.
 951          * @exception SecurityException if a security manager is present and it
 952          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 953          * @since 9
 954          */
 955         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
 956             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
 957             return accessClass(targetClass);
 958         }
 959 
 960         /**
 961          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
 962          * static initializer of the class is not run.
 963          *
 964          * @param targetClass the class to be access-checked
 965          *
 966          * @return the class that has been access-checked
 967          *
 968          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
 969          * modes.
 970          * @exception SecurityException if a security manager is present and it
 971          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 972          * @since 9
 973          */
 974         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
 975             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
 976                 throw new MemberName(targetClass).makeAccessException("access violation", this);
 977             }
 978             checkSecurityManager(targetClass, null);
 979             return targetClass;
 980         }
 981 
 982         /**
 983          * Produces an early-bound method handle for a virtual method.
 984          * It will bypass checks for overriding methods on the receiver,
 985          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
 986          * instruction from within the explicitly specified {@code specialCaller}.
 987          * The type of the method handle will be that of the method,
 988          * with a suitably restricted receiver type prepended.
 989          * (The receiver type will be {@code specialCaller} or a subtype.)
 990          * The method and all its argument types must be accessible
 991          * to the lookup object.
 992          * <p>
 993          * Before method resolution,
 994          * if the explicitly specified caller class is not identical with the
 995          * lookup class, or if this lookup object does not have
 996          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
 997          * privileges, the access fails.
 998          * <p>
 999          * The returned method handle will have
1000          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1001          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1002          * <p style="font-size:smaller;">
1003          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1004          * even though the {@code invokespecial} instruction can refer to them
1005          * in special circumstances.  Use {@link #findConstructor findConstructor}
1006          * to access instance initialization methods in a safe manner.)</em>
1007          * <p><b>Example:</b>
1008          * <blockquote><pre>{@code
1009 import static java.lang.invoke.MethodHandles.*;
1010 import static java.lang.invoke.MethodType.*;
1011 ...
1012 static class Listie extends ArrayList {
1013   public String toString() { return "[wee Listie]"; }
1014   static Lookup lookup() { return MethodHandles.lookup(); }
1015 }
1016 ...
1017 // no access to constructor via invokeSpecial:
1018 MethodHandle MH_newListie = Listie.lookup()
1019   .findConstructor(Listie.class, methodType(void.class));
1020 Listie l = (Listie) MH_newListie.invokeExact();
1021 try { assertEquals("impossible", Listie.lookup().findSpecial(
1022         Listie.class, "<init>", methodType(void.class), Listie.class));
1023  } catch (NoSuchMethodException ex) { } // OK
1024 // access to super and self methods via invokeSpecial:
1025 MethodHandle MH_super = Listie.lookup().findSpecial(
1026   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1027 MethodHandle MH_this = Listie.lookup().findSpecial(
1028   Listie.class, "toString" , methodType(String.class), Listie.class);
1029 MethodHandle MH_duper = Listie.lookup().findSpecial(
1030   Object.class, "toString" , methodType(String.class), Listie.class);
1031 assertEquals("[]", (String) MH_super.invokeExact(l));
1032 assertEquals(""+l, (String) MH_this.invokeExact(l));
1033 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1034 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1035         String.class, "toString", methodType(String.class), Listie.class));
1036  } catch (IllegalAccessException ex) { } // OK
1037 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1038 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1039          * }</pre></blockquote>
1040          *
1041          * @param refc the class or interface from which the method is accessed
1042          * @param name the name of the method (which must not be "&lt;init&gt;")
1043          * @param type the type of the method, with the receiver argument omitted
1044          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1045          * @return the desired method handle
1046          * @throws NoSuchMethodException if the method does not exist
1047          * @throws IllegalAccessException if access checking fails,
1048          *                                or if the method is {@code static},
1049          *                                or if the method's variable arity modifier bit
1050          *                                is set and {@code asVarargsCollector} fails
1051          * @exception SecurityException if a security manager is present and it
1052          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1053          * @throws NullPointerException if any argument is null
1054          */
1055         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1056                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1057             checkSpecialCaller(specialCaller, refc);
1058             Lookup specialLookup = this.in(specialCaller);
1059             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1060             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1061         }
1062 
1063         /**
1064          * Produces a method handle giving read access to a non-static field.
1065          * The type of the method handle will have a return type of the field's
1066          * value type.
1067          * The method handle's single argument will be the instance containing
1068          * the field.
1069          * Access checking is performed immediately on behalf of the lookup class.
1070          * @param refc the class or interface from which the method is accessed
1071          * @param name the field's name
1072          * @param type the field's type
1073          * @return a method handle which can load values from the field
1074          * @throws NoSuchFieldException if the field does not exist
1075          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1076          * @exception SecurityException if a security manager is present and it
1077          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1078          * @throws NullPointerException if any argument is null
1079          */
1080         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1081             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1082             return getDirectField(REF_getField, refc, field);
1083         }
1084 
1085         /**
1086          * Produces a method handle giving write access to a non-static field.
1087          * The type of the method handle will have a void return type.
1088          * The method handle will take two arguments, the instance containing
1089          * the field, and the value to be stored.
1090          * The second argument will be of the field's value type.
1091          * Access checking is performed immediately on behalf of the lookup class.
1092          * @param refc the class or interface from which the method is accessed
1093          * @param name the field's name
1094          * @param type the field's type
1095          * @return a method handle which can store values into the field
1096          * @throws NoSuchFieldException if the field does not exist
1097          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1098          * @exception SecurityException if a security manager is present and it
1099          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1100          * @throws NullPointerException if any argument is null
1101          */
1102         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1103             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1104             return getDirectField(REF_putField, refc, field);
1105         }
1106 
1107         /**
1108          * Produces a method handle giving read access to a static field.
1109          * The type of the method handle will have a return type of the field's
1110          * value type.
1111          * The method handle will take no arguments.
1112          * Access checking is performed immediately on behalf of the lookup class.
1113          * <p>
1114          * If the returned method handle is invoked, the field's class will
1115          * be initialized, if it has not already been initialized.
1116          * @param refc the class or interface from which the method is accessed
1117          * @param name the field's name
1118          * @param type the field's type
1119          * @return a method handle which can load values from the field
1120          * @throws NoSuchFieldException if the field does not exist
1121          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1122          * @exception SecurityException if a security manager is present and it
1123          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1124          * @throws NullPointerException if any argument is null
1125          */
1126         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1127             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1128             return getDirectField(REF_getStatic, refc, field);
1129         }
1130 
1131         /**
1132          * Produces a method handle giving write access to a static field.
1133          * The type of the method handle will have a void return type.
1134          * The method handle will take a single
1135          * argument, of the field's value type, the value to be stored.
1136          * Access checking is performed immediately on behalf of the lookup class.
1137          * <p>
1138          * If the returned method handle is invoked, the field's class will
1139          * be initialized, if it has not already been initialized.
1140          * @param refc the class or interface from which the method is accessed
1141          * @param name the field's name
1142          * @param type the field's type
1143          * @return a method handle which can store values into the field
1144          * @throws NoSuchFieldException if the field does not exist
1145          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1146          * @exception SecurityException if a security manager is present and it
1147          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1148          * @throws NullPointerException if any argument is null
1149          */
1150         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1151             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1152             return getDirectField(REF_putStatic, refc, field);
1153         }
1154 
1155         /**
1156          * Produces an early-bound method handle for a non-static method.
1157          * The receiver must have a supertype {@code defc} in which a method
1158          * of the given name and type is accessible to the lookup class.
1159          * The method and all its argument types must be accessible to the lookup object.
1160          * The type of the method handle will be that of the method,
1161          * without any insertion of an additional receiver parameter.
1162          * The given receiver will be bound into the method handle,
1163          * so that every call to the method handle will invoke the
1164          * requested method on the given receiver.
1165          * <p>
1166          * The returned method handle will have
1167          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1168          * the method's variable arity modifier bit ({@code 0x0080}) is set
1169          * <em>and</em> the trailing array argument is not the only argument.
1170          * (If the trailing array argument is the only argument,
1171          * the given receiver value will be bound to it.)
1172          * <p>
1173          * This is equivalent to the following code:
1174          * <blockquote><pre>{@code
1175 import static java.lang.invoke.MethodHandles.*;
1176 import static java.lang.invoke.MethodType.*;
1177 ...
1178 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1179 MethodHandle mh1 = mh0.bindTo(receiver);
1180 MethodType mt1 = mh1.type();
1181 if (mh0.isVarargsCollector())
1182   mh1 = mh1.asVarargsCollector(mt1.parameterType(mt1.parameterCount()-1));
1183 return mh1;
1184          * }</pre></blockquote>
1185          * where {@code defc} is either {@code receiver.getClass()} or a super
1186          * type of that class, in which the requested method is accessible
1187          * to the lookup class.
1188          * (Note that {@code bindTo} does not preserve variable arity.)
1189          * @param receiver the object from which the method is accessed
1190          * @param name the name of the method
1191          * @param type the type of the method, with the receiver argument omitted
1192          * @return the desired method handle
1193          * @throws NoSuchMethodException if the method does not exist
1194          * @throws IllegalAccessException if access checking fails
1195          *                                or if the method's variable arity modifier bit
1196          *                                is set and {@code asVarargsCollector} fails
1197          * @exception SecurityException if a security manager is present and it
1198          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1199          * @throws NullPointerException if any argument is null
1200          * @see MethodHandle#bindTo
1201          * @see #findVirtual
1202          */
1203         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1204             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1205             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1206             MethodHandle mh = getDirectMethodNoRestrict(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1207             return mh.bindArgumentL(0, receiver).setVarargs(method);
1208         }
1209 
1210         /**
1211          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1212          * to <i>m</i>, if the lookup class has permission.
1213          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1214          * If <i>m</i> is virtual, overriding is respected on every call.
1215          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1216          * The type of the method handle will be that of the method,
1217          * with the receiver type prepended (but only if it is non-static).
1218          * If the method's {@code accessible} flag is not set,
1219          * access checking is performed immediately on behalf of the lookup class.
1220          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1221          * <p>
1222          * The returned method handle will have
1223          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1224          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1225          * <p>
1226          * If <i>m</i> is static, and
1227          * if the returned method handle is invoked, the method's class will
1228          * be initialized, if it has not already been initialized.
1229          * @param m the reflected method
1230          * @return a method handle which can invoke the reflected method
1231          * @throws IllegalAccessException if access checking fails
1232          *                                or if the method's variable arity modifier bit
1233          *                                is set and {@code asVarargsCollector} fails
1234          * @throws NullPointerException if the argument is null
1235          */
1236         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1237             if (m.getDeclaringClass() == MethodHandle.class) {
1238                 MethodHandle mh = unreflectForMH(m);
1239                 if (mh != null)  return mh;
1240             }
1241             MemberName method = new MemberName(m);
1242             byte refKind = method.getReferenceKind();
1243             if (refKind == REF_invokeSpecial)
1244                 refKind = REF_invokeVirtual;
1245             assert(method.isMethod());
1246             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1247             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1248         }
1249         private MethodHandle unreflectForMH(Method m) {
1250             // these names require special lookups because they throw UnsupportedOperationException
1251             if (MemberName.isMethodHandleInvokeName(m.getName()))
1252                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1253             return null;
1254         }
1255 
1256         /**
1257          * Produces a method handle for a reflected method.
1258          * It will bypass checks for overriding methods on the receiver,
1259          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1260          * instruction from within the explicitly specified {@code specialCaller}.
1261          * The type of the method handle will be that of the method,
1262          * with a suitably restricted receiver type prepended.
1263          * (The receiver type will be {@code specialCaller} or a subtype.)
1264          * If the method's {@code accessible} flag is not set,
1265          * access checking is performed immediately on behalf of the lookup class,
1266          * as if {@code invokespecial} instruction were being linked.
1267          * <p>
1268          * Before method resolution,
1269          * if the explicitly specified caller class is not identical with the
1270          * lookup class, or if this lookup object does not have
1271          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1272          * privileges, the access fails.
1273          * <p>
1274          * The returned method handle will have
1275          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1276          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1277          * @param m the reflected method
1278          * @param specialCaller the class nominally calling the method
1279          * @return a method handle which can invoke the reflected method
1280          * @throws IllegalAccessException if access checking fails,
1281          *                                or if the method is {@code static},
1282          *                                or if the method's variable arity modifier bit
1283          *                                is set and {@code asVarargsCollector} fails
1284          * @throws NullPointerException if any argument is null
1285          */
1286         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1287             checkSpecialCaller(specialCaller, null);
1288             Lookup specialLookup = this.in(specialCaller);
1289             MemberName method = new MemberName(m, true);
1290             assert(method.isMethod());
1291             // ignore m.isAccessible:  this is a new kind of access
1292             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1293         }
1294 
1295         /**
1296          * Produces a method handle for a reflected constructor.
1297          * The type of the method handle will be that of the constructor,
1298          * with the return type changed to the declaring class.
1299          * The method handle will perform a {@code newInstance} operation,
1300          * creating a new instance of the constructor's class on the
1301          * arguments passed to the method handle.
1302          * <p>
1303          * If the constructor's {@code accessible} flag is not set,
1304          * access checking is performed immediately on behalf of the lookup class.
1305          * <p>
1306          * The returned method handle will have
1307          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1308          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1309          * <p>
1310          * If the returned method handle is invoked, the constructor's class will
1311          * be initialized, if it has not already been initialized.
1312          * @param c the reflected constructor
1313          * @return a method handle which can invoke the reflected constructor
1314          * @throws IllegalAccessException if access checking fails
1315          *                                or if the method's variable arity modifier bit
1316          *                                is set and {@code asVarargsCollector} fails
1317          * @throws NullPointerException if the argument is null
1318          */
1319         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1320             MemberName ctor = new MemberName(c);
1321             assert(ctor.isConstructor());
1322             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1323             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1324         }
1325 
1326         /**
1327          * Produces a method handle giving read access to a reflected field.
1328          * The type of the method handle will have a return type of the field's
1329          * value type.
1330          * If the field is static, the method handle will take no arguments.
1331          * Otherwise, its single argument will be the instance containing
1332          * the field.
1333          * If the field's {@code accessible} flag is not set,
1334          * access checking is performed immediately on behalf of the lookup class.
1335          * <p>
1336          * If the field is static, and
1337          * if the returned method handle is invoked, the field's class will
1338          * be initialized, if it has not already been initialized.
1339          * @param f the reflected field
1340          * @return a method handle which can load values from the reflected field
1341          * @throws IllegalAccessException if access checking fails
1342          * @throws NullPointerException if the argument is null
1343          */
1344         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1345             return unreflectField(f, false);
1346         }
1347         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1348             MemberName field = new MemberName(f, isSetter);
1349             assert(isSetter
1350                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1351                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1352             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1353             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1354         }
1355 
1356         /**
1357          * Produces a method handle giving write access to a reflected field.
1358          * The type of the method handle will have a void return type.
1359          * If the field is static, the method handle will take a single
1360          * argument, of the field's value type, the value to be stored.
1361          * Otherwise, the two arguments will be the instance containing
1362          * the field, and the value to be stored.
1363          * If the field's {@code accessible} flag is not set,
1364          * access checking is performed immediately on behalf of the lookup class.
1365          * <p>
1366          * If the field is static, and
1367          * if the returned method handle is invoked, the field's class will
1368          * be initialized, if it has not already been initialized.
1369          * @param f the reflected field
1370          * @return a method handle which can store values into the reflected field
1371          * @throws IllegalAccessException if access checking fails
1372          * @throws NullPointerException if the argument is null
1373          */
1374         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1375             return unreflectField(f, true);
1376         }
1377 
1378         /**
1379          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1380          * created by this lookup object or a similar one.
1381          * Security and access checks are performed to ensure that this lookup object
1382          * is capable of reproducing the target method handle.
1383          * This means that the cracking may fail if target is a direct method handle
1384          * but was created by an unrelated lookup object.
1385          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1386          * and was created by a lookup object for a different class.
1387          * @param target a direct method handle to crack into symbolic reference components
1388          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
1389          * @exception SecurityException if a security manager is present and it
1390          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1391          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
1392          * @exception NullPointerException if the target is {@code null}
1393          * @see MethodHandleInfo
1394          * @since 1.8
1395          */
1396         public MethodHandleInfo revealDirect(MethodHandle target) {
1397             MemberName member = target.internalMemberName();
1398             if (member == null || (!member.isResolved() && !member.isMethodHandleInvoke()))
1399                 throw newIllegalArgumentException("not a direct method handle");
1400             Class<?> defc = member.getDeclaringClass();
1401             byte refKind = member.getReferenceKind();
1402             assert(MethodHandleNatives.refKindIsValid(refKind));
1403             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
1404                 // Devirtualized method invocation is usually formally virtual.
1405                 // To avoid creating extra MemberName objects for this common case,
1406                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
1407                 refKind = REF_invokeVirtual;
1408             if (refKind == REF_invokeVirtual && defc.isInterface())
1409                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
1410                 refKind = REF_invokeInterface;
1411             // Check SM permissions and member access before cracking.
1412             try {
1413                 checkAccess(refKind, defc, member);
1414                 checkSecurityManager(defc, member);
1415             } catch (IllegalAccessException ex) {
1416                 throw new IllegalArgumentException(ex);
1417             }
1418             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
1419                 Class<?> callerClass = target.internalCallerClass();
1420                 if (!hasPrivateAccess() || callerClass != lookupClass())
1421                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
1422             }
1423             // Produce the handle to the results.
1424             return new InfoFromMemberName(this, member, refKind);
1425         }
1426 
1427         /// Helper methods, all package-private.
1428 
1429         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1430             checkSymbolicClass(refc);  // do this before attempting to resolve
1431             Objects.requireNonNull(name);
1432             Objects.requireNonNull(type);
1433             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1434                                             NoSuchFieldException.class);
1435         }
1436 
1437         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1438             checkSymbolicClass(refc);  // do this before attempting to resolve
1439             Objects.requireNonNull(name);
1440             Objects.requireNonNull(type);
1441             checkMethodName(refKind, name);  // NPE check on name
1442             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1443                                             NoSuchMethodException.class);
1444         }
1445 
1446         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
1447             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
1448             Objects.requireNonNull(member.getName());
1449             Objects.requireNonNull(member.getType());
1450             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
1451                                             ReflectiveOperationException.class);
1452         }
1453 
1454         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
1455             Objects.requireNonNull(refc);
1456             Class<?> caller = lookupClassOrNull();
1457             if (caller != null && !VerifyAccess.isClassAccessible(refc, caller, allowedModes))
1458                 throw new MemberName(refc).makeAccessException("symbolic reference class is not public", this);
1459         }
1460 
1461         /** Check name for an illegal leading "&lt;" character. */
1462         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
1463             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
1464                 throw new NoSuchMethodException("illegal method name: "+name);
1465         }
1466 
1467 
1468         /**
1469          * Find my trustable caller class if m is a caller sensitive method.
1470          * If this lookup object has private access, then the caller class is the lookupClass.
1471          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
1472          */
1473         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
1474             Class<?> callerClass = null;
1475             if (MethodHandleNatives.isCallerSensitive(m)) {
1476                 // Only lookups with private access are allowed to resolve caller-sensitive methods
1477                 if (hasPrivateAccess()) {
1478                     callerClass = lookupClass;
1479                 } else {
1480                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
1481                 }
1482             }
1483             return callerClass;
1484         }
1485 
1486         private boolean hasPrivateAccess() {
1487             return (allowedModes & PRIVATE) != 0;
1488         }
1489 
1490         /**
1491          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
1492          * Determines a trustable caller class to compare with refc, the symbolic reference class.
1493          * If this lookup object has private access, then the caller class is the lookupClass.
1494          */
1495         void checkSecurityManager(Class<?> refc, MemberName m) {
1496             SecurityManager smgr = System.getSecurityManager();
1497             if (smgr == null)  return;
1498             if (allowedModes == TRUSTED)  return;
1499 
1500             // Step 1:
1501             boolean fullPowerLookup = hasPrivateAccess();
1502             if (!fullPowerLookup ||
1503                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
1504                 ReflectUtil.checkPackageAccess(refc);
1505             }
1506 
1507             if (m == null) {  // findClass or accessClass
1508                 // Step 2b:
1509                 if (!fullPowerLookup) {
1510                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
1511                 }
1512                 return;
1513             }
1514 
1515             // Step 2a:
1516             if (m.isPublic()) return;
1517             if (!fullPowerLookup) {
1518                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
1519             }
1520 
1521             // Step 3:
1522             Class<?> defc = m.getDeclaringClass();
1523             if (!fullPowerLookup && defc != refc) {
1524                 ReflectUtil.checkPackageAccess(defc);
1525             }
1526         }
1527 
1528         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1529             boolean wantStatic = (refKind == REF_invokeStatic);
1530             String message;
1531             if (m.isConstructor())
1532                 message = "expected a method, not a constructor";
1533             else if (!m.isMethod())
1534                 message = "expected a method";
1535             else if (wantStatic != m.isStatic())
1536                 message = wantStatic ? "expected a static method" : "expected a non-static method";
1537             else
1538                 { checkAccess(refKind, refc, m); return; }
1539             throw m.makeAccessException(message, this);
1540         }
1541 
1542         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1543             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
1544             String message;
1545             if (wantStatic != m.isStatic())
1546                 message = wantStatic ? "expected a static field" : "expected a non-static field";
1547             else
1548                 { checkAccess(refKind, refc, m); return; }
1549             throw m.makeAccessException(message, this);
1550         }
1551 
1552         /** Check public/protected/private bits on the symbolic reference class and its member. */
1553         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1554             assert(m.referenceKindIsConsistentWith(refKind) &&
1555                    MethodHandleNatives.refKindIsValid(refKind) &&
1556                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
1557             int allowedModes = this.allowedModes;
1558             if (allowedModes == TRUSTED)  return;
1559             int mods = m.getModifiers();
1560             if (Modifier.isProtected(mods) &&
1561                     refKind == REF_invokeVirtual &&
1562                     m.getDeclaringClass() == Object.class &&
1563                     m.getName().equals("clone") &&
1564                     refc.isArray()) {
1565                 // The JVM does this hack also.
1566                 // (See ClassVerifier::verify_invoke_instructions
1567                 // and LinkResolver::check_method_accessability.)
1568                 // Because the JVM does not allow separate methods on array types,
1569                 // there is no separate method for int[].clone.
1570                 // All arrays simply inherit Object.clone.
1571                 // But for access checking logic, we make Object.clone
1572                 // (normally protected) appear to be public.
1573                 // Later on, when the DirectMethodHandle is created,
1574                 // its leading argument will be restricted to the
1575                 // requested array type.
1576                 // N.B. The return type is not adjusted, because
1577                 // that is *not* the bytecode behavior.
1578                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
1579             }
1580             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
1581                 // cannot "new" a protected ctor in a different package
1582                 mods ^= Modifier.PROTECTED;
1583             }
1584             if (Modifier.isFinal(mods) &&
1585                     MethodHandleNatives.refKindIsSetter(refKind))
1586                 throw m.makeAccessException("unexpected set of a final field", this);
1587             if (Modifier.isPublic(mods) && Modifier.isPublic(refc.getModifiers()) && allowedModes != 0)
1588                 return;  // common case
1589             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
1590             if ((requestedModes & allowedModes) != 0) {
1591                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
1592                                                     mods, lookupClass(), allowedModes))
1593                     return;
1594             } else {
1595                 // Protected members can also be checked as if they were package-private.
1596                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
1597                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
1598                     return;
1599             }
1600             throw m.makeAccessException(accessFailedMessage(refc, m), this);
1601         }
1602 
1603         String accessFailedMessage(Class<?> refc, MemberName m) {
1604             Class<?> defc = m.getDeclaringClass();
1605             int mods = m.getModifiers();
1606             // check the class first:
1607             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
1608                                (defc == refc ||
1609                                 Modifier.isPublic(refc.getModifiers())));
1610             if (!classOK && (allowedModes & PACKAGE) != 0) {
1611                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), ALL_MODES) &&
1612                            (defc == refc ||
1613                             VerifyAccess.isClassAccessible(refc, lookupClass(), ALL_MODES)));
1614             }
1615             if (!classOK)
1616                 return "class is not public";
1617             if (Modifier.isPublic(mods))
1618                 return "access to public member failed";  // (how?)
1619             if (Modifier.isPrivate(mods))
1620                 return "member is private";
1621             if (Modifier.isProtected(mods))
1622                 return "member is protected";
1623             return "member is private to package";
1624         }
1625 
1626         private static final boolean ALLOW_NESTMATE_ACCESS = false;
1627 
1628         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
1629             int allowedModes = this.allowedModes;
1630             if (allowedModes == TRUSTED)  return;
1631             if (!hasPrivateAccess()
1632                 || (specialCaller != lookupClass()
1633                        // ensure non-abstract methods in superinterfaces can be special-invoked
1634                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))
1635                     && !(ALLOW_NESTMATE_ACCESS &&
1636                          VerifyAccess.isSamePackageMember(specialCaller, lookupClass()))))
1637                 throw new MemberName(specialCaller).
1638                     makeAccessException("no private access for invokespecial", this);
1639         }
1640 
1641         private boolean restrictProtectedReceiver(MemberName method) {
1642             // The accessing class only has the right to use a protected member
1643             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
1644             if (!method.isProtected() || method.isStatic()
1645                 || allowedModes == TRUSTED
1646                 || method.getDeclaringClass() == lookupClass()
1647                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())
1648                 || (ALLOW_NESTMATE_ACCESS &&
1649                     VerifyAccess.isSamePackageMember(method.getDeclaringClass(), lookupClass())))
1650                 return false;
1651             return true;
1652         }
1653         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
1654             assert(!method.isStatic());
1655             // receiver type of mh is too wide; narrow to caller
1656             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
1657                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
1658             }
1659             MethodType rawType = mh.type();
1660             if (rawType.parameterType(0) == caller)  return mh;
1661             MethodType narrowType = rawType.changeParameterType(0, caller);
1662             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
1663             assert(mh.viewAsTypeChecks(narrowType, true));
1664             return mh.copyWith(narrowType, mh.form);
1665         }
1666 
1667         /** Check access and get the requested method. */
1668         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1669             final boolean doRestrict    = true;
1670             final boolean checkSecurity = true;
1671             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1672         }
1673         /** Check access and get the requested method, eliding receiver narrowing rules. */
1674         private MethodHandle getDirectMethodNoRestrict(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1675             final boolean doRestrict    = false;
1676             final boolean checkSecurity = true;
1677             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1678         }
1679         /** Check access and get the requested method, eliding security manager checks. */
1680         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1681             final boolean doRestrict    = true;
1682             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1683             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1684         }
1685         /** Common code for all methods; do not call directly except from immediately above. */
1686         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
1687                                                    boolean checkSecurity,
1688                                                    boolean doRestrict, Class<?> callerClass) throws IllegalAccessException {
1689             checkMethod(refKind, refc, method);
1690             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1691             if (checkSecurity)
1692                 checkSecurityManager(refc, method);
1693             assert(!method.isMethodHandleInvoke());
1694 
1695             if (refKind == REF_invokeSpecial &&
1696                 refc != lookupClass() &&
1697                 !refc.isInterface() &&
1698                 refc != lookupClass().getSuperclass() &&
1699                 refc.isAssignableFrom(lookupClass())) {
1700                 assert(!method.getName().equals("<init>"));  // not this code path
1701                 // Per JVMS 6.5, desc. of invokespecial instruction:
1702                 // If the method is in a superclass of the LC,
1703                 // and if our original search was above LC.super,
1704                 // repeat the search (symbolic lookup) from LC.super
1705                 // and continue with the direct superclass of that class,
1706                 // and so forth, until a match is found or no further superclasses exist.
1707                 // FIXME: MemberName.resolve should handle this instead.
1708                 Class<?> refcAsSuper = lookupClass();
1709                 MemberName m2;
1710                 do {
1711                     refcAsSuper = refcAsSuper.getSuperclass();
1712                     m2 = new MemberName(refcAsSuper,
1713                                         method.getName(),
1714                                         method.getMethodType(),
1715                                         REF_invokeSpecial);
1716                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
1717                 } while (m2 == null &&         // no method is found yet
1718                          refc != refcAsSuper); // search up to refc
1719                 if (m2 == null)  throw new InternalError(method.toString());
1720                 method = m2;
1721                 refc = refcAsSuper;
1722                 // redo basic checks
1723                 checkMethod(refKind, refc, method);
1724             }
1725 
1726             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method);
1727             MethodHandle mh = dmh;
1728             // Optionally narrow the receiver argument to refc using restrictReceiver.
1729             if (doRestrict &&
1730                    (refKind == REF_invokeSpecial ||
1731                        (MethodHandleNatives.refKindHasReceiver(refKind) &&
1732                            restrictProtectedReceiver(method)))) {
1733                 mh = restrictReceiver(method, dmh, lookupClass());
1734             }
1735             mh = maybeBindCaller(method, mh, callerClass);
1736             mh = mh.setVarargs(method);
1737             return mh;
1738         }
1739         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
1740                                              Class<?> callerClass)
1741                                              throws IllegalAccessException {
1742             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
1743                 return mh;
1744             Class<?> hostClass = lookupClass;
1745             if (!hasPrivateAccess())  // caller must have private access
1746                 hostClass = callerClass;  // callerClass came from a security manager style stack walk
1747             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
1748             // Note: caller will apply varargs after this step happens.
1749             return cbmh;
1750         }
1751         /** Check access and get the requested field. */
1752         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1753             final boolean checkSecurity = true;
1754             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1755         }
1756         /** Check access and get the requested field, eliding security manager checks. */
1757         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1758             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1759             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1760         }
1761         /** Common code for all fields; do not call directly except from immediately above. */
1762         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
1763                                                   boolean checkSecurity) throws IllegalAccessException {
1764             checkField(refKind, refc, field);
1765             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1766             if (checkSecurity)
1767                 checkSecurityManager(refc, field);
1768             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
1769             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
1770                                     restrictProtectedReceiver(field));
1771             if (doRestrict)
1772                 return restrictReceiver(field, dmh, lookupClass());
1773             return dmh;
1774         }
1775         /** Check access and get the requested constructor. */
1776         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
1777             final boolean checkSecurity = true;
1778             return getDirectConstructorCommon(refc, ctor, checkSecurity);
1779         }
1780         /** Check access and get the requested constructor, eliding security manager checks. */
1781         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
1782             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1783             return getDirectConstructorCommon(refc, ctor, checkSecurity);
1784         }
1785         /** Common code for all constructors; do not call directly except from immediately above. */
1786         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
1787                                                   boolean checkSecurity) throws IllegalAccessException {
1788             assert(ctor.isConstructor());
1789             checkAccess(REF_newInvokeSpecial, refc, ctor);
1790             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1791             if (checkSecurity)
1792                 checkSecurityManager(refc, ctor);
1793             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
1794             return DirectMethodHandle.make(ctor).setVarargs(ctor);
1795         }
1796 
1797         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
1798          */
1799         /*non-public*/
1800         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
1801             if (!(type instanceof Class || type instanceof MethodType))
1802                 throw new InternalError("unresolved MemberName");
1803             MemberName member = new MemberName(refKind, defc, name, type);
1804             MethodHandle mh = LOOKASIDE_TABLE.get(member);
1805             if (mh != null) {
1806                 checkSymbolicClass(defc);
1807                 return mh;
1808             }
1809             // Treat MethodHandle.invoke and invokeExact specially.
1810             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
1811                 mh = findVirtualForMH(member.getName(), member.getMethodType());
1812                 if (mh != null) {
1813                     return mh;
1814                 }
1815             }
1816             MemberName resolved = resolveOrFail(refKind, member);
1817             mh = getDirectMethodForConstant(refKind, defc, resolved);
1818             if (mh instanceof DirectMethodHandle
1819                     && canBeCached(refKind, defc, resolved)) {
1820                 MemberName key = mh.internalMemberName();
1821                 if (key != null) {
1822                     key = key.asNormalOriginal();
1823                 }
1824                 if (member.equals(key)) {  // better safe than sorry
1825                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
1826                 }
1827             }
1828             return mh;
1829         }
1830         private
1831         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
1832             if (refKind == REF_invokeSpecial) {
1833                 return false;
1834             }
1835             if (!Modifier.isPublic(defc.getModifiers()) ||
1836                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
1837                     !member.isPublic() ||
1838                     member.isCallerSensitive()) {
1839                 return false;
1840             }
1841             ClassLoader loader = defc.getClassLoader();
1842             if (!jdk.internal.misc.VM.isSystemDomainLoader(loader)) {
1843                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
1844                 boolean found = false;
1845                 while (sysl != null) {
1846                     if (loader == sysl) { found = true; break; }
1847                     sysl = sysl.getParent();
1848                 }
1849                 if (!found) {
1850                     return false;
1851                 }
1852             }
1853             try {
1854                 MemberName resolved2 = publicLookup().resolveOrFail(refKind,
1855                     new MemberName(refKind, defc, member.getName(), member.getType()));
1856                 checkSecurityManager(defc, resolved2);
1857             } catch (ReflectiveOperationException | SecurityException ex) {
1858                 return false;
1859             }
1860             return true;
1861         }
1862         private
1863         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
1864                 throws ReflectiveOperationException {
1865             if (MethodHandleNatives.refKindIsField(refKind)) {
1866                 return getDirectFieldNoSecurityManager(refKind, defc, member);
1867             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
1868                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
1869             } else if (refKind == REF_newInvokeSpecial) {
1870                 return getDirectConstructorNoSecurityManager(defc, member);
1871             }
1872             // oops
1873             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
1874         }
1875 
1876         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
1877     }
1878 
1879     /**
1880      * Produces a method handle giving read access to elements of an array.
1881      * The type of the method handle will have a return type of the array's
1882      * element type.  Its first argument will be the array type,
1883      * and the second will be {@code int}.
1884      * @param arrayClass an array type
1885      * @return a method handle which can load values from the given array type
1886      * @throws NullPointerException if the argument is null
1887      * @throws  IllegalArgumentException if arrayClass is not an array type
1888      */
1889     public static
1890     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
1891         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, false);
1892     }
1893 
1894     /**
1895      * Produces a method handle giving write access to elements of an array.
1896      * The type of the method handle will have a void return type.
1897      * Its last argument will be the array's element type.
1898      * The first and second arguments will be the array type and int.
1899      * @param arrayClass the class of an array
1900      * @return a method handle which can store values into the array type
1901      * @throws NullPointerException if the argument is null
1902      * @throws IllegalArgumentException if arrayClass is not an array type
1903      */
1904     public static
1905     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
1906         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, true);
1907     }
1908 
1909     /// method handle invocation (reflective style)
1910 
1911     /**
1912      * Produces a method handle which will invoke any method handle of the
1913      * given {@code type}, with a given number of trailing arguments replaced by
1914      * a single trailing {@code Object[]} array.
1915      * The resulting invoker will be a method handle with the following
1916      * arguments:
1917      * <ul>
1918      * <li>a single {@code MethodHandle} target
1919      * <li>zero or more leading values (counted by {@code leadingArgCount})
1920      * <li>an {@code Object[]} array containing trailing arguments
1921      * </ul>
1922      * <p>
1923      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
1924      * the indicated {@code type}.
1925      * That is, if the target is exactly of the given {@code type}, it will behave
1926      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
1927      * is used to convert the target to the required {@code type}.
1928      * <p>
1929      * The type of the returned invoker will not be the given {@code type}, but rather
1930      * will have all parameters except the first {@code leadingArgCount}
1931      * replaced by a single array of type {@code Object[]}, which will be
1932      * the final parameter.
1933      * <p>
1934      * Before invoking its target, the invoker will spread the final array, apply
1935      * reference casts as necessary, and unbox and widen primitive arguments.
1936      * If, when the invoker is called, the supplied array argument does
1937      * not have the correct number of elements, the invoker will throw
1938      * an {@link IllegalArgumentException} instead of invoking the target.
1939      * <p>
1940      * This method is equivalent to the following code (though it may be more efficient):
1941      * <blockquote><pre>{@code
1942 MethodHandle invoker = MethodHandles.invoker(type);
1943 int spreadArgCount = type.parameterCount() - leadingArgCount;
1944 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
1945 return invoker;
1946      * }</pre></blockquote>
1947      * This method throws no reflective or security exceptions.
1948      * @param type the desired target type
1949      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
1950      * @return a method handle suitable for invoking any method handle of the given type
1951      * @throws NullPointerException if {@code type} is null
1952      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
1953      *                  the range from 0 to {@code type.parameterCount()} inclusive,
1954      *                  or if the resulting method handle's type would have
1955      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
1956      */
1957     public static
1958     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
1959         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
1960             throw newIllegalArgumentException("bad argument count", leadingArgCount);
1961         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
1962         return type.invokers().spreadInvoker(leadingArgCount);
1963     }
1964 
1965     /**
1966      * Produces a special <em>invoker method handle</em> which can be used to
1967      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
1968      * The resulting invoker will have a type which is
1969      * exactly equal to the desired type, except that it will accept
1970      * an additional leading argument of type {@code MethodHandle}.
1971      * <p>
1972      * This method is equivalent to the following code (though it may be more efficient):
1973      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
1974      *
1975      * <p style="font-size:smaller;">
1976      * <em>Discussion:</em>
1977      * Invoker method handles can be useful when working with variable method handles
1978      * of unknown types.
1979      * For example, to emulate an {@code invokeExact} call to a variable method
1980      * handle {@code M}, extract its type {@code T},
1981      * look up the invoker method {@code X} for {@code T},
1982      * and call the invoker method, as {@code X.invoke(T, A...)}.
1983      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
1984      * is unknown.)
1985      * If spreading, collecting, or other argument transformations are required,
1986      * they can be applied once to the invoker {@code X} and reused on many {@code M}
1987      * method handle values, as long as they are compatible with the type of {@code X}.
1988      * <p style="font-size:smaller;">
1989      * <em>(Note:  The invoker method is not available via the Core Reflection API.
1990      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
1991      * on the declared {@code invokeExact} or {@code invoke} method will raise an
1992      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
1993      * <p>
1994      * This method throws no reflective or security exceptions.
1995      * @param type the desired target type
1996      * @return a method handle suitable for invoking any method handle of the given type
1997      * @throws IllegalArgumentException if the resulting method handle's type would have
1998      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
1999      */
2000     public static
2001     MethodHandle exactInvoker(MethodType type) {
2002         return type.invokers().exactInvoker();
2003     }
2004 
2005     /**
2006      * Produces a special <em>invoker method handle</em> which can be used to
2007      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
2008      * The resulting invoker will have a type which is
2009      * exactly equal to the desired type, except that it will accept
2010      * an additional leading argument of type {@code MethodHandle}.
2011      * <p>
2012      * Before invoking its target, if the target differs from the expected type,
2013      * the invoker will apply reference casts as
2014      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
2015      * Similarly, the return value will be converted as necessary.
2016      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
2017      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
2018      * <p>
2019      * This method is equivalent to the following code (though it may be more efficient):
2020      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
2021      * <p style="font-size:smaller;">
2022      * <em>Discussion:</em>
2023      * A {@linkplain MethodType#genericMethodType general method type} is one which
2024      * mentions only {@code Object} arguments and return values.
2025      * An invoker for such a type is capable of calling any method handle
2026      * of the same arity as the general type.
2027      * <p style="font-size:smaller;">
2028      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2029      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2030      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2031      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2032      * <p>
2033      * This method throws no reflective or security exceptions.
2034      * @param type the desired target type
2035      * @return a method handle suitable for invoking any method handle convertible to the given type
2036      * @throws IllegalArgumentException if the resulting method handle's type would have
2037      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2038      */
2039     public static
2040     MethodHandle invoker(MethodType type) {
2041         return type.invokers().genericInvoker();
2042     }
2043 
2044     static /*non-public*/
2045     MethodHandle basicInvoker(MethodType type) {
2046         return type.invokers().basicInvoker();
2047     }
2048 
2049      /// method handle modification (creation from other method handles)
2050 
2051     /**
2052      * Produces a method handle which adapts the type of the
2053      * given method handle to a new type by pairwise argument and return type conversion.
2054      * The original type and new type must have the same number of arguments.
2055      * The resulting method handle is guaranteed to report a type
2056      * which is equal to the desired new type.
2057      * <p>
2058      * If the original type and new type are equal, returns target.
2059      * <p>
2060      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
2061      * and some additional conversions are also applied if those conversions fail.
2062      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
2063      * if possible, before or instead of any conversions done by {@code asType}:
2064      * <ul>
2065      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
2066      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
2067      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
2068      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
2069      *     the boolean is converted to a byte value, 1 for true, 0 for false.
2070      *     (This treatment follows the usage of the bytecode verifier.)
2071      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
2072      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
2073      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
2074      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
2075      *     then a Java casting conversion (JLS 5.5) is applied.
2076      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
2077      *     widening and/or narrowing.)
2078      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
2079      *     conversion will be applied at runtime, possibly followed
2080      *     by a Java casting conversion (JLS 5.5) on the primitive value,
2081      *     possibly followed by a conversion from byte to boolean by testing
2082      *     the low-order bit.
2083      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
2084      *     and if the reference is null at runtime, a zero value is introduced.
2085      * </ul>
2086      * @param target the method handle to invoke after arguments are retyped
2087      * @param newType the expected type of the new method handle
2088      * @return a method handle which delegates to the target after performing
2089      *           any necessary argument conversions, and arranges for any
2090      *           necessary return value conversions
2091      * @throws NullPointerException if either argument is null
2092      * @throws WrongMethodTypeException if the conversion cannot be made
2093      * @see MethodHandle#asType
2094      */
2095     public static
2096     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
2097         explicitCastArgumentsChecks(target, newType);
2098         // use the asTypeCache when possible:
2099         MethodType oldType = target.type();
2100         if (oldType == newType)  return target;
2101         if (oldType.explicitCastEquivalentToAsType(newType)) {
2102             return target.asFixedArity().asType(newType);
2103         }
2104         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
2105     }
2106 
2107     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
2108         if (target.type().parameterCount() != newType.parameterCount()) {
2109             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
2110         }
2111     }
2112 
2113     /**
2114      * Produces a method handle which adapts the calling sequence of the
2115      * given method handle to a new type, by reordering the arguments.
2116      * The resulting method handle is guaranteed to report a type
2117      * which is equal to the desired new type.
2118      * <p>
2119      * The given array controls the reordering.
2120      * Call {@code #I} the number of incoming parameters (the value
2121      * {@code newType.parameterCount()}, and call {@code #O} the number
2122      * of outgoing parameters (the value {@code target.type().parameterCount()}).
2123      * Then the length of the reordering array must be {@code #O},
2124      * and each element must be a non-negative number less than {@code #I}.
2125      * For every {@code N} less than {@code #O}, the {@code N}-th
2126      * outgoing argument will be taken from the {@code I}-th incoming
2127      * argument, where {@code I} is {@code reorder[N]}.
2128      * <p>
2129      * No argument or return value conversions are applied.
2130      * The type of each incoming argument, as determined by {@code newType},
2131      * must be identical to the type of the corresponding outgoing parameter
2132      * or parameters in the target method handle.
2133      * The return type of {@code newType} must be identical to the return
2134      * type of the original target.
2135      * <p>
2136      * The reordering array need not specify an actual permutation.
2137      * An incoming argument will be duplicated if its index appears
2138      * more than once in the array, and an incoming argument will be dropped
2139      * if its index does not appear in the array.
2140      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
2141      * incoming arguments which are not mentioned in the reordering array
2142      * are may be any type, as determined only by {@code newType}.
2143      * <blockquote><pre>{@code
2144 import static java.lang.invoke.MethodHandles.*;
2145 import static java.lang.invoke.MethodType.*;
2146 ...
2147 MethodType intfn1 = methodType(int.class, int.class);
2148 MethodType intfn2 = methodType(int.class, int.class, int.class);
2149 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
2150 assert(sub.type().equals(intfn2));
2151 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
2152 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
2153 assert((int)rsub.invokeExact(1, 100) == 99);
2154 MethodHandle add = ... (int x, int y) -> (x+y) ...;
2155 assert(add.type().equals(intfn2));
2156 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
2157 assert(twice.type().equals(intfn1));
2158 assert((int)twice.invokeExact(21) == 42);
2159      * }</pre></blockquote>
2160      * @param target the method handle to invoke after arguments are reordered
2161      * @param newType the expected type of the new method handle
2162      * @param reorder an index array which controls the reordering
2163      * @return a method handle which delegates to the target after it
2164      *           drops unused arguments and moves and/or duplicates the other arguments
2165      * @throws NullPointerException if any argument is null
2166      * @throws IllegalArgumentException if the index array length is not equal to
2167      *                  the arity of the target, or if any index array element
2168      *                  not a valid index for a parameter of {@code newType},
2169      *                  or if two corresponding parameter types in
2170      *                  {@code target.type()} and {@code newType} are not identical,
2171      */
2172     public static
2173     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
2174         reorder = reorder.clone();  // get a private copy
2175         MethodType oldType = target.type();
2176         permuteArgumentChecks(reorder, newType, oldType);
2177         // first detect dropped arguments and handle them separately
2178         int[] originalReorder = reorder;
2179         BoundMethodHandle result = target.rebind();
2180         LambdaForm form = result.form;
2181         int newArity = newType.parameterCount();
2182         // Normalize the reordering into a real permutation,
2183         // by removing duplicates and adding dropped elements.
2184         // This somewhat improves lambda form caching, as well
2185         // as simplifying the transform by breaking it up into steps.
2186         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
2187             if (ddIdx > 0) {
2188                 // We found a duplicated entry at reorder[ddIdx].
2189                 // Example:  (x,y,z)->asList(x,y,z)
2190                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
2191                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
2192                 // The starred element corresponds to the argument
2193                 // deleted by the dupArgumentForm transform.
2194                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
2195                 boolean killFirst = false;
2196                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
2197                     // Set killFirst if the dup is larger than an intervening position.
2198                     // This will remove at least one inversion from the permutation.
2199                     if (dupVal > val) killFirst = true;
2200                 }
2201                 if (!killFirst) {
2202                     srcPos = dstPos;
2203                     dstPos = ddIdx;
2204                 }
2205                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
2206                 assert (reorder[srcPos] == reorder[dstPos]);
2207                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
2208                 // contract the reordering by removing the element at dstPos
2209                 int tailPos = dstPos + 1;
2210                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
2211                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
2212             } else {
2213                 int dropVal = ~ddIdx, insPos = 0;
2214                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
2215                     // Find first element of reorder larger than dropVal.
2216                     // This is where we will insert the dropVal.
2217                     insPos += 1;
2218                 }
2219                 Class<?> ptype = newType.parameterType(dropVal);
2220                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
2221                 oldType = oldType.insertParameterTypes(insPos, ptype);
2222                 // expand the reordering by inserting an element at insPos
2223                 int tailPos = insPos + 1;
2224                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
2225                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
2226                 reorder[insPos] = dropVal;
2227             }
2228             assert (permuteArgumentChecks(reorder, newType, oldType));
2229         }
2230         assert (reorder.length == newArity);  // a perfect permutation
2231         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
2232         form = form.editor().permuteArgumentsForm(1, reorder);
2233         if (newType == result.type() && form == result.internalForm())
2234             return result;
2235         return result.copyWith(newType, form);
2236     }
2237 
2238     /**
2239      * Return an indication of any duplicate or omission in reorder.
2240      * If the reorder contains a duplicate entry, return the index of the second occurrence.
2241      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
2242      * Otherwise, return zero.
2243      * If an element not in [0..newArity-1] is encountered, return reorder.length.
2244      */
2245     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
2246         final int BIT_LIMIT = 63;  // max number of bits in bit mask
2247         if (newArity < BIT_LIMIT) {
2248             long mask = 0;
2249             for (int i = 0; i < reorder.length; i++) {
2250                 int arg = reorder[i];
2251                 if (arg >= newArity) {
2252                     return reorder.length;
2253                 }
2254                 long bit = 1L << arg;
2255                 if ((mask & bit) != 0) {
2256                     return i;  // >0 indicates a dup
2257                 }
2258                 mask |= bit;
2259             }
2260             if (mask == (1L << newArity) - 1) {
2261                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
2262                 return 0;
2263             }
2264             // find first zero
2265             long zeroBit = Long.lowestOneBit(~mask);
2266             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
2267             assert(zeroPos <= newArity);
2268             if (zeroPos == newArity) {
2269                 return 0;
2270             }
2271             return ~zeroPos;
2272         } else {
2273             // same algorithm, different bit set
2274             BitSet mask = new BitSet(newArity);
2275             for (int i = 0; i < reorder.length; i++) {
2276                 int arg = reorder[i];
2277                 if (arg >= newArity) {
2278                     return reorder.length;
2279                 }
2280                 if (mask.get(arg)) {
2281                     return i;  // >0 indicates a dup
2282                 }
2283                 mask.set(arg);
2284             }
2285             int zeroPos = mask.nextClearBit(0);
2286             assert(zeroPos <= newArity);
2287             if (zeroPos == newArity) {
2288                 return 0;
2289             }
2290             return ~zeroPos;
2291         }
2292     }
2293 
2294     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
2295         if (newType.returnType() != oldType.returnType())
2296             throw newIllegalArgumentException("return types do not match",
2297                     oldType, newType);
2298         if (reorder.length == oldType.parameterCount()) {
2299             int limit = newType.parameterCount();
2300             boolean bad = false;
2301             for (int j = 0; j < reorder.length; j++) {
2302                 int i = reorder[j];
2303                 if (i < 0 || i >= limit) {
2304                     bad = true; break;
2305                 }
2306                 Class<?> src = newType.parameterType(i);
2307                 Class<?> dst = oldType.parameterType(j);
2308                 if (src != dst)
2309                     throw newIllegalArgumentException("parameter types do not match after reorder",
2310                             oldType, newType);
2311             }
2312             if (!bad)  return true;
2313         }
2314         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
2315     }
2316 
2317     /**
2318      * Produces a method handle of the requested return type which returns the given
2319      * constant value every time it is invoked.
2320      * <p>
2321      * Before the method handle is returned, the passed-in value is converted to the requested type.
2322      * If the requested type is primitive, widening primitive conversions are attempted,
2323      * else reference conversions are attempted.
2324      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
2325      * @param type the return type of the desired method handle
2326      * @param value the value to return
2327      * @return a method handle of the given return type and no arguments, which always returns the given value
2328      * @throws NullPointerException if the {@code type} argument is null
2329      * @throws ClassCastException if the value cannot be converted to the required return type
2330      * @throws IllegalArgumentException if the given type is {@code void.class}
2331      */
2332     public static
2333     MethodHandle constant(Class<?> type, Object value) {
2334         if (type.isPrimitive()) {
2335             if (type == void.class)
2336                 throw newIllegalArgumentException("void type");
2337             Wrapper w = Wrapper.forPrimitiveType(type);
2338             value = w.convert(value, type);
2339             if (w.zero().equals(value))
2340                 return zero(w, type);
2341             return insertArguments(identity(type), 0, value);
2342         } else {
2343             if (value == null)
2344                 return zero(Wrapper.OBJECT, type);
2345             return identity(type).bindTo(value);
2346         }
2347     }
2348 
2349     /**
2350      * Produces a method handle which returns its sole argument when invoked.
2351      * @param type the type of the sole parameter and return value of the desired method handle
2352      * @return a unary method handle which accepts and returns the given type
2353      * @throws NullPointerException if the argument is null
2354      * @throws IllegalArgumentException if the given type is {@code void.class}
2355      */
2356     public static
2357     MethodHandle identity(Class<?> type) {
2358         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
2359         int pos = btw.ordinal();
2360         MethodHandle ident = IDENTITY_MHS[pos];
2361         if (ident == null) {
2362             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
2363         }
2364         if (ident.type().returnType() == type)
2365             return ident;
2366         // something like identity(Foo.class); do not bother to intern these
2367         assert(btw == Wrapper.OBJECT);
2368         return makeIdentity(type);
2369     }
2370     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.values().length];
2371     private static MethodHandle makeIdentity(Class<?> ptype) {
2372         MethodType mtype = MethodType.methodType(ptype, ptype);
2373         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
2374         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
2375     }
2376 
2377     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
2378         int pos = btw.ordinal();
2379         MethodHandle zero = ZERO_MHS[pos];
2380         if (zero == null) {
2381             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
2382         }
2383         if (zero.type().returnType() == rtype)
2384             return zero;
2385         assert(btw == Wrapper.OBJECT);
2386         return makeZero(rtype);
2387     }
2388     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.values().length];
2389     private static MethodHandle makeZero(Class<?> rtype) {
2390         MethodType mtype = MethodType.methodType(rtype);
2391         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
2392         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
2393     }
2394 
2395     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
2396         // Simulate a CAS, to avoid racy duplication of results.
2397         MethodHandle prev = cache[pos];
2398         if (prev != null) return prev;
2399         return cache[pos] = value;
2400     }
2401 
2402     /**
2403      * Provides a target method handle with one or more <em>bound arguments</em>
2404      * in advance of the method handle's invocation.
2405      * The formal parameters to the target corresponding to the bound
2406      * arguments are called <em>bound parameters</em>.
2407      * Returns a new method handle which saves away the bound arguments.
2408      * When it is invoked, it receives arguments for any non-bound parameters,
2409      * binds the saved arguments to their corresponding parameters,
2410      * and calls the original target.
2411      * <p>
2412      * The type of the new method handle will drop the types for the bound
2413      * parameters from the original target type, since the new method handle
2414      * will no longer require those arguments to be supplied by its callers.
2415      * <p>
2416      * Each given argument object must match the corresponding bound parameter type.
2417      * If a bound parameter type is a primitive, the argument object
2418      * must be a wrapper, and will be unboxed to produce the primitive value.
2419      * <p>
2420      * The {@code pos} argument selects which parameters are to be bound.
2421      * It may range between zero and <i>N-L</i> (inclusively),
2422      * where <i>N</i> is the arity of the target method handle
2423      * and <i>L</i> is the length of the values array.
2424      * @param target the method handle to invoke after the argument is inserted
2425      * @param pos where to insert the argument (zero for the first)
2426      * @param values the series of arguments to insert
2427      * @return a method handle which inserts an additional argument,
2428      *         before calling the original method handle
2429      * @throws NullPointerException if the target or the {@code values} array is null
2430      * @see MethodHandle#bindTo
2431      */
2432     public static
2433     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
2434         int insCount = values.length;
2435         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
2436         if (insCount == 0)  return target;
2437         BoundMethodHandle result = target.rebind();
2438         for (int i = 0; i < insCount; i++) {
2439             Object value = values[i];
2440             Class<?> ptype = ptypes[pos+i];
2441             if (ptype.isPrimitive()) {
2442                 result = insertArgumentPrimitive(result, pos, ptype, value);
2443             } else {
2444                 value = ptype.cast(value);  // throw CCE if needed
2445                 result = result.bindArgumentL(pos, value);
2446             }
2447         }
2448         return result;
2449     }
2450 
2451     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
2452                                                              Class<?> ptype, Object value) {
2453         Wrapper w = Wrapper.forPrimitiveType(ptype);
2454         // perform unboxing and/or primitive conversion
2455         value = w.convert(value, ptype);
2456         switch (w) {
2457         case INT:     return result.bindArgumentI(pos, (int)value);
2458         case LONG:    return result.bindArgumentJ(pos, (long)value);
2459         case FLOAT:   return result.bindArgumentF(pos, (float)value);
2460         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
2461         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
2462         }
2463     }
2464 
2465     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
2466         MethodType oldType = target.type();
2467         int outargs = oldType.parameterCount();
2468         int inargs  = outargs - insCount;
2469         if (inargs < 0)
2470             throw newIllegalArgumentException("too many values to insert");
2471         if (pos < 0 || pos > inargs)
2472             throw newIllegalArgumentException("no argument type to append");
2473         return oldType.ptypes();
2474     }
2475 
2476     /**
2477      * Produces a method handle which will discard some dummy arguments
2478      * before calling some other specified <i>target</i> method handle.
2479      * The type of the new method handle will be the same as the target's type,
2480      * except it will also include the dummy argument types,
2481      * at some given position.
2482      * <p>
2483      * The {@code pos} argument may range between zero and <i>N</i>,
2484      * where <i>N</i> is the arity of the target.
2485      * If {@code pos} is zero, the dummy arguments will precede
2486      * the target's real arguments; if {@code pos} is <i>N</i>
2487      * they will come after.
2488      * <p>
2489      * <b>Example:</b>
2490      * <blockquote><pre>{@code
2491 import static java.lang.invoke.MethodHandles.*;
2492 import static java.lang.invoke.MethodType.*;
2493 ...
2494 MethodHandle cat = lookup().findVirtual(String.class,
2495   "concat", methodType(String.class, String.class));
2496 assertEquals("xy", (String) cat.invokeExact("x", "y"));
2497 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
2498 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
2499 assertEquals(bigType, d0.type());
2500 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
2501      * }</pre></blockquote>
2502      * <p>
2503      * This method is also equivalent to the following code:
2504      * <blockquote><pre>
2505      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
2506      * </pre></blockquote>
2507      * @param target the method handle to invoke after the arguments are dropped
2508      * @param valueTypes the type(s) of the argument(s) to drop
2509      * @param pos position of first argument to drop (zero for the leftmost)
2510      * @return a method handle which drops arguments of the given types,
2511      *         before calling the original method handle
2512      * @throws NullPointerException if the target is null,
2513      *                              or if the {@code valueTypes} list or any of its elements is null
2514      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
2515      *                  or if {@code pos} is negative or greater than the arity of the target,
2516      *                  or if the new method handle's type would have too many parameters
2517      */
2518     public static
2519     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
2520         MethodType oldType = target.type();  // get NPE
2521         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
2522         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
2523         if (dropped == 0)  return target;
2524         BoundMethodHandle result = target.rebind();
2525         LambdaForm lform = result.form;
2526         int insertFormArg = 1 + pos;
2527         for (Class<?> ptype : valueTypes) {
2528             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
2529         }
2530         result = result.copyWith(newType, lform);
2531         return result;
2532     }
2533 
2534     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
2535         int dropped = valueTypes.size();
2536         MethodType.checkSlotCount(dropped);
2537         int outargs = oldType.parameterCount();
2538         int inargs  = outargs + dropped;
2539         if (pos < 0 || pos > outargs)
2540             throw newIllegalArgumentException("no argument type to remove"
2541                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
2542                     );
2543         return dropped;
2544     }
2545 
2546     /**
2547      * Produces a method handle which will discard some dummy arguments
2548      * before calling some other specified <i>target</i> method handle.
2549      * The type of the new method handle will be the same as the target's type,
2550      * except it will also include the dummy argument types,
2551      * at some given position.
2552      * <p>
2553      * The {@code pos} argument may range between zero and <i>N</i>,
2554      * where <i>N</i> is the arity of the target.
2555      * If {@code pos} is zero, the dummy arguments will precede
2556      * the target's real arguments; if {@code pos} is <i>N</i>
2557      * they will come after.
2558      * <p>
2559      * <b>Example:</b>
2560      * <blockquote><pre>{@code
2561 import static java.lang.invoke.MethodHandles.*;
2562 import static java.lang.invoke.MethodType.*;
2563 ...
2564 MethodHandle cat = lookup().findVirtual(String.class,
2565   "concat", methodType(String.class, String.class));
2566 assertEquals("xy", (String) cat.invokeExact("x", "y"));
2567 MethodHandle d0 = dropArguments(cat, 0, String.class);
2568 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
2569 MethodHandle d1 = dropArguments(cat, 1, String.class);
2570 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
2571 MethodHandle d2 = dropArguments(cat, 2, String.class);
2572 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
2573 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
2574 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
2575      * }</pre></blockquote>
2576      * <p>
2577      * This method is also equivalent to the following code:
2578      * <blockquote><pre>
2579      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
2580      * </pre></blockquote>
2581      * @param target the method handle to invoke after the arguments are dropped
2582      * @param valueTypes the type(s) of the argument(s) to drop
2583      * @param pos position of first argument to drop (zero for the leftmost)
2584      * @return a method handle which drops arguments of the given types,
2585      *         before calling the original method handle
2586      * @throws NullPointerException if the target is null,
2587      *                              or if the {@code valueTypes} array or any of its elements is null
2588      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
2589      *                  or if {@code pos} is negative or greater than the arity of the target,
2590      *                  or if the new method handle's type would have
2591      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
2592      */
2593     public static
2594     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
2595         return dropArguments(target, pos, Arrays.asList(valueTypes));
2596     }
2597 
2598     /**
2599      * Adapts a target method handle by pre-processing
2600      * one or more of its arguments, each with its own unary filter function,
2601      * and then calling the target with each pre-processed argument
2602      * replaced by the result of its corresponding filter function.
2603      * <p>
2604      * The pre-processing is performed by one or more method handles,
2605      * specified in the elements of the {@code filters} array.
2606      * The first element of the filter array corresponds to the {@code pos}
2607      * argument of the target, and so on in sequence.
2608      * <p>
2609      * Null arguments in the array are treated as identity functions,
2610      * and the corresponding arguments left unchanged.
2611      * (If there are no non-null elements in the array, the original target is returned.)
2612      * Each filter is applied to the corresponding argument of the adapter.
2613      * <p>
2614      * If a filter {@code F} applies to the {@code N}th argument of
2615      * the target, then {@code F} must be a method handle which
2616      * takes exactly one argument.  The type of {@code F}'s sole argument
2617      * replaces the corresponding argument type of the target
2618      * in the resulting adapted method handle.
2619      * The return type of {@code F} must be identical to the corresponding
2620      * parameter type of the target.
2621      * <p>
2622      * It is an error if there are elements of {@code filters}
2623      * (null or not)
2624      * which do not correspond to argument positions in the target.
2625      * <p><b>Example:</b>
2626      * <blockquote><pre>{@code
2627 import static java.lang.invoke.MethodHandles.*;
2628 import static java.lang.invoke.MethodType.*;
2629 ...
2630 MethodHandle cat = lookup().findVirtual(String.class,
2631   "concat", methodType(String.class, String.class));
2632 MethodHandle upcase = lookup().findVirtual(String.class,
2633   "toUpperCase", methodType(String.class));
2634 assertEquals("xy", (String) cat.invokeExact("x", "y"));
2635 MethodHandle f0 = filterArguments(cat, 0, upcase);
2636 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
2637 MethodHandle f1 = filterArguments(cat, 1, upcase);
2638 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
2639 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
2640 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
2641      * }</pre></blockquote>
2642      * <p> Here is pseudocode for the resulting adapter:
2643      * <blockquote><pre>{@code
2644      * V target(P... p, A[i]... a[i], B... b);
2645      * A[i] filter[i](V[i]);
2646      * T adapter(P... p, V[i]... v[i], B... b) {
2647      *   return target(p..., f[i](v[i])..., b...);
2648      * }
2649      * }</pre></blockquote>
2650      *
2651      * @param target the method handle to invoke after arguments are filtered
2652      * @param pos the position of the first argument to filter
2653      * @param filters method handles to call initially on filtered arguments
2654      * @return method handle which incorporates the specified argument filtering logic
2655      * @throws NullPointerException if the target is null
2656      *                              or if the {@code filters} array is null
2657      * @throws IllegalArgumentException if a non-null element of {@code filters}
2658      *          does not match a corresponding argument type of target as described above,
2659      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
2660      *          or if the resulting method handle's type would have
2661      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2662      */
2663     public static
2664     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
2665         filterArgumentsCheckArity(target, pos, filters);
2666         MethodHandle adapter = target;
2667         int curPos = pos-1;  // pre-incremented
2668         for (MethodHandle filter : filters) {
2669             curPos += 1;
2670             if (filter == null)  continue;  // ignore null elements of filters
2671             adapter = filterArgument(adapter, curPos, filter);
2672         }
2673         return adapter;
2674     }
2675 
2676     /*non-public*/ static
2677     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
2678         filterArgumentChecks(target, pos, filter);
2679         MethodType targetType = target.type();
2680         MethodType filterType = filter.type();
2681         BoundMethodHandle result = target.rebind();
2682         Class<?> newParamType = filterType.parameterType(0);
2683         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
2684         MethodType newType = targetType.changeParameterType(pos, newParamType);
2685         result = result.copyWithExtendL(newType, lform, filter);
2686         return result;
2687     }
2688 
2689     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
2690         MethodType targetType = target.type();
2691         int maxPos = targetType.parameterCount();
2692         if (pos + filters.length > maxPos)
2693             throw newIllegalArgumentException("too many filters");
2694     }
2695 
2696     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
2697         MethodType targetType = target.type();
2698         MethodType filterType = filter.type();
2699         if (filterType.parameterCount() != 1
2700             || filterType.returnType() != targetType.parameterType(pos))
2701             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2702     }
2703 
2704     /**
2705      * Adapts a target method handle by pre-processing
2706      * a sub-sequence of its arguments with a filter (another method handle).
2707      * The pre-processed arguments are replaced by the result (if any) of the
2708      * filter function.
2709      * The target is then called on the modified (usually shortened) argument list.
2710      * <p>
2711      * If the filter returns a value, the target must accept that value as
2712      * its argument in position {@code pos}, preceded and/or followed by
2713      * any arguments not passed to the filter.
2714      * If the filter returns void, the target must accept all arguments
2715      * not passed to the filter.
2716      * No arguments are reordered, and a result returned from the filter
2717      * replaces (in order) the whole subsequence of arguments originally
2718      * passed to the adapter.
2719      * <p>
2720      * The argument types (if any) of the filter
2721      * replace zero or one argument types of the target, at position {@code pos},
2722      * in the resulting adapted method handle.
2723      * The return type of the filter (if any) must be identical to the
2724      * argument type of the target at position {@code pos}, and that target argument
2725      * is supplied by the return value of the filter.
2726      * <p>
2727      * In all cases, {@code pos} must be greater than or equal to zero, and
2728      * {@code pos} must also be less than or equal to the target's arity.
2729      * <p><b>Example:</b>
2730      * <blockquote><pre>{@code
2731 import static java.lang.invoke.MethodHandles.*;
2732 import static java.lang.invoke.MethodType.*;
2733 ...
2734 MethodHandle deepToString = publicLookup()
2735   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
2736 
2737 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
2738 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
2739 
2740 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
2741 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
2742 
2743 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
2744 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
2745 assertEquals("[top, [up, down], strange]",
2746              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
2747 
2748 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
2749 assertEquals("[top, [up, down], [strange]]",
2750              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
2751 
2752 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
2753 assertEquals("[top, [[up, down, strange], charm], bottom]",
2754              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
2755      * }</pre></blockquote>
2756      * <p> Here is pseudocode for the resulting adapter:
2757      * <blockquote><pre>{@code
2758      * T target(A...,V,C...);
2759      * V filter(B...);
2760      * T adapter(A... a,B... b,C... c) {
2761      *   V v = filter(b...);
2762      *   return target(a...,v,c...);
2763      * }
2764      * // and if the filter has no arguments:
2765      * T target2(A...,V,C...);
2766      * V filter2();
2767      * T adapter2(A... a,C... c) {
2768      *   V v = filter2();
2769      *   return target2(a...,v,c...);
2770      * }
2771      * // and if the filter has a void return:
2772      * T target3(A...,C...);
2773      * void filter3(B...);
2774      * void adapter3(A... a,B... b,C... c) {
2775      *   filter3(b...);
2776      *   return target3(a...,c...);
2777      * }
2778      * }</pre></blockquote>
2779      * <p>
2780      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
2781      * one which first "folds" the affected arguments, and then drops them, in separate
2782      * steps as follows:
2783      * <blockquote><pre>{@code
2784      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
2785      * mh = MethodHandles.foldArguments(mh, coll); //step 1
2786      * }</pre></blockquote>
2787      * If the target method handle consumes no arguments besides than the result
2788      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
2789      * is equivalent to {@code filterReturnValue(coll, mh)}.
2790      * If the filter method handle {@code coll} consumes one argument and produces
2791      * a non-void result, then {@code collectArguments(mh, N, coll)}
2792      * is equivalent to {@code filterArguments(mh, N, coll)}.
2793      * Other equivalences are possible but would require argument permutation.
2794      *
2795      * @param target the method handle to invoke after filtering the subsequence of arguments
2796      * @param pos the position of the first adapter argument to pass to the filter,
2797      *            and/or the target argument which receives the result of the filter
2798      * @param filter method handle to call on the subsequence of arguments
2799      * @return method handle which incorporates the specified argument subsequence filtering logic
2800      * @throws NullPointerException if either argument is null
2801      * @throws IllegalArgumentException if the return type of {@code filter}
2802      *          is non-void and is not the same as the {@code pos} argument of the target,
2803      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
2804      *          or if the resulting method handle's type would have
2805      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2806      * @see MethodHandles#foldArguments
2807      * @see MethodHandles#filterArguments
2808      * @see MethodHandles#filterReturnValue
2809      */
2810     public static
2811     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
2812         MethodType newType = collectArgumentsChecks(target, pos, filter);
2813         MethodType collectorType = filter.type();
2814         BoundMethodHandle result = target.rebind();
2815         LambdaForm lform;
2816         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
2817             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
2818             if (lform != null) {
2819                 return result.copyWith(newType, lform);
2820             }
2821         }
2822         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
2823         return result.copyWithExtendL(newType, lform, filter);
2824     }
2825 
2826     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
2827         MethodType targetType = target.type();
2828         MethodType filterType = filter.type();
2829         Class<?> rtype = filterType.returnType();
2830         List<Class<?>> filterArgs = filterType.parameterList();
2831         if (rtype == void.class) {
2832             return targetType.insertParameterTypes(pos, filterArgs);
2833         }
2834         if (rtype != targetType.parameterType(pos)) {
2835             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2836         }
2837         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
2838     }
2839 
2840     /**
2841      * Adapts a target method handle by post-processing
2842      * its return value (if any) with a filter (another method handle).
2843      * The result of the filter is returned from the adapter.
2844      * <p>
2845      * If the target returns a value, the filter must accept that value as
2846      * its only argument.
2847      * If the target returns void, the filter must accept no arguments.
2848      * <p>
2849      * The return type of the filter
2850      * replaces the return type of the target
2851      * in the resulting adapted method handle.
2852      * The argument type of the filter (if any) must be identical to the
2853      * return type of the target.
2854      * <p><b>Example:</b>
2855      * <blockquote><pre>{@code
2856 import static java.lang.invoke.MethodHandles.*;
2857 import static java.lang.invoke.MethodType.*;
2858 ...
2859 MethodHandle cat = lookup().findVirtual(String.class,
2860   "concat", methodType(String.class, String.class));
2861 MethodHandle length = lookup().findVirtual(String.class,
2862   "length", methodType(int.class));
2863 System.out.println((String) cat.invokeExact("x", "y")); // xy
2864 MethodHandle f0 = filterReturnValue(cat, length);
2865 System.out.println((int) f0.invokeExact("x", "y")); // 2
2866      * }</pre></blockquote>
2867      * <p> Here is pseudocode for the resulting adapter:
2868      * <blockquote><pre>{@code
2869      * V target(A...);
2870      * T filter(V);
2871      * T adapter(A... a) {
2872      *   V v = target(a...);
2873      *   return filter(v);
2874      * }
2875      * // and if the target has a void return:
2876      * void target2(A...);
2877      * T filter2();
2878      * T adapter2(A... a) {
2879      *   target2(a...);
2880      *   return filter2();
2881      * }
2882      * // and if the filter has a void return:
2883      * V target3(A...);
2884      * void filter3(V);
2885      * void adapter3(A... a) {
2886      *   V v = target3(a...);
2887      *   filter3(v);
2888      * }
2889      * }</pre></blockquote>
2890      * @param target the method handle to invoke before filtering the return value
2891      * @param filter method handle to call on the return value
2892      * @return method handle which incorporates the specified return value filtering logic
2893      * @throws NullPointerException if either argument is null
2894      * @throws IllegalArgumentException if the argument list of {@code filter}
2895      *          does not match the return type of target as described above
2896      */
2897     public static
2898     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
2899         MethodType targetType = target.type();
2900         MethodType filterType = filter.type();
2901         filterReturnValueChecks(targetType, filterType);
2902         BoundMethodHandle result = target.rebind();
2903         BasicType rtype = BasicType.basicType(filterType.returnType());
2904         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
2905         MethodType newType = targetType.changeReturnType(filterType.returnType());
2906         result = result.copyWithExtendL(newType, lform, filter);
2907         return result;
2908     }
2909 
2910     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
2911         Class<?> rtype = targetType.returnType();
2912         int filterValues = filterType.parameterCount();
2913         if (filterValues == 0
2914                 ? (rtype != void.class)
2915                 : (rtype != filterType.parameterType(0)))
2916             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2917     }
2918 
2919     /**
2920      * Adapts a target method handle by pre-processing
2921      * some of its arguments, and then calling the target with
2922      * the result of the pre-processing, inserted into the original
2923      * sequence of arguments.
2924      * <p>
2925      * The pre-processing is performed by {@code combiner}, a second method handle.
2926      * Of the arguments passed to the adapter, the first {@code N} arguments
2927      * are copied to the combiner, which is then called.
2928      * (Here, {@code N} is defined as the parameter count of the combiner.)
2929      * After this, control passes to the target, with any result
2930      * from the combiner inserted before the original {@code N} incoming
2931      * arguments.
2932      * <p>
2933      * If the combiner returns a value, the first parameter type of the target
2934      * must be identical with the return type of the combiner, and the next
2935      * {@code N} parameter types of the target must exactly match the parameters
2936      * of the combiner.
2937      * <p>
2938      * If the combiner has a void return, no result will be inserted,
2939      * and the first {@code N} parameter types of the target
2940      * must exactly match the parameters of the combiner.
2941      * <p>
2942      * The resulting adapter is the same type as the target, except that the
2943      * first parameter type is dropped,
2944      * if it corresponds to the result of the combiner.
2945      * <p>
2946      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
2947      * that either the combiner or the target does not wish to receive.
2948      * If some of the incoming arguments are destined only for the combiner,
2949      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
2950      * arguments will not need to be live on the stack on entry to the
2951      * target.)
2952      * <p><b>Example:</b>
2953      * <blockquote><pre>{@code
2954 import static java.lang.invoke.MethodHandles.*;
2955 import static java.lang.invoke.MethodType.*;
2956 ...
2957 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
2958   "println", methodType(void.class, String.class))
2959     .bindTo(System.out);
2960 MethodHandle cat = lookup().findVirtual(String.class,
2961   "concat", methodType(String.class, String.class));
2962 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
2963 MethodHandle catTrace = foldArguments(cat, trace);
2964 // also prints "boo":
2965 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
2966      * }</pre></blockquote>
2967      * <p> Here is pseudocode for the resulting adapter:
2968      * <blockquote><pre>{@code
2969      * // there are N arguments in A...
2970      * T target(V, A[N]..., B...);
2971      * V combiner(A...);
2972      * T adapter(A... a, B... b) {
2973      *   V v = combiner(a...);
2974      *   return target(v, a..., b...);
2975      * }
2976      * // and if the combiner has a void return:
2977      * T target2(A[N]..., B...);
2978      * void combiner2(A...);
2979      * T adapter2(A... a, B... b) {
2980      *   combiner2(a...);
2981      *   return target2(a..., b...);
2982      * }
2983      * }</pre></blockquote>
2984      * @param target the method handle to invoke after arguments are combined
2985      * @param combiner method handle to call initially on the incoming arguments
2986      * @return method handle which incorporates the specified argument folding logic
2987      * @throws NullPointerException if either argument is null
2988      * @throws IllegalArgumentException if {@code combiner}'s return type
2989      *          is non-void and not the same as the first argument type of
2990      *          the target, or if the initial {@code N} argument types
2991      *          of the target
2992      *          (skipping one matching the {@code combiner}'s return type)
2993      *          are not identical with the argument types of {@code combiner}
2994      */
2995     public static
2996     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
2997         return foldArguments(target, 0, combiner);
2998     }
2999 
3000     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
3001         int foldArgs   = combinerType.parameterCount();
3002         Class<?> rtype = combinerType.returnType();
3003         int foldVals = rtype == void.class ? 0 : 1;
3004         int afterInsertPos = foldPos + foldVals;
3005         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
3006         if (ok && !(combinerType.parameterList()
3007                     .equals(targetType.parameterList().subList(afterInsertPos,
3008                                                                afterInsertPos + foldArgs))))
3009             ok = false;
3010         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
3011             ok = false;
3012         if (!ok)
3013             throw misMatchedTypes("target and combiner types", targetType, combinerType);
3014         return rtype;
3015     }
3016 
3017     /**
3018      * Makes a method handle which adapts a target method handle,
3019      * by guarding it with a test, a boolean-valued method handle.
3020      * If the guard fails, a fallback handle is called instead.
3021      * All three method handles must have the same corresponding
3022      * argument and return types, except that the return type
3023      * of the test must be boolean, and the test is allowed
3024      * to have fewer arguments than the other two method handles.
3025      * <p> Here is pseudocode for the resulting adapter:
3026      * <blockquote><pre>{@code
3027      * boolean test(A...);
3028      * T target(A...,B...);
3029      * T fallback(A...,B...);
3030      * T adapter(A... a,B... b) {
3031      *   if (test(a...))
3032      *     return target(a..., b...);
3033      *   else
3034      *     return fallback(a..., b...);
3035      * }
3036      * }</pre></blockquote>
3037      * Note that the test arguments ({@code a...} in the pseudocode) cannot
3038      * be modified by execution of the test, and so are passed unchanged
3039      * from the caller to the target or fallback as appropriate.
3040      * @param test method handle used for test, must return boolean
3041      * @param target method handle to call if test passes
3042      * @param fallback method handle to call if test fails
3043      * @return method handle which incorporates the specified if/then/else logic
3044      * @throws NullPointerException if any argument is null
3045      * @throws IllegalArgumentException if {@code test} does not return boolean,
3046      *          or if all three method types do not match (with the return
3047      *          type of {@code test} changed to match that of the target).
3048      */
3049     public static
3050     MethodHandle guardWithTest(MethodHandle test,
3051                                MethodHandle target,
3052                                MethodHandle fallback) {
3053         MethodType gtype = test.type();
3054         MethodType ttype = target.type();
3055         MethodType ftype = fallback.type();
3056         if (!ttype.equals(ftype))
3057             throw misMatchedTypes("target and fallback types", ttype, ftype);
3058         if (gtype.returnType() != boolean.class)
3059             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
3060         List<Class<?>> targs = ttype.parameterList();
3061         List<Class<?>> gargs = gtype.parameterList();
3062         if (!targs.equals(gargs)) {
3063             int gpc = gargs.size(), tpc = targs.size();
3064             if (gpc >= tpc || !targs.subList(0, gpc).equals(gargs))
3065                 throw misMatchedTypes("target and test types", ttype, gtype);
3066             test = dropArguments(test, gpc, targs.subList(gpc, tpc));
3067             gtype = test.type();
3068         }
3069         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
3070     }
3071 
3072     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
3073         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
3074     }
3075 
3076     /**
3077      * Makes a method handle which adapts a target method handle,
3078      * by running it inside an exception handler.
3079      * If the target returns normally, the adapter returns that value.
3080      * If an exception matching the specified type is thrown, the fallback
3081      * handle is called instead on the exception, plus the original arguments.
3082      * <p>
3083      * The target and handler must have the same corresponding
3084      * argument and return types, except that handler may omit trailing arguments
3085      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
3086      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
3087      * <p> Here is pseudocode for the resulting adapter:
3088      * <blockquote><pre>{@code
3089      * T target(A..., B...);
3090      * T handler(ExType, A...);
3091      * T adapter(A... a, B... b) {
3092      *   try {
3093      *     return target(a..., b...);
3094      *   } catch (ExType ex) {
3095      *     return handler(ex, a...);
3096      *   }
3097      * }
3098      * }</pre></blockquote>
3099      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
3100      * be modified by execution of the target, and so are passed unchanged
3101      * from the caller to the handler, if the handler is invoked.
3102      * <p>
3103      * The target and handler must return the same type, even if the handler
3104      * always throws.  (This might happen, for instance, because the handler
3105      * is simulating a {@code finally} clause).
3106      * To create such a throwing handler, compose the handler creation logic
3107      * with {@link #throwException throwException},
3108      * in order to create a method handle of the correct return type.
3109      * @param target method handle to call
3110      * @param exType the type of exception which the handler will catch
3111      * @param handler method handle to call if a matching exception is thrown
3112      * @return method handle which incorporates the specified try/catch logic
3113      * @throws NullPointerException if any argument is null
3114      * @throws IllegalArgumentException if {@code handler} does not accept
3115      *          the given exception type, or if the method handle types do
3116      *          not match in their return types and their
3117      *          corresponding parameters
3118      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
3119      */
3120     public static
3121     MethodHandle catchException(MethodHandle target,
3122                                 Class<? extends Throwable> exType,
3123                                 MethodHandle handler) {
3124         MethodType ttype = target.type();
3125         MethodType htype = handler.type();
3126         if (!Throwable.class.isAssignableFrom(exType))
3127             throw new ClassCastException(exType.getName());
3128         if (htype.parameterCount() < 1 ||
3129             !htype.parameterType(0).isAssignableFrom(exType))
3130             throw newIllegalArgumentException("handler does not accept exception type "+exType);
3131         if (htype.returnType() != ttype.returnType())
3132             throw misMatchedTypes("target and handler return types", ttype, htype);
3133         List<Class<?>> targs = ttype.parameterList();
3134         List<Class<?>> hargs = htype.parameterList();
3135         hargs = hargs.subList(1, hargs.size());  // omit leading parameter from handler
3136         if (!targs.equals(hargs)) {
3137             int hpc = hargs.size(), tpc = targs.size();
3138             if (hpc >= tpc || !targs.subList(0, hpc).equals(hargs))
3139                 throw misMatchedTypes("target and handler types", ttype, htype);
3140             handler = dropArguments(handler, 1+hpc, targs.subList(hpc, tpc));
3141             htype = handler.type();
3142         }
3143         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
3144     }
3145 
3146     /**
3147      * Produces a method handle which will throw exceptions of the given {@code exType}.
3148      * The method handle will accept a single argument of {@code exType},
3149      * and immediately throw it as an exception.
3150      * The method type will nominally specify a return of {@code returnType}.
3151      * The return type may be anything convenient:  It doesn't matter to the
3152      * method handle's behavior, since it will never return normally.
3153      * @param returnType the return type of the desired method handle
3154      * @param exType the parameter type of the desired method handle
3155      * @return method handle which can throw the given exceptions
3156      * @throws NullPointerException if either argument is null
3157      */
3158     public static
3159     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
3160         if (!Throwable.class.isAssignableFrom(exType))
3161             throw new ClassCastException(exType.getName());
3162         return MethodHandleImpl.throwException(MethodType.methodType(returnType, exType));
3163     }
3164 
3165     /**
3166      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
3167      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
3168      * delivers the loop's result, which is the return value of the resulting handle.
3169      * <p>
3170      * Intuitively, every loop is formed by one or more "clauses", each specifying a local iteration value and/or a loop
3171      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
3172      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
3173      * terms of method handles, each clause will determine four actions:<ul>
3174      * <li>Before the loop executes, the initialization of an iteration variable or loop invariant local.
3175      * <li>When a clause executes, an update step for the iteration variable.
3176      * <li>When a clause executes, a predicate execution to test for loop exit.
3177      * <li>If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
3178      * </ul>
3179      * <p>
3180      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
3181      * this case. See below for a detailed description.
3182      * <p>
3183      * Each clause function, with the exception of clause initializers, is able to observe the entire loop state,
3184      * because it will be passed <em>all</em> current iteration variable values, as well as all incoming loop
3185      * parameters. Most clause functions will not need all of this information, but they will be formally connected as
3186      * if by {@link #dropArguments}.
3187      * <p>
3188      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
3189      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
3190      * corresponds to a place where {@link IllegalArgumentException} may be thrown if the required constraint is not met
3191      * by the inputs to the loop combinator. The term "effectively identical", applied to parameter type lists, means
3192      * that they must be identical, or else one list must be a proper prefix of the other.
3193      * <p>
3194      * <em>Step 0: Determine clause structure.</em><ol type="a">
3195      * <li>The clause array (of type {@code MethodHandle[][]} must be non-{@code null} and contain at least one element.
3196      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
3197      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
3198      * four. Padding takes place by appending elements to the array.
3199      * <li>Clauses with all {@code null}s are disregarded.
3200      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
3201      * </ol>
3202      * <p>
3203      * <em>Step 1A: Determine iteration variables.</em><ol type="a">
3204      * <li>Examine init and step function return types, pairwise, to determine each clause's iteration variable type.
3205      * <li>If both functions are omitted, use {@code void}; else if one is omitted, use the other's return type; else
3206      * use the common return type (they must be identical).
3207      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
3208      * <li>This list of types is called the "common prefix".
3209      * </ol>
3210      * <p>
3211      * <em>Step 1B: Determine loop parameters.</em><ol type="a">
3212      * <li>Examine init function parameter lists.
3213      * <li>Omitted init functions are deemed to have {@code null} parameter lists.
3214      * <li>All init function parameter lists must be effectively identical.
3215      * <li>The longest parameter list (which is necessarily unique) is called the "common suffix".
3216      * </ol>
3217      * <p>
3218      * <em>Step 1C: Determine loop return type.</em><ol type="a">
3219      * <li>Examine fini function return types, disregarding omitted fini functions.
3220      * <li>If there are no fini functions, use {@code void} as the loop return type.
3221      * <li>Otherwise, use the common return type of the fini functions; they must all be identical.
3222      * </ol>
3223      * <p>
3224      * <em>Step 1D: Check other types.</em><ol type="a">
3225      * <li>There must be at least one non-omitted pred function.
3226      * <li>Every non-omitted pred function must have a {@code boolean} return type.
3227      * </ol>
3228      * <p>
3229      * (Implementation Note: Steps 1A, 1B, 1C, 1D are logically independent of each other, and may be performed in any
3230      * order.)
3231      * <p>
3232      * <em>Step 2: Determine parameter lists.</em><ol type="a">
3233      * <li>The parameter list for the resulting loop handle will be the "common suffix".
3234      * <li>The parameter list for init functions will be adjusted to the "common suffix". (Note that their parameter
3235      * lists are already effectively identical to the common suffix.)
3236      * <li>The parameter list for non-init (step, pred, and fini) functions will be adjusted to the common prefix
3237      * followed by the common suffix, called the "common parameter sequence".
3238      * <li>Every non-init, non-omitted function parameter list must be effectively identical to the common parameter
3239      * sequence.
3240      * </ol>
3241      * <p>
3242      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
3243      * <li>If an init function is omitted, use a {@linkplain #constant constant function} of the appropriate
3244      * {@code null}/zero/{@code false}/{@code void} type. (For this purpose, a constant {@code void} is simply a
3245      * function which does nothing and returns {@code void}; it can be obtained from another constant function by
3246      * {@linkplain MethodHandle#asType type conversion}.)
3247      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
3248      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
3249      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
3250      * <li>If a pred function is omitted, the corresponding fini function must also be omitted.
3251      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
3252      * as this clause is concerned.)
3253      * <li>If a fini function is omitted, use a constant {@code null}/zero/{@code false}/{@code void} function of the
3254      * loop return type.
3255      * </ol>
3256      * <p>
3257      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
3258      * <li>At this point, every init function parameter list is effectively identical to the common suffix, but some
3259      * lists may be shorter. For every init function with a short parameter list, pad out the end of the list by
3260      * {@linkplain #dropArguments dropping arguments}.
3261      * <li>At this point, every non-init function parameter list is effectively identical to the common parameter
3262      * sequence, but some lists may be shorter. For every non-init function with a short parameter list, pad out the end
3263      * of the list by {@linkplain #dropArguments dropping arguments}.
3264      * </ol>
3265      * <p>
3266      * <em>Final observations.</em><ol type="a">
3267      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
3268      * <li>All init functions have a common parameter type list, which the final loop handle will also have.
3269      * <li>All fini functions have a common return type, which the final loop handle will also have.
3270      * <li>All non-init functions have a common parameter type list, which is the common parameter sequence, of
3271      * (non-{@code void}) iteration variables followed by loop parameters.
3272      * <li>Each pair of init and step functions agrees in their return types.
3273      * <li>Each non-init function will be able to observe the current values of all iteration variables, by means of the
3274      * common prefix.
3275      * </ol>
3276      * <p>
3277      * <em>Loop execution.</em><ol type="a">
3278      * <li>When the loop is called, the loop input values are saved in locals, to be passed (as the common suffix) to
3279      * every clause function. These locals are loop invariant.
3280      * <li>Each init function is executed in clause order (passing the common suffix) and the non-{@code void} values
3281      * are saved (as the common prefix) into locals. These locals are loop varying (unless their steps are identity
3282      * functions, as noted above).
3283      * <li>All function executions (except init functions) will be passed the common parameter sequence, consisting of
3284      * the non-{@code void} iteration values (in clause order) and then the loop inputs (in argument order).
3285      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
3286      * returns {@code false}.
3287      * <li>The non-{@code void} result from a step function call is used to update the corresponding loop variable. The
3288      * updated value is immediately visible to all subsequent function calls.
3289      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
3290      * is returned from the loop as a whole.
3291      * </ol>
3292      * <p>
3293      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the types / values
3294      * of loop variables; {@code A}/{@code a}, those of arguments passed to the resulting loop; and {@code R}, the
3295      * result types of finalizers as well as of the resulting loop.
3296      * <blockquote><pre>{@code
3297      * V... init...(A...);
3298      * boolean pred...(V..., A...);
3299      * V... step...(V..., A...);
3300      * R fini...(V..., A...);
3301      * R loop(A... a) {
3302      *   V... v... = init...(a...);
3303      *   for (;;) {
3304      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
3305      *       v = s(v..., a...);
3306      *       if (!p(v..., a...)) {
3307      *         return f(v..., a...);
3308      *       }
3309      *     }
3310      *   }
3311      * }
3312      * }</pre></blockquote>
3313      * <p>
3314      * @apiNote Example:
3315      * <blockquote><pre>{@code
3316      * // iterative implementation of the factorial function as a loop handle
3317      * static int one(int k) { return 1; }
3318      * int inc(int i, int acc, int k) { return i + 1; }
3319      * int mult(int i, int acc, int k) { return i * acc; }
3320      * boolean pred(int i, int acc, int k) { return i < k; }
3321      * int fin(int i, int acc, int k) { return acc; }
3322      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
3323      * // null initializer for counter, should initialize to 0
3324      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
3325      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
3326      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
3327      * assertEquals(120, loop.invoke(5));
3328      * }</pre></blockquote>
3329      *
3330      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
3331      *
3332      * @return a method handle embodying the looping behavior as defined by the arguments.
3333      *
3334      * @throws IllegalArgumentException in case any of the constraints described above is violated.
3335      *
3336      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
3337      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
3338      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
3339      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
3340      * @since 9
3341      */
3342     public static MethodHandle loop(MethodHandle[]... clauses) {
3343         // Step 0: determine clause structure.
3344         checkLoop0(clauses);
3345 
3346         List<MethodHandle> init = new ArrayList<>();
3347         List<MethodHandle> step = new ArrayList<>();
3348         List<MethodHandle> pred = new ArrayList<>();
3349         List<MethodHandle> fini = new ArrayList<>();
3350 
3351         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
3352             init.add(clause[0]); // all clauses have at least length 1
3353             step.add(clause.length <= 1 ? null : clause[1]);
3354             pred.add(clause.length <= 2 ? null : clause[2]);
3355             fini.add(clause.length <= 3 ? null : clause[3]);
3356         });
3357 
3358         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
3359         final int nclauses = init.size();
3360 
3361         // Step 1A: determine iteration variables.
3362         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
3363         for (int i = 0; i < nclauses; ++i) {
3364             MethodHandle in = init.get(i);
3365             MethodHandle st = step.get(i);
3366             if (in == null && st == null) {
3367                 iterationVariableTypes.add(void.class);
3368             } else if (in != null && st != null) {
3369                 checkLoop1a(i, in, st);
3370                 iterationVariableTypes.add(in.type().returnType());
3371             } else {
3372                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
3373             }
3374         }
3375         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
3376                 collect(Collectors.toList());
3377 
3378         // Step 1B: determine loop parameters.
3379         final List<Class<?>> empty = new ArrayList<>();
3380         final List<Class<?>> commonSuffix = init.stream().filter(Objects::nonNull).map(MethodHandle::type).
3381                 map(MethodType::parameterList).reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
3382         checkLoop1b(init, commonSuffix);
3383 
3384         // Step 1C: determine loop return type.
3385         // Step 1D: check other types.
3386         final Class<?> loopReturnType = fini.stream().filter(Objects::nonNull).map(MethodHandle::type).
3387                 map(MethodType::returnType).findFirst().orElse(void.class);
3388         checkLoop1cd(pred, fini, loopReturnType);
3389 
3390         // Step 2: determine parameter lists.
3391         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
3392         commonParameterSequence.addAll(commonSuffix);
3393         checkLoop2(step, pred, fini, commonParameterSequence);
3394 
3395         // Step 3: fill in omitted functions.
3396         for (int i = 0; i < nclauses; ++i) {
3397             Class<?> t = iterationVariableTypes.get(i);
3398             if (init.get(i) == null) {
3399                 init.set(i, zeroHandle(t));
3400             }
3401             if (step.get(i) == null) {
3402                 step.set(i, dropArguments(t == void.class ? zeroHandle(t) : identity(t), 0, commonPrefix.subList(0, i)));
3403             }
3404             if (pred.get(i) == null) {
3405                 pred.set(i, constant(boolean.class, true));
3406             }
3407             if (fini.get(i) == null) {
3408                 fini.set(i, zeroHandle(t));
3409             }
3410         }
3411 
3412         // Step 4: fill in missing parameter types.
3413         List<MethodHandle> finit = fillParameterTypes(init, commonSuffix);
3414         List<MethodHandle> fstep = fillParameterTypes(step, commonParameterSequence);
3415         List<MethodHandle> fpred = fillParameterTypes(pred, commonParameterSequence);
3416         List<MethodHandle> ffini = fillParameterTypes(fini, commonParameterSequence);
3417 
3418         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
3419                 allMatch(pl -> pl.equals(commonSuffix));
3420         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
3421                 allMatch(pl -> pl.equals(commonParameterSequence));
3422 
3423         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, commonPrefix, finit, fstep, fpred, ffini);
3424     }
3425 
3426     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
3427         return hs.stream().map(h -> {
3428             int pc = h.type().parameterCount();
3429             int tpsize = targetParams.size();
3430             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
3431         }).collect(Collectors.toList());
3432     }
3433 
3434     /**
3435      * Constructs a {@code while} loop from an initializer, a body, and a predicate. This is a convenience wrapper for
3436      * the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
3437      * <p>
3438      * The loop handle's result type is the same as the sole loop variable's, i.e., the result type of {@code init}.
3439      * The parameter type list of {@code init} also determines that of the resulting handle. The {@code pred} handle
3440      * must have an additional leading parameter of the same type as {@code init}'s result, and so must the {@code
3441      * body}. These constraints follow directly from those described for the {@linkplain MethodHandles#loop(MethodHandle[][])
3442      * generic loop combinator}.
3443      * <p>
3444      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
3445      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
3446      * passed to the loop.
3447      * <blockquote><pre>{@code
3448      * V init(A);
3449      * boolean pred(V, A);
3450      * V body(V, A);
3451      * V whileLoop(A a) {
3452      *   V v = init(a);
3453      *   while (pred(v, a)) {
3454      *     v = body(v, a);
3455      *   }
3456      *   return v;
3457      * }
3458      * }</pre></blockquote>
3459      * <p>
3460      * @apiNote Example:
3461      * <blockquote><pre>{@code
3462      * // implement the zip function for lists as a loop handle
3463      * List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
3464      * boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
3465      * List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
3466      *   zip.add(a.next());
3467      *   zip.add(b.next());
3468      *   return zip;
3469      * }
3470      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
3471      * MethodHandle loop = MethodHandles.doWhileLoop(MH_initZip, MH_zipStep, MH_zipPred);
3472      * List<String> a = Arrays.asList("a", "b", "c", "d");
3473      * List<String> b = Arrays.asList("e", "f", "g", "h");
3474      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
3475      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
3476      * }</pre></blockquote>
3477      *
3478      * <p>
3479      * @implSpec The implementation of this method is equivalent to:
3480      * <blockquote><pre>{@code
3481      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
3482      *     MethodHandle[]
3483      *         checkExit = {null, null, pred, identity(init.type().returnType())},
3484      *         varBody = {init, body};
3485      *     return loop(checkExit, varBody);
3486      * }
3487      * }</pre></blockquote>
3488      *
3489      * @param init initializer: it should provide the initial value of the loop variable. This controls the loop's
3490      *             result type. Passing {@code null} or a {@code void} init function will make the loop's result type
3491      *             {@code void}.
3492      * @param pred condition for the loop, which may not be {@code null}.
3493      * @param body body of the loop, which may not be {@code null}.
3494      *
3495      * @return the value of the loop variable as the loop terminates.
3496      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
3497      *
3498      * @see MethodHandles#loop(MethodHandle[][])
3499      * @since 9
3500      */
3501     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
3502         MethodHandle fin = init == null ? zeroHandle(void.class) : identity(init.type().returnType());
3503         MethodHandle[] checkExit = {null, null, pred, fin};
3504         MethodHandle[] varBody = {init, body};
3505         return loop(checkExit, varBody);
3506     }
3507 
3508     /**
3509      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate. This is a convenience wrapper
3510      * for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
3511      * <p>
3512      * The loop handle's result type is the same as the sole loop variable's, i.e., the result type of {@code init}.
3513      * The parameter type list of {@code init} also determines that of the resulting handle. The {@code pred} handle
3514      * must have an additional leading parameter of the same type as {@code init}'s result, and so must the {@code
3515      * body}. These constraints follow directly from those described for the {@linkplain MethodHandles#loop(MethodHandle[][])
3516      * generic loop combinator}.
3517      * <p>
3518      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
3519      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
3520      * passed to the loop.
3521      * <blockquote><pre>{@code
3522      * V init(A);
3523      * boolean pred(V, A);
3524      * V body(V, A);
3525      * V doWhileLoop(A a) {
3526      *   V v = init(a);
3527      *   do {
3528      *     v = body(v, a);
3529      *   } while (pred(v, a));
3530      *   return v;
3531      * }
3532      * }</pre></blockquote>
3533      * <p>
3534      * @apiNote Example:
3535      * <blockquote><pre>{@code
3536      * // int i = 0; while (i < limit) { ++i; } return i; => limit
3537      * int zero(int limit) { return 0; }
3538      * int step(int i, int limit) { return i + 1; }
3539      * boolean pred(int i, int limit) { return i < limit; }
3540      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
3541      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
3542      * assertEquals(23, loop.invoke(23));
3543      * }</pre></blockquote>
3544      *
3545      * <p>
3546      * @implSpec The implementation of this method is equivalent to:
3547      * <blockquote><pre>{@code
3548      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
3549      *     MethodHandle[] clause = { init, body, pred, identity(init.type().returnType()) };
3550      *     return loop(clause);
3551      * }
3552      * }</pre></blockquote>
3553      *
3554      *
3555      * @param init initializer: it should provide the initial value of the loop variable. This controls the loop's
3556      *             result type. Passing {@code null} or a {@code void} init function will make the loop's result type
3557      *             {@code void}.
3558      * @param pred condition for the loop, which may not be {@code null}.
3559      * @param body body of the loop, which may not be {@code null}.
3560      *
3561      * @return the value of the loop variable as the loop terminates.
3562      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
3563      *
3564      * @see MethodHandles#loop(MethodHandle[][])
3565      * @since 9
3566      */
3567     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
3568         MethodHandle fin = init == null ? zeroHandle(void.class) : identity(init.type().returnType());
3569         MethodHandle[] clause = {init, body, pred, fin};
3570         return loop(clause);
3571     }
3572 
3573     /**
3574      * Constructs a loop that runs a given number of iterations. The loop counter is an {@code int} initialized from the
3575      * {@code iterations} handle evaluation result. The counter is passed to the {@code body} function, so that must
3576      * accept an initial {@code int} argument. The result of the loop execution is the final value of the additional
3577      * local state. This is a convenience wrapper for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop
3578      * combinator}.
3579      * <p>
3580      * The result type and parameter type list of {@code init} determine those of the resulting handle. The {@code
3581      * iterations} handle must accept the same parameter types as {@code init} but return an {@code int}. The {@code
3582      * body} handle must accept the same parameter types as well, preceded by an {@code int} parameter for the counter,
3583      * and a parameter of the same type as {@code init}'s result. These constraints follow directly from those described
3584      * for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
3585      * <p>
3586      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
3587      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
3588      * passed to the loop.
3589      * <blockquote><pre>{@code
3590      * int iterations(A);
3591      * V init(A);
3592      * V body(int, V, A);
3593      * V countedLoop(A a) {
3594      *   int end = iterations(a);
3595      *   V v = init(a);
3596      *   for (int i = 0; i < end; ++i) {
3597      *     v = body(i, v, a);
3598      *   }
3599      *   return v;
3600      * }
3601      * }</pre></blockquote>
3602      * <p>
3603      * @apiNote Example:
3604      * <blockquote><pre>{@code
3605      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
3606      * // => a variation on a well known theme
3607      * String start(String arg) { return arg; }
3608      * String step(int counter, String v, String arg) { return "na " + v; }
3609      * // assume MH_start and MH_step are handles to the two methods above
3610      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
3611      * MethodHandle loop = MethodHandles.countedLoop(fit13, MH_start, MH_step);
3612      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
3613      * }</pre></blockquote>
3614      *
3615      * <p>
3616      * @implSpec The implementation of this method is equivalent to:
3617      * <blockquote><pre>{@code
3618      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
3619      *     return countedLoop(null, iterations, init, body);  // null => constant zero
3620      * }
3621      * }</pre></blockquote>
3622      *
3623      * @param iterations a handle to return the number of iterations this loop should run.
3624      * @param init initializer for additional loop state. This determines the loop's result type.
3625      *             Passing {@code null} or a {@code void} init function will make the loop's result type
3626      *             {@code void}.
3627      * @param body the body of the loop, which must not be {@code null}.
3628      *             It must accept an initial {@code int} parameter (for the counter), and then any
3629      *             additional loop-local variable plus loop parameters.
3630      *
3631      * @return a method handle representing the loop.
3632      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
3633      *
3634      * @since 9
3635      */
3636     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
3637         return countedLoop(null, iterations, init, body);
3638     }
3639 
3640     /**
3641      * Constructs a loop that counts over a range of numbers. The loop counter is an {@code int} that will be
3642      * initialized to the {@code int} value returned from the evaluation of the {@code start} handle and run to the
3643      * value returned from {@code end} (exclusively) with a step width of 1. The counter value is passed to the {@code
3644      * body} function in each iteration; it has to accept an initial {@code int} parameter
3645      * for that. The result of the loop execution is the final value of the additional local state
3646      * obtained by running {@code init}.
3647      * This is a
3648      * convenience wrapper for the {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}.
3649      * <p>
3650      * The constraints for the {@code init} and {@code body} handles are the same as for {@link
3651      * #countedLoop(MethodHandle, MethodHandle, MethodHandle)}. Additionally, the {@code start} and {@code end} handles
3652      * must return an {@code int} and accept the same parameters as {@code init}.
3653      * <p>
3654      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
3655      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
3656      * passed to the loop.
3657      * <blockquote><pre>{@code
3658      * int start(A);
3659      * int end(A);
3660      * V init(A);
3661      * V body(int, V, A);
3662      * V countedLoop(A a) {
3663      *   int s = start(a);
3664      *   int e = end(a);
3665      *   V v = init(a);
3666      *   for (int i = s; i < e; ++i) {
3667      *     v = body(i, v, a);
3668      *   }
3669      *   return v;
3670      * }
3671      * }</pre></blockquote>
3672      *
3673      * <p>
3674      * @implSpec The implementation of this method is equivalent to:
3675      * <blockquote><pre>{@code
3676      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
3677      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
3678      *     // assume MH_increment and MH_lessThan are handles to x+1 and x<y of type int
3679      *     MethodHandle[]
3680      *         indexVar = {start, MH_increment}, // i = start; i = i+1
3681      *         loopLimit = {end, null, MH_lessThan, returnVar }, // i<end
3682      *         bodyClause = {init, dropArguments(body, 1, int.class)};  // v = body(i, v);
3683      *     return loop(indexVar, loopLimit, bodyClause);
3684      * }
3685      * }</pre></blockquote>
3686      *
3687      * @param start a handle to return the start value of the loop counter.
3688      *              If it is {@code null}, a constant zero is assumed.
3689      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to {@code end-1}).
3690      * @param init initializer for additional loop state. This determines the loop's result type.
3691      *             Passing {@code null} or a {@code void} init function will make the loop's result type
3692      *             {@code void}.
3693      * @param body the body of the loop, which must not be {@code null}.
3694      *             It must accept an initial {@code int} parameter (for the counter), and then any
3695      *             additional loop-local variable plus loop parameters.
3696      *
3697      * @return a method handle representing the loop.
3698      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
3699      *
3700      * @since 9
3701      */
3702     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
3703         MethodHandle returnVar = dropArguments(init == null ? zeroHandle(void.class) : identity(init.type().returnType()),
3704                 0, int.class, int.class);
3705         MethodHandle[] indexVar = {start, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep)};
3706         MethodHandle[] loopLimit = {end, null, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred), returnVar};
3707         MethodHandle[] bodyClause = {init, dropArguments(body, 1, int.class)};
3708         return loop(indexVar, loopLimit, bodyClause);
3709     }
3710 
3711     /**
3712      * Constructs a loop that ranges over the elements produced by an {@code Iterator<T>}.
3713      * The iterator will be produced by the evaluation of the {@code iterator} handle.
3714      * If this handle is passed as {@code null} the method {@link Iterable#iterator} will be used instead,
3715      * and will be applied to a leading argument of the loop handle.
3716      * Each value produced by the iterator is passed to the {@code body}, which must accept an initial {@code T} parameter.
3717      * The result of the loop execution is the final value of the additional local state
3718      * obtained by running {@code init}.
3719      * <p>
3720      * This is a convenience wrapper for the
3721      * {@linkplain MethodHandles#loop(MethodHandle[][]) generic loop combinator}, and the constraints imposed on the {@code body}
3722      * handle follow directly from those described for the latter.
3723      * <p>
3724      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
3725      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
3726      * structure the loop iterates over, and {@code A}/{@code a}, that of the argument passed to the loop.
3727      * <blockquote><pre>{@code
3728      * Iterator<T> iterator(A);  // defaults to Iterable::iterator
3729      * V init(A);
3730      * V body(T,V,A);
3731      * V iteratedLoop(A a) {
3732      *   Iterator<T> it = iterator(a);
3733      *   V v = init(a);
3734      *   for (T t : it) {
3735      *     v = body(t, v, a);
3736      *   }
3737      *   return v;
3738      * }
3739      * }</pre></blockquote>
3740      * <p>
3741      * The type {@code T} may be either a primitive or reference.
3742      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type
3743      * {@code Iterator}, the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body}
3744      * to {@code Object} as if by the {@link MethodHandle#asType asType} conversion method.
3745      * Therefore, if an iterator of the wrong type appears as the loop is executed,
3746      * runtime exceptions may occur as the result of dynamic conversions performed by {@code asType}.
3747      * <p>
3748      * @apiNote Example:
3749      * <blockquote><pre>{@code
3750      * // reverse a list
3751      * List<String> reverseStep(String e, List<String> r, List<String> l) {
3752      *   r.add(0, e);
3753      *   return r;
3754      * }
3755      * List<String> newArrayList(List<String> l) { return new ArrayList<>(); }
3756      * // assume MH_reverseStep, MH_newArrayList are handles to the above methods
3757      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
3758      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
3759      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
3760      * assertEquals(reversedList, (List<String>) loop.invoke(list));
3761      * }</pre></blockquote>
3762      * <p>
3763      * @implSpec The implementation of this method is equivalent to:
3764      * <blockquote><pre>{@code
3765      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
3766      *     // assume MH_next and MH_hasNext are handles to methods of Iterator
3767      *     Class<?> itype = iterator.type().returnType();
3768      *     Class<?> ttype = body.type().parameterType(0);
3769      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, itype);
3770      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
3771      *     MethodHandle[]
3772      *         iterVar = {iterator, null, MH_hasNext, returnVar}, // it = iterator(); while (it.hasNext)
3773      *         bodyClause = {init, filterArgument(body, 0, nextVal)};  // v = body(t, v, a);
3774      *     return loop(iterVar, bodyClause);
3775      * }
3776      * }</pre></blockquote>
3777      *
3778      * @param iterator a handle to return the iterator to start the loop.
3779      *             Passing {@code null} will make the loop call {@link Iterable#iterator()} on the first
3780      *             incoming value.
3781      * @param init initializer for additional loop state. This determines the loop's result type.
3782      *             Passing {@code null} or a {@code void} init function will make the loop's result type
3783      *             {@code void}.
3784      * @param body the body of the loop, which must not be {@code null}.
3785      *             It must accept an initial {@code T} parameter (for the iterated values), and then any
3786      *             additional loop-local variable plus loop parameters.
3787      *
3788      * @return a method handle embodying the iteration loop functionality.
3789      * @throws IllegalArgumentException if any argument has a type inconsistent with the loop structure
3790      *
3791      * @since 9
3792      */
3793     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
3794         checkIteratedLoop(body);
3795 
3796         MethodHandle initit = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
3797         MethodHandle initIterator = iterator == null ?
3798                 initit.asType(initit.type().changeParameterType(0, body.type().parameterType(init == null ? 1 : 2))) :
3799                 iterator;
3800         Class<?> itype = initIterator.type().returnType();
3801         Class<?> ttype = body.type().parameterType(0);
3802 
3803         MethodHandle returnVar =
3804                 dropArguments(init == null ? zeroHandle(void.class) : identity(init.type().returnType()), 0, itype);
3805         MethodHandle initnx = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
3806         MethodHandle nextVal = initnx.asType(initnx.type().changeReturnType(ttype));
3807 
3808         MethodHandle[] iterVar = {initIterator, null, MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred), returnVar};
3809         MethodHandle[] bodyClause = {init, filterArgument(body, 0, nextVal)};
3810 
3811         return loop(iterVar, bodyClause);
3812     }
3813 
3814     /**
3815      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
3816      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
3817      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
3818      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
3819      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
3820      * {@code try-finally} handle.
3821      * <p>
3822      * The {@code cleanup} handle will be passed one or two additional leading arguments.
3823      * The first is the exception thrown during the
3824      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
3825      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
3826      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
3827      * The second argument is not present if the {@code target} handle has a {@code void} return type.
3828      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
3829      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
3830      * <p>
3831      * The {@code target} and {@code cleanup} handles' return types must be the same. Their parameter type lists also
3832      * must be the same, but the {@code cleanup} handle must accept one or two more leading parameters:<ul>
3833      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
3834      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
3835      * the result from the execution of the {@code target} handle.
3836      * This parameter is not present if the {@code target} returns {@code void}.
3837      * </ul>
3838      * <p>
3839      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
3840      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
3841      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
3842      * the cleanup.
3843      * <blockquote><pre>{@code
3844      * V target(A..., B...);
3845      * V cleanup(Throwable, V, A...);
3846      * V adapter(A... a, B... b) {
3847      *   V result = (zero value for V);
3848      *   Throwable throwable = null;
3849      *   try {
3850      *     result = target(a..., b...);
3851      *   } catch (Throwable t) {
3852      *     throwable = t;
3853      *     throw t;
3854      *   } finally {
3855      *     result = cleanup(throwable, result, a...);
3856      *   }
3857      *   return result;
3858      * }
3859      * }</pre></blockquote>
3860      * <p>
3861      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
3862      * be modified by execution of the target, and so are passed unchanged
3863      * from the caller to the cleanup, if it is invoked.
3864      * <p>
3865      * The target and cleanup must return the same type, even if the cleanup
3866      * always throws.
3867      * To create such a throwing cleanup, compose the cleanup logic
3868      * with {@link #throwException throwException},
3869      * in order to create a method handle of the correct return type.
3870      * <p>
3871      * Note that {@code tryFinally} never converts exceptions into normal returns.
3872      * In rare cases where exceptions must be converted in that way, first wrap
3873      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
3874      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
3875      *
3876      * @param target the handle whose execution is to be wrapped in a {@code try} block.
3877      * @param cleanup the handle that is invoked in the finally block.
3878      *
3879      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
3880      * @throws NullPointerException if any argument is null
3881      * @throws IllegalArgumentException if {@code cleanup} does not accept
3882      *          the required leading arguments, or if the method handle types do
3883      *          not match in their return types and their
3884      *          corresponding trailing parameters
3885      *
3886      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
3887      * @since 9
3888      */
3889     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
3890         List<Class<?>> targetParamTypes = target.type().parameterList();
3891         List<Class<?>> cleanupParamTypes = cleanup.type().parameterList();
3892         Class<?> rtype = target.type().returnType();
3893 
3894         checkTryFinally(target, cleanup);
3895 
3896         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
3897         int tpSize = targetParamTypes.size();
3898         int cpPrefixLength = rtype == void.class ? 1 : 2;
3899         int cpSize = cleanupParamTypes.size();
3900         MethodHandle aCleanup = cpSize - cpPrefixLength < tpSize ?
3901                 dropArguments(cleanup, cpSize, targetParamTypes.subList(tpSize - (cpSize - cpPrefixLength), tpSize)) :
3902                 cleanup;
3903 
3904         MethodHandle aTarget = target.asSpreader(Object[].class, target.type().parameterCount());
3905         aCleanup = aCleanup.asSpreader(Object[].class, tpSize);
3906 
3907         return MethodHandleImpl.makeTryFinally(aTarget, aCleanup, rtype, targetParamTypes);
3908     }
3909 
3910     /**
3911      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
3912      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
3913      * before the folded arguments.
3914      * <p>
3915      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
3916      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
3917      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
3918      * 0.
3919      * <p>
3920      * @apiNote Example:
3921      * <blockquote><pre>{@code
3922     import static java.lang.invoke.MethodHandles.*;
3923     import static java.lang.invoke.MethodType.*;
3924     ...
3925     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
3926     "println", methodType(void.class, String.class))
3927     .bindTo(System.out);
3928     MethodHandle cat = lookup().findVirtual(String.class,
3929     "concat", methodType(String.class, String.class));
3930     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
3931     MethodHandle catTrace = foldArguments(cat, 1, trace);
3932     // also prints "jum":
3933     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
3934      * }</pre></blockquote>
3935      * <p> Here is pseudocode for the resulting adapter:
3936      * <blockquote><pre>{@code
3937      * // there are N arguments in A...
3938      * T target(Z..., V, A[N]..., B...);
3939      * V combiner(A...);
3940      * T adapter(Z... z, A... a, B... b) {
3941      *   V v = combiner(a...);
3942      *   return target(z..., v, a..., b...);
3943      * }
3944      * // and if the combiner has a void return:
3945      * T target2(Z..., A[N]..., B...);
3946      * void combiner2(A...);
3947      * T adapter2(Z... z, A... a, B... b) {
3948      *   combiner2(a...);
3949      *   return target2(z..., a..., b...);
3950      * }
3951      * }</pre></blockquote>
3952      *
3953      * @param target the method handle to invoke after arguments are combined
3954      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
3955      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
3956      * @param combiner method handle to call initially on the incoming arguments
3957      * @return method handle which incorporates the specified argument folding logic
3958      * @throws NullPointerException if either argument is null
3959      * @throws IllegalArgumentException if {@code combiner}'s return type
3960      *          is non-void and not the same as the argument type at position {@code pos} of
3961      *          the target signature, or if the {@code N} argument types at position {@code pos}
3962      *          of the target signature
3963      *          (skipping one matching the {@code combiner}'s return type)
3964      *          are not identical with the argument types of {@code combiner}
3965      *
3966      * @see #foldArguments(MethodHandle, MethodHandle)
3967      * @since 9
3968      */
3969     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
3970         MethodType targetType = target.type();
3971         MethodType combinerType = combiner.type();
3972         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
3973         BoundMethodHandle result = target.rebind();
3974         boolean dropResult = rtype == void.class;
3975         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
3976         MethodType newType = targetType;
3977         if (!dropResult) {
3978             newType = newType.dropParameterTypes(pos, pos + 1);
3979         }
3980         result = result.copyWithExtendL(newType, lform, combiner);
3981         return result;
3982     }
3983 
3984     /**
3985      * Wrap creation of a proper zero handle for a given type.
3986      *
3987      * @param type the type.
3988      *
3989      * @return a zero value for the given type.
3990      */
3991     static MethodHandle zeroHandle(Class<?> type) {
3992         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
3993     }
3994 
3995     private static void checkLoop0(MethodHandle[][] clauses) {
3996         if (clauses == null || clauses.length == 0) {
3997             throw newIllegalArgumentException("null or no clauses passed");
3998         }
3999         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
4000             throw newIllegalArgumentException("null clauses are not allowed");
4001         }
4002         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
4003             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
4004         }
4005     }
4006 
4007     private static void checkLoop1a(int i, MethodHandle in, MethodHandle st) {
4008         if (in.type().returnType() != st.type().returnType()) {
4009             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
4010                     st.type().returnType());
4011         }
4012     }
4013 
4014     private static void checkLoop1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
4015         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::parameterList).
4016                 anyMatch(pl -> !pl.equals(commonSuffix.subList(0, pl.size())))) {
4017             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
4018                     " (common suffix: " + commonSuffix + ")");
4019         }
4020     }
4021 
4022     private static void checkLoop1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
4023         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4024                 anyMatch(t -> t != loopReturnType)) {
4025             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
4026                     loopReturnType + ")");
4027         }
4028 
4029         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
4030             throw newIllegalArgumentException("no predicate found", pred);
4031         }
4032         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
4033                 anyMatch(t -> t != boolean.class)) {
4034             throw newIllegalArgumentException("predicates must have boolean return type", pred);
4035         }
4036     }
4037 
4038     private static void checkLoop2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
4039         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
4040                 map(MethodType::parameterList).anyMatch(pl -> !pl.equals(commonParameterSequence.subList(0, pl.size())))) {
4041             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
4042                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
4043         }
4044     }
4045 
4046     private static void checkIteratedLoop(MethodHandle body) {
4047         if (null == body) {
4048             throw newIllegalArgumentException("iterated loop body must not be null");
4049         }
4050     }
4051 
4052     private static void checkTryFinally(MethodHandle target, MethodHandle cleanup) {
4053         Class<?> rtype = target.type().returnType();
4054         if (rtype != cleanup.type().returnType()) {
4055             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
4056         }
4057         List<Class<?>> cleanupParamTypes = cleanup.type().parameterList();
4058         if (!Throwable.class.isAssignableFrom(cleanupParamTypes.get(0))) {
4059             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
4060         }
4061         if (rtype != void.class && cleanupParamTypes.get(1) != rtype) {
4062             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
4063         }
4064         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
4065         // target parameter list.
4066         int cleanupArgIndex = rtype == void.class ? 1 : 2;
4067         if (!cleanupParamTypes.subList(cleanupArgIndex, cleanupParamTypes.size()).
4068                 equals(target.type().parameterList().subList(0, cleanupParamTypes.size() - cleanupArgIndex))) {
4069             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
4070                     cleanup.type(), target.type());
4071         }
4072     }
4073 
4074 }