1 /*
   2  * Copyright (c) 2008, 2013, 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.BitSet;
  30 import java.util.List;
  31 import java.util.Arrays;
  32 
  33 import sun.invoke.util.ValueConversions;
  34 import sun.invoke.util.VerifyAccess;
  35 import sun.invoke.util.Wrapper;
  36 import sun.reflect.CallerSensitive;
  37 import sun.reflect.Reflection;
  38 import sun.reflect.misc.ReflectUtil;
  39 import sun.security.util.SecurityConstants;
  40 import java.lang.invoke.LambdaForm.BasicType;
  41 import static java.lang.invoke.LambdaForm.BasicType.*;
  42 import static java.lang.invoke.MethodHandleStatics.*;
  43 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  44 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  45 
  46 import java.util.concurrent.ConcurrentHashMap;
  47 
  48 /**
  49  * This class consists exclusively of static methods that operate on or return
  50  * method handles. They fall into several categories:
  51  * <ul>
  52  * <li>Lookup methods which help create method handles for methods and fields.
  53  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  54  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  55  * </ul>
  56  * <p>
  57  * @author John Rose, JSR 292 EG
  58  * @since 1.7
  59  */
  60 public class MethodHandles {
  61 
  62     private MethodHandles() { }  // do not instantiate
  63 
  64     private static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  65     static { MethodHandleImpl.initStatics(); }
  66     // See IMPL_LOOKUP below.
  67 
  68     //// Method handle creation from ordinary methods.
  69 
  70     /**
  71      * Returns a {@link Lookup lookup object} with
  72      * full capabilities to emulate all supported bytecode behaviors of the caller.
  73      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  74      * Factory methods on the lookup object can create
  75      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  76      * for any member that the caller has access to via bytecodes,
  77      * including protected and private fields and methods.
  78      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
  79      * Do not store it in place where untrusted code can access it.
  80      * <p>
  81      * This method is caller sensitive, which means that it may return different
  82      * values to different callers.
  83      * <p>
  84      * For any given caller class {@code C}, the lookup object returned by this call
  85      * has equivalent capabilities to any lookup object
  86      * supplied by the JVM to the bootstrap method of an
  87      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
  88      * executing in the same caller class {@code C}.
  89      * @return a lookup object for the caller of this method, with private access
  90      */
  91     @CallerSensitive
  92     public static Lookup lookup() {
  93         return new Lookup(Reflection.getCallerClass());
  94     }
  95 
  96     /**
  97      * Returns a {@link Lookup lookup object} which is trusted minimally.
  98      * It can only be used to create method handles to
  99      * publicly accessible fields and methods.
 100      * <p>
 101      * As a matter of pure convention, the {@linkplain Lookup#lookupClass lookup class}
 102      * of this lookup object will be {@link java.lang.Object}.
 103      *
 104      * <p style="font-size:smaller;">
 105      * <em>Discussion:</em>
 106      * The lookup class can be changed to any other class {@code C} using an expression of the form
 107      * {@link Lookup#in publicLookup().in(C.class)}.
 108      * Since all classes have equal access to public names,
 109      * such a change would confer no new access rights.
 110      * A public lookup object is always subject to
 111      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 112      * Also, it cannot access
 113      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 114      * @return a lookup object which is trusted minimally
 115      */
 116     public static Lookup publicLookup() {
 117         return Lookup.PUBLIC_LOOKUP;
 118     }
 119 
 120     /**
 121      * Performs an unchecked "crack" of a
 122      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 123      * The result is as if the user had obtained a lookup object capable enough
 124      * to crack the target method handle, called
 125      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 126      * on the target to obtain its symbolic reference, and then called
 127      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 128      * to resolve the symbolic reference to a member.
 129      * <p>
 130      * If there is a security manager, its {@code checkPermission} method
 131      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 132      * @param <T> the desired type of the result, either {@link Member} or a subtype
 133      * @param target a direct method handle to crack into symbolic reference components
 134      * @param expected a class object representing the desired result type {@code T}
 135      * @return a reference to the method, constructor, or field object
 136      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 137      * @exception NullPointerException if either argument is {@code null}
 138      * @exception IllegalArgumentException if the target is not a direct method handle
 139      * @exception ClassCastException if the member is not of the expected type
 140      * @since 1.8
 141      */
 142     public static <T extends Member> T
 143     reflectAs(Class<T> expected, MethodHandle target) {
 144         SecurityManager smgr = System.getSecurityManager();
 145         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 146         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 147         return lookup.revealDirect(target).reflectAs(expected, lookup);
 148     }
 149     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 150     static final private java.security.Permission ACCESS_PERMISSION =
 151         new ReflectPermission("suppressAccessChecks");
 152 
 153     /**
 154      * A <em>lookup object</em> is a factory for creating method handles,
 155      * when the creation requires access checking.
 156      * Method handles do not perform
 157      * access checks when they are called, but rather when they are created.
 158      * Therefore, method handle access
 159      * restrictions must be enforced when a method handle is created.
 160      * The caller class against which those restrictions are enforced
 161      * is known as the {@linkplain #lookupClass lookup class}.
 162      * <p>
 163      * A lookup class which needs to create method handles will call
 164      * {@link MethodHandles#lookup MethodHandles.lookup} to create a factory for itself.
 165      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 166      * determined, and securely stored in the {@code Lookup} object.
 167      * The lookup class (or its delegates) may then use factory methods
 168      * on the {@code Lookup} object to create method handles for access-checked members.
 169      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 170      * even private ones.
 171      *
 172      * <h1><a name="lookups"></a>Lookup Factory Methods</h1>
 173      * The factory methods on a {@code Lookup} object correspond to all major
 174      * use cases for methods, constructors, and fields.
 175      * Each method handle created by a factory method is the functional
 176      * equivalent of a particular <em>bytecode behavior</em>.
 177      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 178      * Here is a summary of the correspondence between these factory methods and
 179      * the behavior the resulting method handles:
 180      * <table border=1 cellpadding=5 summary="lookup method behaviors">
 181      * <tr>
 182      *     <th><a name="equiv"></a>lookup expression</th>
 183      *     <th>member</th>
 184      *     <th>bytecode behavior</th>
 185      * </tr>
 186      * <tr>
 187      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</td>
 188      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 189      * </tr>
 190      * <tr>
 191      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</td>
 192      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 193      * </tr>
 194      * <tr>
 195      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</td>
 196      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 197      * </tr>
 198      * <tr>
 199      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</td>
 200      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 201      * </tr>
 202      * <tr>
 203      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</td>
 204      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 205      * </tr>
 206      * <tr>
 207      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</td>
 208      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 209      * </tr>
 210      * <tr>
 211      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</td>
 212      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 213      * </tr>
 214      * <tr>
 215      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</td>
 216      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 217      * </tr>
 218      * <tr>
 219      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</td>
 220      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 221      * </tr>
 222      * <tr>
 223      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</td>
 224      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 225      * </tr>
 226      * <tr>
 227      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 228      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 229      * </tr>
 230      * <tr>
 231      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</td>
 232      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 233      * </tr>
 234      * <tr>
 235      *     <td>{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
 236      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 237      * </tr>
 238      * </table>
 239      *
 240      * Here, the type {@code C} is the class or interface being searched for a member,
 241      * documented as a parameter named {@code refc} in the lookup methods.
 242      * The method type {@code MT} is composed from the return type {@code T}
 243      * and the sequence of argument types {@code A*}.
 244      * The constructor also has a sequence of argument types {@code A*} and
 245      * is deemed to return the newly-created object of type {@code C}.
 246      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 247      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 248      * if it is present, it is always the leading argument to the method handle invocation.
 249      * (In the case of some {@code protected} members, {@code this} may be
 250      * restricted in type to the lookup class; see below.)
 251      * The name {@code arg} stands for all the other method handle arguments.
 252      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 253      * stands for a null reference if the accessed method or field is static,
 254      * and {@code this} otherwise.
 255      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 256      * for reflective objects corresponding to the given members.
 257      * <p>
 258      * In cases where the given member is of variable arity (i.e., a method or constructor)
 259      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 260      * In all other cases, the returned method handle will be of fixed arity.
 261      * <p style="font-size:smaller;">
 262      * <em>Discussion:</em>
 263      * The equivalence between looked-up method handles and underlying
 264      * class members and bytecode behaviors
 265      * can break down in a few ways:
 266      * <ul style="font-size:smaller;">
 267      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 268      * the lookup can still succeed, even when there is no equivalent
 269      * Java expression or bytecoded constant.
 270      * <li>Likewise, if {@code T} or {@code MT}
 271      * is not symbolically accessible from the lookup class's loader,
 272      * the lookup can still succeed.
 273      * For example, lookups for {@code MethodHandle.invokeExact} and
 274      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 275      * <li>If there is a security manager installed, it can forbid the lookup
 276      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 277      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 278      * constant is not subject to security manager checks.
 279      * <li>If the looked-up method has a
 280      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 281      * the method handle creation may fail, due to the method handle
 282      * type having too many parameters.
 283      * </ul>
 284      *
 285      * <h1><a name="access"></a>Access checking</h1>
 286      * Access checks are applied in the factory methods of {@code Lookup},
 287      * when a method handle is created.
 288      * This is a key difference from the Core Reflection API, since
 289      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 290      * performs access checking against every caller, on every call.
 291      * <p>
 292      * All access checks start from a {@code Lookup} object, which
 293      * compares its recorded lookup class against all requests to
 294      * create method handles.
 295      * A single {@code Lookup} object can be used to create any number
 296      * of access-checked method handles, all checked against a single
 297      * lookup class.
 298      * <p>
 299      * A {@code Lookup} object can be shared with other trusted code,
 300      * such as a metaobject protocol.
 301      * A shared {@code Lookup} object delegates the capability
 302      * to create method handles on private members of the lookup class.
 303      * Even if privileged code uses the {@code Lookup} object,
 304      * the access checking is confined to the privileges of the
 305      * original lookup class.
 306      * <p>
 307      * A lookup can fail, because
 308      * the containing class is not accessible to the lookup class, or
 309      * because the desired class member is missing, or because the
 310      * desired class member is not accessible to the lookup class, or
 311      * because the lookup object is not trusted enough to access the member.
 312      * In any of these cases, a {@code ReflectiveOperationException} will be
 313      * thrown from the attempted lookup.  The exact class will be one of
 314      * the following:
 315      * <ul>
 316      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 317      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 318      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 319      * </ul>
 320      * <p>
 321      * In general, the conditions under which a method handle may be
 322      * looked up for a method {@code M} are no more restrictive than the conditions
 323      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 324      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 325      * a method handle lookup will generally raise a corresponding
 326      * checked exception, such as {@code NoSuchMethodException}.
 327      * And the effect of invoking the method handle resulting from the lookup
 328      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 329      * to executing the compiled, verified, and resolved call to {@code M}.
 330      * The same point is true of fields and constructors.
 331      * <p style="font-size:smaller;">
 332      * <em>Discussion:</em>
 333      * Access checks only apply to named and reflected methods,
 334      * constructors, and fields.
 335      * Other method handle creation methods, such as
 336      * {@link MethodHandle#asType MethodHandle.asType},
 337      * do not require any access checks, and are used
 338      * independently of any {@code Lookup} object.
 339      * <p>
 340      * If the desired member is {@code protected}, the usual JVM rules apply,
 341      * including the requirement that the lookup class must be either be in the
 342      * same package as the desired member, or must inherit that member.
 343      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 344      * In addition, if the desired member is a non-static field or method
 345      * in a different package, the resulting method handle may only be applied
 346      * to objects of the lookup class or one of its subclasses.
 347      * This requirement is enforced by narrowing the type of the leading
 348      * {@code this} parameter from {@code C}
 349      * (which will necessarily be a superclass of the lookup class)
 350      * to the lookup class itself.
 351      * <p>
 352      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 353      * that the receiver argument must match both the resolved method <em>and</em>
 354      * the current class.  Again, this requirement is enforced by narrowing the
 355      * type of the leading parameter to the resulting method handle.
 356      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 357      * <p>
 358      * The JVM represents constructors and static initializer blocks as internal methods
 359      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 360      * The internal syntax of invocation instructions allows them to refer to such internal
 361      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 362      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 363      * <p>
 364      * In some cases, access between nested classes is obtained by the Java compiler by creating
 365      * an wrapper method to access a private method of another class
 366      * in the same top-level declaration.
 367      * For example, a nested class {@code C.D}
 368      * can access private members within other related classes such as
 369      * {@code C}, {@code C.D.E}, or {@code C.B},
 370      * but the Java compiler may need to generate wrapper methods in
 371      * those related classes.  In such cases, a {@code Lookup} object on
 372      * {@code C.E} would be unable to those private members.
 373      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 374      * which can transform a lookup on {@code C.E} into one on any of those other
 375      * classes, without special elevation of privilege.
 376      * <p>
 377      * The accesses permitted to a given lookup object may be limited,
 378      * according to its set of {@link #lookupModes lookupModes},
 379      * to a subset of members normally accessible to the lookup class.
 380      * For example, the {@link MethodHandles#publicLookup publicLookup}
 381      * method produces a lookup object which is only allowed to access
 382      * public members in public classes.
 383      * The caller sensitive method {@link MethodHandles#lookup lookup}
 384      * produces a lookup object with full capabilities relative to
 385      * its caller class, to emulate all supported bytecode behaviors.
 386      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 387      * with fewer access modes than the original lookup object.
 388      *
 389      * <p style="font-size:smaller;">
 390      * <a name="privacc"></a>
 391      * <em>Discussion of private access:</em>
 392      * We say that a lookup has <em>private access</em>
 393      * if its {@linkplain #lookupModes lookup modes}
 394      * include the possibility of accessing {@code private} members.
 395      * As documented in the relevant methods elsewhere,
 396      * only lookups with private access possess the following capabilities:
 397      * <ul style="font-size:smaller;">
 398      * <li>access private fields, methods, and constructors of the lookup class
 399      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 400      *     such as {@code Class.forName}
 401      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 402      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 403      *     for classes accessible to the lookup class
 404      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 405      *     within the same package member
 406      * </ul>
 407      * <p style="font-size:smaller;">
 408      * Each of these permissions is a consequence of the fact that a lookup object
 409      * with private access can be securely traced back to an originating class,
 410      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 411      * can be reliably determined and emulated by method handles.
 412      *
 413      * <h1><a name="secmgr"></a>Security manager interactions</h1>
 414      * Although bytecode instructions can only refer to classes in
 415      * a related class loader, this API can search for methods in any
 416      * class, as long as a reference to its {@code Class} object is
 417      * available.  Such cross-loader references are also possible with the
 418      * Core Reflection API, and are impossible to bytecode instructions
 419      * such as {@code invokestatic} or {@code getfield}.
 420      * There is a {@linkplain java.lang.SecurityManager security manager API}
 421      * to allow applications to check such cross-loader references.
 422      * These checks apply to both the {@code MethodHandles.Lookup} API
 423      * and the Core Reflection API
 424      * (as found on {@link java.lang.Class Class}).
 425      * <p>
 426      * If a security manager is present, member lookups are subject to
 427      * additional checks.
 428      * From one to three calls are made to the security manager.
 429      * Any of these calls can refuse access by throwing a
 430      * {@link java.lang.SecurityException SecurityException}.
 431      * Define {@code smgr} as the security manager,
 432      * {@code lookc} as the lookup class of the current lookup object,
 433      * {@code refc} as the containing class in which the member
 434      * is being sought, and {@code defc} as the class in which the
 435      * member is actually defined.
 436      * The value {@code lookc} is defined as <em>not present</em>
 437      * if the current lookup object does not have
 438      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 439      * The calls are made according to the following rules:
 440      * <ul>
 441      * <li><b>Step 1:</b>
 442      *     If {@code lookc} is not present, or if its class loader is not
 443      *     the same as or an ancestor of the class loader of {@code refc},
 444      *     then {@link SecurityManager#checkPackageAccess
 445      *     smgr.checkPackageAccess(refcPkg)} is called,
 446      *     where {@code refcPkg} is the package of {@code refc}.
 447      * <li><b>Step 2:</b>
 448      *     If the retrieved member is not public and
 449      *     {@code lookc} is not present, then
 450      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 451      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 452      * <li><b>Step 3:</b>
 453      *     If the retrieved member is not public,
 454      *     and if {@code lookc} is not present,
 455      *     and if {@code defc} and {@code refc} are different,
 456      *     then {@link SecurityManager#checkPackageAccess
 457      *     smgr.checkPackageAccess(defcPkg)} is called,
 458      *     where {@code defcPkg} is the package of {@code defc}.
 459      * </ul>
 460      * Security checks are performed after other access checks have passed.
 461      * Therefore, the above rules presuppose a member that is public,
 462      * or else that is being accessed from a lookup class that has
 463      * rights to access the member.
 464      *
 465      * <h1><a name="callsens"></a>Caller sensitive methods</h1>
 466      * A small number of Java methods have a special property called caller sensitivity.
 467      * A <em>caller-sensitive</em> method can behave differently depending on the
 468      * identity of its immediate caller.
 469      * <p>
 470      * If a method handle for a caller-sensitive method is requested,
 471      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 472      * but they take account of the lookup class in a special way.
 473      * The resulting method handle behaves as if it were called
 474      * from an instruction contained in the lookup class,
 475      * so that the caller-sensitive method detects the lookup class.
 476      * (By contrast, the invoker of the method handle is disregarded.)
 477      * Thus, in the case of caller-sensitive methods,
 478      * different lookup classes may give rise to
 479      * differently behaving method handles.
 480      * <p>
 481      * In cases where the lookup object is
 482      * {@link MethodHandles#publicLookup() publicLookup()},
 483      * or some other lookup object without
 484      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 485      * the lookup class is disregarded.
 486      * In such cases, no caller-sensitive method handle can be created,
 487      * access is forbidden, and the lookup fails with an
 488      * {@code IllegalAccessException}.
 489      * <p style="font-size:smaller;">
 490      * <em>Discussion:</em>
 491      * For example, the caller-sensitive method
 492      * {@link java.lang.Class#forName(String) Class.forName(x)}
 493      * can return varying classes or throw varying exceptions,
 494      * depending on the class loader of the class that calls it.
 495      * A public lookup of {@code Class.forName} will fail, because
 496      * there is no reasonable way to determine its bytecode behavior.
 497      * <p style="font-size:smaller;">
 498      * If an application caches method handles for broad sharing,
 499      * it should use {@code publicLookup()} to create them.
 500      * If there is a lookup of {@code Class.forName}, it will fail,
 501      * and the application must take appropriate action in that case.
 502      * It may be that a later lookup, perhaps during the invocation of a
 503      * bootstrap method, can incorporate the specific identity
 504      * of the caller, making the method accessible.
 505      * <p style="font-size:smaller;">
 506      * The function {@code MethodHandles.lookup} is caller sensitive
 507      * so that there can be a secure foundation for lookups.
 508      * Nearly all other methods in the JSR 292 API rely on lookup
 509      * objects to check access requests.
 510      */
 511     public static final
 512     class Lookup {
 513         /** The class on behalf of whom the lookup is being performed. */
 514         private final Class<?> lookupClass;
 515 
 516         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 517         private final int allowedModes;
 518 
 519         /** A single-bit mask representing {@code public} access,
 520          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 521          *  The value, {@code 0x01}, happens to be the same as the value of the
 522          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 523          */
 524         public static final int PUBLIC = Modifier.PUBLIC;
 525 
 526         /** A single-bit mask representing {@code private} access,
 527          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 528          *  The value, {@code 0x02}, happens to be the same as the value of the
 529          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 530          */
 531         public static final int PRIVATE = Modifier.PRIVATE;
 532 
 533         /** A single-bit mask representing {@code protected} access,
 534          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 535          *  The value, {@code 0x04}, happens to be the same as the value of the
 536          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 537          */
 538         public static final int PROTECTED = Modifier.PROTECTED;
 539 
 540         /** A single-bit mask representing {@code package} access (default access),
 541          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 542          *  The value is {@code 0x08}, which does not correspond meaningfully to
 543          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 544          */
 545         public static final int PACKAGE = Modifier.STATIC;
 546 
 547         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE);
 548         private static final int TRUSTED   = -1;
 549 
 550         private static int fixmods(int mods) {
 551             mods &= (ALL_MODES - PACKAGE);
 552             return (mods != 0) ? mods : PACKAGE;
 553         }
 554 
 555         /** Tells which class is performing the lookup.  It is this class against
 556          *  which checks are performed for visibility and access permissions.
 557          *  <p>
 558          *  The class implies a maximum level of access permission,
 559          *  but the permissions may be additionally limited by the bitmask
 560          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 561          *  can be accessed.
 562          *  @return the lookup class, on behalf of which this lookup object finds members
 563          */
 564         public Class<?> lookupClass() {
 565             return lookupClass;
 566         }
 567 
 568         // This is just for calling out to MethodHandleImpl.
 569         private Class<?> lookupClassOrNull() {
 570             return (allowedModes == TRUSTED) ? null : lookupClass;
 571         }
 572 
 573         /** Tells which access-protection classes of members this lookup object can produce.
 574          *  The result is a bit-mask of the bits
 575          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 576          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 577          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 578          *  and {@linkplain #PACKAGE PACKAGE (0x08)}.
 579          *  <p>
 580          *  A freshly-created lookup object
 581          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class}
 582          *  has all possible bits set, since the caller class can access all its own members.
 583          *  A lookup object on a new lookup class
 584          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 585          *  may have some mode bits set to zero.
 586          *  The purpose of this is to restrict access via the new lookup object,
 587          *  so that it can access only names which can be reached by the original
 588          *  lookup object, and also by the new lookup class.
 589          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 590          */
 591         public int lookupModes() {
 592             return allowedModes & ALL_MODES;
 593         }
 594 
 595         /** Embody the current class (the lookupClass) as a lookup class
 596          * for method handle creation.
 597          * Must be called by from a method in this package,
 598          * which in turn is called by a method not in this package.
 599          */
 600         Lookup(Class<?> lookupClass) {
 601             this(lookupClass, ALL_MODES);
 602             // make sure we haven't accidentally picked up a privileged class:
 603             checkUnprivilegedlookupClass(lookupClass, ALL_MODES);
 604         }
 605 
 606         private Lookup(Class<?> lookupClass, int allowedModes) {
 607             this.lookupClass = lookupClass;
 608             this.allowedModes = allowedModes;
 609         }
 610 
 611         /**
 612          * Creates a lookup on the specified new lookup class.
 613          * The resulting object will report the specified
 614          * class as its own {@link #lookupClass lookupClass}.
 615          * <p>
 616          * However, the resulting {@code Lookup} object is guaranteed
 617          * to have no more access capabilities than the original.
 618          * In particular, access capabilities can be lost as follows:<ul>
 619          * <li>If the new lookup class differs from the old one,
 620          * protected members will not be accessible by virtue of inheritance.
 621          * (Protected members may continue to be accessible because of package sharing.)
 622          * <li>If the new lookup class is in a different package
 623          * than the old one, protected and default (package) members will not be accessible.
 624          * <li>If the new lookup class is not within the same package member
 625          * as the old one, private members will not be accessible.
 626          * <li>If the new lookup class is not accessible to the old lookup class,
 627          * then no members, not even public members, will be accessible.
 628          * (In all other cases, public members will continue to be accessible.)
 629          * </ul>
 630          *
 631          * @param requestedLookupClass the desired lookup class for the new lookup object
 632          * @return a lookup object which reports the desired lookup class
 633          * @throws NullPointerException if the argument is null
 634          */
 635         public Lookup in(Class<?> requestedLookupClass) {
 636             requestedLookupClass.getClass();  // null check
 637             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 638                 return new Lookup(requestedLookupClass, ALL_MODES);
 639             if (requestedLookupClass == this.lookupClass)
 640                 return this;  // keep same capabilities
 641             int newModes = (allowedModes & (ALL_MODES & ~PROTECTED));
 642             if ((newModes & PACKAGE) != 0
 643                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 644                 newModes &= ~(PACKAGE|PRIVATE);
 645             }
 646             // Allow nestmate lookups to be created without special privilege:
 647             if ((newModes & PRIVATE) != 0
 648                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 649                 newModes &= ~PRIVATE;
 650             }
 651             if ((newModes & PUBLIC) != 0
 652                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 653                 // The requested class it not accessible from the lookup class.
 654                 // No permissions.
 655                 newModes = 0;
 656             }
 657             checkUnprivilegedlookupClass(requestedLookupClass, newModes);
 658             return new Lookup(requestedLookupClass, newModes);
 659         }
 660 
 661         // Make sure outer class is initialized first.
 662         static { IMPL_NAMES.getClass(); }
 663 
 664         /** Version of lookup which is trusted minimally.
 665          *  It can only be used to create method handles to
 666          *  publicly accessible members.
 667          */
 668         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, PUBLIC);
 669 
 670         /** Package-private version of lookup which is trusted. */
 671         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
 672 
 673         private static void checkUnprivilegedlookupClass(Class<?> lookupClass, int allowedModes) {
 674             String name = lookupClass.getName();
 675             if (name.startsWith("java.lang.invoke."))
 676                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
 677 
 678             // For caller-sensitive MethodHandles.lookup()
 679             // disallow lookup more restricted packages
 680             if (allowedModes == ALL_MODES && lookupClass.getClassLoader() == null) {
 681                 if (name.startsWith("java.") ||
 682                         (name.startsWith("sun.") && !name.startsWith("sun.invoke."))) {
 683                     throw newIllegalArgumentException("illegal lookupClass: " + lookupClass);
 684                 }
 685             }
 686         }
 687 
 688         /**
 689          * Displays the name of the class from which lookups are to be made.
 690          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
 691          * If there are restrictions on the access permitted to this lookup,
 692          * this is indicated by adding a suffix to the class name, consisting
 693          * of a slash and a keyword.  The keyword represents the strongest
 694          * allowed access, and is chosen as follows:
 695          * <ul>
 696          * <li>If no access is allowed, the suffix is "/noaccess".
 697          * <li>If only public access is allowed, the suffix is "/public".
 698          * <li>If only public and package access are allowed, the suffix is "/package".
 699          * <li>If only public, package, and private access are allowed, the suffix is "/private".
 700          * </ul>
 701          * If none of the above cases apply, it is the case that full
 702          * access (public, package, private, and protected) is allowed.
 703          * In this case, no suffix is added.
 704          * This is true only of an object obtained originally from
 705          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
 706          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
 707          * always have restricted access, and will display a suffix.
 708          * <p>
 709          * (It may seem strange that protected access should be
 710          * stronger than private access.  Viewed independently from
 711          * package access, protected access is the first to be lost,
 712          * because it requires a direct subclass relationship between
 713          * caller and callee.)
 714          * @see #in
 715          */
 716         @Override
 717         public String toString() {
 718             String cname = lookupClass.getName();
 719             switch (allowedModes) {
 720             case 0:  // no privileges
 721                 return cname + "/noaccess";
 722             case PUBLIC:
 723                 return cname + "/public";
 724             case PUBLIC|PACKAGE:
 725                 return cname + "/package";
 726             case ALL_MODES & ~PROTECTED:
 727                 return cname + "/private";
 728             case ALL_MODES:
 729                 return cname;
 730             case TRUSTED:
 731                 return "/trusted";  // internal only; not exported
 732             default:  // Should not happen, but it's a bitfield...
 733                 cname = cname + "/" + Integer.toHexString(allowedModes);
 734                 assert(false) : cname;
 735                 return cname;
 736             }
 737         }
 738 
 739         /**
 740          * Produces a method handle for a static method.
 741          * The type of the method handle will be that of the method.
 742          * (Since static methods do not take receivers, there is no
 743          * additional receiver argument inserted into the method handle type,
 744          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
 745          * The method and all its argument types must be accessible to the lookup object.
 746          * <p>
 747          * The returned method handle will have
 748          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 749          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 750          * <p>
 751          * If the returned method handle is invoked, the method's class will
 752          * be initialized, if it has not already been initialized.
 753          * <p><b>Example:</b>
 754          * <blockquote><pre>{@code
 755 import static java.lang.invoke.MethodHandles.*;
 756 import static java.lang.invoke.MethodType.*;
 757 ...
 758 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
 759   "asList", methodType(List.class, Object[].class));
 760 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
 761          * }</pre></blockquote>
 762          * @param refc the class from which the method is accessed
 763          * @param name the name of the method
 764          * @param type the type of the method
 765          * @return the desired method handle
 766          * @throws NoSuchMethodException if the method does not exist
 767          * @throws IllegalAccessException if access checking fails,
 768          *                                or if the method is not {@code static},
 769          *                                or if the method's variable arity modifier bit
 770          *                                is set and {@code asVarargsCollector} fails
 771          * @exception SecurityException if a security manager is present and it
 772          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 773          * @throws NullPointerException if any argument is null
 774          */
 775         public
 776         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 777             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
 778             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
 779         }
 780 
 781         /**
 782          * Produces a method handle for a virtual method.
 783          * The type of the method handle will be that of the method,
 784          * with the receiver type (usually {@code refc}) prepended.
 785          * The method and all its argument types must be accessible to the lookup object.
 786          * <p>
 787          * When called, the handle will treat the first argument as a receiver
 788          * and dispatch on the receiver's type to determine which method
 789          * implementation to enter.
 790          * (The dispatching action is identical with that performed by an
 791          * {@code invokevirtual} or {@code invokeinterface} instruction.)
 792          * <p>
 793          * The first argument will be of type {@code refc} if the lookup
 794          * class has full privileges to access the member.  Otherwise
 795          * the member must be {@code protected} and the first argument
 796          * will be restricted in type to the lookup class.
 797          * <p>
 798          * The returned method handle will have
 799          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 800          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 801          * <p>
 802          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
 803          * instructions and method handles produced by {@code findVirtual},
 804          * if the class is {@code MethodHandle} and the name string is
 805          * {@code invokeExact} or {@code invoke}, the resulting
 806          * method handle is equivalent to one produced by
 807          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
 808          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
 809          * with the same {@code type} argument.
 810          *
 811          * <b>Example:</b>
 812          * <blockquote><pre>{@code
 813 import static java.lang.invoke.MethodHandles.*;
 814 import static java.lang.invoke.MethodType.*;
 815 ...
 816 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
 817   "concat", methodType(String.class, String.class));
 818 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
 819   "hashCode", methodType(int.class));
 820 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
 821   "hashCode", methodType(int.class));
 822 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
 823 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
 824 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
 825 // interface method:
 826 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
 827   "subSequence", methodType(CharSequence.class, int.class, int.class));
 828 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
 829 // constructor "internal method" must be accessed differently:
 830 MethodType MT_newString = methodType(void.class); //()V for new String()
 831 try { assertEquals("impossible", lookup()
 832         .findVirtual(String.class, "<init>", MT_newString));
 833  } catch (NoSuchMethodException ex) { } // OK
 834 MethodHandle MH_newString = publicLookup()
 835   .findConstructor(String.class, MT_newString);
 836 assertEquals("", (String) MH_newString.invokeExact());
 837          * }</pre></blockquote>
 838          *
 839          * @param refc the class or interface from which the method is accessed
 840          * @param name the name of the method
 841          * @param type the type of the method, with the receiver argument omitted
 842          * @return the desired method handle
 843          * @throws NoSuchMethodException if the method does not exist
 844          * @throws IllegalAccessException if access checking fails,
 845          *                                or if the method is {@code static}
 846          *                                or if the method's variable arity modifier bit
 847          *                                is set and {@code asVarargsCollector} fails
 848          * @exception SecurityException if a security manager is present and it
 849          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 850          * @throws NullPointerException if any argument is null
 851          */
 852         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 853             if (refc == MethodHandle.class) {
 854                 MethodHandle mh = findVirtualForMH(name, type);
 855                 if (mh != null)  return mh;
 856             }
 857             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
 858             MemberName method = resolveOrFail(refKind, refc, name, type);
 859             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
 860         }
 861         private MethodHandle findVirtualForMH(String name, MethodType type) {
 862             // these names require special lookups because of the implicit MethodType argument
 863             if ("invoke".equals(name))
 864                 return invoker(type);
 865             if ("invokeExact".equals(name))
 866                 return exactInvoker(type);
 867             if ("invokeBasic".equals(name))
 868                 return basicInvoker(type);
 869             assert(!MemberName.isMethodHandleInvokeName(name));
 870             return null;
 871         }
 872 
 873         /**
 874          * Produces a method handle which creates an object and initializes it, using
 875          * the constructor of the specified type.
 876          * The parameter types of the method handle will be those of the constructor,
 877          * while the return type will be a reference to the constructor's class.
 878          * The constructor and all its argument types must be accessible to the lookup object.
 879          * <p>
 880          * The requested type must have a return type of {@code void}.
 881          * (This is consistent with the JVM's treatment of constructor type descriptors.)
 882          * <p>
 883          * The returned method handle will have
 884          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 885          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
 886          * <p>
 887          * If the returned method handle is invoked, the constructor's class will
 888          * be initialized, if it has not already been initialized.
 889          * <p><b>Example:</b>
 890          * <blockquote><pre>{@code
 891 import static java.lang.invoke.MethodHandles.*;
 892 import static java.lang.invoke.MethodType.*;
 893 ...
 894 MethodHandle MH_newArrayList = publicLookup().findConstructor(
 895   ArrayList.class, methodType(void.class, Collection.class));
 896 Collection orig = Arrays.asList("x", "y");
 897 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
 898 assert(orig != copy);
 899 assertEquals(orig, copy);
 900 // a variable-arity constructor:
 901 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
 902   ProcessBuilder.class, methodType(void.class, String[].class));
 903 ProcessBuilder pb = (ProcessBuilder)
 904   MH_newProcessBuilder.invoke("x", "y", "z");
 905 assertEquals("[x, y, z]", pb.command().toString());
 906          * }</pre></blockquote>
 907          * @param refc the class or interface from which the method is accessed
 908          * @param type the type of the method, with the receiver argument omitted, and a void return type
 909          * @return the desired method handle
 910          * @throws NoSuchMethodException if the constructor does not exist
 911          * @throws IllegalAccessException if access checking fails
 912          *                                or if the method's variable arity modifier bit
 913          *                                is set and {@code asVarargsCollector} fails
 914          * @exception SecurityException if a security manager is present and it
 915          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 916          * @throws NullPointerException if any argument is null
 917          */
 918         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
 919             String name = "<init>";
 920             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
 921             return getDirectConstructor(refc, ctor);
 922         }
 923 
 924         /**
 925          * Produces an early-bound method handle for a virtual method.
 926          * It will bypass checks for overriding methods on the receiver,
 927          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
 928          * instruction from within the explicitly specified {@code specialCaller}.
 929          * The type of the method handle will be that of the method,
 930          * with a suitably restricted receiver type prepended.
 931          * (The receiver type will be {@code specialCaller} or a subtype.)
 932          * The method and all its argument types must be accessible
 933          * to the lookup object.
 934          * <p>
 935          * Before method resolution,
 936          * if the explicitly specified caller class is not identical with the
 937          * lookup class, or if this lookup object does not have
 938          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
 939          * privileges, the access fails.
 940          * <p>
 941          * The returned method handle will have
 942          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
 943          * the method's variable arity modifier bit ({@code 0x0080}) is set.
 944          * <p style="font-size:smaller;">
 945          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
 946          * even though the {@code invokespecial} instruction can refer to them
 947          * in special circumstances.  Use {@link #findConstructor findConstructor}
 948          * to access instance initialization methods in a safe manner.)</em>
 949          * <p><b>Example:</b>
 950          * <blockquote><pre>{@code
 951 import static java.lang.invoke.MethodHandles.*;
 952 import static java.lang.invoke.MethodType.*;
 953 ...
 954 static class Listie extends ArrayList {
 955   public String toString() { return "[wee Listie]"; }
 956   static Lookup lookup() { return MethodHandles.lookup(); }
 957 }
 958 ...
 959 // no access to constructor via invokeSpecial:
 960 MethodHandle MH_newListie = Listie.lookup()
 961   .findConstructor(Listie.class, methodType(void.class));
 962 Listie l = (Listie) MH_newListie.invokeExact();
 963 try { assertEquals("impossible", Listie.lookup().findSpecial(
 964         Listie.class, "<init>", methodType(void.class), Listie.class));
 965  } catch (NoSuchMethodException ex) { } // OK
 966 // access to super and self methods via invokeSpecial:
 967 MethodHandle MH_super = Listie.lookup().findSpecial(
 968   ArrayList.class, "toString" , methodType(String.class), Listie.class);
 969 MethodHandle MH_this = Listie.lookup().findSpecial(
 970   Listie.class, "toString" , methodType(String.class), Listie.class);
 971 MethodHandle MH_duper = Listie.lookup().findSpecial(
 972   Object.class, "toString" , methodType(String.class), Listie.class);
 973 assertEquals("[]", (String) MH_super.invokeExact(l));
 974 assertEquals(""+l, (String) MH_this.invokeExact(l));
 975 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
 976 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
 977         String.class, "toString", methodType(String.class), Listie.class));
 978  } catch (IllegalAccessException ex) { } // OK
 979 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
 980 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
 981          * }</pre></blockquote>
 982          *
 983          * @param refc the class or interface from which the method is accessed
 984          * @param name the name of the method (which must not be "&lt;init&gt;")
 985          * @param type the type of the method, with the receiver argument omitted
 986          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
 987          * @return the desired method handle
 988          * @throws NoSuchMethodException if the method does not exist
 989          * @throws IllegalAccessException if access checking fails
 990          *                                or if the method's variable arity modifier bit
 991          *                                is set and {@code asVarargsCollector} fails
 992          * @exception SecurityException if a security manager is present and it
 993          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
 994          * @throws NullPointerException if any argument is null
 995          */
 996         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
 997                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
 998             checkSpecialCaller(specialCaller);
 999             Lookup specialLookup = this.in(specialCaller);
1000             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1001             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1002         }
1003 
1004         /**
1005          * Produces a method handle giving read access to a non-static field.
1006          * The type of the method handle will have a return type of the field's
1007          * value type.
1008          * The method handle's single argument will be the instance containing
1009          * the field.
1010          * Access checking is performed immediately on behalf of the lookup class.
1011          * @param refc the class or interface from which the method is accessed
1012          * @param name the field's name
1013          * @param type the field's type
1014          * @return a method handle which can load values from the field
1015          * @throws NoSuchFieldException if the field does not exist
1016          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1017          * @exception SecurityException if a security manager is present and it
1018          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1019          * @throws NullPointerException if any argument is null
1020          */
1021         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1022             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1023             return getDirectField(REF_getField, refc, field);
1024         }
1025 
1026         /**
1027          * Produces a method handle giving write access to a non-static field.
1028          * The type of the method handle will have a void return type.
1029          * The method handle will take two arguments, the instance containing
1030          * the field, and the value to be stored.
1031          * The second argument will be of the field's value type.
1032          * Access checking is performed immediately on behalf of the lookup class.
1033          * @param refc the class or interface from which the method is accessed
1034          * @param name the field's name
1035          * @param type the field's type
1036          * @return a method handle which can store values into the field
1037          * @throws NoSuchFieldException if the field does not exist
1038          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1039          * @exception SecurityException if a security manager is present and it
1040          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1041          * @throws NullPointerException if any argument is null
1042          */
1043         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1044             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1045             return getDirectField(REF_putField, refc, field);
1046         }
1047 
1048         /**
1049          * Produces a method handle giving read access to a static field.
1050          * The type of the method handle will have a return type of the field's
1051          * value type.
1052          * The method handle will take no arguments.
1053          * Access checking is performed immediately on behalf of the lookup class.
1054          * <p>
1055          * If the returned method handle is invoked, the field's class will
1056          * be initialized, if it has not already been initialized.
1057          * @param refc the class or interface from which the method is accessed
1058          * @param name the field's name
1059          * @param type the field's type
1060          * @return a method handle which can load values from the field
1061          * @throws NoSuchFieldException if the field does not exist
1062          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1063          * @exception SecurityException if a security manager is present and it
1064          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1065          * @throws NullPointerException if any argument is null
1066          */
1067         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1068             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1069             return getDirectField(REF_getStatic, refc, field);
1070         }
1071 
1072         /**
1073          * Produces a method handle giving write access to a static field.
1074          * The type of the method handle will have a void return type.
1075          * The method handle will take a single
1076          * argument, of the field's value type, the value to be stored.
1077          * Access checking is performed immediately on behalf of the lookup class.
1078          * <p>
1079          * If the returned method handle is invoked, the field's class will
1080          * be initialized, if it has not already been initialized.
1081          * @param refc the class or interface from which the method is accessed
1082          * @param name the field's name
1083          * @param type the field's type
1084          * @return a method handle which can store values into the field
1085          * @throws NoSuchFieldException if the field does not exist
1086          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1087          * @exception SecurityException if a security manager is present and it
1088          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1089          * @throws NullPointerException if any argument is null
1090          */
1091         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1092             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1093             return getDirectField(REF_putStatic, refc, field);
1094         }
1095 
1096         /**
1097          * Produces an early-bound method handle for a non-static method.
1098          * The receiver must have a supertype {@code defc} in which a method
1099          * of the given name and type is accessible to the lookup class.
1100          * The method and all its argument types must be accessible to the lookup object.
1101          * The type of the method handle will be that of the method,
1102          * without any insertion of an additional receiver parameter.
1103          * The given receiver will be bound into the method handle,
1104          * so that every call to the method handle will invoke the
1105          * requested method on the given receiver.
1106          * <p>
1107          * The returned method handle will have
1108          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1109          * the method's variable arity modifier bit ({@code 0x0080}) is set
1110          * <em>and</em> the trailing array argument is not the only argument.
1111          * (If the trailing array argument is the only argument,
1112          * the given receiver value will be bound to it.)
1113          * <p>
1114          * This is equivalent to the following code:
1115          * <blockquote><pre>{@code
1116 import static java.lang.invoke.MethodHandles.*;
1117 import static java.lang.invoke.MethodType.*;
1118 ...
1119 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1120 MethodHandle mh1 = mh0.bindTo(receiver);
1121 MethodType mt1 = mh1.type();
1122 if (mh0.isVarargsCollector())
1123   mh1 = mh1.asVarargsCollector(mt1.parameterType(mt1.parameterCount()-1));
1124 return mh1;
1125          * }</pre></blockquote>
1126          * where {@code defc} is either {@code receiver.getClass()} or a super
1127          * type of that class, in which the requested method is accessible
1128          * to the lookup class.
1129          * (Note that {@code bindTo} does not preserve variable arity.)
1130          * @param receiver the object from which the method is accessed
1131          * @param name the name of the method
1132          * @param type the type of the method, with the receiver argument omitted
1133          * @return the desired method handle
1134          * @throws NoSuchMethodException if the method does not exist
1135          * @throws IllegalAccessException if access checking fails
1136          *                                or if the method's variable arity modifier bit
1137          *                                is set and {@code asVarargsCollector} fails
1138          * @exception SecurityException if a security manager is present and it
1139          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1140          * @throws NullPointerException if any argument is null
1141          * @see MethodHandle#bindTo
1142          * @see #findVirtual
1143          */
1144         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1145             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1146             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1147             MethodHandle mh = getDirectMethodNoRestrict(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1148             return mh.bindArgumentL(0, receiver).setVarargs(method);
1149         }
1150 
1151         /**
1152          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1153          * to <i>m</i>, if the lookup class has permission.
1154          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1155          * If <i>m</i> is virtual, overriding is respected on every call.
1156          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1157          * The type of the method handle will be that of the method,
1158          * with the receiver type prepended (but only if it is non-static).
1159          * If the method's {@code accessible} flag is not set,
1160          * access checking is performed immediately on behalf of the lookup class.
1161          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1162          * <p>
1163          * The returned method handle will have
1164          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1165          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1166          * <p>
1167          * If <i>m</i> is static, and
1168          * if the returned method handle is invoked, the method's class will
1169          * be initialized, if it has not already been initialized.
1170          * @param m the reflected method
1171          * @return a method handle which can invoke the reflected method
1172          * @throws IllegalAccessException if access checking fails
1173          *                                or if the method's variable arity modifier bit
1174          *                                is set and {@code asVarargsCollector} fails
1175          * @throws NullPointerException if the argument is null
1176          */
1177         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1178             if (m.getDeclaringClass() == MethodHandle.class) {
1179                 MethodHandle mh = unreflectForMH(m);
1180                 if (mh != null)  return mh;
1181             }
1182             MemberName method = new MemberName(m);
1183             byte refKind = method.getReferenceKind();
1184             if (refKind == REF_invokeSpecial)
1185                 refKind = REF_invokeVirtual;
1186             assert(method.isMethod());
1187             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1188             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1189         }
1190         private MethodHandle unreflectForMH(Method m) {
1191             // these names require special lookups because they throw UnsupportedOperationException
1192             if (MemberName.isMethodHandleInvokeName(m.getName()))
1193                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1194             return null;
1195         }
1196 
1197         /**
1198          * Produces a method handle for a reflected method.
1199          * It will bypass checks for overriding methods on the receiver,
1200          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1201          * instruction from within the explicitly specified {@code specialCaller}.
1202          * The type of the method handle will be that of the method,
1203          * with a suitably restricted receiver type prepended.
1204          * (The receiver type will be {@code specialCaller} or a subtype.)
1205          * If the method's {@code accessible} flag is not set,
1206          * access checking is performed immediately on behalf of the lookup class,
1207          * as if {@code invokespecial} instruction were being linked.
1208          * <p>
1209          * Before method resolution,
1210          * if the explicitly specified caller class is not identical with the
1211          * lookup class, or if this lookup object does not have
1212          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1213          * privileges, the access fails.
1214          * <p>
1215          * The returned method handle will have
1216          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1217          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1218          * @param m the reflected method
1219          * @param specialCaller the class nominally calling the method
1220          * @return a method handle which can invoke the reflected method
1221          * @throws IllegalAccessException if access checking fails
1222          *                                or if the method's variable arity modifier bit
1223          *                                is set and {@code asVarargsCollector} fails
1224          * @throws NullPointerException if any argument is null
1225          */
1226         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1227             checkSpecialCaller(specialCaller);
1228             Lookup specialLookup = this.in(specialCaller);
1229             MemberName method = new MemberName(m, true);
1230             assert(method.isMethod());
1231             // ignore m.isAccessible:  this is a new kind of access
1232             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1233         }
1234 
1235         /**
1236          * Produces a method handle for a reflected constructor.
1237          * The type of the method handle will be that of the constructor,
1238          * with the return type changed to the declaring class.
1239          * The method handle will perform a {@code newInstance} operation,
1240          * creating a new instance of the constructor's class on the
1241          * arguments passed to the method handle.
1242          * <p>
1243          * If the constructor's {@code accessible} flag is not set,
1244          * access checking is performed immediately on behalf of the lookup class.
1245          * <p>
1246          * The returned method handle will have
1247          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1248          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1249          * <p>
1250          * If the returned method handle is invoked, the constructor's class will
1251          * be initialized, if it has not already been initialized.
1252          * @param c the reflected constructor
1253          * @return a method handle which can invoke the reflected constructor
1254          * @throws IllegalAccessException if access checking fails
1255          *                                or if the method's variable arity modifier bit
1256          *                                is set and {@code asVarargsCollector} fails
1257          * @throws NullPointerException if the argument is null
1258          */
1259         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1260             MemberName ctor = new MemberName(c);
1261             assert(ctor.isConstructor());
1262             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1263             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1264         }
1265 
1266         /**
1267          * Produces a method handle giving read access to a reflected field.
1268          * The type of the method handle will have a return type of the field's
1269          * value type.
1270          * If the field is static, the method handle will take no arguments.
1271          * Otherwise, its single argument will be the instance containing
1272          * the field.
1273          * If the field's {@code accessible} flag is not set,
1274          * access checking is performed immediately on behalf of the lookup class.
1275          * <p>
1276          * If the field is static, and
1277          * if the returned method handle is invoked, the field's class will
1278          * be initialized, if it has not already been initialized.
1279          * @param f the reflected field
1280          * @return a method handle which can load values from the reflected field
1281          * @throws IllegalAccessException if access checking fails
1282          * @throws NullPointerException if the argument is null
1283          */
1284         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1285             return unreflectField(f, false);
1286         }
1287         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1288             MemberName field = new MemberName(f, isSetter);
1289             assert(isSetter
1290                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1291                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1292             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1293             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1294         }
1295 
1296         /**
1297          * Produces a method handle giving write access to a reflected field.
1298          * The type of the method handle will have a void return type.
1299          * If the field is static, the method handle will take a single
1300          * argument, of the field's value type, the value to be stored.
1301          * Otherwise, the two arguments will be the instance containing
1302          * the field, and the value to be stored.
1303          * If the field's {@code accessible} flag is not set,
1304          * access checking is performed immediately on behalf of the lookup class.
1305          * <p>
1306          * If the field is static, and
1307          * if the returned method handle is invoked, the field's class will
1308          * be initialized, if it has not already been initialized.
1309          * @param f the reflected field
1310          * @return a method handle which can store values into the reflected field
1311          * @throws IllegalAccessException if access checking fails
1312          * @throws NullPointerException if the argument is null
1313          */
1314         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1315             return unreflectField(f, true);
1316         }
1317 
1318         /**
1319          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1320          * created by this lookup object or a similar one.
1321          * Security and access checks are performed to ensure that this lookup object
1322          * is capable of reproducing the target method handle.
1323          * This means that the cracking may fail if target is a direct method handle
1324          * but was created by an unrelated lookup object.
1325          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1326          * and was created by a lookup object for a different class.
1327          * @param target a direct method handle to crack into symbolic reference components
1328          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
1329          * @exception SecurityException if a security manager is present and it
1330          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1331          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
1332          * @exception NullPointerException if the target is {@code null}
1333          * @see MethodHandleInfo
1334          * @since 1.8
1335          */
1336         public MethodHandleInfo revealDirect(MethodHandle target) {
1337             MemberName member = target.internalMemberName();
1338             if (member == null || (!member.isResolved() && !member.isMethodHandleInvoke()))
1339                 throw newIllegalArgumentException("not a direct method handle");
1340             Class<?> defc = member.getDeclaringClass();
1341             byte refKind = member.getReferenceKind();
1342             assert(MethodHandleNatives.refKindIsValid(refKind));
1343             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
1344                 // Devirtualized method invocation is usually formally virtual.
1345                 // To avoid creating extra MemberName objects for this common case,
1346                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
1347                 refKind = REF_invokeVirtual;
1348             if (refKind == REF_invokeVirtual && defc.isInterface())
1349                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
1350                 refKind = REF_invokeInterface;
1351             // Check SM permissions and member access before cracking.
1352             try {
1353                 checkAccess(refKind, defc, member);
1354                 checkSecurityManager(defc, member);
1355             } catch (IllegalAccessException ex) {
1356                 throw new IllegalArgumentException(ex);
1357             }
1358             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
1359                 Class<?> callerClass = target.internalCallerClass();
1360                 if (!hasPrivateAccess() || callerClass != lookupClass())
1361                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
1362             }
1363             // Produce the handle to the results.
1364             return new InfoFromMemberName(this, member, refKind);
1365         }
1366 
1367         /// Helper methods, all package-private.
1368 
1369         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1370             checkSymbolicClass(refc);  // do this before attempting to resolve
1371             name.getClass();  // NPE
1372             type.getClass();  // NPE
1373             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1374                                             NoSuchFieldException.class);
1375         }
1376 
1377         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1378             checkSymbolicClass(refc);  // do this before attempting to resolve
1379             name.getClass();  // NPE
1380             type.getClass();  // NPE
1381             checkMethodName(refKind, name);  // NPE check on name
1382             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
1383                                             NoSuchMethodException.class);
1384         }
1385 
1386         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
1387             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
1388             member.getName().getClass();  // NPE
1389             member.getType().getClass();  // NPE
1390             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
1391                                             ReflectiveOperationException.class);
1392         }
1393 
1394         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
1395             refc.getClass();  // NPE
1396             Class<?> caller = lookupClassOrNull();
1397             if (caller != null && !VerifyAccess.isClassAccessible(refc, caller, allowedModes))
1398                 throw new MemberName(refc).makeAccessException("symbolic reference class is not public", this);
1399         }
1400 
1401         /** Check name for an illegal leading "&lt;" character. */
1402         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
1403             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
1404                 throw new NoSuchMethodException("illegal method name: "+name);
1405         }
1406 
1407 
1408         /**
1409          * Find my trustable caller class if m is a caller sensitive method.
1410          * If this lookup object has private access, then the caller class is the lookupClass.
1411          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
1412          */
1413         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
1414             Class<?> callerClass = null;
1415             if (MethodHandleNatives.isCallerSensitive(m)) {
1416                 // Only lookups with private access are allowed to resolve caller-sensitive methods
1417                 if (hasPrivateAccess()) {
1418                     callerClass = lookupClass;
1419                 } else {
1420                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
1421                 }
1422             }
1423             return callerClass;
1424         }
1425 
1426         private boolean hasPrivateAccess() {
1427             return (allowedModes & PRIVATE) != 0;
1428         }
1429 
1430         /**
1431          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
1432          * Determines a trustable caller class to compare with refc, the symbolic reference class.
1433          * If this lookup object has private access, then the caller class is the lookupClass.
1434          */
1435         void checkSecurityManager(Class<?> refc, MemberName m) {
1436             SecurityManager smgr = System.getSecurityManager();
1437             if (smgr == null)  return;
1438             if (allowedModes == TRUSTED)  return;
1439 
1440             // Step 1:
1441             boolean fullPowerLookup = hasPrivateAccess();
1442             if (!fullPowerLookup ||
1443                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
1444                 ReflectUtil.checkPackageAccess(refc);
1445             }
1446 
1447             // Step 2:
1448             if (m.isPublic()) return;
1449             if (!fullPowerLookup) {
1450                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
1451             }
1452 
1453             // Step 3:
1454             Class<?> defc = m.getDeclaringClass();
1455             if (!fullPowerLookup && defc != refc) {
1456                 ReflectUtil.checkPackageAccess(defc);
1457             }
1458         }
1459 
1460         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1461             boolean wantStatic = (refKind == REF_invokeStatic);
1462             String message;
1463             if (m.isConstructor())
1464                 message = "expected a method, not a constructor";
1465             else if (!m.isMethod())
1466                 message = "expected a method";
1467             else if (wantStatic != m.isStatic())
1468                 message = wantStatic ? "expected a static method" : "expected a non-static method";
1469             else
1470                 { checkAccess(refKind, refc, m); return; }
1471             throw m.makeAccessException(message, this);
1472         }
1473 
1474         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1475             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
1476             String message;
1477             if (wantStatic != m.isStatic())
1478                 message = wantStatic ? "expected a static field" : "expected a non-static field";
1479             else
1480                 { checkAccess(refKind, refc, m); return; }
1481             throw m.makeAccessException(message, this);
1482         }
1483 
1484         /** Check public/protected/private bits on the symbolic reference class and its member. */
1485         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
1486             assert(m.referenceKindIsConsistentWith(refKind) &&
1487                    MethodHandleNatives.refKindIsValid(refKind) &&
1488                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
1489             int allowedModes = this.allowedModes;
1490             if (allowedModes == TRUSTED)  return;
1491             int mods = m.getModifiers();
1492             if (Modifier.isProtected(mods) &&
1493                     refKind == REF_invokeVirtual &&
1494                     m.getDeclaringClass() == Object.class &&
1495                     m.getName().equals("clone") &&
1496                     refc.isArray()) {
1497                 // The JVM does this hack also.
1498                 // (See ClassVerifier::verify_invoke_instructions
1499                 // and LinkResolver::check_method_accessability.)
1500                 // Because the JVM does not allow separate methods on array types,
1501                 // there is no separate method for int[].clone.
1502                 // All arrays simply inherit Object.clone.
1503                 // But for access checking logic, we make Object.clone
1504                 // (normally protected) appear to be public.
1505                 // Later on, when the DirectMethodHandle is created,
1506                 // its leading argument will be restricted to the
1507                 // requested array type.
1508                 // N.B. The return type is not adjusted, because
1509                 // that is *not* the bytecode behavior.
1510                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
1511             }
1512             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
1513                 // cannot "new" a protected ctor in a different package
1514                 mods ^= Modifier.PROTECTED;
1515             }
1516             if (Modifier.isFinal(mods) &&
1517                     MethodHandleNatives.refKindIsSetter(refKind))
1518                 throw m.makeAccessException("unexpected set of a final field", this);
1519             if (Modifier.isPublic(mods) && Modifier.isPublic(refc.getModifiers()) && allowedModes != 0)
1520                 return;  // common case
1521             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
1522             if ((requestedModes & allowedModes) != 0) {
1523                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
1524                                                     mods, lookupClass(), allowedModes))
1525                     return;
1526             } else {
1527                 // Protected members can also be checked as if they were package-private.
1528                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
1529                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
1530                     return;
1531             }
1532             throw m.makeAccessException(accessFailedMessage(refc, m), this);
1533         }
1534 
1535         String accessFailedMessage(Class<?> refc, MemberName m) {
1536             Class<?> defc = m.getDeclaringClass();
1537             int mods = m.getModifiers();
1538             // check the class first:
1539             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
1540                                (defc == refc ||
1541                                 Modifier.isPublic(refc.getModifiers())));
1542             if (!classOK && (allowedModes & PACKAGE) != 0) {
1543                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), ALL_MODES) &&
1544                            (defc == refc ||
1545                             VerifyAccess.isClassAccessible(refc, lookupClass(), ALL_MODES)));
1546             }
1547             if (!classOK)
1548                 return "class is not public";
1549             if (Modifier.isPublic(mods))
1550                 return "access to public member failed";  // (how?)
1551             if (Modifier.isPrivate(mods))
1552                 return "member is private";
1553             if (Modifier.isProtected(mods))
1554                 return "member is protected";
1555             return "member is private to package";
1556         }
1557 
1558         private static final boolean ALLOW_NESTMATE_ACCESS = false;
1559 
1560         private void checkSpecialCaller(Class<?> specialCaller) throws IllegalAccessException {
1561             int allowedModes = this.allowedModes;
1562             if (allowedModes == TRUSTED)  return;
1563             if (!hasPrivateAccess()
1564                 || (specialCaller != lookupClass()
1565                     && !(ALLOW_NESTMATE_ACCESS &&
1566                          VerifyAccess.isSamePackageMember(specialCaller, lookupClass()))))
1567                 throw new MemberName(specialCaller).
1568                     makeAccessException("no private access for invokespecial", this);
1569         }
1570 
1571         private boolean restrictProtectedReceiver(MemberName method) {
1572             // The accessing class only has the right to use a protected member
1573             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
1574             if (!method.isProtected() || method.isStatic()
1575                 || allowedModes == TRUSTED
1576                 || method.getDeclaringClass() == lookupClass()
1577                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())
1578                 || (ALLOW_NESTMATE_ACCESS &&
1579                     VerifyAccess.isSamePackageMember(method.getDeclaringClass(), lookupClass())))
1580                 return false;
1581             return true;
1582         }
1583         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
1584             assert(!method.isStatic());
1585             // receiver type of mh is too wide; narrow to caller
1586             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
1587                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
1588             }
1589             MethodType rawType = mh.type();
1590             if (rawType.parameterType(0) == caller)  return mh;
1591             MethodType narrowType = rawType.changeParameterType(0, caller);
1592             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
1593             assert(mh.viewAsTypeChecks(narrowType, true));
1594             return mh.copyWith(narrowType, mh.form);
1595         }
1596 
1597         /** Check access and get the requested method. */
1598         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1599             final boolean doRestrict    = true;
1600             final boolean checkSecurity = true;
1601             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1602         }
1603         /** Check access and get the requested method, eliding receiver narrowing rules. */
1604         private MethodHandle getDirectMethodNoRestrict(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1605             final boolean doRestrict    = false;
1606             final boolean checkSecurity = true;
1607             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1608         }
1609         /** Check access and get the requested method, eliding security manager checks. */
1610         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> callerClass) throws IllegalAccessException {
1611             final boolean doRestrict    = true;
1612             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1613             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerClass);
1614         }
1615         /** Common code for all methods; do not call directly except from immediately above. */
1616         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
1617                                                    boolean checkSecurity,
1618                                                    boolean doRestrict, Class<?> callerClass) throws IllegalAccessException {
1619             checkMethod(refKind, refc, method);
1620             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1621             if (checkSecurity)
1622                 checkSecurityManager(refc, method);
1623             assert(!method.isMethodHandleInvoke());
1624 
1625             if (refKind == REF_invokeSpecial &&
1626                 refc != lookupClass() &&
1627                 !refc.isInterface() &&
1628                 refc != lookupClass().getSuperclass() &&
1629                 refc.isAssignableFrom(lookupClass())) {
1630                 assert(!method.getName().equals("<init>"));  // not this code path
1631                 // Per JVMS 6.5, desc. of invokespecial instruction:
1632                 // If the method is in a superclass of the LC,
1633                 // and if our original search was above LC.super,
1634                 // repeat the search (symbolic lookup) from LC.super
1635                 // and continue with the direct superclass of that class,
1636                 // and so forth, until a match is found or no further superclasses exist.
1637                 // FIXME: MemberName.resolve should handle this instead.
1638                 Class<?> refcAsSuper = lookupClass();
1639                 MemberName m2;
1640                 do {
1641                     refcAsSuper = refcAsSuper.getSuperclass();
1642                     m2 = new MemberName(refcAsSuper,
1643                                         method.getName(),
1644                                         method.getMethodType(),
1645                                         REF_invokeSpecial);
1646                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
1647                 } while (m2 == null &&         // no method is found yet
1648                          refc != refcAsSuper); // search up to refc
1649                 if (m2 == null)  throw new InternalError(method.toString());
1650                 method = m2;
1651                 refc = refcAsSuper;
1652                 // redo basic checks
1653                 checkMethod(refKind, refc, method);
1654             }
1655 
1656             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method);
1657             MethodHandle mh = dmh;
1658             // Optionally narrow the receiver argument to refc using restrictReceiver.
1659             if (doRestrict &&
1660                    (refKind == REF_invokeSpecial ||
1661                        (MethodHandleNatives.refKindHasReceiver(refKind) &&
1662                            restrictProtectedReceiver(method)))) {
1663                 mh = restrictReceiver(method, dmh, lookupClass());
1664             }
1665             mh = maybeBindCaller(method, mh, callerClass);
1666             mh = mh.setVarargs(method);
1667             return mh;
1668         }
1669         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
1670                                              Class<?> callerClass)
1671                                              throws IllegalAccessException {
1672             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
1673                 return mh;
1674             Class<?> hostClass = lookupClass;
1675             if (!hasPrivateAccess())  // caller must have private access
1676                 hostClass = callerClass;  // callerClass came from a security manager style stack walk
1677             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
1678             // Note: caller will apply varargs after this step happens.
1679             return cbmh;
1680         }
1681         /** Check access and get the requested field. */
1682         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1683             final boolean checkSecurity = true;
1684             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1685         }
1686         /** Check access and get the requested field, eliding security manager checks. */
1687         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
1688             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1689             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
1690         }
1691         /** Common code for all fields; do not call directly except from immediately above. */
1692         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
1693                                                   boolean checkSecurity) throws IllegalAccessException {
1694             checkField(refKind, refc, field);
1695             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1696             if (checkSecurity)
1697                 checkSecurityManager(refc, field);
1698             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
1699             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
1700                                     restrictProtectedReceiver(field));
1701             if (doRestrict)
1702                 return restrictReceiver(field, dmh, lookupClass());
1703             return dmh;
1704         }
1705         /** Check access and get the requested constructor. */
1706         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
1707             final boolean checkSecurity = true;
1708             return getDirectConstructorCommon(refc, ctor, checkSecurity);
1709         }
1710         /** Check access and get the requested constructor, eliding security manager checks. */
1711         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
1712             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
1713             return getDirectConstructorCommon(refc, ctor, checkSecurity);
1714         }
1715         /** Common code for all constructors; do not call directly except from immediately above. */
1716         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
1717                                                   boolean checkSecurity) throws IllegalAccessException {
1718             assert(ctor.isConstructor());
1719             checkAccess(REF_newInvokeSpecial, refc, ctor);
1720             // Optionally check with the security manager; this isn't needed for unreflect* calls.
1721             if (checkSecurity)
1722                 checkSecurityManager(refc, ctor);
1723             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
1724             return DirectMethodHandle.make(ctor).setVarargs(ctor);
1725         }
1726 
1727         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
1728          */
1729         /*non-public*/
1730         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
1731             if (!(type instanceof Class || type instanceof MethodType))
1732                 throw new InternalError("unresolved MemberName");
1733             MemberName member = new MemberName(refKind, defc, name, type);
1734             MethodHandle mh = LOOKASIDE_TABLE.get(member);
1735             if (mh != null) {
1736                 checkSymbolicClass(defc);
1737                 return mh;
1738             }
1739             // Treat MethodHandle.invoke and invokeExact specially.
1740             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
1741                 mh = findVirtualForMH(member.getName(), member.getMethodType());
1742                 if (mh != null) {
1743                     return mh;
1744                 }
1745             }
1746             MemberName resolved = resolveOrFail(refKind, member);
1747             mh = getDirectMethodForConstant(refKind, defc, resolved);
1748             if (mh instanceof DirectMethodHandle
1749                     && canBeCached(refKind, defc, resolved)) {
1750                 MemberName key = mh.internalMemberName();
1751                 if (key != null) {
1752                     key = key.asNormalOriginal();
1753                 }
1754                 if (member.equals(key)) {  // better safe than sorry
1755                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
1756                 }
1757             }
1758             return mh;
1759         }
1760         private
1761         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
1762             if (refKind == REF_invokeSpecial) {
1763                 return false;
1764             }
1765             if (!Modifier.isPublic(defc.getModifiers()) ||
1766                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
1767                     !member.isPublic() ||
1768                     member.isCallerSensitive()) {
1769                 return false;
1770             }
1771             ClassLoader loader = defc.getClassLoader();
1772             if (!sun.misc.VM.isSystemDomainLoader(loader)) {
1773                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
1774                 boolean found = false;
1775                 while (sysl != null) {
1776                     if (loader == sysl) { found = true; break; }
1777                     sysl = sysl.getParent();
1778                 }
1779                 if (!found) {
1780                     return false;
1781                 }
1782             }
1783             try {
1784                 MemberName resolved2 = publicLookup().resolveOrFail(refKind,
1785                     new MemberName(refKind, defc, member.getName(), member.getType()));
1786                 checkSecurityManager(defc, resolved2);
1787             } catch (ReflectiveOperationException | SecurityException ex) {
1788                 return false;
1789             }
1790             return true;
1791         }
1792         private
1793         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
1794                 throws ReflectiveOperationException {
1795             if (MethodHandleNatives.refKindIsField(refKind)) {
1796                 return getDirectFieldNoSecurityManager(refKind, defc, member);
1797             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
1798                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
1799             } else if (refKind == REF_newInvokeSpecial) {
1800                 return getDirectConstructorNoSecurityManager(defc, member);
1801             }
1802             // oops
1803             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
1804         }
1805 
1806         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
1807     }
1808 
1809     /**
1810      * Produces a method handle giving read access to elements of an array.
1811      * The type of the method handle will have a return type of the array's
1812      * element type.  Its first argument will be the array type,
1813      * and the second will be {@code int}.
1814      * @param arrayClass an array type
1815      * @return a method handle which can load values from the given array type
1816      * @throws NullPointerException if the argument is null
1817      * @throws  IllegalArgumentException if arrayClass is not an array type
1818      */
1819     public static
1820     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
1821         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, false);
1822     }
1823 
1824     /**
1825      * Produces a method handle giving write access to elements of an array.
1826      * The type of the method handle will have a void return type.
1827      * Its last argument will be the array's element type.
1828      * The first and second arguments will be the array type and int.
1829      * @param arrayClass the class of an array
1830      * @return a method handle which can store values into the array type
1831      * @throws NullPointerException if the argument is null
1832      * @throws IllegalArgumentException if arrayClass is not an array type
1833      */
1834     public static
1835     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
1836         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, true);
1837     }
1838 
1839     /// method handle invocation (reflective style)
1840 
1841     /**
1842      * Produces a method handle which will invoke any method handle of the
1843      * given {@code type}, with a given number of trailing arguments replaced by
1844      * a single trailing {@code Object[]} array.
1845      * The resulting invoker will be a method handle with the following
1846      * arguments:
1847      * <ul>
1848      * <li>a single {@code MethodHandle} target
1849      * <li>zero or more leading values (counted by {@code leadingArgCount})
1850      * <li>an {@code Object[]} array containing trailing arguments
1851      * </ul>
1852      * <p>
1853      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
1854      * the indicated {@code type}.
1855      * That is, if the target is exactly of the given {@code type}, it will behave
1856      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
1857      * is used to convert the target to the required {@code type}.
1858      * <p>
1859      * The type of the returned invoker will not be the given {@code type}, but rather
1860      * will have all parameters except the first {@code leadingArgCount}
1861      * replaced by a single array of type {@code Object[]}, which will be
1862      * the final parameter.
1863      * <p>
1864      * Before invoking its target, the invoker will spread the final array, apply
1865      * reference casts as necessary, and unbox and widen primitive arguments.
1866      * If, when the invoker is called, the supplied array argument does
1867      * not have the correct number of elements, the invoker will throw
1868      * an {@link IllegalArgumentException} instead of invoking the target.
1869      * <p>
1870      * This method is equivalent to the following code (though it may be more efficient):
1871      * <blockquote><pre>{@code
1872 MethodHandle invoker = MethodHandles.invoker(type);
1873 int spreadArgCount = type.parameterCount() - leadingArgCount;
1874 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
1875 return invoker;
1876      * }</pre></blockquote>
1877      * This method throws no reflective or security exceptions.
1878      * @param type the desired target type
1879      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
1880      * @return a method handle suitable for invoking any method handle of the given type
1881      * @throws NullPointerException if {@code type} is null
1882      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
1883      *                  the range from 0 to {@code type.parameterCount()} inclusive,
1884      *                  or if the resulting method handle's type would have
1885      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
1886      */
1887     static public
1888     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
1889         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
1890             throw newIllegalArgumentException("bad argument count", leadingArgCount);
1891         type = type.asSpreaderType(Object[].class, type.parameterCount() - leadingArgCount);
1892         return type.invokers().spreadInvoker(leadingArgCount);
1893     }
1894 
1895     /**
1896      * Produces a special <em>invoker method handle</em> which can be used to
1897      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
1898      * The resulting invoker will have a type which is
1899      * exactly equal to the desired type, except that it will accept
1900      * an additional leading argument of type {@code MethodHandle}.
1901      * <p>
1902      * This method is equivalent to the following code (though it may be more efficient):
1903      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
1904      *
1905      * <p style="font-size:smaller;">
1906      * <em>Discussion:</em>
1907      * Invoker method handles can be useful when working with variable method handles
1908      * of unknown types.
1909      * For example, to emulate an {@code invokeExact} call to a variable method
1910      * handle {@code M}, extract its type {@code T},
1911      * look up the invoker method {@code X} for {@code T},
1912      * and call the invoker method, as {@code X.invoke(T, A...)}.
1913      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
1914      * is unknown.)
1915      * If spreading, collecting, or other argument transformations are required,
1916      * they can be applied once to the invoker {@code X} and reused on many {@code M}
1917      * method handle values, as long as they are compatible with the type of {@code X}.
1918      * <p style="font-size:smaller;">
1919      * <em>(Note:  The invoker method is not available via the Core Reflection API.
1920      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
1921      * on the declared {@code invokeExact} or {@code invoke} method will raise an
1922      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
1923      * <p>
1924      * This method throws no reflective or security exceptions.
1925      * @param type the desired target type
1926      * @return a method handle suitable for invoking any method handle of the given type
1927      * @throws IllegalArgumentException if the resulting method handle's type would have
1928      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
1929      */
1930     static public
1931     MethodHandle exactInvoker(MethodType type) {
1932         return type.invokers().exactInvoker();
1933     }
1934 
1935     /**
1936      * Produces a special <em>invoker method handle</em> which can be used to
1937      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
1938      * The resulting invoker will have a type which is
1939      * exactly equal to the desired type, except that it will accept
1940      * an additional leading argument of type {@code MethodHandle}.
1941      * <p>
1942      * Before invoking its target, if the target differs from the expected type,
1943      * the invoker will apply reference casts as
1944      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
1945      * Similarly, the return value will be converted as necessary.
1946      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
1947      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
1948      * <p>
1949      * This method is equivalent to the following code (though it may be more efficient):
1950      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
1951      * <p style="font-size:smaller;">
1952      * <em>Discussion:</em>
1953      * A {@linkplain MethodType#genericMethodType general method type} is one which
1954      * mentions only {@code Object} arguments and return values.
1955      * An invoker for such a type is capable of calling any method handle
1956      * of the same arity as the general type.
1957      * <p style="font-size:smaller;">
1958      * <em>(Note:  The invoker method is not available via the Core Reflection API.
1959      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
1960      * on the declared {@code invokeExact} or {@code invoke} method will raise an
1961      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
1962      * <p>
1963      * This method throws no reflective or security exceptions.
1964      * @param type the desired target type
1965      * @return a method handle suitable for invoking any method handle convertible to the given type
1966      * @throws IllegalArgumentException if the resulting method handle's type would have
1967      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
1968      */
1969     static public
1970     MethodHandle invoker(MethodType type) {
1971         return type.invokers().genericInvoker();
1972     }
1973 
1974     static /*non-public*/
1975     MethodHandle basicInvoker(MethodType type) {
1976         return type.invokers().basicInvoker();
1977     }
1978 
1979      /// method handle modification (creation from other method handles)
1980 
1981     /**
1982      * Produces a method handle which adapts the type of the
1983      * given method handle to a new type by pairwise argument and return type conversion.
1984      * The original type and new type must have the same number of arguments.
1985      * The resulting method handle is guaranteed to report a type
1986      * which is equal to the desired new type.
1987      * <p>
1988      * If the original type and new type are equal, returns target.
1989      * <p>
1990      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
1991      * and some additional conversions are also applied if those conversions fail.
1992      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
1993      * if possible, before or instead of any conversions done by {@code asType}:
1994      * <ul>
1995      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
1996      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
1997      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
1998      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
1999      *     the boolean is converted to a byte value, 1 for true, 0 for false.
2000      *     (This treatment follows the usage of the bytecode verifier.)
2001      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
2002      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
2003      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
2004      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
2005      *     then a Java casting conversion (JLS 5.5) is applied.
2006      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
2007      *     widening and/or narrowing.)
2008      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
2009      *     conversion will be applied at runtime, possibly followed
2010      *     by a Java casting conversion (JLS 5.5) on the primitive value,
2011      *     possibly followed by a conversion from byte to boolean by testing
2012      *     the low-order bit.
2013      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
2014      *     and if the reference is null at runtime, a zero value is introduced.
2015      * </ul>
2016      * @param target the method handle to invoke after arguments are retyped
2017      * @param newType the expected type of the new method handle
2018      * @return a method handle which delegates to the target after performing
2019      *           any necessary argument conversions, and arranges for any
2020      *           necessary return value conversions
2021      * @throws NullPointerException if either argument is null
2022      * @throws WrongMethodTypeException if the conversion cannot be made
2023      * @see MethodHandle#asType
2024      */
2025     public static
2026     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
2027         explicitCastArgumentsChecks(target, newType);
2028         // use the asTypeCache when possible:
2029         MethodType oldType = target.type();
2030         if (oldType == newType)  return target;
2031         if (oldType.explicitCastEquivalentToAsType(newType)) {
2032             return target.asFixedArity().asType(newType);
2033         }
2034         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
2035     }
2036 
2037     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
2038         if (target.type().parameterCount() != newType.parameterCount()) {
2039             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
2040         }
2041     }
2042 
2043     /**
2044      * Produces a method handle which adapts the calling sequence of the
2045      * given method handle to a new type, by reordering the arguments.
2046      * The resulting method handle is guaranteed to report a type
2047      * which is equal to the desired new type.
2048      * <p>
2049      * The given array controls the reordering.
2050      * Call {@code #I} the number of incoming parameters (the value
2051      * {@code newType.parameterCount()}, and call {@code #O} the number
2052      * of outgoing parameters (the value {@code target.type().parameterCount()}).
2053      * Then the length of the reordering array must be {@code #O},
2054      * and each element must be a non-negative number less than {@code #I}.
2055      * For every {@code N} less than {@code #O}, the {@code N}-th
2056      * outgoing argument will be taken from the {@code I}-th incoming
2057      * argument, where {@code I} is {@code reorder[N]}.
2058      * <p>
2059      * No argument or return value conversions are applied.
2060      * The type of each incoming argument, as determined by {@code newType},
2061      * must be identical to the type of the corresponding outgoing parameter
2062      * or parameters in the target method handle.
2063      * The return type of {@code newType} must be identical to the return
2064      * type of the original target.
2065      * <p>
2066      * The reordering array need not specify an actual permutation.
2067      * An incoming argument will be duplicated if its index appears
2068      * more than once in the array, and an incoming argument will be dropped
2069      * if its index does not appear in the array.
2070      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
2071      * incoming arguments which are not mentioned in the reordering array
2072      * are may be any type, as determined only by {@code newType}.
2073      * <blockquote><pre>{@code
2074 import static java.lang.invoke.MethodHandles.*;
2075 import static java.lang.invoke.MethodType.*;
2076 ...
2077 MethodType intfn1 = methodType(int.class, int.class);
2078 MethodType intfn2 = methodType(int.class, int.class, int.class);
2079 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
2080 assert(sub.type().equals(intfn2));
2081 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
2082 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
2083 assert((int)rsub.invokeExact(1, 100) == 99);
2084 MethodHandle add = ... (int x, int y) -> (x+y) ...;
2085 assert(add.type().equals(intfn2));
2086 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
2087 assert(twice.type().equals(intfn1));
2088 assert((int)twice.invokeExact(21) == 42);
2089      * }</pre></blockquote>
2090      * @param target the method handle to invoke after arguments are reordered
2091      * @param newType the expected type of the new method handle
2092      * @param reorder an index array which controls the reordering
2093      * @return a method handle which delegates to the target after it
2094      *           drops unused arguments and moves and/or duplicates the other arguments
2095      * @throws NullPointerException if any argument is null
2096      * @throws IllegalArgumentException if the index array length is not equal to
2097      *                  the arity of the target, or if any index array element
2098      *                  not a valid index for a parameter of {@code newType},
2099      *                  or if two corresponding parameter types in
2100      *                  {@code target.type()} and {@code newType} are not identical,
2101      */
2102     public static
2103     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
2104         reorder = reorder.clone();  // get a private copy
2105         MethodType oldType = target.type();
2106         permuteArgumentChecks(reorder, newType, oldType);
2107         // first detect dropped arguments and handle them separately
2108         int[] originalReorder = reorder;
2109         BoundMethodHandle result = target.rebind();
2110         LambdaForm form = result.form;
2111         int newArity = newType.parameterCount();
2112         // Normalize the reordering into a real permutation,
2113         // by removing duplicates and adding dropped elements.
2114         // This somewhat improves lambda form caching, as well
2115         // as simplifying the transform by breaking it up into steps.
2116         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
2117             if (ddIdx > 0) {
2118                 // We found a duplicated entry at reorder[ddIdx].
2119                 // Example:  (x,y,z)->asList(x,y,z)
2120                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
2121                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
2122                 // The starred element corresponds to the argument
2123                 // deleted by the dupArgumentForm transform.
2124                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
2125                 boolean killFirst = false;
2126                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
2127                     // Set killFirst if the dup is larger than an intervening position.
2128                     // This will remove at least one inversion from the permutation.
2129                     if (dupVal > val) killFirst = true;
2130                 }
2131                 if (!killFirst) {
2132                     srcPos = dstPos;
2133                     dstPos = ddIdx;
2134                 }
2135                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
2136                 assert (reorder[srcPos] == reorder[dstPos]);
2137                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
2138                 // contract the reordering by removing the element at dstPos
2139                 int tailPos = dstPos + 1;
2140                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
2141                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
2142             } else {
2143                 int dropVal = ~ddIdx, insPos = 0;
2144                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
2145                     // Find first element of reorder larger than dropVal.
2146                     // This is where we will insert the dropVal.
2147                     insPos += 1;
2148                 }
2149                 Class<?> ptype = newType.parameterType(dropVal);
2150                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
2151                 oldType = oldType.insertParameterTypes(insPos, ptype);
2152                 // expand the reordering by inserting an element at insPos
2153                 int tailPos = insPos + 1;
2154                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
2155                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
2156                 reorder[insPos] = dropVal;
2157             }
2158             assert (permuteArgumentChecks(reorder, newType, oldType));
2159         }
2160         assert (reorder.length == newArity);  // a perfect permutation
2161         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
2162         form = form.editor().permuteArgumentsForm(1, reorder);
2163         if (newType == result.type() && form == result.internalForm())
2164             return result;
2165         return result.copyWith(newType, form);
2166     }
2167 
2168     /**
2169      * Return an indication of any duplicate or omission in reorder.
2170      * If the reorder contains a duplicate entry, return the index of the second occurrence.
2171      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
2172      * Otherwise, return zero.
2173      * If an element not in [0..newArity-1] is encountered, return reorder.length.
2174      */
2175     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
2176         final int BIT_LIMIT = 63;  // max number of bits in bit mask
2177         if (newArity < BIT_LIMIT) {
2178             long mask = 0;
2179             for (int i = 0; i < reorder.length; i++) {
2180                 int arg = reorder[i];
2181                 if (arg >= newArity) {
2182                     return reorder.length;
2183                 }
2184                 long bit = 1L << arg;
2185                 if ((mask & bit) != 0) {
2186                     return i;  // >0 indicates a dup
2187                 }
2188                 mask |= bit;
2189             }
2190             if (mask == (1L << newArity) - 1) {
2191                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
2192                 return 0;
2193             }
2194             // find first zero
2195             long zeroBit = Long.lowestOneBit(~mask);
2196             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
2197             assert(zeroPos <= newArity);
2198             if (zeroPos == newArity) {
2199                 return 0;
2200             }
2201             return ~zeroPos;
2202         } else {
2203             // same algorithm, different bit set
2204             BitSet mask = new BitSet(newArity);
2205             for (int i = 0; i < reorder.length; i++) {
2206                 int arg = reorder[i];
2207                 if (arg >= newArity) {
2208                     return reorder.length;
2209                 }
2210                 if (mask.get(arg)) {
2211                     return i;  // >0 indicates a dup
2212                 }
2213                 mask.set(arg);
2214             }
2215             int zeroPos = mask.nextClearBit(0);
2216             assert(zeroPos <= newArity);
2217             if (zeroPos == newArity) {
2218                 return 0;
2219             }
2220             return ~zeroPos;
2221         }
2222     }
2223 
2224     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
2225         if (newType.returnType() != oldType.returnType())
2226             throw newIllegalArgumentException("return types do not match",
2227                     oldType, newType);
2228         if (reorder.length == oldType.parameterCount()) {
2229             int limit = newType.parameterCount();
2230             boolean bad = false;
2231             for (int j = 0; j < reorder.length; j++) {
2232                 int i = reorder[j];
2233                 if (i < 0 || i >= limit) {
2234                     bad = true; break;
2235                 }
2236                 Class<?> src = newType.parameterType(i);
2237                 Class<?> dst = oldType.parameterType(j);
2238                 if (src != dst)
2239                     throw newIllegalArgumentException("parameter types do not match after reorder",
2240                             oldType, newType);
2241             }
2242             if (!bad)  return true;
2243         }
2244         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
2245     }
2246 
2247     /**
2248      * Produces a method handle of the requested return type which returns the given
2249      * constant value every time it is invoked.
2250      * <p>
2251      * Before the method handle is returned, the passed-in value is converted to the requested type.
2252      * If the requested type is primitive, widening primitive conversions are attempted,
2253      * else reference conversions are attempted.
2254      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
2255      * @param type the return type of the desired method handle
2256      * @param value the value to return
2257      * @return a method handle of the given return type and no arguments, which always returns the given value
2258      * @throws NullPointerException if the {@code type} argument is null
2259      * @throws ClassCastException if the value cannot be converted to the required return type
2260      * @throws IllegalArgumentException if the given type is {@code void.class}
2261      */
2262     public static
2263     MethodHandle constant(Class<?> type, Object value) {
2264         if (type.isPrimitive()) {
2265             if (type == void.class)
2266                 throw newIllegalArgumentException("void type");
2267             Wrapper w = Wrapper.forPrimitiveType(type);
2268             value = w.convert(value, type);
2269             if (w.zero().equals(value))
2270                 return zero(w, type);
2271             return insertArguments(identity(type), 0, value);
2272         } else {
2273             if (value == null)
2274                 return zero(Wrapper.OBJECT, type);
2275             return identity(type).bindTo(value);
2276         }
2277     }
2278 
2279     /**
2280      * Produces a method handle which returns its sole argument when invoked.
2281      * @param type the type of the sole parameter and return value of the desired method handle
2282      * @return a unary method handle which accepts and returns the given type
2283      * @throws NullPointerException if the argument is null
2284      * @throws IllegalArgumentException if the given type is {@code void.class}
2285      */
2286     public static
2287     MethodHandle identity(Class<?> type) {
2288         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
2289         int pos = btw.ordinal();
2290         MethodHandle ident = IDENTITY_MHS[pos];
2291         if (ident == null) {
2292             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
2293         }
2294         if (ident.type().returnType() == type)
2295             return ident;
2296         // something like identity(Foo.class); do not bother to intern these
2297         assert(btw == Wrapper.OBJECT);
2298         return makeIdentity(type);
2299     }
2300     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.values().length];
2301     private static MethodHandle makeIdentity(Class<?> ptype) {
2302         MethodType mtype = MethodType.methodType(ptype, ptype);
2303         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
2304         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
2305     }
2306 
2307     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
2308         int pos = btw.ordinal();
2309         MethodHandle zero = ZERO_MHS[pos];
2310         if (zero == null) {
2311             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
2312         }
2313         if (zero.type().returnType() == rtype)
2314             return zero;
2315         assert(btw == Wrapper.OBJECT);
2316         return makeZero(rtype);
2317     }
2318     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.values().length];
2319     private static MethodHandle makeZero(Class<?> rtype) {
2320         MethodType mtype = MethodType.methodType(rtype);
2321         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
2322         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
2323     }
2324 
2325     synchronized private static MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
2326         // Simulate a CAS, to avoid racy duplication of results.
2327         MethodHandle prev = cache[pos];
2328         if (prev != null) return prev;
2329         return cache[pos] = value;
2330     }
2331 
2332     /**
2333      * Provides a target method handle with one or more <em>bound arguments</em>
2334      * in advance of the method handle's invocation.
2335      * The formal parameters to the target corresponding to the bound
2336      * arguments are called <em>bound parameters</em>.
2337      * Returns a new method handle which saves away the bound arguments.
2338      * When it is invoked, it receives arguments for any non-bound parameters,
2339      * binds the saved arguments to their corresponding parameters,
2340      * and calls the original target.
2341      * <p>
2342      * The type of the new method handle will drop the types for the bound
2343      * parameters from the original target type, since the new method handle
2344      * will no longer require those arguments to be supplied by its callers.
2345      * <p>
2346      * Each given argument object must match the corresponding bound parameter type.
2347      * If a bound parameter type is a primitive, the argument object
2348      * must be a wrapper, and will be unboxed to produce the primitive value.
2349      * <p>
2350      * The {@code pos} argument selects which parameters are to be bound.
2351      * It may range between zero and <i>N-L</i> (inclusively),
2352      * where <i>N</i> is the arity of the target method handle
2353      * and <i>L</i> is the length of the values array.
2354      * @param target the method handle to invoke after the argument is inserted
2355      * @param pos where to insert the argument (zero for the first)
2356      * @param values the series of arguments to insert
2357      * @return a method handle which inserts an additional argument,
2358      *         before calling the original method handle
2359      * @throws NullPointerException if the target or the {@code values} array is null
2360      * @see MethodHandle#bindTo
2361      */
2362     public static
2363     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
2364         int insCount = values.length;
2365         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
2366         if (insCount == 0)  return target;
2367         BoundMethodHandle result = target.rebind();
2368         for (int i = 0; i < insCount; i++) {
2369             Object value = values[i];
2370             Class<?> ptype = ptypes[pos+i];
2371             if (ptype.isPrimitive()) {
2372                 result = insertArgumentPrimitive(result, pos, ptype, value);
2373             } else {
2374                 value = ptype.cast(value);  // throw CCE if needed
2375                 result = result.bindArgumentL(pos, value);
2376             }
2377         }
2378         return result;
2379     }
2380 
2381     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
2382                                                              Class<?> ptype, Object value) {
2383         Wrapper w = Wrapper.forPrimitiveType(ptype);
2384         // perform unboxing and/or primitive conversion
2385         value = w.convert(value, ptype);
2386         switch (w) {
2387         case INT:     return result.bindArgumentI(pos, (int)value);
2388         case LONG:    return result.bindArgumentJ(pos, (long)value);
2389         case FLOAT:   return result.bindArgumentF(pos, (float)value);
2390         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
2391         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
2392         }
2393     }
2394 
2395     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
2396         MethodType oldType = target.type();
2397         int outargs = oldType.parameterCount();
2398         int inargs  = outargs - insCount;
2399         if (inargs < 0)
2400             throw newIllegalArgumentException("too many values to insert");
2401         if (pos < 0 || pos > inargs)
2402             throw newIllegalArgumentException("no argument type to append");
2403         return oldType.ptypes();
2404     }
2405 
2406     /**
2407      * Produces a method handle which will discard some dummy arguments
2408      * before calling some other specified <i>target</i> method handle.
2409      * The type of the new method handle will be the same as the target's type,
2410      * except it will also include the dummy argument types,
2411      * at some given position.
2412      * <p>
2413      * The {@code pos} argument may range between zero and <i>N</i>,
2414      * where <i>N</i> is the arity of the target.
2415      * If {@code pos} is zero, the dummy arguments will precede
2416      * the target's real arguments; if {@code pos} is <i>N</i>
2417      * they will come after.
2418      * <p>
2419      * <b>Example:</b>
2420      * <blockquote><pre>{@code
2421 import static java.lang.invoke.MethodHandles.*;
2422 import static java.lang.invoke.MethodType.*;
2423 ...
2424 MethodHandle cat = lookup().findVirtual(String.class,
2425   "concat", methodType(String.class, String.class));
2426 assertEquals("xy", (String) cat.invokeExact("x", "y"));
2427 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
2428 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
2429 assertEquals(bigType, d0.type());
2430 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
2431      * }</pre></blockquote>
2432      * <p>
2433      * This method is also equivalent to the following code:
2434      * <blockquote><pre>
2435      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
2436      * </pre></blockquote>
2437      * @param target the method handle to invoke after the arguments are dropped
2438      * @param valueTypes the type(s) of the argument(s) to drop
2439      * @param pos position of first argument to drop (zero for the leftmost)
2440      * @return a method handle which drops arguments of the given types,
2441      *         before calling the original method handle
2442      * @throws NullPointerException if the target is null,
2443      *                              or if the {@code valueTypes} list or any of its elements is null
2444      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
2445      *                  or if {@code pos} is negative or greater than the arity of the target,
2446      *                  or if the new method handle's type would have too many parameters
2447      */
2448     public static
2449     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
2450         MethodType oldType = target.type();  // get NPE
2451         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
2452         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
2453         if (dropped == 0)  return target;
2454         BoundMethodHandle result = target.rebind();
2455         LambdaForm lform = result.form;
2456         int insertFormArg = 1 + pos;
2457         for (Class<?> ptype : valueTypes) {
2458             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
2459         }
2460         result = result.copyWith(newType, lform);
2461         return result;
2462     }
2463 
2464     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
2465         int dropped = valueTypes.size();
2466         MethodType.checkSlotCount(dropped);
2467         int outargs = oldType.parameterCount();
2468         int inargs  = outargs + dropped;
2469         if (pos < 0 || pos > outargs)
2470             throw newIllegalArgumentException("no argument type to remove"
2471                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
2472                     );
2473         return dropped;
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 MethodHandle d0 = dropArguments(cat, 0, String.class);
2498 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
2499 MethodHandle d1 = dropArguments(cat, 1, String.class);
2500 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
2501 MethodHandle d2 = dropArguments(cat, 2, String.class);
2502 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
2503 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
2504 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
2505      * }</pre></blockquote>
2506      * <p>
2507      * This method is also equivalent to the following code:
2508      * <blockquote><pre>
2509      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
2510      * </pre></blockquote>
2511      * @param target the method handle to invoke after the arguments are dropped
2512      * @param valueTypes the type(s) of the argument(s) to drop
2513      * @param pos position of first argument to drop (zero for the leftmost)
2514      * @return a method handle which drops arguments of the given types,
2515      *         before calling the original method handle
2516      * @throws NullPointerException if the target is null,
2517      *                              or if the {@code valueTypes} array or any of its elements is null
2518      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
2519      *                  or if {@code pos} is negative or greater than the arity of the target,
2520      *                  or if the new method handle's type would have
2521      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
2522      */
2523     public static
2524     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
2525         return dropArguments(target, pos, Arrays.asList(valueTypes));
2526     }
2527 
2528     /**
2529      * Adapts a target method handle by pre-processing
2530      * one or more of its arguments, each with its own unary filter function,
2531      * and then calling the target with each pre-processed argument
2532      * replaced by the result of its corresponding filter function.
2533      * <p>
2534      * The pre-processing is performed by one or more method handles,
2535      * specified in the elements of the {@code filters} array.
2536      * The first element of the filter array corresponds to the {@code pos}
2537      * argument of the target, and so on in sequence.
2538      * <p>
2539      * Null arguments in the array are treated as identity functions,
2540      * and the corresponding arguments left unchanged.
2541      * (If there are no non-null elements in the array, the original target is returned.)
2542      * Each filter is applied to the corresponding argument of the adapter.
2543      * <p>
2544      * If a filter {@code F} applies to the {@code N}th argument of
2545      * the target, then {@code F} must be a method handle which
2546      * takes exactly one argument.  The type of {@code F}'s sole argument
2547      * replaces the corresponding argument type of the target
2548      * in the resulting adapted method handle.
2549      * The return type of {@code F} must be identical to the corresponding
2550      * parameter type of the target.
2551      * <p>
2552      * It is an error if there are elements of {@code filters}
2553      * (null or not)
2554      * which do not correspond to argument positions in the target.
2555      * <p><b>Example:</b>
2556      * <blockquote><pre>{@code
2557 import static java.lang.invoke.MethodHandles.*;
2558 import static java.lang.invoke.MethodType.*;
2559 ...
2560 MethodHandle cat = lookup().findVirtual(String.class,
2561   "concat", methodType(String.class, String.class));
2562 MethodHandle upcase = lookup().findVirtual(String.class,
2563   "toUpperCase", methodType(String.class));
2564 assertEquals("xy", (String) cat.invokeExact("x", "y"));
2565 MethodHandle f0 = filterArguments(cat, 0, upcase);
2566 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
2567 MethodHandle f1 = filterArguments(cat, 1, upcase);
2568 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
2569 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
2570 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
2571      * }</pre></blockquote>
2572      * <p> Here is pseudocode for the resulting adapter:
2573      * <blockquote><pre>{@code
2574      * V target(P... p, A[i]... a[i], B... b);
2575      * A[i] filter[i](V[i]);
2576      * T adapter(P... p, V[i]... v[i], B... b) {
2577      *   return target(p..., f[i](v[i])..., b...);
2578      * }
2579      * }</pre></blockquote>
2580      *
2581      * @param target the method handle to invoke after arguments are filtered
2582      * @param pos the position of the first argument to filter
2583      * @param filters method handles to call initially on filtered arguments
2584      * @return method handle which incorporates the specified argument filtering logic
2585      * @throws NullPointerException if the target is null
2586      *                              or if the {@code filters} array is null
2587      * @throws IllegalArgumentException if a non-null element of {@code filters}
2588      *          does not match a corresponding argument type of target as described above,
2589      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
2590      *          or if the resulting method handle's type would have
2591      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2592      */
2593     public static
2594     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
2595         filterArgumentsCheckArity(target, pos, filters);
2596         MethodHandle adapter = target;
2597         int curPos = pos-1;  // pre-incremented
2598         for (MethodHandle filter : filters) {
2599             curPos += 1;
2600             if (filter == null)  continue;  // ignore null elements of filters
2601             adapter = filterArgument(adapter, curPos, filter);
2602         }
2603         return adapter;
2604     }
2605 
2606     /*non-public*/ static
2607     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
2608         filterArgumentChecks(target, pos, filter);
2609         MethodType targetType = target.type();
2610         MethodType filterType = filter.type();
2611         BoundMethodHandle result = target.rebind();
2612         Class<?> newParamType = filterType.parameterType(0);
2613         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
2614         MethodType newType = targetType.changeParameterType(pos, newParamType);
2615         result = result.copyWithExtendL(newType, lform, filter);
2616         return result;
2617     }
2618 
2619     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
2620         MethodType targetType = target.type();
2621         int maxPos = targetType.parameterCount();
2622         if (pos + filters.length > maxPos)
2623             throw newIllegalArgumentException("too many filters");
2624     }
2625 
2626     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
2627         MethodType targetType = target.type();
2628         MethodType filterType = filter.type();
2629         if (filterType.parameterCount() != 1
2630             || filterType.returnType() != targetType.parameterType(pos))
2631             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2632     }
2633 
2634     /**
2635      * Adapts a target method handle by pre-processing
2636      * a sub-sequence of its arguments with a filter (another method handle).
2637      * The pre-processed arguments are replaced by the result (if any) of the
2638      * filter function.
2639      * The target is then called on the modified (usually shortened) argument list.
2640      * <p>
2641      * If the filter returns a value, the target must accept that value as
2642      * its argument in position {@code pos}, preceded and/or followed by
2643      * any arguments not passed to the filter.
2644      * If the filter returns void, the target must accept all arguments
2645      * not passed to the filter.
2646      * No arguments are reordered, and a result returned from the filter
2647      * replaces (in order) the whole subsequence of arguments originally
2648      * passed to the adapter.
2649      * <p>
2650      * The argument types (if any) of the filter
2651      * replace zero or one argument types of the target, at position {@code pos},
2652      * in the resulting adapted method handle.
2653      * The return type of the filter (if any) must be identical to the
2654      * argument type of the target at position {@code pos}, and that target argument
2655      * is supplied by the return value of the filter.
2656      * <p>
2657      * In all cases, {@code pos} must be greater than or equal to zero, and
2658      * {@code pos} must also be less than or equal to the target's arity.
2659      * <p><b>Example:</b>
2660      * <blockquote><pre>{@code
2661 import static java.lang.invoke.MethodHandles.*;
2662 import static java.lang.invoke.MethodType.*;
2663 ...
2664 MethodHandle deepToString = publicLookup()
2665   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
2666 
2667 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
2668 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
2669 
2670 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
2671 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
2672 
2673 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
2674 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
2675 assertEquals("[top, [up, down], strange]",
2676              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
2677 
2678 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
2679 assertEquals("[top, [up, down], [strange]]",
2680              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
2681 
2682 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
2683 assertEquals("[top, [[up, down, strange], charm], bottom]",
2684              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
2685      * }</pre></blockquote>
2686      * <p> Here is pseudocode for the resulting adapter:
2687      * <blockquote><pre>{@code
2688      * T target(A...,V,C...);
2689      * V filter(B...);
2690      * T adapter(A... a,B... b,C... c) {
2691      *   V v = filter(b...);
2692      *   return target(a...,v,c...);
2693      * }
2694      * // and if the filter has no arguments:
2695      * T target2(A...,V,C...);
2696      * V filter2();
2697      * T adapter2(A... a,C... c) {
2698      *   V v = filter2();
2699      *   return target2(a...,v,c...);
2700      * }
2701      * // and if the filter has a void return:
2702      * T target3(A...,C...);
2703      * void filter3(B...);
2704      * void adapter3(A... a,B... b,C... c) {
2705      *   filter3(b...);
2706      *   return target3(a...,c...);
2707      * }
2708      * }</pre></blockquote>
2709      * <p>
2710      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
2711      * one which first "folds" the affected arguments, and then drops them, in separate
2712      * steps as follows:
2713      * <blockquote><pre>{@code
2714      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
2715      * mh = MethodHandles.foldArguments(mh, coll); //step 1
2716      * }</pre></blockquote>
2717      * If the target method handle consumes no arguments besides than the result
2718      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
2719      * is equivalent to {@code filterReturnValue(coll, mh)}.
2720      * If the filter method handle {@code coll} consumes one argument and produces
2721      * a non-void result, then {@code collectArguments(mh, N, coll)}
2722      * is equivalent to {@code filterArguments(mh, N, coll)}.
2723      * Other equivalences are possible but would require argument permutation.
2724      *
2725      * @param target the method handle to invoke after filtering the subsequence of arguments
2726      * @param pos the position of the first adapter argument to pass to the filter,
2727      *            and/or the target argument which receives the result of the filter
2728      * @param filter method handle to call on the subsequence of arguments
2729      * @return method handle which incorporates the specified argument subsequence filtering logic
2730      * @throws NullPointerException if either argument is null
2731      * @throws IllegalArgumentException if the return type of {@code filter}
2732      *          is non-void and is not the same as the {@code pos} argument of the target,
2733      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
2734      *          or if the resulting method handle's type would have
2735      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2736      * @see MethodHandles#foldArguments
2737      * @see MethodHandles#filterArguments
2738      * @see MethodHandles#filterReturnValue
2739      */
2740     public static
2741     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
2742         MethodType newType = collectArgumentsChecks(target, pos, filter);
2743         MethodType collectorType = filter.type();
2744         BoundMethodHandle result = target.rebind();
2745         LambdaForm lform;
2746         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
2747             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
2748             if (lform != null) {
2749                 return result.copyWith(newType, lform);
2750             }
2751         }
2752         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
2753         return result.copyWithExtendL(newType, lform, filter);
2754     }
2755 
2756     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
2757         MethodType targetType = target.type();
2758         MethodType filterType = filter.type();
2759         Class<?> rtype = filterType.returnType();
2760         List<Class<?>> filterArgs = filterType.parameterList();
2761         if (rtype == void.class) {
2762             return targetType.insertParameterTypes(pos, filterArgs);
2763         }
2764         if (rtype != targetType.parameterType(pos)) {
2765             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2766         }
2767         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
2768     }
2769 
2770     /**
2771      * Adapts a target method handle by post-processing
2772      * its return value (if any) with a filter (another method handle).
2773      * The result of the filter is returned from the adapter.
2774      * <p>
2775      * If the target returns a value, the filter must accept that value as
2776      * its only argument.
2777      * If the target returns void, the filter must accept no arguments.
2778      * <p>
2779      * The return type of the filter
2780      * replaces the return type of the target
2781      * in the resulting adapted method handle.
2782      * The argument type of the filter (if any) must be identical to the
2783      * return type of the target.
2784      * <p><b>Example:</b>
2785      * <blockquote><pre>{@code
2786 import static java.lang.invoke.MethodHandles.*;
2787 import static java.lang.invoke.MethodType.*;
2788 ...
2789 MethodHandle cat = lookup().findVirtual(String.class,
2790   "concat", methodType(String.class, String.class));
2791 MethodHandle length = lookup().findVirtual(String.class,
2792   "length", methodType(int.class));
2793 System.out.println((String) cat.invokeExact("x", "y")); // xy
2794 MethodHandle f0 = filterReturnValue(cat, length);
2795 System.out.println((int) f0.invokeExact("x", "y")); // 2
2796      * }</pre></blockquote>
2797      * <p> Here is pseudocode for the resulting adapter:
2798      * <blockquote><pre>{@code
2799      * V target(A...);
2800      * T filter(V);
2801      * T adapter(A... a) {
2802      *   V v = target(a...);
2803      *   return filter(v);
2804      * }
2805      * // and if the target has a void return:
2806      * void target2(A...);
2807      * T filter2();
2808      * T adapter2(A... a) {
2809      *   target2(a...);
2810      *   return filter2();
2811      * }
2812      * // and if the filter has a void return:
2813      * V target3(A...);
2814      * void filter3(V);
2815      * void adapter3(A... a) {
2816      *   V v = target3(a...);
2817      *   filter3(v);
2818      * }
2819      * }</pre></blockquote>
2820      * @param target the method handle to invoke before filtering the return value
2821      * @param filter method handle to call on the return value
2822      * @return method handle which incorporates the specified return value filtering logic
2823      * @throws NullPointerException if either argument is null
2824      * @throws IllegalArgumentException if the argument list of {@code filter}
2825      *          does not match the return type of target as described above
2826      */
2827     public static
2828     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
2829         MethodType targetType = target.type();
2830         MethodType filterType = filter.type();
2831         filterReturnValueChecks(targetType, filterType);
2832         BoundMethodHandle result = target.rebind();
2833         BasicType rtype = BasicType.basicType(filterType.returnType());
2834         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
2835         MethodType newType = targetType.changeReturnType(filterType.returnType());
2836         result = result.copyWithExtendL(newType, lform, filter);
2837         return result;
2838     }
2839 
2840     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
2841         Class<?> rtype = targetType.returnType();
2842         int filterValues = filterType.parameterCount();
2843         if (filterValues == 0
2844                 ? (rtype != void.class)
2845                 : (rtype != filterType.parameterType(0)))
2846             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
2847     }
2848 
2849     /**
2850      * Adapts a target method handle by pre-processing
2851      * some of its arguments, and then calling the target with
2852      * the result of the pre-processing, inserted into the original
2853      * sequence of arguments.
2854      * <p>
2855      * The pre-processing is performed by {@code combiner}, a second method handle.
2856      * Of the arguments passed to the adapter, the first {@code N} arguments
2857      * are copied to the combiner, which is then called.
2858      * (Here, {@code N} is defined as the parameter count of the combiner.)
2859      * After this, control passes to the target, with any result
2860      * from the combiner inserted before the original {@code N} incoming
2861      * arguments.
2862      * <p>
2863      * If the combiner returns a value, the first parameter type of the target
2864      * must be identical with the return type of the combiner, and the next
2865      * {@code N} parameter types of the target must exactly match the parameters
2866      * of the combiner.
2867      * <p>
2868      * If the combiner has a void return, no result will be inserted,
2869      * and the first {@code N} parameter types of the target
2870      * must exactly match the parameters of the combiner.
2871      * <p>
2872      * The resulting adapter is the same type as the target, except that the
2873      * first parameter type is dropped,
2874      * if it corresponds to the result of the combiner.
2875      * <p>
2876      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
2877      * that either the combiner or the target does not wish to receive.
2878      * If some of the incoming arguments are destined only for the combiner,
2879      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
2880      * arguments will not need to be live on the stack on entry to the
2881      * target.)
2882      * <p><b>Example:</b>
2883      * <blockquote><pre>{@code
2884 import static java.lang.invoke.MethodHandles.*;
2885 import static java.lang.invoke.MethodType.*;
2886 ...
2887 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
2888   "println", methodType(void.class, String.class))
2889     .bindTo(System.out);
2890 MethodHandle cat = lookup().findVirtual(String.class,
2891   "concat", methodType(String.class, String.class));
2892 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
2893 MethodHandle catTrace = foldArguments(cat, trace);
2894 // also prints "boo":
2895 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
2896      * }</pre></blockquote>
2897      * <p> Here is pseudocode for the resulting adapter:
2898      * <blockquote><pre>{@code
2899      * // there are N arguments in A...
2900      * T target(V, A[N]..., B...);
2901      * V combiner(A...);
2902      * T adapter(A... a, B... b) {
2903      *   V v = combiner(a...);
2904      *   return target(v, a..., b...);
2905      * }
2906      * // and if the combiner has a void return:
2907      * T target2(A[N]..., B...);
2908      * void combiner2(A...);
2909      * T adapter2(A... a, B... b) {
2910      *   combiner2(a...);
2911      *   return target2(a..., b...);
2912      * }
2913      * }</pre></blockquote>
2914      * @param target the method handle to invoke after arguments are combined
2915      * @param combiner method handle to call initially on the incoming arguments
2916      * @return method handle which incorporates the specified argument folding logic
2917      * @throws NullPointerException if either argument is null
2918      * @throws IllegalArgumentException if {@code combiner}'s return type
2919      *          is non-void and not the same as the first argument type of
2920      *          the target, or if the initial {@code N} argument types
2921      *          of the target
2922      *          (skipping one matching the {@code combiner}'s return type)
2923      *          are not identical with the argument types of {@code combiner}
2924      */
2925     public static
2926     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
2927         int foldPos = 0;
2928         MethodType targetType = target.type();
2929         MethodType combinerType = combiner.type();
2930         Class<?> rtype = foldArgumentChecks(foldPos, targetType, combinerType);
2931         BoundMethodHandle result = target.rebind();
2932         boolean dropResult = (rtype == void.class);
2933         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
2934         LambdaForm lform = result.editor().foldArgumentsForm(1 + foldPos, dropResult, combinerType.basicType());
2935         MethodType newType = targetType;
2936         if (!dropResult)
2937             newType = newType.dropParameterTypes(foldPos, foldPos + 1);
2938         result = result.copyWithExtendL(newType, lform, combiner);
2939         return result;
2940     }
2941 
2942     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
2943         int foldArgs   = combinerType.parameterCount();
2944         Class<?> rtype = combinerType.returnType();
2945         int foldVals = rtype == void.class ? 0 : 1;
2946         int afterInsertPos = foldPos + foldVals;
2947         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
2948         if (ok && !(combinerType.parameterList()
2949                     .equals(targetType.parameterList().subList(afterInsertPos,
2950                                                                afterInsertPos + foldArgs))))
2951             ok = false;
2952         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(0))
2953             ok = false;
2954         if (!ok)
2955             throw misMatchedTypes("target and combiner types", targetType, combinerType);
2956         return rtype;
2957     }
2958 
2959     /**
2960      * Makes a method handle which adapts a target method handle,
2961      * by guarding it with a test, a boolean-valued method handle.
2962      * If the guard fails, a fallback handle is called instead.
2963      * All three method handles must have the same corresponding
2964      * argument and return types, except that the return type
2965      * of the test must be boolean, and the test is allowed
2966      * to have fewer arguments than the other two method handles.
2967      * <p> Here is pseudocode for the resulting adapter:
2968      * <blockquote><pre>{@code
2969      * boolean test(A...);
2970      * T target(A...,B...);
2971      * T fallback(A...,B...);
2972      * T adapter(A... a,B... b) {
2973      *   if (test(a...))
2974      *     return target(a..., b...);
2975      *   else
2976      *     return fallback(a..., b...);
2977      * }
2978      * }</pre></blockquote>
2979      * Note that the test arguments ({@code a...} in the pseudocode) cannot
2980      * be modified by execution of the test, and so are passed unchanged
2981      * from the caller to the target or fallback as appropriate.
2982      * @param test method handle used for test, must return boolean
2983      * @param target method handle to call if test passes
2984      * @param fallback method handle to call if test fails
2985      * @return method handle which incorporates the specified if/then/else logic
2986      * @throws NullPointerException if any argument is null
2987      * @throws IllegalArgumentException if {@code test} does not return boolean,
2988      *          or if all three method types do not match (with the return
2989      *          type of {@code test} changed to match that of the target).
2990      */
2991     public static
2992     MethodHandle guardWithTest(MethodHandle test,
2993                                MethodHandle target,
2994                                MethodHandle fallback) {
2995         MethodType gtype = test.type();
2996         MethodType ttype = target.type();
2997         MethodType ftype = fallback.type();
2998         if (!ttype.equals(ftype))
2999             throw misMatchedTypes("target and fallback types", ttype, ftype);
3000         if (gtype.returnType() != boolean.class)
3001             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
3002         List<Class<?>> targs = ttype.parameterList();
3003         List<Class<?>> gargs = gtype.parameterList();
3004         if (!targs.equals(gargs)) {
3005             int gpc = gargs.size(), tpc = targs.size();
3006             if (gpc >= tpc || !targs.subList(0, gpc).equals(gargs))
3007                 throw misMatchedTypes("target and test types", ttype, gtype);
3008             test = dropArguments(test, gpc, targs.subList(gpc, tpc));
3009             gtype = test.type();
3010         }
3011         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
3012     }
3013 
3014     static RuntimeException misMatchedTypes(String what, MethodType t1, MethodType t2) {
3015         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
3016     }
3017 
3018     /**
3019      * Makes a method handle which adapts a target method handle,
3020      * by running it inside an exception handler.
3021      * If the target returns normally, the adapter returns that value.
3022      * If an exception matching the specified type is thrown, the fallback
3023      * handle is called instead on the exception, plus the original arguments.
3024      * <p>
3025      * The target and handler must have the same corresponding
3026      * argument and return types, except that handler may omit trailing arguments
3027      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
3028      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
3029      * <p> Here is pseudocode for the resulting adapter:
3030      * <blockquote><pre>{@code
3031      * T target(A..., B...);
3032      * T handler(ExType, A...);
3033      * T adapter(A... a, B... b) {
3034      *   try {
3035      *     return target(a..., b...);
3036      *   } catch (ExType ex) {
3037      *     return handler(ex, a...);
3038      *   }
3039      * }
3040      * }</pre></blockquote>
3041      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
3042      * be modified by execution of the target, and so are passed unchanged
3043      * from the caller to the handler, if the handler is invoked.
3044      * <p>
3045      * The target and handler must return the same type, even if the handler
3046      * always throws.  (This might happen, for instance, because the handler
3047      * is simulating a {@code finally} clause).
3048      * To create such a throwing handler, compose the handler creation logic
3049      * with {@link #throwException throwException},
3050      * in order to create a method handle of the correct return type.
3051      * @param target method handle to call
3052      * @param exType the type of exception which the handler will catch
3053      * @param handler method handle to call if a matching exception is thrown
3054      * @return method handle which incorporates the specified try/catch logic
3055      * @throws NullPointerException if any argument is null
3056      * @throws IllegalArgumentException if {@code handler} does not accept
3057      *          the given exception type, or if the method handle types do
3058      *          not match in their return types and their
3059      *          corresponding parameters
3060      */
3061     public static
3062     MethodHandle catchException(MethodHandle target,
3063                                 Class<? extends Throwable> exType,
3064                                 MethodHandle handler) {
3065         MethodType ttype = target.type();
3066         MethodType htype = handler.type();
3067         if (htype.parameterCount() < 1 ||
3068             !htype.parameterType(0).isAssignableFrom(exType))
3069             throw newIllegalArgumentException("handler does not accept exception type "+exType);
3070         if (htype.returnType() != ttype.returnType())
3071             throw misMatchedTypes("target and handler return types", ttype, htype);
3072         List<Class<?>> targs = ttype.parameterList();
3073         List<Class<?>> hargs = htype.parameterList();
3074         hargs = hargs.subList(1, hargs.size());  // omit leading parameter from handler
3075         if (!targs.equals(hargs)) {
3076             int hpc = hargs.size(), tpc = targs.size();
3077             if (hpc >= tpc || !targs.subList(0, hpc).equals(hargs))
3078                 throw misMatchedTypes("target and handler types", ttype, htype);
3079             handler = dropArguments(handler, 1+hpc, targs.subList(hpc, tpc));
3080             htype = handler.type();
3081         }
3082         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
3083     }
3084 
3085     /**
3086      * Produces a method handle which will throw exceptions of the given {@code exType}.
3087      * The method handle will accept a single argument of {@code exType},
3088      * and immediately throw it as an exception.
3089      * The method type will nominally specify a return of {@code returnType}.
3090      * The return type may be anything convenient:  It doesn't matter to the
3091      * method handle's behavior, since it will never return normally.
3092      * @param returnType the return type of the desired method handle
3093      * @param exType the parameter type of the desired method handle
3094      * @return method handle which can throw the given exceptions
3095      * @throws NullPointerException if either argument is null
3096      */
3097     public static
3098     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
3099         if (!Throwable.class.isAssignableFrom(exType))
3100             throw new ClassCastException(exType.getName());
3101         return MethodHandleImpl.throwException(MethodType.methodType(returnType, exType));
3102     }
3103 }