1 /*
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   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
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  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).
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  25 
  26 package java.lang.invoke;
  27 
  28 
  29 import java.util.*;
  30 import sun.invoke.util.*;
  31 import sun.misc.Unsafe;
  32 
  33 import static java.lang.invoke.MethodHandleStatics.*;
  34 
  35 /**
  36  * A method handle is a typed, directly executable reference to an underlying method,
  37  * constructor, field, or similar low-level operation, with optional
  38  * transformations of arguments or return values.
  39  * These transformations are quite general, and include such patterns as
  40  * {@linkplain #asType conversion},
  41  * {@linkplain #bindTo insertion},
  42  * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
  43  * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
  44  *
  45  * <h1>Method handle contents</h1>
  46  * Method handles are dynamically and strongly typed according to their parameter and return types.
  47  * They are not distinguished by the name or the defining class of their underlying methods.
  48  * A method handle must be invoked using a symbolic type descriptor which matches
  49  * the method handle's own {@linkplain #type type descriptor}.
  50  * <p>
  51  * Every method handle reports its type descriptor via the {@link #type type} accessor.
  52  * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
  53  * whose structure is a series of classes, one of which is
  54  * the return type of the method (or {@code void.class} if none).
  55  * <p>
  56  * A method handle's type controls the types of invocations it accepts,
  57  * and the kinds of transformations that apply to it.
  58  * <p>
  59  * A method handle contains a pair of special invoker methods
  60  * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
  61  * Both invoker methods provide direct access to the method handle's
  62  * underlying method, constructor, field, or other operation,
  63  * as modified by transformations of arguments and return values.
  64  * Both invokers accept calls which exactly match the method handle's own type.
  65  * The plain, inexact invoker also accepts a range of other call types.
  66  * <p>
  67  * Method handles are immutable and have no visible state.
  68  * Of course, they can be bound to underlying methods or data which exhibit state.
  69  * With respect to the Java Memory Model, any method handle will behave
  70  * as if all of its (internal) fields are final variables.  This means that any method
  71  * handle made visible to the application will always be fully formed.
  72  * This is true even if the method handle is published through a shared
  73  * variable in a data race.
  74  * <p>
  75  * Method handles cannot be subclassed by the user.
  76  * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
  77  * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
  78  * operation.  The programmer should not draw conclusions about a method handle
  79  * from its specific class, as the method handle class hierarchy (if any)
  80  * may change from time to time or across implementations from different vendors.
  81  *
  82  * <h1>Method handle compilation</h1>
  83  * A Java method call expression naming {@code invokeExact} or {@code invoke}
  84  * can invoke a method handle from Java source code.
  85  * From the viewpoint of source code, these methods can take any arguments
  86  * and their result can be cast to any return type.
  87  * Formally this is accomplished by giving the invoker methods
  88  * {@code Object} return types and variable arity {@code Object} arguments,
  89  * but they have an additional quality called <em>signature polymorphism</em>
  90  * which connects this freedom of invocation directly to the JVM execution stack.
  91  * <p>
  92  * As is usual with virtual methods, source-level calls to {@code invokeExact}
  93  * and {@code invoke} compile to an {@code invokevirtual} instruction.
  94  * More unusually, the compiler must record the actual argument types,
  95  * and may not perform method invocation conversions on the arguments.
  96  * Instead, it must push them on the stack according to their own unconverted types.
  97  * The method handle object itself is pushed on the stack before the arguments.
  98  * The compiler then calls the method handle with a symbolic type descriptor which
  99  * describes the argument and return types.
 100  * <p>
 101  * To issue a complete symbolic type descriptor, the compiler must also determine
 102  * the return type.  This is based on a cast on the method invocation expression,
 103  * if there is one, or else {@code Object} if the invocation is an expression
 104  * or else {@code void} if the invocation is a statement.
 105  * The cast may be to a primitive type (but not {@code void}).
 106  * <p>
 107  * As a corner case, an uncasted {@code null} argument is given
 108  * a symbolic type descriptor of {@code java.lang.Void}.
 109  * The ambiguity with the type {@code Void} is harmless, since there are no references of type
 110  * {@code Void} except the null reference.
 111  *
 112  * <h1>Method handle invocation</h1>
 113  * The first time a {@code invokevirtual} instruction is executed
 114  * it is linked, by symbolically resolving the names in the instruction
 115  * and verifying that the method call is statically legal.
 116  * This is true of calls to {@code invokeExact} and {@code invoke}.
 117  * In this case, the symbolic type descriptor emitted by the compiler is checked for
 118  * correct syntax and names it contains are resolved.
 119  * Thus, an {@code invokevirtual} instruction which invokes
 120  * a method handle will always link, as long
 121  * as the symbolic type descriptor is syntactically well-formed
 122  * and the types exist.
 123  * <p>
 124  * When the {@code invokevirtual} is executed after linking,
 125  * the receiving method handle's type is first checked by the JVM
 126  * to ensure that it matches the symbolic type descriptor.
 127  * If the type match fails, it means that the method which the
 128  * caller is invoking is not present on the individual
 129  * method handle being invoked.
 130  * <p>
 131  * In the case of {@code invokeExact}, the type descriptor of the invocation
 132  * (after resolving symbolic type names) must exactly match the method type
 133  * of the receiving method handle.
 134  * In the case of plain, inexact {@code invoke}, the resolved type descriptor
 135  * must be a valid argument to the receiver's {@link #asType asType} method.
 136  * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
 137  * <p>
 138  * After type matching, a call to {@code invokeExact} directly
 139  * and immediately invoke the method handle's underlying method
 140  * (or other behavior, as the case may be).
 141  * <p>
 142  * A call to plain {@code invoke} works the same as a call to
 143  * {@code invokeExact}, if the symbolic type descriptor specified by the caller
 144  * exactly matches the method handle's own type.
 145  * If there is a type mismatch, {@code invoke} attempts
 146  * to adjust the type of the receiving method handle,
 147  * as if by a call to {@link #asType asType},
 148  * to obtain an exactly invokable method handle {@code M2}.
 149  * This allows a more powerful negotiation of method type
 150  * between caller and callee.
 151  * <p>
 152  * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
 153  * and implementations are therefore not required to materialize it.)
 154  *
 155  * <h1>Invocation checking</h1>
 156  * In typical programs, method handle type matching will usually succeed.
 157  * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
 158  * either directly (in the case of {@code invokeExact}) or indirectly as if
 159  * by a failed call to {@code asType} (in the case of {@code invoke}).
 160  * <p>
 161  * Thus, a method type mismatch which might show up as a linkage error
 162  * in a statically typed program can show up as
 163  * a dynamic {@code WrongMethodTypeException}
 164  * in a program which uses method handles.
 165  * <p>
 166  * Because method types contain "live" {@code Class} objects,
 167  * method type matching takes into account both types names and class loaders.
 168  * Thus, even if a method handle {@code M} is created in one
 169  * class loader {@code L1} and used in another {@code L2},
 170  * method handle calls are type-safe, because the caller's symbolic type
 171  * descriptor, as resolved in {@code L2},
 172  * is matched against the original callee method's symbolic type descriptor,
 173  * as resolved in {@code L1}.
 174  * The resolution in {@code L1} happens when {@code M} is created
 175  * and its type is assigned, while the resolution in {@code L2} happens
 176  * when the {@code invokevirtual} instruction is linked.
 177  * <p>
 178  * Apart from the checking of type descriptors,
 179  * a method handle's capability to call its underlying method is unrestricted.
 180  * If a method handle is formed on a non-public method by a class
 181  * that has access to that method, the resulting handle can be used
 182  * in any place by any caller who receives a reference to it.
 183  * <p>
 184  * Unlike with the Core Reflection API, where access is checked every time
 185  * a reflective method is invoked,
 186  * method handle access checking is performed
 187  * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
 188  * In the case of {@code ldc} (see below), access checking is performed as part of linking
 189  * the constant pool entry underlying the constant method handle.
 190  * <p>
 191  * Thus, handles to non-public methods, or to methods in non-public classes,
 192  * should generally be kept secret.
 193  * They should not be passed to untrusted code unless their use from
 194  * the untrusted code would be harmless.
 195  *
 196  * <h1>Method handle creation</h1>
 197  * Java code can create a method handle that directly accesses
 198  * any method, constructor, or field that is accessible to that code.
 199  * This is done via a reflective, capability-based API called
 200  * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
 201  * For example, a static method handle can be obtained
 202  * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
 203  * There are also conversion methods from Core Reflection API objects,
 204  * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 205  * <p>
 206  * Like classes and strings, method handles that correspond to accessible
 207  * fields, methods, and constructors can also be represented directly
 208  * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
 209  * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
 210  * refers directly to an associated {@code CONSTANT_Methodref},
 211  * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
 212  * constant pool entry.
 213  * (For full details on method handle constants,
 214  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
 215  * <p>
 216  * Method handles produced by lookups or constant loads from methods or
 217  * constructors with the variable arity modifier bit ({@code 0x0080})
 218  * have a corresponding variable arity, as if they were defined with
 219  * the help of {@link #asVarargsCollector asVarargsCollector}.
 220  * <p>
 221  * A method reference may refer either to a static or non-static method.
 222  * In the non-static case, the method handle type includes an explicit
 223  * receiver argument, prepended before any other arguments.
 224  * In the method handle's type, the initial receiver argument is typed
 225  * according to the class under which the method was initially requested.
 226  * (E.g., if a non-static method handle is obtained via {@code ldc},
 227  * the type of the receiver is the class named in the constant pool entry.)
 228  * <p>
 229  * Method handle constants are subject to the same link-time access checks
 230  * their corresponding bytecode instructions, and the {@code ldc} instruction
 231  * will throw corresponding linkage errors if the bytecode behaviors would
 232  * throw such errors.
 233  * <p>
 234  * As a corollary of this, access to protected members is restricted
 235  * to receivers only of the accessing class, or one of its subclasses,
 236  * and the accessing class must in turn be a subclass (or package sibling)
 237  * of the protected member's defining class.
 238  * If a method reference refers to a protected non-static method or field
 239  * of a class outside the current package, the receiver argument will
 240  * be narrowed to the type of the accessing class.
 241  * <p>
 242  * When a method handle to a virtual method is invoked, the method is
 243  * always looked up in the receiver (that is, the first argument).
 244  * <p>
 245  * A non-virtual method handle to a specific virtual method implementation
 246  * can also be created.  These do not perform virtual lookup based on
 247  * receiver type.  Such a method handle simulates the effect of
 248  * an {@code invokespecial} instruction to the same method.
 249  *
 250  * <h1>Usage examples</h1>
 251  * Here are some examples of usage:
 252  * <blockquote><pre>{@code
 253 Object x, y; String s; int i;
 254 MethodType mt; MethodHandle mh;
 255 MethodHandles.Lookup lookup = MethodHandles.lookup();
 256 // mt is (char,char)String
 257 mt = MethodType.methodType(String.class, char.class, char.class);
 258 mh = lookup.findVirtual(String.class, "replace", mt);
 259 s = (String) mh.invokeExact("daddy",'d','n');
 260 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
 261 assertEquals(s, "nanny");
 262 // weakly typed invocation (using MHs.invoke)
 263 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
 264 assertEquals(s, "savvy");
 265 // mt is (Object[])List
 266 mt = MethodType.methodType(java.util.List.class, Object[].class);
 267 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
 268 assert(mh.isVarargsCollector());
 269 x = mh.invoke("one", "two");
 270 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
 271 assertEquals(x, java.util.Arrays.asList("one","two"));
 272 // mt is (Object,Object,Object)Object
 273 mt = MethodType.genericMethodType(3);
 274 mh = mh.asType(mt);
 275 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
 276 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 277 assertEquals(x, java.util.Arrays.asList(1,2,3));
 278 // mt is ()int
 279 mt = MethodType.methodType(int.class);
 280 mh = lookup.findVirtual(java.util.List.class, "size", mt);
 281 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
 282 // invokeExact(Ljava/util/List;)I
 283 assert(i == 3);
 284 mt = MethodType.methodType(void.class, String.class);
 285 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
 286 mh.invokeExact(System.out, "Hello, world.");
 287 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
 288  * }</pre></blockquote>
 289  * Each of the above calls to {@code invokeExact} or plain {@code invoke}
 290  * generates a single invokevirtual instruction with
 291  * the symbolic type descriptor indicated in the following comment.
 292  * In these examples, the helper method {@code assertEquals} is assumed to
 293  * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
 294  * on its arguments, and asserts that the result is true.
 295  *
 296  * <h1>Exceptions</h1>
 297  * The methods {@code invokeExact} and {@code invoke} are declared
 298  * to throw {@link java.lang.Throwable Throwable},
 299  * which is to say that there is no static restriction on what a method handle
 300  * can throw.  Since the JVM does not distinguish between checked
 301  * and unchecked exceptions (other than by their class, of course),
 302  * there is no particular effect on bytecode shape from ascribing
 303  * checked exceptions to method handle invocations.  But in Java source
 304  * code, methods which perform method handle calls must either explicitly
 305  * throw {@code Throwable}, or else must catch all
 306  * throwables locally, rethrowing only those which are legal in the context,
 307  * and wrapping ones which are illegal.
 308  *
 309  * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
 310  * The unusual compilation and linkage behavior of
 311  * {@code invokeExact} and plain {@code invoke}
 312  * is referenced by the term <em>signature polymorphism</em>.
 313  * As defined in the Java Language Specification,
 314  * a signature polymorphic method is one which can operate with
 315  * any of a wide range of call signatures and return types.
 316  * <p>
 317  * In source code, a call to a signature polymorphic method will
 318  * compile, regardless of the requested symbolic type descriptor.
 319  * As usual, the Java compiler emits an {@code invokevirtual}
 320  * instruction with the given symbolic type descriptor against the named method.
 321  * The unusual part is that the symbolic type descriptor is derived from
 322  * the actual argument and return types, not from the method declaration.
 323  * <p>
 324  * When the JVM processes bytecode containing signature polymorphic calls,
 325  * it will successfully link any such call, regardless of its symbolic type descriptor.
 326  * (In order to retain type safety, the JVM will guard such calls with suitable
 327  * dynamic type checks, as described elsewhere.)
 328  * <p>
 329  * Bytecode generators, including the compiler back end, are required to emit
 330  * untransformed symbolic type descriptors for these methods.
 331  * Tools which determine symbolic linkage are required to accept such
 332  * untransformed descriptors, without reporting linkage errors.
 333  *
 334  * <h1>Interoperation between method handles and the Core Reflection API</h1>
 335  * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
 336  * any class member represented by a Core Reflection API object
 337  * can be converted to a behaviorally equivalent method handle.
 338  * For example, a reflective {@link java.lang.reflect.Method Method} can
 339  * be converted to a method handle using
 340  * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 341  * The resulting method handles generally provide more direct and efficient
 342  * access to the underlying class members.
 343  * <p>
 344  * As a special case,
 345  * when the Core Reflection API is used to view the signature polymorphic
 346  * methods {@code invokeExact} or plain {@code invoke} in this class,
 347  * they appear as ordinary non-polymorphic methods.
 348  * Their reflective appearance, as viewed by
 349  * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
 350  * is unaffected by their special status in this API.
 351  * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
 352  * will report exactly those modifier bits required for any similarly
 353  * declared method, including in this case {@code native} and {@code varargs} bits.
 354  * <p>
 355  * As with any reflected method, these methods (when reflected) may be
 356  * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
 357  * However, such reflective calls do not result in method handle invocations.
 358  * Such a call, if passed the required argument
 359  * (a single one, of type {@code Object[]}), will ignore the argument and
 360  * will throw an {@code UnsupportedOperationException}.
 361  * <p>
 362  * Since {@code invokevirtual} instructions can natively
 363  * invoke method handles under any symbolic type descriptor, this reflective view conflicts
 364  * with the normal presentation of these methods via bytecodes.
 365  * Thus, these two native methods, when reflectively viewed by
 366  * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
 367  * <p>
 368  * In order to obtain an invoker method for a particular type descriptor,
 369  * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
 370  * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
 371  * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
 372  * API is also able to return a method handle
 373  * to call {@code invokeExact} or plain {@code invoke},
 374  * for any specified type descriptor .
 375  *
 376  * <h1>Interoperation between method handles and Java generics</h1>
 377  * A method handle can be obtained on a method, constructor, or field
 378  * which is declared with Java generic types.
 379  * As with the Core Reflection API, the type of the method handle
 380  * will constructed from the erasure of the source-level type.
 381  * When a method handle is invoked, the types of its arguments
 382  * or the return value cast type may be generic types or type instances.
 383  * If this occurs, the compiler will replace those
 384  * types by their erasures when it constructs the symbolic type descriptor
 385  * for the {@code invokevirtual} instruction.
 386  * <p>
 387  * Method handles do not represent
 388  * their function-like types in terms of Java parameterized (generic) types,
 389  * because there are three mismatches between function-like types and parameterized
 390  * Java types.
 391  * <ul>
 392  * <li>Method types range over all possible arities,
 393  * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
 394  * Generics are not variadic, and so cannot represent this.</li>
 395  * <li>Method types can specify arguments of primitive types,
 396  * which Java generic types cannot range over.</li>
 397  * <li>Higher order functions over method handles (combinators) are
 398  * often generic across a wide range of function types, including
 399  * those of multiple arities.  It is impossible to represent such
 400  * genericity with a Java type parameter.</li>
 401  * </ul>
 402  *
 403  * <h1><a name="maxarity"></a>Arity limits</h1>
 404  * The JVM imposes on all methods and constructors of any kind an absolute
 405  * limit of 255 stacked arguments.  This limit can appear more restrictive
 406  * in certain cases:
 407  * <ul>
 408  * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
 409  * <li>A non-static method consumes an extra argument for the object on which the method is called.
 410  * <li>A constructor consumes an extra argument for the object which is being constructed.
 411  * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
 412  *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
 413  * </ul>
 414  * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
 415  * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
 416  * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
 417  * In particular, a method handle&rsquo;s type must not have an arity of the exact maximum 255.
 418  *
 419  * @see MethodType
 420  * @see MethodHandles
 421  * @author John Rose, JSR 292 EG
 422  */
 423 public abstract class MethodHandle {
 424     static { MethodHandleImpl.initStatics(); }
 425 
 426     /**
 427      * Internal marker interface which distinguishes (to the Java compiler)
 428      * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
 429      */
 430     @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
 431     @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
 432     @interface PolymorphicSignature { }
 433 
 434     private final MethodType type;
 435     /*private*/ final LambdaForm form;
 436     // form is not private so that invokers can easily fetch it
 437     /*private*/ MethodHandle asTypeCache;
 438     // asTypeCache is not private so that invokers can easily fetch it
 439 
 440     /**
 441      * Reports the type of this method handle.
 442      * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
 443      * @return the method handle type
 444      */
 445     public MethodType type() {
 446         return type;
 447     }
 448 
 449     /**
 450      * Package-private constructor for the method handle implementation hierarchy.
 451      * Method handle inheritance will be contained completely within
 452      * the {@code java.lang.invoke} package.
 453      */
 454     // @param type type (permanently assigned) of the new method handle
 455     /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
 456         type.getClass();  // explicit NPE
 457         form.getClass();  // explicit NPE
 458         this.type = type;
 459         this.form = form;
 460 
 461         form.prepare();  // TO DO:  Try to delay this step until just before invocation.
 462     }
 463 
 464     /**
 465      * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
 466      * The symbolic type descriptor at the call site of {@code invokeExact} must
 467      * exactly match this method handle's {@link #type type}.
 468      * No conversions are allowed on arguments or return values.
 469      * <p>
 470      * When this method is observed via the Core Reflection API,
 471      * it will appear as a single native method, taking an object array and returning an object.
 472      * If this native method is invoked directly via
 473      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 474      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 475      * it will throw an {@code UnsupportedOperationException}.
 476      * @param args the signature-polymorphic parameter list, statically represented using varargs
 477      * @return the signature-polymorphic result, statically represented using {@code Object}
 478      * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
 479      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 480      */
 481     public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
 482 
 483     /**
 484      * Invokes the method handle, allowing any caller type descriptor,
 485      * and optionally performing conversions on arguments and return values.
 486      * <p>
 487      * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
 488      * the call proceeds as if by {@link #invokeExact invokeExact}.
 489      * <p>
 490      * Otherwise, the call proceeds as if this method handle were first
 491      * adjusted by calling {@link #asType asType} to adjust this method handle
 492      * to the required type, and then the call proceeds as if by
 493      * {@link #invokeExact invokeExact} on the adjusted method handle.
 494      * <p>
 495      * There is no guarantee that the {@code asType} call is actually made.
 496      * If the JVM can predict the results of making the call, it may perform
 497      * adaptations directly on the caller's arguments,
 498      * and call the target method handle according to its own exact type.
 499      * <p>
 500      * The resolved type descriptor at the call site of {@code invoke} must
 501      * be a valid argument to the receivers {@code asType} method.
 502      * In particular, the caller must specify the same argument arity
 503      * as the callee's type,
 504      * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
 505      * <p>
 506      * When this method is observed via the Core Reflection API,
 507      * it will appear as a single native method, taking an object array and returning an object.
 508      * If this native method is invoked directly via
 509      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 510      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 511      * it will throw an {@code UnsupportedOperationException}.
 512      * @param args the signature-polymorphic parameter list, statically represented using varargs
 513      * @return the signature-polymorphic result, statically represented using {@code Object}
 514      * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
 515      * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
 516      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 517      */
 518     public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
 519 
 520     /**
 521      * Private method for trusted invocation of a method handle respecting simplified signatures.
 522      * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
 523      * <p>
 524      * The caller signature is restricted to the following basic types:
 525      * Object, int, long, float, double, and void return.
 526      * <p>
 527      * The caller is responsible for maintaining type correctness by ensuring
 528      * that the each outgoing argument value is a member of the range of the corresponding
 529      * callee argument type.
 530      * (The caller should therefore issue appropriate casts and integer narrowing
 531      * operations on outgoing argument values.)
 532      * The caller can assume that the incoming result value is part of the range
 533      * of the callee's return type.
 534      * @param args the signature-polymorphic parameter list, statically represented using varargs
 535      * @return the signature-polymorphic result, statically represented using {@code Object}
 536      */
 537     /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
 538 
 539     /**
 540      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
 541      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 542      * The trailing (not leading) argument must be a MemberName.
 543      * @param args the signature-polymorphic parameter list, statically represented using varargs
 544      * @return the signature-polymorphic result, statically represented using {@code Object}
 545      */
 546     /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
 547 
 548     /**
 549      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
 550      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 551      * The trailing (not leading) argument must be a MemberName.
 552      * @param args the signature-polymorphic parameter list, statically represented using varargs
 553      * @return the signature-polymorphic result, statically represented using {@code Object}
 554      */
 555     /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
 556 
 557     /**
 558      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
 559      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 560      * The trailing (not leading) argument must be a MemberName.
 561      * @param args the signature-polymorphic parameter list, statically represented using varargs
 562      * @return the signature-polymorphic result, statically represented using {@code Object}
 563      */
 564     /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
 565 
 566     /**
 567      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
 568      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 569      * The trailing (not leading) argument must be a MemberName.
 570      * @param args the signature-polymorphic parameter list, statically represented using varargs
 571      * @return the signature-polymorphic result, statically represented using {@code Object}
 572      */
 573     /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
 574 
 575     /**
 576      * Performs a variable arity invocation, passing the arguments in the given list
 577      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 578      * which mentions only the type {@code Object}, and whose arity is the length
 579      * of the argument list.
 580      * <p>
 581      * Specifically, execution proceeds as if by the following steps,
 582      * although the methods are not guaranteed to be called if the JVM
 583      * can predict their effects.
 584      * <ul>
 585      * <li>Determine the length of the argument array as {@code N}.
 586      *     For a null reference, {@code N=0}. </li>
 587      * <li>Determine the general type {@code TN} of {@code N} arguments as
 588      *     as {@code TN=MethodType.genericMethodType(N)}.</li>
 589      * <li>Force the original target method handle {@code MH0} to the
 590      *     required type, as {@code MH1 = MH0.asType(TN)}. </li>
 591      * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
 592      * <li>Invoke the type-adjusted method handle on the unpacked arguments:
 593      *     MH1.invokeExact(A0, ...). </li>
 594      * <li>Take the return value as an {@code Object} reference. </li>
 595      * </ul>
 596      * <p>
 597      * Because of the action of the {@code asType} step, the following argument
 598      * conversions are applied as necessary:
 599      * <ul>
 600      * <li>reference casting
 601      * <li>unboxing
 602      * <li>widening primitive conversions
 603      * </ul>
 604      * <p>
 605      * The result returned by the call is boxed if it is a primitive,
 606      * or forced to null if the return type is void.
 607      * <p>
 608      * This call is equivalent to the following code:
 609      * <blockquote><pre>{@code
 610      * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
 611      * Object result = invoker.invokeExact(this, arguments);
 612      * }</pre></blockquote>
 613      * <p>
 614      * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
 615      * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
 616      * It can therefore be used as a bridge between native or reflective code and method handles.
 617      *
 618      * @param arguments the arguments to pass to the target
 619      * @return the result returned by the target
 620      * @throws ClassCastException if an argument cannot be converted by reference casting
 621      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 622      * @throws Throwable anything thrown by the target method invocation
 623      * @see MethodHandles#spreadInvoker
 624      */
 625     public Object invokeWithArguments(Object... arguments) throws Throwable {
 626         int argc = arguments == null ? 0 : arguments.length;
 627         @SuppressWarnings("LocalVariableHidesMemberVariable")
 628         MethodType type = type();
 629         if (type.parameterCount() != argc || isVarargsCollector()) {
 630             // simulate invoke
 631             return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments);
 632         }
 633         MethodHandle invoker = type.invokers().varargsInvoker();
 634         return invoker.invokeExact(this, arguments);
 635     }
 636 
 637     /**
 638      * Performs a variable arity invocation, passing the arguments in the given array
 639      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 640      * which mentions only the type {@code Object}, and whose arity is the length
 641      * of the argument array.
 642      * <p>
 643      * This method is also equivalent to the following code:
 644      * <blockquote><pre>{@code
 645      *   invokeWithArguments(arguments.toArray()
 646      * }</pre></blockquote>
 647      *
 648      * @param arguments the arguments to pass to the target
 649      * @return the result returned by the target
 650      * @throws NullPointerException if {@code arguments} is a null reference
 651      * @throws ClassCastException if an argument cannot be converted by reference casting
 652      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 653      * @throws Throwable anything thrown by the target method invocation
 654      */
 655     public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
 656         return invokeWithArguments(arguments.toArray());
 657     }
 658 
 659     /**
 660      * Produces an adapter method handle which adapts the type of the
 661      * current method handle to a new type.
 662      * The resulting method handle is guaranteed to report a type
 663      * which is equal to the desired new type.
 664      * <p>
 665      * If the original type and new type are equal, returns {@code this}.
 666      * <p>
 667      * The new method handle, when invoked, will perform the following
 668      * steps:
 669      * <ul>
 670      * <li>Convert the incoming argument list to match the original
 671      *     method handle's argument list.
 672      * <li>Invoke the original method handle on the converted argument list.
 673      * <li>Convert any result returned by the original method handle
 674      *     to the return type of new method handle.
 675      * </ul>
 676      * <p>
 677      * This method provides the crucial behavioral difference between
 678      * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
 679      * The two methods
 680      * perform the same steps when the caller's type descriptor exactly m atches
 681      * the callee's, but when the types differ, plain {@link #invoke invoke}
 682      * also calls {@code asType} (or some internal equivalent) in order
 683      * to match up the caller's and callee's types.
 684      * <p>
 685      * If the current method is a variable arity method handle
 686      * argument list conversion may involve the conversion and collection
 687      * of several arguments into an array, as
 688      * {@linkplain #asVarargsCollector described elsewhere}.
 689      * In every other case, all conversions are applied <em>pairwise</em>,
 690      * which means that each argument or return value is converted to
 691      * exactly one argument or return value (or no return value).
 692      * The applied conversions are defined by consulting the
 693      * the corresponding component types of the old and new
 694      * method handle types.
 695      * <p>
 696      * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
 697      * or old and new return types.  Specifically, for some valid index {@code i}, let
 698      * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
 699      * Or else, going the other way for return values, let
 700      * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
 701      * If the types are the same, the new method handle makes no change
 702      * to the corresponding argument or return value (if any).
 703      * Otherwise, one of the following conversions is applied
 704      * if possible:
 705      * <ul>
 706      * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
 707      *     (The types do not need to be related in any particular way.
 708      *     This is because a dynamic value of null can convert to any reference type.)
 709      * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
 710      *     conversion (JLS 5.3) is applied, if one exists.
 711      *     (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
 712      * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
 713      *     a Java casting conversion (JLS 5.5) is applied if one exists.
 714      *     (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
 715      *     which is then widened as needed to <em>T1</em>.)
 716      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
 717      *     conversion will be applied at runtime, possibly followed
 718      *     by a Java method invocation conversion (JLS 5.3)
 719      *     on the primitive value.  (These are the primitive widening conversions.)
 720      *     <em>T0</em> must be a wrapper class or a supertype of one.
 721      *     (In the case where <em>T0</em> is Object, these are the conversions
 722      *     allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
 723      *     The unboxing conversion must have a possibility of success, which means that
 724      *     if <em>T0</em> is not itself a wrapper class, there must exist at least one
 725      *     wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
 726      *     primitive value can be widened to <em>T1</em>.
 727      * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
 728      * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
 729      * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
 730      *     a zero value is introduced.
 731      * </ul>
 732     * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
 733      * because neither corresponds specifically to the <em>dynamic type</em> of any
 734      * actual argument or return value.)
 735      * <p>
 736      * The method handle conversion cannot be made if any one of the required
 737      * pairwise conversions cannot be made.
 738      * <p>
 739      * At runtime, the conversions applied to reference arguments
 740      * or return values may require additional runtime checks which can fail.
 741      * An unboxing operation may fail because the original reference is null,
 742      * causing a {@link java.lang.NullPointerException NullPointerException}.
 743      * An unboxing operation or a reference cast may also fail on a reference
 744      * to an object of the wrong type,
 745      * causing a {@link java.lang.ClassCastException ClassCastException}.
 746      * Although an unboxing operation may accept several kinds of wrappers,
 747      * if none are available, a {@code ClassCastException} will be thrown.
 748      *
 749      * @param newType the expected type of the new method handle
 750      * @return a method handle which delegates to {@code this} after performing
 751      *           any necessary argument conversions, and arranges for any
 752      *           necessary return value conversions
 753      * @throws NullPointerException if {@code newType} is a null reference
 754      * @throws WrongMethodTypeException if the conversion cannot be made
 755      * @see MethodHandles#explicitCastArguments
 756      */
 757     public MethodHandle asType(MethodType newType) {
 758         // Fast path alternative to a heavyweight {@code asType} call.
 759         // Return 'this' if the conversion will be a no-op.
 760         if (newType == type) {
 761             return this;
 762         }
 763         // Return 'this.asTypeCache' if the conversion is already memoized.
 764         MethodHandle atc = asTypeCache;
 765         if (atc != null && newType == atc.type) {
 766             return atc;
 767         }
 768         return asTypeUncached(newType);
 769     }
 770 
 771     /** Override this to change asType behavior. */
 772     /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
 773         if (!type.isConvertibleTo(newType))
 774             throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
 775         return asTypeCache = convertArguments(newType);
 776     }
 777 
 778     /**
 779      * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
 780      * and spreads its elements as positional arguments.
 781      * The new method handle adapts, as its <i>target</i>,
 782      * the current method handle.  The type of the adapter will be
 783      * the same as the type of the target, except that the final
 784      * {@code arrayLength} parameters of the target's type are replaced
 785      * by a single array parameter of type {@code arrayType}.
 786      * <p>
 787      * If the array element type differs from any of the corresponding
 788      * argument types on the original target,
 789      * the original target is adapted to take the array elements directly,
 790      * as if by a call to {@link #asType asType}.
 791      * <p>
 792      * When called, the adapter replaces a trailing array argument
 793      * by the array's elements, each as its own argument to the target.
 794      * (The order of the arguments is preserved.)
 795      * They are converted pairwise by casting and/or unboxing
 796      * to the types of the trailing parameters of the target.
 797      * Finally the target is called.
 798      * What the target eventually returns is returned unchanged by the adapter.
 799      * <p>
 800      * Before calling the target, the adapter verifies that the array
 801      * contains exactly enough elements to provide a correct argument count
 802      * to the target method handle.
 803      * (The array may also be null when zero elements are required.)
 804      * <p>
 805      * If, when the adapter is called, the supplied array argument does
 806      * not have the correct number of elements, the adapter will throw
 807      * an {@link IllegalArgumentException} instead of invoking the target.
 808      * <p>
 809      * Here are some simple examples of array-spreading method handles:
 810      * <blockquote><pre>{@code
 811 MethodHandle equals = publicLookup()
 812   .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
 813 assert( (boolean) equals.invokeExact("me", (Object)"me"));
 814 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
 815 // spread both arguments from a 2-array:
 816 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
 817 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
 818 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
 819 // try to spread from anything but a 2-array:
 820 for (int n = 0; n <= 10; n++) {
 821   Object[] badArityArgs = (n == 2 ? null : new Object[n]);
 822   try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
 823   catch (IllegalArgumentException ex) { } // OK
 824 }
 825 // spread both arguments from a String array:
 826 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
 827 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
 828 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
 829 // spread second arguments from a 1-array:
 830 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
 831 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
 832 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
 833 // spread no arguments from a 0-array or null:
 834 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
 835 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
 836 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
 837 // asSpreader and asCollector are approximate inverses:
 838 for (int n = 0; n <= 2; n++) {
 839     for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
 840         MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
 841         assert( (boolean) equals2.invokeWithArguments("me", "me"));
 842         assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
 843     }
 844 }
 845 MethodHandle caToString = publicLookup()
 846   .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
 847 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
 848 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
 849 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
 850 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
 851 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
 852      * }</pre></blockquote>
 853      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
 854      * @param arrayLength the number of arguments to spread from an incoming array argument
 855      * @return a new method handle which spreads its final array argument,
 856      *         before calling the original method handle
 857      * @throws NullPointerException if {@code arrayType} is a null reference
 858      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
 859      *         or if target does not have at least
 860      *         {@code arrayLength} parameter types,
 861      *         or if {@code arrayLength} is negative,
 862      *         or if the resulting method handle's type would have
 863      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 864      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 865      * @see #asCollector
 866      */
 867     public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
 868         asSpreaderChecks(arrayType, arrayLength);
 869         int spreadArgPos = type.parameterCount() - arrayLength;
 870         return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength);
 871     }
 872 
 873     private void asSpreaderChecks(Class<?> arrayType, int arrayLength) {
 874         spreadArrayChecks(arrayType, arrayLength);
 875         int nargs = type().parameterCount();
 876         if (nargs < arrayLength || arrayLength < 0)
 877             throw newIllegalArgumentException("bad spread array length");
 878         if (arrayType != Object[].class && arrayLength != 0) {
 879             boolean sawProblem = false;
 880             Class<?> arrayElement = arrayType.getComponentType();
 881             for (int i = nargs - arrayLength; i < nargs; i++) {
 882                 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) {
 883                     sawProblem = true;
 884                     break;
 885                 }
 886             }
 887             if (sawProblem) {
 888                 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList());
 889                 for (int i = nargs - arrayLength; i < nargs; i++) {
 890                     ptypes.set(i, arrayElement);
 891                 }
 892                 // elicit an error:
 893                 this.asType(MethodType.methodType(type().returnType(), ptypes));
 894             }
 895         }
 896     }
 897 
 898     private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
 899         Class<?> arrayElement = arrayType.getComponentType();
 900         if (arrayElement == null)
 901             throw newIllegalArgumentException("not an array type", arrayType);
 902         if ((arrayLength & 0x7F) != arrayLength) {
 903             if ((arrayLength & 0xFF) != arrayLength)
 904                 throw newIllegalArgumentException("array length is not legal", arrayLength);
 905             assert(arrayLength >= 128);
 906             if (arrayElement == long.class ||
 907                 arrayElement == double.class)
 908                 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
 909         }
 910     }
 911 
 912     /**
 913      * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
 914      * positional arguments and collects them into an array argument.
 915      * The new method handle adapts, as its <i>target</i>,
 916      * the current method handle.  The type of the adapter will be
 917      * the same as the type of the target, except that a single trailing
 918      * parameter (usually of type {@code arrayType}) is replaced by
 919      * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
 920      * <p>
 921      * If the array type differs from the final argument type on the original target,
 922      * the original target is adapted to take the array type directly,
 923      * as if by a call to {@link #asType asType}.
 924      * <p>
 925      * When called, the adapter replaces its trailing {@code arrayLength}
 926      * arguments by a single new array of type {@code arrayType}, whose elements
 927      * comprise (in order) the replaced arguments.
 928      * Finally the target is called.
 929      * What the target eventually returns is returned unchanged by the adapter.
 930      * <p>
 931      * (The array may also be a shared constant when {@code arrayLength} is zero.)
 932      * <p>
 933      * (<em>Note:</em> The {@code arrayType} is often identical to the last
 934      * parameter type of the original target.
 935      * It is an explicit argument for symmetry with {@code asSpreader}, and also
 936      * to allow the target to use a simple {@code Object} as its last parameter type.)
 937      * <p>
 938      * In order to create a collecting adapter which is not restricted to a particular
 939      * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
 940      * <p>
 941      * Here are some examples of array-collecting method handles:
 942      * <blockquote><pre>{@code
 943 MethodHandle deepToString = publicLookup()
 944   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
 945 assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
 946 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
 947 assertEquals(methodType(String.class, Object.class), ts1.type());
 948 //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
 949 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
 950 // arrayType can be a subtype of Object[]
 951 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
 952 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
 953 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
 954 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
 955 assertEquals("[]", (String) ts0.invokeExact());
 956 // collectors can be nested, Lisp-style
 957 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
 958 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
 959 // arrayType can be any primitive array type
 960 MethodHandle bytesToString = publicLookup()
 961   .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
 962   .asCollector(byte[].class, 3);
 963 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
 964 MethodHandle longsToString = publicLookup()
 965   .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
 966   .asCollector(long[].class, 1);
 967 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
 968      * }</pre></blockquote>
 969      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
 970      * @param arrayLength the number of arguments to collect into a new array argument
 971      * @return a new method handle which collects some trailing argument
 972      *         into an array, before calling the original method handle
 973      * @throws NullPointerException if {@code arrayType} is a null reference
 974      * @throws IllegalArgumentException if {@code arrayType} is not an array type
 975      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type,
 976      *         or {@code arrayLength} is not a legal array size,
 977      *         or the resulting method handle's type would have
 978      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 979      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 980      * @see #asSpreader
 981      * @see #asVarargsCollector
 982      */
 983     public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
 984         asCollectorChecks(arrayType, arrayLength);
 985         int collectArgPos = type().parameterCount()-1;
 986         MethodHandle target = this;
 987         if (arrayType != type().parameterType(collectArgPos))
 988             target = convertArguments(type().changeParameterType(collectArgPos, arrayType));
 989         MethodHandle collector = ValueConversions.varargsArray(arrayType, arrayLength);
 990         return MethodHandles.collectArguments(target, collectArgPos, collector);
 991     }
 992 
 993     // private API: return true if last param exactly matches arrayType
 994     private boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
 995         spreadArrayChecks(arrayType, arrayLength);
 996         int nargs = type().parameterCount();
 997         if (nargs != 0) {
 998             Class<?> lastParam = type().parameterType(nargs-1);
 999             if (lastParam == arrayType)  return true;
1000             if (lastParam.isAssignableFrom(arrayType))  return false;
1001         }
1002         throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1003     }
1004 
1005     /**
1006      * Makes a <em>variable arity</em> adapter which is able to accept
1007      * any number of trailing positional arguments and collect them
1008      * into an array argument.
1009      * <p>
1010      * The type and behavior of the adapter will be the same as
1011      * the type and behavior of the target, except that certain
1012      * {@code invoke} and {@code asType} requests can lead to
1013      * trailing positional arguments being collected into target's
1014      * trailing parameter.
1015      * Also, the last parameter type of the adapter will be
1016      * {@code arrayType}, even if the target has a different
1017      * last parameter type.
1018      * <p>
1019      * This transformation may return {@code this} if the method handle is
1020      * already of variable arity and its trailing parameter type
1021      * is identical to {@code arrayType}.
1022      * <p>
1023      * When called with {@link #invokeExact invokeExact}, the adapter invokes
1024      * the target with no argument changes.
1025      * (<em>Note:</em> This behavior is different from a
1026      * {@linkplain #asCollector fixed arity collector},
1027      * since it accepts a whole array of indeterminate length,
1028      * rather than a fixed number of arguments.)
1029      * <p>
1030      * When called with plain, inexact {@link #invoke invoke}, if the caller
1031      * type is the same as the adapter, the adapter invokes the target as with
1032      * {@code invokeExact}.
1033      * (This is the normal behavior for {@code invoke} when types match.)
1034      * <p>
1035      * Otherwise, if the caller and adapter arity are the same, and the
1036      * trailing parameter type of the caller is a reference type identical to
1037      * or assignable to the trailing parameter type of the adapter,
1038      * the arguments and return values are converted pairwise,
1039      * as if by {@link #asType asType} on a fixed arity
1040      * method handle.
1041      * <p>
1042      * Otherwise, the arities differ, or the adapter's trailing parameter
1043      * type is not assignable from the corresponding caller type.
1044      * In this case, the adapter replaces all trailing arguments from
1045      * the original trailing argument position onward, by
1046      * a new array of type {@code arrayType}, whose elements
1047      * comprise (in order) the replaced arguments.
1048      * <p>
1049      * The caller type must provides as least enough arguments,
1050      * and of the correct type, to satisfy the target's requirement for
1051      * positional arguments before the trailing array argument.
1052      * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1053      * where {@code N} is the arity of the target.
1054      * Also, there must exist conversions from the incoming arguments
1055      * to the target's arguments.
1056      * As with other uses of plain {@code invoke}, if these basic
1057      * requirements are not fulfilled, a {@code WrongMethodTypeException}
1058      * may be thrown.
1059      * <p>
1060      * In all cases, what the target eventually returns is returned unchanged by the adapter.
1061      * <p>
1062      * In the final case, it is exactly as if the target method handle were
1063      * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1064      * to the arity required by the caller type.
1065      * (As with {@code asCollector}, if the array length is zero,
1066      * a shared constant may be used instead of a new array.
1067      * If the implied call to {@code asCollector} would throw
1068      * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1069      * the call to the variable arity adapter must throw
1070      * {@code WrongMethodTypeException}.)
1071      * <p>
1072      * The behavior of {@link #asType asType} is also specialized for
1073      * variable arity adapters, to maintain the invariant that
1074      * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1075      * call to adjust the target type, followed by {@code invokeExact}.
1076      * Therefore, a variable arity adapter responds
1077      * to an {@code asType} request by building a fixed arity collector,
1078      * if and only if the adapter and requested type differ either
1079      * in arity or trailing argument type.
1080      * The resulting fixed arity collector has its type further adjusted
1081      * (if necessary) to the requested type by pairwise conversion,
1082      * as if by another application of {@code asType}.
1083      * <p>
1084      * When a method handle is obtained by executing an {@code ldc} instruction
1085      * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1086      * as a variable arity method (with the modifier bit {@code 0x0080}),
1087      * the method handle will accept multiple arities, as if the method handle
1088      * constant were created by means of a call to {@code asVarargsCollector}.
1089      * <p>
1090      * In order to create a collecting adapter which collects a predetermined
1091      * number of arguments, and whose type reflects this predetermined number,
1092      * use {@link #asCollector asCollector} instead.
1093      * <p>
1094      * No method handle transformations produce new method handles with
1095      * variable arity, unless they are documented as doing so.
1096      * Therefore, besides {@code asVarargsCollector},
1097      * all methods in {@code MethodHandle} and {@code MethodHandles}
1098      * will return a method handle with fixed arity,
1099      * except in the cases where they are specified to return their original
1100      * operand (e.g., {@code asType} of the method handle's own type).
1101      * <p>
1102      * Calling {@code asVarargsCollector} on a method handle which is already
1103      * of variable arity will produce a method handle with the same type and behavior.
1104      * It may (or may not) return the original variable arity method handle.
1105      * <p>
1106      * Here is an example, of a list-making variable arity method handle:
1107      * <blockquote><pre>{@code
1108 MethodHandle deepToString = publicLookup()
1109   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1110 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1111 assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
1112 assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
1113 assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
1114 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1115 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1116 MethodHandle asList = publicLookup()
1117   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1118 assertEquals(methodType(List.class, Object[].class), asList.type());
1119 assert(asList.isVarargsCollector());
1120 assertEquals("[]", asList.invoke().toString());
1121 assertEquals("[1]", asList.invoke(1).toString());
1122 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1123 String[] argv = { "three", "thee", "tee" };
1124 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1125 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1126 List ls = (List) asList.invoke((Object)argv);
1127 assertEquals(1, ls.size());
1128 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1129      * }</pre></blockquote>
1130      * <p style="font-size:smaller;">
1131      * <em>Discussion:</em>
1132      * These rules are designed as a dynamically-typed variation
1133      * of the Java rules for variable arity methods.
1134      * In both cases, callers to a variable arity method or method handle
1135      * can either pass zero or more positional arguments, or else pass
1136      * pre-collected arrays of any length.  Users should be aware of the
1137      * special role of the final argument, and of the effect of a
1138      * type match on that final argument, which determines whether
1139      * or not a single trailing argument is interpreted as a whole
1140      * array or a single element of an array to be collected.
1141      * Note that the dynamic type of the trailing argument has no
1142      * effect on this decision, only a comparison between the symbolic
1143      * type descriptor of the call site and the type descriptor of the method handle.)
1144      *
1145      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1146      * @return a new method handle which can collect any number of trailing arguments
1147      *         into an array, before calling the original method handle
1148      * @throws NullPointerException if {@code arrayType} is a null reference
1149      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1150      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type
1151      * @see #asCollector
1152      * @see #isVarargsCollector
1153      * @see #asFixedArity
1154      */
1155     public MethodHandle asVarargsCollector(Class<?> arrayType) {
1156         Class<?> arrayElement = arrayType.getComponentType();
1157         boolean lastMatch = asCollectorChecks(arrayType, 0);
1158         if (isVarargsCollector() && lastMatch)
1159             return this;
1160         return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1161     }
1162 
1163     /**
1164      * Determines if this method handle
1165      * supports {@linkplain #asVarargsCollector variable arity} calls.
1166      * Such method handles arise from the following sources:
1167      * <ul>
1168      * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1169      * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1170      *     which resolves to a variable arity Java method or constructor
1171      * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1172      *     which resolves to a variable arity Java method or constructor
1173      * </ul>
1174      * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1175      * @see #asVarargsCollector
1176      * @see #asFixedArity
1177      */
1178     public boolean isVarargsCollector() {
1179         return false;
1180     }
1181 
1182     /**
1183      * Makes a <em>fixed arity</em> method handle which is otherwise
1184      * equivalent to the current method handle.
1185      * <p>
1186      * If the current method handle is not of
1187      * {@linkplain #asVarargsCollector variable arity},
1188      * the current method handle is returned.
1189      * This is true even if the current method handle
1190      * could not be a valid input to {@code asVarargsCollector}.
1191      * <p>
1192      * Otherwise, the resulting fixed-arity method handle has the same
1193      * type and behavior of the current method handle,
1194      * except that {@link #isVarargsCollector isVarargsCollector}
1195      * will be false.
1196      * The fixed-arity method handle may (or may not) be the
1197      * a previous argument to {@code asVarargsCollector}.
1198      * <p>
1199      * Here is an example, of a list-making variable arity method handle:
1200      * <blockquote><pre>{@code
1201 MethodHandle asListVar = publicLookup()
1202   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1203   .asVarargsCollector(Object[].class);
1204 MethodHandle asListFix = asListVar.asFixedArity();
1205 assertEquals("[1]", asListVar.invoke(1).toString());
1206 Exception caught = null;
1207 try { asListFix.invoke((Object)1); }
1208 catch (Exception ex) { caught = ex; }
1209 assert(caught instanceof ClassCastException);
1210 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1211 try { asListFix.invoke("two", "too"); }
1212 catch (Exception ex) { caught = ex; }
1213 assert(caught instanceof WrongMethodTypeException);
1214 Object[] argv = { "three", "thee", "tee" };
1215 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1216 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1217 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1218 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1219      * }</pre></blockquote>
1220      *
1221      * @return a new method handle which accepts only a fixed number of arguments
1222      * @see #asVarargsCollector
1223      * @see #isVarargsCollector
1224      */
1225     public MethodHandle asFixedArity() {
1226         assert(!isVarargsCollector());
1227         return this;
1228     }
1229 
1230     /**
1231      * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1232      * The new method handle adapts, as its <i>target</i>,
1233      * the current method handle by binding it to the given argument.
1234      * The type of the bound handle will be
1235      * the same as the type of the target, except that a single leading
1236      * reference parameter will be omitted.
1237      * <p>
1238      * When called, the bound handle inserts the given value {@code x}
1239      * as a new leading argument to the target.  The other arguments are
1240      * also passed unchanged.
1241      * What the target eventually returns is returned unchanged by the bound handle.
1242      * <p>
1243      * The reference {@code x} must be convertible to the first parameter
1244      * type of the target.
1245      * <p>
1246      * (<em>Note:</em>  Because method handles are immutable, the target method handle
1247      * retains its original type and behavior.)
1248      * @param x  the value to bind to the first argument of the target
1249      * @return a new method handle which prepends the given value to the incoming
1250      *         argument list, before calling the original method handle
1251      * @throws IllegalArgumentException if the target does not have a
1252      *         leading parameter type that is a reference type
1253      * @throws ClassCastException if {@code x} cannot be converted
1254      *         to the leading parameter type of the target
1255      * @see MethodHandles#insertArguments
1256      */
1257     public MethodHandle bindTo(Object x) {
1258         Class<?> ptype;
1259         @SuppressWarnings("LocalVariableHidesMemberVariable")
1260         MethodType type = type();
1261         if (type.parameterCount() == 0 ||
1262             (ptype = type.parameterType(0)).isPrimitive())
1263             throw newIllegalArgumentException("no leading reference parameter", x);
1264         x = ptype.cast(x);  // throw CCE if needed
1265         return bindReceiver(x);
1266     }
1267 
1268     /**
1269      * Returns a string representation of the method handle,
1270      * starting with the string {@code "MethodHandle"} and
1271      * ending with the string representation of the method handle's type.
1272      * In other words, this method returns a string equal to the value of:
1273      * <blockquote><pre>{@code
1274      * "MethodHandle" + type().toString()
1275      * }</pre></blockquote>
1276      * <p>
1277      * (<em>Note:</em>  Future releases of this API may add further information
1278      * to the string representation.
1279      * Therefore, the present syntax should not be parsed by applications.)
1280      *
1281      * @return a string representation of the method handle
1282      */
1283     @Override
1284     public String toString() {
1285         if (DEBUG_METHOD_HANDLE_NAMES)  return debugString();
1286         return standardString();
1287     }
1288     String standardString() {
1289         return "MethodHandle"+type;
1290     }
1291     String debugString() {
1292         return standardString()+"/LF="+internalForm()+internalProperties();
1293     }
1294 
1295     //// Implementation methods.
1296     //// Sub-classes can override these default implementations.
1297     //// All these methods assume arguments are already validated.
1298 
1299     // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch
1300 
1301     /*non-public*/
1302     MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1303         if (!member.isVarargs())  return this;
1304         int argc = type().parameterCount();
1305         if (argc != 0) {
1306             Class<?> arrayType = type().parameterType(argc-1);
1307             if (arrayType.isArray()) {
1308                 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1309             }
1310         }
1311         throw member.makeAccessException("cannot make variable arity", null);
1312     }
1313     /*non-public*/
1314     MethodHandle viewAsType(MethodType newType) {
1315         // No actual conversions, just a new view of the same method.
1316         return MethodHandleImpl.makePairwiseConvert(this, newType, 0);
1317     }
1318 
1319     // Decoding
1320 
1321     /*non-public*/
1322     LambdaForm internalForm() {
1323         return form;
1324     }
1325 
1326     /*non-public*/
1327     MemberName internalMemberName() {
1328         return null;  // DMH returns DMH.member
1329     }
1330 
1331     /*non-public*/
1332     Class<?> internalCallerClass() {
1333         return null;  // caller-bound MH for @CallerSensitive method returns caller
1334     }
1335 
1336     /*non-public*/
1337     MethodHandle withInternalMemberName(MemberName member) {
1338         if (member != null) {
1339             return MethodHandleImpl.makeWrappedMember(this, member);
1340         } else if (internalMemberName() == null) {
1341             // The required internaMemberName is null, and this MH (like most) doesn't have one.
1342             return this;
1343         } else {
1344             // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1345             MethodHandle result = rebind();
1346             assert (result.internalMemberName() == null);
1347             return result;
1348         }
1349     }
1350 
1351     /*non-public*/
1352     boolean isInvokeSpecial() {
1353         return false;  // DMH.Special returns true
1354     }
1355 
1356     /*non-public*/
1357     Object internalValues() {
1358         return null;
1359     }
1360 
1361     /*non-public*/
1362     Object internalProperties() {
1363         // Override to something like "/FOO=bar"
1364         return "";
1365     }
1366 
1367     //// Method handle implementation methods.
1368     //// Sub-classes can override these default implementations.
1369     //// All these methods assume arguments are already validated.
1370 
1371     /*non-public*/ MethodHandle convertArguments(MethodType newType) {
1372         // Override this if it can be improved.
1373         return MethodHandleImpl.makePairwiseConvert(this, newType, 1);
1374     }
1375 
1376     /*non-public*/
1377     MethodHandle bindArgument(int pos, char basicType, Object value) {
1378         // Override this if it can be improved.
1379         return rebind().bindArgument(pos, basicType, value);
1380     }
1381 
1382     /*non-public*/
1383     MethodHandle bindReceiver(Object receiver) {
1384         // Override this if it can be improved.
1385         return bindArgument(0, 'L', receiver);
1386     }
1387 
1388     /*non-public*/
1389     MethodHandle bindImmediate(int pos, char basicType, Object value) {
1390         // Bind an immediate value to a position in the arguments.
1391         // This means, elide the respective argument,
1392         // and replace all references to it in NamedFunction args with the specified value.
1393 
1394         // CURRENT RESTRICTIONS
1395         // * only for pos 0 and UNSAFE (position is adjusted in MHImpl to make API usable for others)
1396         assert pos == 0 && basicType == 'L' && value instanceof Unsafe;
1397         MethodType type2 = type.dropParameterTypes(pos, pos + 1); // adjustment: ignore receiver!
1398         LambdaForm form2 = form.bindImmediate(pos + 1, basicType, value); // adjust pos to form-relative pos
1399         return copyWith(type2, form2);
1400     }
1401 
1402     /*non-public*/
1403     MethodHandle copyWith(MethodType mt, LambdaForm lf) {
1404         throw new InternalError("copyWith: " + this.getClass());
1405     }
1406 
1407     /*non-public*/
1408     MethodHandle dropArguments(MethodType srcType, int pos, int drops) {
1409         // Override this if it can be improved.
1410         return rebind().dropArguments(srcType, pos, drops);
1411     }
1412 
1413     /*non-public*/
1414     MethodHandle permuteArguments(MethodType newType, int[] reorder) {
1415         // Override this if it can be improved.
1416         return rebind().permuteArguments(newType, reorder);
1417     }
1418 
1419     /*non-public*/
1420     MethodHandle rebind() {
1421         // Bind 'this' into a new invoker, of the known class BMH.
1422         MethodType type2 = type();
1423         LambdaForm form2 = reinvokerForm(this);
1424         // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) }
1425         return BoundMethodHandle.bindSingle(type2, form2, this);
1426     }
1427 
1428     /*non-public*/
1429     MethodHandle reinvokerTarget() {
1430         throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this);
1431     }
1432 
1433     /** Create a LF which simply reinvokes a target of the given basic type.
1434      *  The target MH must override {@link #reinvokerTarget} to provide the target.
1435      */
1436     static LambdaForm reinvokerForm(MethodHandle target) {
1437         MethodType mtype = target.type().basicType();
1438         LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE);
1439         if (reinvoker != null)  return reinvoker;
1440         if (mtype.parameterSlotCount() >= MethodType.MAX_MH_ARITY)
1441             return makeReinvokerForm(target.type(), target);  // cannot cache this
1442         reinvoker = makeReinvokerForm(mtype, null);
1443         return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, reinvoker);
1444     }
1445     private static LambdaForm makeReinvokerForm(MethodType mtype, MethodHandle customTargetOrNull) {
1446         boolean customized = (customTargetOrNull != null);
1447         MethodHandle MH_invokeBasic = customized ? null : MethodHandles.basicInvoker(mtype);
1448         final int THIS_BMH    = 0;
1449         final int ARG_BASE    = 1;
1450         final int ARG_LIMIT   = ARG_BASE + mtype.parameterCount();
1451         int nameCursor = ARG_LIMIT;
1452         final int NEXT_MH     = customized ? -1 : nameCursor++;
1453         final int REINVOKE    = nameCursor++;
1454         LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType());
1455         Object[] targetArgs;
1456         MethodHandle targetMH;
1457         if (customized) {
1458             targetArgs = Arrays.copyOfRange(names, ARG_BASE, ARG_LIMIT, Object[].class);
1459             targetMH = customTargetOrNull;
1460         } else {
1461             names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]);
1462             targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class);
1463             targetArgs[0] = names[NEXT_MH];  // overwrite this MH with next MH
1464             targetMH = MethodHandles.basicInvoker(mtype);
1465         }
1466         names[REINVOKE] = new LambdaForm.Name(targetMH, targetArgs);
1467         return new LambdaForm("BMH.reinvoke", ARG_LIMIT, names);
1468     }
1469 
1470     private static final LambdaForm.NamedFunction NF_reinvokerTarget;
1471     static {
1472         try {
1473             NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class
1474                 .getDeclaredMethod("reinvokerTarget"));
1475         } catch (ReflectiveOperationException ex) {
1476             throw newInternalError(ex);
1477         }
1478     }
1479 
1480     /**
1481      * Replace the old lambda form of this method handle with a new one.
1482      * The new one must be functionally equivalent to the old one.
1483      * Threads may continue running the old form indefinitely,
1484      * but it is likely that the new one will be preferred for new executions.
1485      * Use with discretion.
1486      */
1487     /*non-public*/
1488     void updateForm(LambdaForm newForm) {
1489         if (form == newForm)  return;
1490         // ISSUE: Should we have a memory fence here?
1491         UNSAFE.putObject(this, FORM_OFFSET, newForm);
1492         this.form.prepare();  // as in MethodHandle.<init>
1493     }
1494 
1495     private static final long FORM_OFFSET;
1496     static {
1497         try {
1498             FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1499         } catch (ReflectiveOperationException ex) {
1500             throw newInternalError(ex);
1501         }
1502     }
1503 }