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