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   8  * particular file as subject to the "Classpath" exception as provided
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  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 jdk.internal.HotSpotIntrinsicCandidate;
  30 
  31 import java.util.Arrays;
  32 import java.util.Objects;
  33 
  34 import static java.lang.invoke.MethodHandleStatics.*;
  35 
  36 /**
  37  * A method handle is a typed, directly executable reference to an underlying method,
  38  * constructor, field, or similar low-level operation, with optional
  39  * transformations of arguments or return values.
  40  * These transformations are quite general, and include such patterns as
  41  * {@linkplain #asType conversion},
  42  * {@linkplain #bindTo insertion},
  43  * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
  44  * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
  45  *
  46  * <h1>Method handle contents</h1>
  47  * Method handles are dynamically and strongly typed according to their parameter and return types.
  48  * They are not distinguished by the name or the defining class of their underlying methods.
  49  * A method handle must be invoked using a symbolic type descriptor which matches
  50  * the method handle's own {@linkplain #type type descriptor}.
  51  * <p>
  52  * Every method handle reports its type descriptor via the {@link #type type} accessor.
  53  * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
  54  * whose structure is a series of classes, one of which is
  55  * the return type of the method (or {@code void.class} if none).
  56  * <p>
  57  * A method handle's type controls the types of invocations it accepts,
  58  * and the kinds of transformations that apply to it.
  59  * <p>
  60  * A method handle contains a pair of special invoker methods
  61  * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
  62  * Both invoker methods provide direct access to the method handle's
  63  * underlying method, constructor, field, or other operation,
  64  * as modified by transformations of arguments and return values.
  65  * Both invokers accept calls which exactly match the method handle's own type.
  66  * The plain, inexact invoker also accepts a range of other call types.
  67  * <p>
  68  * Method handles are immutable and have no visible state.
  69  * Of course, they can be bound to underlying methods or data which exhibit state.
  70  * With respect to the Java Memory Model, any method handle will behave
  71  * as if all of its (internal) fields are final variables.  This means that any method
  72  * handle made visible to the application will always be fully formed.
  73  * This is true even if the method handle is published through a shared
  74  * variable in a data race.
  75  * <p>
  76  * Method handles cannot be subclassed by the user.
  77  * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
  78  * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
  79  * operation.  The programmer should not draw conclusions about a method handle
  80  * from its specific class, as the method handle class hierarchy (if any)
  81  * may change from time to time or across implementations from different vendors.
  82  *
  83  * <h1>Method handle compilation</h1>
  84  * A Java method call expression naming {@code invokeExact} or {@code invoke}
  85  * can invoke a method handle from Java source code.
  86  * From the viewpoint of source code, these methods can take any arguments
  87  * and their result can be cast to any return type.
  88  * Formally this is accomplished by giving the invoker methods
  89  * {@code Object} return types and variable arity {@code Object} arguments,
  90  * but they have an additional quality called <em>signature polymorphism</em>
  91  * which connects this freedom of invocation directly to the JVM execution stack.
  92  * <p>
  93  * As is usual with virtual methods, source-level calls to {@code invokeExact}
  94  * and {@code invoke} compile to an {@code invokevirtual} instruction.
  95  * More unusually, the compiler must record the actual argument types,
  96  * and may not perform method invocation conversions on the arguments.
  97  * Instead, it must generate instructions that push them on the stack according
  98  * to their own unconverted types.  The method handle object itself is pushed on
  99  * the stack before the arguments.
 100  * The compiler then generates an {@code invokevirtual} instruction that invokes
 101  * the method handle with a symbolic type descriptor which describes the argument
 102  * and return types.
 103  * <p>
 104  * To issue a complete symbolic type descriptor, the compiler must also determine
 105  * the return type.  This is based on a cast on the method invocation expression,
 106  * if there is one, or else {@code Object} if the invocation is an expression,
 107  * or else {@code void} if the invocation is a statement.
 108  * The cast may be to a primitive type (but not {@code void}).
 109  * <p>
 110  * As a corner case, an uncasted {@code null} argument is given
 111  * a symbolic type descriptor of {@code java.lang.Void}.
 112  * The ambiguity with the type {@code Void} is harmless, since there are no references of type
 113  * {@code Void} except the null reference.
 114  *
 115  * <h1>Method handle invocation</h1>
 116  * The first time an {@code invokevirtual} instruction is executed
 117  * it is linked by symbolically resolving the names in the instruction
 118  * and verifying that the method call is statically legal.
 119  * This also holds for calls to {@code invokeExact} and {@code invoke}.
 120  * In this case, the symbolic type descriptor emitted by the compiler is checked for
 121  * correct syntax, and names it contains are resolved.
 122  * Thus, an {@code invokevirtual} instruction which invokes
 123  * a method handle will always link, as long
 124  * as the symbolic type descriptor is syntactically well-formed
 125  * and the types exist.
 126  * <p>
 127  * When the {@code invokevirtual} is executed after linking,
 128  * the receiving method handle's type is first checked by the JVM
 129  * to ensure that it matches the symbolic type descriptor.
 130  * If the type match fails, it means that the method which the
 131  * caller is invoking is not present on the individual
 132  * method handle being invoked.
 133  * <p>
 134  * In the case of {@code invokeExact}, the type descriptor of the invocation
 135  * (after resolving symbolic type names) must exactly match the method type
 136  * of the receiving method handle.
 137  * In the case of plain, inexact {@code invoke}, the resolved type descriptor
 138  * must be a valid argument to the receiver's {@link #asType asType} method.
 139  * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
 140  * <p>
 141  * After type matching, a call to {@code invokeExact} directly
 142  * and immediately invoke the method handle's underlying method
 143  * (or other behavior, as the case may be).
 144  * <p>
 145  * A call to plain {@code invoke} works the same as a call to
 146  * {@code invokeExact}, if the symbolic type descriptor specified by the caller
 147  * exactly matches the method handle's own type.
 148  * If there is a type mismatch, {@code invoke} attempts
 149  * to adjust the type of the receiving method handle,
 150  * as if by a call to {@link #asType asType},
 151  * to obtain an exactly invokable method handle {@code M2}.
 152  * This allows a more powerful negotiation of method type
 153  * between caller and callee.
 154  * <p>
 155  * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
 156  * and implementations are therefore not required to materialize it.)
 157  *
 158  * <h1>Invocation checking</h1>
 159  * In typical programs, method handle type matching will usually succeed.
 160  * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
 161  * either directly (in the case of {@code invokeExact}) or indirectly as if
 162  * by a failed call to {@code asType} (in the case of {@code invoke}).
 163  * <p>
 164  * Thus, a method type mismatch which might show up as a linkage error
 165  * in a statically typed program can show up as
 166  * a dynamic {@code WrongMethodTypeException}
 167  * in a program which uses method handles.
 168  * <p>
 169  * Because method types contain "live" {@code Class} objects,
 170  * method type matching takes into account both type names and class loaders.
 171  * Thus, even if a method handle {@code M} is created in one
 172  * class loader {@code L1} and used in another {@code L2},
 173  * method handle calls are type-safe, because the caller's symbolic type
 174  * descriptor, as resolved in {@code L2},
 175  * is matched against the original callee method's symbolic type descriptor,
 176  * as resolved in {@code L1}.
 177  * The resolution in {@code L1} happens when {@code M} is created
 178  * and its type is assigned, while the resolution in {@code L2} happens
 179  * when the {@code invokevirtual} instruction is linked.
 180  * <p>
 181  * Apart from type descriptor checks,
 182  * a method handle's capability to call its underlying method is unrestricted.
 183  * If a method handle is formed on a non-public method by a class
 184  * that has access to that method, the resulting handle can be used
 185  * in any place by any caller who receives a reference to it.
 186  * <p>
 187  * Unlike with the Core Reflection API, where access is checked every time
 188  * a reflective method is invoked,
 189  * method handle access checking is performed
 190  * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
 191  * In the case of {@code ldc} (see below), access checking is performed as part of linking
 192  * the constant pool entry underlying the constant method handle.
 193  * <p>
 194  * Thus, handles to non-public methods, or to methods in non-public classes,
 195  * should generally be kept secret.
 196  * They should not be passed to untrusted code unless their use from
 197  * the untrusted code would be harmless.
 198  *
 199  * <h1>Method handle creation</h1>
 200  * Java code can create a method handle that directly accesses
 201  * any method, constructor, or field that is accessible to that code.
 202  * This is done via a reflective, capability-based API called
 203  * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}.
 204  * For example, a static method handle can be obtained
 205  * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
 206  * There are also conversion methods from Core Reflection API objects,
 207  * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 208  * <p>
 209  * Like classes and strings, method handles that correspond to accessible
 210  * fields, methods, and constructors can also be represented directly
 211  * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
 212  * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
 213  * refers directly to an associated {@code CONSTANT_Methodref},
 214  * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
 215  * constant pool entry.
 216  * (For full details on method handle constants,
 217  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
 218  * <p>
 219  * Method handles produced by lookups or constant loads from methods or
 220  * constructors with the variable arity modifier bit ({@code 0x0080})
 221  * have a corresponding variable arity, as if they were defined with
 222  * the help of {@link #asVarargsCollector asVarargsCollector}
 223  * or {@link #withVarargs withVarargs}.
 224  * <p>
 225  * A method reference may refer either to a static or non-static method.
 226  * In the non-static case, the method handle type includes an explicit
 227  * receiver argument, prepended before any other arguments.
 228  * In the method handle's type, the initial receiver argument is typed
 229  * according to the class under which the method was initially requested.
 230  * (E.g., if a non-static method handle is obtained via {@code ldc},
 231  * the type of the receiver is the class named in the constant pool entry.)
 232  * <p>
 233  * Method handle constants are subject to the same link-time access checks
 234  * their corresponding bytecode instructions, and the {@code ldc} instruction
 235  * will throw corresponding linkage errors if the bytecode behaviors would
 236  * throw such errors.
 237  * <p>
 238  * As a corollary of this, access to protected members is restricted
 239  * to receivers only of the accessing class, or one of its subclasses,
 240  * and the accessing class must in turn be a subclass (or package sibling)
 241  * of the protected member's defining class.
 242  * If a method reference refers to a protected non-static method or field
 243  * of a class outside the current package, the receiver argument will
 244  * be narrowed to the type of the accessing class.
 245  * <p>
 246  * When a method handle to a virtual method is invoked, the method is
 247  * always looked up in the receiver (that is, the first argument).
 248  * <p>
 249  * A non-virtual method handle to a specific virtual method implementation
 250  * can also be created.  These do not perform virtual lookup based on
 251  * receiver type.  Such a method handle simulates the effect of
 252  * an {@code invokespecial} instruction to the same method.
 253  *
 254  * <h1>Usage examples</h1>
 255  * Here are some examples of usage:
 256  * <blockquote><pre>{@code
 257 Object x, y; String s; int i;
 258 MethodType mt; MethodHandle mh;
 259 MethodHandles.Lookup lookup = MethodHandles.lookup();
 260 // mt is (char,char)String
 261 mt = MethodType.methodType(String.class, char.class, char.class);
 262 mh = lookup.findVirtual(String.class, "replace", mt);
 263 s = (String) mh.invokeExact("daddy",'d','n');
 264 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
 265 assertEquals(s, "nanny");
 266 // weakly typed invocation (using MHs.invoke)
 267 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
 268 assertEquals(s, "savvy");
 269 // mt is (Object[])List
 270 mt = MethodType.methodType(java.util.List.class, Object[].class);
 271 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
 272 assert(mh.isVarargsCollector());
 273 x = mh.invoke("one", "two");
 274 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
 275 assertEquals(x, java.util.Arrays.asList("one","two"));
 276 // mt is (Object,Object,Object)Object
 277 mt = MethodType.genericMethodType(3);
 278 mh = mh.asType(mt);
 279 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
 280 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 281 assertEquals(x, java.util.Arrays.asList(1,2,3));
 282 // mt is ()int
 283 mt = MethodType.methodType(int.class);
 284 mh = lookup.findVirtual(java.util.List.class, "size", mt);
 285 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
 286 // invokeExact(Ljava/util/List;)I
 287 assert(i == 3);
 288 mt = MethodType.methodType(void.class, String.class);
 289 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
 290 mh.invokeExact(System.out, "Hello, world.");
 291 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
 292  * }</pre></blockquote>
 293  * Each of the above calls to {@code invokeExact} or plain {@code invoke}
 294  * generates a single invokevirtual instruction with
 295  * the symbolic type descriptor indicated in the following comment.
 296  * In these examples, the helper method {@code assertEquals} is assumed to
 297  * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
 298  * on its arguments, and asserts that the result is true.
 299  *
 300  * <h1>Exceptions</h1>
 301  * The methods {@code invokeExact} and {@code invoke} are declared
 302  * to throw {@link java.lang.Throwable Throwable},
 303  * which is to say that there is no static restriction on what a method handle
 304  * can throw.  Since the JVM does not distinguish between checked
 305  * and unchecked exceptions (other than by their class, of course),
 306  * there is no particular effect on bytecode shape from ascribing
 307  * checked exceptions to method handle invocations.  But in Java source
 308  * code, methods which perform method handle calls must either explicitly
 309  * throw {@code Throwable}, or else must catch all
 310  * throwables locally, rethrowing only those which are legal in the context,
 311  * and wrapping ones which are illegal.
 312  *
 313  * <h1><a id="sigpoly"></a>Signature polymorphism</h1>
 314  * The unusual compilation and linkage behavior of
 315  * {@code invokeExact} and plain {@code invoke}
 316  * is referenced by the term <em>signature polymorphism</em>.
 317  * As defined in the Java Language Specification,
 318  * a signature polymorphic method is one which can operate with
 319  * any of a wide range of call signatures and return types.
 320  * <p>
 321  * In source code, a call to a signature polymorphic method will
 322  * compile, regardless of the requested symbolic type descriptor.
 323  * As usual, the Java compiler emits an {@code invokevirtual}
 324  * instruction with the given symbolic type descriptor against the named method.
 325  * The unusual part is that the symbolic type descriptor is derived from
 326  * the actual argument and return types, not from the method declaration.
 327  * <p>
 328  * When the JVM processes bytecode containing signature polymorphic calls,
 329  * it will successfully link any such call, regardless of its symbolic type descriptor.
 330  * (In order to retain type safety, the JVM will guard such calls with suitable
 331  * dynamic type checks, as described elsewhere.)
 332  * <p>
 333  * Bytecode generators, including the compiler back end, are required to emit
 334  * untransformed symbolic type descriptors for these methods.
 335  * Tools which determine symbolic linkage are required to accept such
 336  * untransformed descriptors, without reporting linkage errors.
 337  *
 338  * <h1>Interoperation between method handles and the Core Reflection API</h1>
 339  * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
 340  * any class member represented by a Core Reflection API object
 341  * can be converted to a behaviorally equivalent method handle.
 342  * For example, a reflective {@link java.lang.reflect.Method Method} can
 343  * be converted to a method handle using
 344  * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 345  * The resulting method handles generally provide more direct and efficient
 346  * access to the underlying class members.
 347  * <p>
 348  * As a special case,
 349  * when the Core Reflection API is used to view the signature polymorphic
 350  * methods {@code invokeExact} or plain {@code invoke} in this class,
 351  * they appear as ordinary non-polymorphic methods.
 352  * Their reflective appearance, as viewed by
 353  * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
 354  * is unaffected by their special status in this API.
 355  * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
 356  * will report exactly those modifier bits required for any similarly
 357  * declared method, including in this case {@code native} and {@code varargs} bits.
 358  * <p>
 359  * As with any reflected method, these methods (when reflected) may be
 360  * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
 361  * However, such reflective calls do not result in method handle invocations.
 362  * Such a call, if passed the required argument
 363  * (a single one, of type {@code Object[]}), will ignore the argument and
 364  * will throw an {@code UnsupportedOperationException}.
 365  * <p>
 366  * Since {@code invokevirtual} instructions can natively
 367  * invoke method handles under any symbolic type descriptor, this reflective view conflicts
 368  * with the normal presentation of these methods via bytecodes.
 369  * Thus, these two native methods, when reflectively viewed by
 370  * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
 371  * <p>
 372  * In order to obtain an invoker method for a particular type descriptor,
 373  * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
 374  * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
 375  * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
 376  * API is also able to return a method handle
 377  * to call {@code invokeExact} or plain {@code invoke},
 378  * for any specified type descriptor .
 379  *
 380  * <h1>Interoperation between method handles and Java generics</h1>
 381  * A method handle can be obtained on a method, constructor, or field
 382  * which is declared with Java generic types.
 383  * As with the Core Reflection API, the type of the method handle
 384  * will constructed from the erasure of the source-level type.
 385  * When a method handle is invoked, the types of its arguments
 386  * or the return value cast type may be generic types or type instances.
 387  * If this occurs, the compiler will replace those
 388  * types by their erasures when it constructs the symbolic type descriptor
 389  * for the {@code invokevirtual} instruction.
 390  * <p>
 391  * Method handles do not represent
 392  * their function-like types in terms of Java parameterized (generic) types,
 393  * because there are three mismatches between function-like types and parameterized
 394  * Java types.
 395  * <ul>
 396  * <li>Method types range over all possible arities,
 397  * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
 398  * Generics are not variadic, and so cannot represent this.</li>
 399  * <li>Method types can specify arguments of primitive types,
 400  * which Java generic types cannot range over.</li>
 401  * <li>Higher order functions over method handles (combinators) are
 402  * often generic across a wide range of function types, including
 403  * those of multiple arities.  It is impossible to represent such
 404  * genericity with a Java type parameter.</li>
 405  * </ul>
 406  *
 407  * <h1><a id="maxarity"></a>Arity limits</h1>
 408  * The JVM imposes on all methods and constructors of any kind an absolute
 409  * limit of 255 stacked arguments.  This limit can appear more restrictive
 410  * in certain cases:
 411  * <ul>
 412  * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
 413  * <li>A non-static method consumes an extra argument for the object on which the method is called.
 414  * <li>A constructor consumes an extra argument for the object which is being constructed.
 415  * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
 416  *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
 417  * </ul>
 418  * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
 419  * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
 420  * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
 421  * In particular, a method handle&rsquo;s type must not have an arity of the exact maximum 255.
 422  *
 423  * @see MethodType
 424  * @see MethodHandles
 425  * @author John Rose, JSR 292 EG
 426  * @since 1.7
 427  */
 428 public abstract class MethodHandle {
 429 
 430     /**
 431      * Internal marker interface which distinguishes (to the Java compiler)
 432      * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
 433      */
 434     @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
 435     @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
 436     @interface PolymorphicSignature { }
 437 
 438     private final MethodType type;
 439     /*private*/ final LambdaForm form;
 440     // form is not private so that invokers can easily fetch it
 441     /*private*/ MethodHandle asTypeCache;
 442     // asTypeCache is not private so that invokers can easily fetch it
 443     /*non-public*/ byte customizationCount;
 444     // customizationCount should be accessible from invokers
 445 
 446     /**
 447      * Reports the type of this method handle.
 448      * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
 449      * @return the method handle type
 450      */
 451     public MethodType type() {
 452         return type;
 453     }
 454 
 455     /**
 456      * Package-private constructor for the method handle implementation hierarchy.
 457      * Method handle inheritance will be contained completely within
 458      * the {@code java.lang.invoke} package.
 459      */
 460     // @param type type (permanently assigned) of the new method handle
 461     /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
 462         this.type = Objects.requireNonNull(type);
 463         this.form = Objects.requireNonNull(form).uncustomize();
 464 
 465         this.form.prepare();  // TO DO:  Try to delay this step until just before invocation.
 466     }
 467 
 468     /**
 469      * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
 470      * The symbolic type descriptor at the call site of {@code invokeExact} must
 471      * exactly match this method handle's {@link #type type}.
 472      * No conversions are allowed on arguments or return values.
 473      * <p>
 474      * When this method is observed via the Core Reflection API,
 475      * it will appear as a single native method, taking an object array and returning an object.
 476      * If this native method is invoked directly via
 477      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 478      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 479      * it will throw an {@code UnsupportedOperationException}.
 480      * @param args the signature-polymorphic parameter list, statically represented using varargs
 481      * @return the signature-polymorphic result, statically represented using {@code Object}
 482      * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
 483      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 484      */
 485     @HotSpotIntrinsicCandidate
 486     public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
 487 
 488     /**
 489      * Invokes the method handle, allowing any caller type descriptor,
 490      * and optionally performing conversions on arguments and return values.
 491      * <p>
 492      * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
 493      * the call proceeds as if by {@link #invokeExact invokeExact}.
 494      * <p>
 495      * Otherwise, the call proceeds as if this method handle were first
 496      * adjusted by calling {@link #asType asType} to adjust this method handle
 497      * to the required type, and then the call proceeds as if by
 498      * {@link #invokeExact invokeExact} on the adjusted method handle.
 499      * <p>
 500      * There is no guarantee that the {@code asType} call is actually made.
 501      * If the JVM can predict the results of making the call, it may perform
 502      * adaptations directly on the caller's arguments,
 503      * and call the target method handle according to its own exact type.
 504      * <p>
 505      * The resolved type descriptor at the call site of {@code invoke} must
 506      * be a valid argument to the receivers {@code asType} method.
 507      * In particular, the caller must specify the same argument arity
 508      * as the callee's type,
 509      * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
 510      * <p>
 511      * When this method is observed via the Core Reflection API,
 512      * it will appear as a single native method, taking an object array and returning an object.
 513      * If this native method is invoked directly via
 514      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 515      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 516      * it will throw an {@code UnsupportedOperationException}.
 517      * @param args the signature-polymorphic parameter list, statically represented using varargs
 518      * @return the signature-polymorphic result, statically represented using {@code Object}
 519      * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
 520      * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
 521      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 522      */
 523     @HotSpotIntrinsicCandidate
 524     public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
 525 
 526     /**
 527      * Private method for trusted invocation of a method handle respecting simplified signatures.
 528      * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
 529      * <p>
 530      * The caller signature is restricted to the following basic types:
 531      * Object, int, long, float, double, and void return.
 532      * <p>
 533      * The caller is responsible for maintaining type correctness by ensuring
 534      * that the each outgoing argument value is a member of the range of the corresponding
 535      * callee argument type.
 536      * (The caller should therefore issue appropriate casts and integer narrowing
 537      * operations on outgoing argument values.)
 538      * The caller can assume that the incoming result value is part of the range
 539      * of the callee's return type.
 540      * @param args the signature-polymorphic parameter list, statically represented using varargs
 541      * @return the signature-polymorphic result, statically represented using {@code Object}
 542      */
 543     @HotSpotIntrinsicCandidate
 544     /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
 545 
 546     /**
 547      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
 548      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 549      * The trailing (not leading) argument must be a MemberName.
 550      * @param args the signature-polymorphic parameter list, statically represented using varargs
 551      * @return the signature-polymorphic result, statically represented using {@code Object}
 552      */
 553     @HotSpotIntrinsicCandidate
 554     /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
 555 
 556     /**
 557      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
 558      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 559      * The trailing (not leading) argument must be a MemberName.
 560      * @param args the signature-polymorphic parameter list, statically represented using varargs
 561      * @return the signature-polymorphic result, statically represented using {@code Object}
 562      */
 563     @HotSpotIntrinsicCandidate
 564     /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
 565 
 566     /**
 567      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
 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     @HotSpotIntrinsicCandidate
 574     /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
 575 
 576     /**
 577      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
 578      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 579      * The trailing (not leading) argument must be a MemberName.
 580      * @param args the signature-polymorphic parameter list, statically represented using varargs
 581      * @return the signature-polymorphic result, statically represented using {@code Object}
 582      */
 583     @HotSpotIntrinsicCandidate
 584     /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
 585 
 586     /**
 587      * Performs a variable arity invocation, passing the arguments in the given list
 588      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 589      * which mentions only the type {@code Object}, and whose arity is the length
 590      * of the argument list.
 591      * <p>
 592      * Specifically, execution proceeds as if by the following steps,
 593      * although the methods are not guaranteed to be called if the JVM
 594      * can predict their effects.
 595      * <ul>
 596      * <li>Determine the length of the argument array as {@code N}.
 597      *     For a null reference, {@code N=0}. </li>
 598      * <li>Determine the general type {@code TN} of {@code N} arguments as
 599      *     as {@code TN=MethodType.genericMethodType(N)}.</li>
 600      * <li>Force the original target method handle {@code MH0} to the
 601      *     required type, as {@code MH1 = MH0.asType(TN)}. </li>
 602      * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
 603      * <li>Invoke the type-adjusted method handle on the unpacked arguments:
 604      *     MH1.invokeExact(A0, ...). </li>
 605      * <li>Take the return value as an {@code Object} reference. </li>
 606      * </ul>
 607      * <p>
 608      * Because of the action of the {@code asType} step, the following argument
 609      * conversions are applied as necessary:
 610      * <ul>
 611      * <li>reference casting
 612      * <li>unboxing
 613      * <li>widening primitive conversions
 614      * </ul>
 615      * <p>
 616      * The result returned by the call is boxed if it is a primitive,
 617      * or forced to null if the return type is void.
 618      * <p>
 619      * This call is equivalent to the following code:
 620      * <blockquote><pre>{@code
 621      * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
 622      * Object result = invoker.invokeExact(this, arguments);
 623      * }</pre></blockquote>
 624      * <p>
 625      * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
 626      * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
 627      * It can therefore be used as a bridge between native or reflective code and method handles.
 628      *
 629      * @param arguments the arguments to pass to the target
 630      * @return the result returned by the target
 631      * @throws ClassCastException if an argument cannot be converted by reference casting
 632      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 633      * @throws Throwable anything thrown by the target method invocation
 634      * @see MethodHandles#spreadInvoker
 635      */
 636     public Object invokeWithArguments(Object... arguments) throws Throwable {
 637         MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length);
 638         return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments);
 639     }
 640 
 641     /**
 642      * Performs a variable arity invocation, passing the arguments in the given array
 643      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 644      * which mentions only the type {@code Object}, and whose arity is the length
 645      * of the argument array.
 646      * <p>
 647      * This method is also equivalent to the following code:
 648      * <blockquote><pre>{@code
 649      *   invokeWithArguments(arguments.toArray())
 650      * }</pre></blockquote>
 651      *
 652      * @param arguments the arguments to pass to the target
 653      * @return the result returned by the target
 654      * @throws NullPointerException if {@code arguments} is a null reference
 655      * @throws ClassCastException if an argument cannot be converted by reference casting
 656      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 657      * @throws Throwable anything thrown by the target method invocation
 658      */
 659     public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
 660         return invokeWithArguments(arguments.toArray());
 661     }
 662 
 663     /**
 664      * Produces an adapter method handle which adapts the type of the
 665      * current method handle to a new type.
 666      * The resulting method handle is guaranteed to report a type
 667      * which is equal to the desired new type.
 668      * <p>
 669      * If the original type and new type are equal, returns {@code this}.
 670      * <p>
 671      * The new method handle, when invoked, will perform the following
 672      * steps:
 673      * <ul>
 674      * <li>Convert the incoming argument list to match the original
 675      *     method handle's argument list.
 676      * <li>Invoke the original method handle on the converted argument list.
 677      * <li>Convert any result returned by the original method handle
 678      *     to the return type of new method handle.
 679      * </ul>
 680      * <p>
 681      * This method provides the crucial behavioral difference between
 682      * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
 683      * The two methods
 684      * perform the same steps when the caller's type descriptor exactly matches
 685      * the callee's, but when the types differ, plain {@link #invoke invoke}
 686      * also calls {@code asType} (or some internal equivalent) in order
 687      * to match up the caller's and callee's types.
 688      * <p>
 689      * If the current method is a variable arity method handle
 690      * argument list conversion may involve the conversion and collection
 691      * of several arguments into an array, as
 692      * {@linkplain #asVarargsCollector described elsewhere}.
 693      * In every other case, all conversions are applied <em>pairwise</em>,
 694      * which means that each argument or return value is converted to
 695      * exactly one argument or return value (or no return value).
 696      * The applied conversions are defined by consulting the
 697      * the corresponding component types of the old and new
 698      * method handle types.
 699      * <p>
 700      * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
 701      * or old and new return types.  Specifically, for some valid index {@code i}, let
 702      * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
 703      * Or else, going the other way for return values, let
 704      * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
 705      * If the types are the same, the new method handle makes no change
 706      * to the corresponding argument or return value (if any).
 707      * Otherwise, one of the following conversions is applied
 708      * if possible:
 709      * <ul>
 710      * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
 711      *     (The types do not need to be related in any particular way.
 712      *     This is because a dynamic value of null can convert to any reference type.)
 713      * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
 714      *     conversion (JLS 5.3) is applied, if one exists.
 715      *     (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
 716      * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
 717      *     a Java casting conversion (JLS 5.5) is applied if one exists.
 718      *     (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
 719      *     which is then widened as needed to <em>T1</em>.)
 720      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
 721      *     conversion will be applied at runtime, possibly followed
 722      *     by a Java method invocation conversion (JLS 5.3)
 723      *     on the primitive value.  (These are the primitive widening conversions.)
 724      *     <em>T0</em> must be a wrapper class or a supertype of one.
 725      *     (In the case where <em>T0</em> is Object, these are the conversions
 726      *     allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
 727      *     The unboxing conversion must have a possibility of success, which means that
 728      *     if <em>T0</em> is not itself a wrapper class, there must exist at least one
 729      *     wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
 730      *     primitive value can be widened to <em>T1</em>.
 731      * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
 732      * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
 733      * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
 734      *     a zero value is introduced.
 735      * </ul>
 736      * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
 737      * because neither corresponds specifically to the <em>dynamic type</em> of any
 738      * actual argument or return value.)
 739      * <p>
 740      * The method handle conversion cannot be made if any one of the required
 741      * pairwise conversions cannot be made.
 742      * <p>
 743      * At runtime, the conversions applied to reference arguments
 744      * or return values may require additional runtime checks which can fail.
 745      * An unboxing operation may fail because the original reference is null,
 746      * causing a {@link java.lang.NullPointerException NullPointerException}.
 747      * An unboxing operation or a reference cast may also fail on a reference
 748      * to an object of the wrong type,
 749      * causing a {@link java.lang.ClassCastException ClassCastException}.
 750      * Although an unboxing operation may accept several kinds of wrappers,
 751      * if none are available, a {@code ClassCastException} will be thrown.
 752      *
 753      * @param newType the expected type of the new method handle
 754      * @return a method handle which delegates to {@code this} after performing
 755      *           any necessary argument conversions, and arranges for any
 756      *           necessary return value conversions
 757      * @throws NullPointerException if {@code newType} is a null reference
 758      * @throws WrongMethodTypeException if the conversion cannot be made
 759      * @see MethodHandles#explicitCastArguments
 760      */
 761     public MethodHandle asType(MethodType newType) {
 762         // Fast path alternative to a heavyweight {@code asType} call.
 763         // Return 'this' if the conversion will be a no-op.
 764         if (newType == type) {
 765             return this;
 766         }
 767         // Return 'this.asTypeCache' if the conversion is already memoized.
 768         MethodHandle atc = asTypeCached(newType);
 769         if (atc != null) {
 770             return atc;
 771         }
 772         return asTypeUncached(newType);
 773     }
 774 
 775     private MethodHandle asTypeCached(MethodType newType) {
 776         MethodHandle atc = asTypeCache;
 777         if (atc != null && newType == atc.type) {
 778             return atc;
 779         }
 780         return null;
 781     }
 782 
 783     /** Override this to change asType behavior. */
 784     /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
 785         if (!type.isConvertibleTo(newType))
 786             throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
 787         return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
 788     }
 789 
 790     /**
 791      * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
 792      * and spreads its elements as positional arguments.
 793      * The new method handle adapts, as its <i>target</i>,
 794      * the current method handle.  The type of the adapter will be
 795      * the same as the type of the target, except that the final
 796      * {@code arrayLength} parameters of the target's type are replaced
 797      * by a single array parameter of type {@code arrayType}.
 798      * <p>
 799      * If the array element type differs from any of the corresponding
 800      * argument types on the original target,
 801      * the original target is adapted to take the array elements directly,
 802      * as if by a call to {@link #asType asType}.
 803      * <p>
 804      * When called, the adapter replaces a trailing array argument
 805      * by the array's elements, each as its own argument to the target.
 806      * (The order of the arguments is preserved.)
 807      * They are converted pairwise by casting and/or unboxing
 808      * to the types of the trailing parameters of the target.
 809      * Finally the target is called.
 810      * What the target eventually returns is returned unchanged by the adapter.
 811      * <p>
 812      * Before calling the target, the adapter verifies that the array
 813      * contains exactly enough elements to provide a correct argument count
 814      * to the target method handle.
 815      * (The array may also be null when zero elements are required.)
 816      * <p>
 817      * If, when the adapter is called, the supplied array argument does
 818      * not have the correct number of elements, the adapter will throw
 819      * an {@link IllegalArgumentException} instead of invoking the target.
 820      * <p>
 821      * Here are some simple examples of array-spreading method handles:
 822      * <blockquote><pre>{@code
 823 MethodHandle equals = publicLookup()
 824   .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
 825 assert( (boolean) equals.invokeExact("me", (Object)"me"));
 826 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
 827 // spread both arguments from a 2-array:
 828 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
 829 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
 830 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
 831 // try to spread from anything but a 2-array:
 832 for (int n = 0; n <= 10; n++) {
 833   Object[] badArityArgs = (n == 2 ? null : new Object[n]);
 834   try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
 835   catch (IllegalArgumentException ex) { } // OK
 836 }
 837 // spread both arguments from a String array:
 838 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
 839 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
 840 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
 841 // spread second arguments from a 1-array:
 842 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
 843 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
 844 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
 845 // spread no arguments from a 0-array or null:
 846 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
 847 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
 848 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
 849 // asSpreader and asCollector are approximate inverses:
 850 for (int n = 0; n <= 2; n++) {
 851     for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
 852         MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
 853         assert( (boolean) equals2.invokeWithArguments("me", "me"));
 854         assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
 855     }
 856 }
 857 MethodHandle caToString = publicLookup()
 858   .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
 859 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
 860 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
 861 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
 862 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
 863 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
 864      * }</pre></blockquote>
 865      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
 866      * @param arrayLength the number of arguments to spread from an incoming array argument
 867      * @return a new method handle which spreads its final array argument,
 868      *         before calling the original method handle
 869      * @throws NullPointerException if {@code arrayType} is a null reference
 870      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
 871      *         or if target does not have at least
 872      *         {@code arrayLength} parameter types,
 873      *         or if {@code arrayLength} is negative,
 874      *         or if the resulting method handle's type would have
 875      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 876      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 877      * @see #asCollector
 878      */
 879     public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
 880         return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength);
 881     }
 882 
 883     /**
 884      * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads
 885      * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>,
 886      * the current method handle. The type of the adapter will be the same as the type of the target, except that the
 887      * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos},
 888      * are replaced by a single array parameter of type {@code arrayType}.
 889      * <p>
 890      * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos}
 891      * argument to indicate at which position in the parameter list the spreading should take place.
 892      *
 893      * @apiNote Example:
 894      * <blockquote><pre>{@code
 895     MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class));
 896     MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2);
 897     Object[] ints = new Object[]{3, 9, 7, 7};
 898     Comparator<Integer> cmp = (a, b) -> a - b;
 899     assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0);
 900     assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0);
 901     assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0);
 902      * }</pre></blockquote>
 903      * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start.
 904      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
 905      * @param arrayLength the number of arguments to spread from an incoming array argument
 906      * @return a new method handle which spreads an array argument at a given position,
 907      *         before calling the original method handle
 908      * @throws NullPointerException if {@code arrayType} is a null reference
 909      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
 910      *         or if target does not have at least
 911      *         {@code arrayLength} parameter types,
 912      *         or if {@code arrayLength} is negative,
 913      *         or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the
 914      *         number of arguments),
 915      *         or if the resulting method handle's type would have
 916      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 917      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 918      *
 919      * @see #asSpreader(Class, int)
 920      * @since 9
 921      */
 922     public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) {
 923         MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength);
 924         MethodHandle afterSpread = this.asType(postSpreadType);
 925         BoundMethodHandle mh = afterSpread.rebind();
 926         LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
 927         MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType);
 928         return mh.copyWith(preSpreadType, lform);
 929     }
 930 
 931     /**
 932      * See if {@code asSpreader} can be validly called with the given arguments.
 933      * Return the type of the method handle call after spreading but before conversions.
 934      */
 935     private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) {
 936         spreadArrayChecks(arrayType, arrayLength);
 937         int nargs = type().parameterCount();
 938         if (nargs < arrayLength || arrayLength < 0)
 939             throw newIllegalArgumentException("bad spread array length");
 940         if (pos < 0 || pos + arrayLength > nargs) {
 941             throw newIllegalArgumentException("bad spread position");
 942         }
 943         Class<?> arrayElement = arrayType.getComponentType();
 944         MethodType mtype = type();
 945         boolean match = true, fail = false;
 946         for (int i = pos; i < pos + arrayLength; i++) {
 947             Class<?> ptype = mtype.parameterType(i);
 948             if (ptype != arrayElement) {
 949                 match = false;
 950                 if (!MethodType.canConvert(arrayElement, ptype)) {
 951                     fail = true;
 952                     break;
 953                 }
 954             }
 955         }
 956         if (match)  return mtype;
 957         MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength);
 958         if (!fail)  return needType;
 959         // elicit an error:
 960         this.asType(needType);
 961         throw newInternalError("should not return");
 962     }
 963 
 964     private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
 965         Class<?> arrayElement = arrayType.getComponentType();
 966         if (arrayElement == null)
 967             throw newIllegalArgumentException("not an array type", arrayType);
 968         if ((arrayLength & 0x7F) != arrayLength) {
 969             if ((arrayLength & 0xFF) != arrayLength)
 970                 throw newIllegalArgumentException("array length is not legal", arrayLength);
 971             assert(arrayLength >= 128);
 972             if (arrayElement == long.class ||
 973                 arrayElement == double.class)
 974                 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
 975         }
 976     }
 977     /**
 978       * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity}
 979       * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}.
 980       * If the method handle is already of the proper arity mode, it is returned
 981       * unchanged.
 982       * @apiNote
 983       * <p>This method is sometimes useful when adapting a method handle that
 984       * may be variable arity, to ensure that the resulting adapter is also
 985       * variable arity if and only if the original handle was.  For example,
 986       * this code changes the first argument of a handle {@code mh} to {@code int} without
 987       * disturbing its variable arity property:
 988       * {@code mh.asType(mh.type().changeParameterType(0,int.class))
 989       *     .withVarargs(mh.isVarargsCollector())}
 990       * @param makeVarargs true if the return method handle should have variable arity behavior
 991       * @return a method handle of the same type, with possibly adjusted variable arity behavior
 992       * @throws IllegalArgumentException if {@code makeVarargs} is true and
 993       *         this method handle does not have a trailing array parameter
 994       * @since 9
 995       * @see #asVarargsCollector
 996       * @see #asFixedArity
 997      */
 998      public MethodHandle withVarargs(boolean makeVarargs) {
 999         if (!makeVarargs) {
1000             return asFixedArity();
1001         } else if (!isVarargsCollector()) {
1002             return asVarargsCollector(type().lastParameterType());
1003         } else {
1004             return this;
1005         }
1006     }
1007 
1008     /**
1009      * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
1010      * positional arguments and collects them into an array argument.
1011      * The new method handle adapts, as its <i>target</i>,
1012      * the current method handle.  The type of the adapter will be
1013      * the same as the type of the target, except that a single trailing
1014      * parameter (usually of type {@code arrayType}) is replaced by
1015      * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1016      * <p>
1017      * If the array type differs from the final argument type on the original target,
1018      * the original target is adapted to take the array type directly,
1019      * as if by a call to {@link #asType asType}.
1020      * <p>
1021      * When called, the adapter replaces its trailing {@code arrayLength}
1022      * arguments by a single new array of type {@code arrayType}, whose elements
1023      * comprise (in order) the replaced arguments.
1024      * Finally the target is called.
1025      * What the target eventually returns is returned unchanged by the adapter.
1026      * <p>
1027      * (The array may also be a shared constant when {@code arrayLength} is zero.)
1028      * <p>
1029      * (<em>Note:</em> The {@code arrayType} is often identical to the last
1030      * parameter type of the original target.
1031      * It is an explicit argument for symmetry with {@code asSpreader}, and also
1032      * to allow the target to use a simple {@code Object} as its last parameter type.)
1033      * <p>
1034      * In order to create a collecting adapter which is not restricted to a particular
1035      * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector}
1036      * or {@link #withVarargs withVarargs} instead.
1037      * <p>
1038      * Here are some examples of array-collecting method handles:
1039      * <blockquote><pre>{@code
1040 MethodHandle deepToString = publicLookup()
1041   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1042 assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
1043 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
1044 assertEquals(methodType(String.class, Object.class), ts1.type());
1045 //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
1046 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
1047 // arrayType can be a subtype of Object[]
1048 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
1049 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
1050 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
1051 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
1052 assertEquals("[]", (String) ts0.invokeExact());
1053 // collectors can be nested, Lisp-style
1054 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
1055 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
1056 // arrayType can be any primitive array type
1057 MethodHandle bytesToString = publicLookup()
1058   .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
1059   .asCollector(byte[].class, 3);
1060 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
1061 MethodHandle longsToString = publicLookup()
1062   .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
1063   .asCollector(long[].class, 1);
1064 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
1065      * }</pre></blockquote>
1066      * <p>
1067      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1068      * variable-arity method handle}, even if the original target method handle was.
1069      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1070      * @param arrayLength the number of arguments to collect into a new array argument
1071      * @return a new method handle which collects some trailing argument
1072      *         into an array, before calling the original method handle
1073      * @throws NullPointerException if {@code arrayType} is a null reference
1074      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1075      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type,
1076      *         or {@code arrayLength} is not a legal array size,
1077      *         or the resulting method handle's type would have
1078      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
1079      * @throws WrongMethodTypeException if the implied {@code asType} call fails
1080      * @see #asSpreader
1081      * @see #asVarargsCollector
1082      */
1083     public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1084         return asCollector(type().parameterCount() - 1, arrayType, arrayLength);
1085     }
1086 
1087     /**
1088      * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting
1089      * at a given position, and collects them into an array argument. The new method handle adapts, as its
1090      * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target,
1091      * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType})
1092      * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1093      * <p>
1094      * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code
1095      * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This
1096      * index is zero-based.
1097      *
1098      * @apiNote Examples:
1099      * <blockquote><pre>{@code
1100     StringWriter swr = new StringWriter();
1101     MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr);
1102     MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4);
1103     swWrite4.invoke('A', 'B', 'C', 'D', 1, 2);
1104     assertEquals("BC", swr.toString());
1105     swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4);
1106     assertEquals("BCPQRS", swr.toString());
1107     swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1);
1108     assertEquals("BCPQRSZ", swr.toString());
1109      * }</pre></blockquote>
1110      * <p>
1111      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1112      * variable-arity method handle}, even if the original target method handle was.
1113      * @param collectArgPos the zero-based position in the parameter list at which to start collecting.
1114      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1115      * @param arrayLength the number of arguments to collect into a new array argument
1116      * @return a new method handle which collects some arguments
1117      *         into an array, before calling the original method handle
1118      * @throws NullPointerException if {@code arrayType} is a null reference
1119      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1120      *         or {@code arrayType} is not assignable to this method handle's array parameter type,
1121      *         or {@code arrayLength} is not a legal array size,
1122      *         or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments),
1123      *         or the resulting method handle's type would have
1124      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
1125      * @throws WrongMethodTypeException if the implied {@code asType} call fails
1126      *
1127      * @see #asCollector(Class, int)
1128      * @since 9
1129      */
1130     public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) {
1131         asCollectorChecks(arrayType, collectArgPos, arrayLength);
1132         BoundMethodHandle mh = rebind();
1133         MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength);
1134         MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
1135         LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
1136         if (lform != null) {
1137             return mh.copyWith(resultType, lform);
1138         }
1139         lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
1140         return mh.copyWithExtendL(resultType, lform, newArray);
1141     }
1142 
1143     /**
1144      * See if {@code asCollector} can be validly called with the given arguments.
1145      * Return false if the last parameter is not an exact match to arrayType.
1146      */
1147     /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) {
1148         spreadArrayChecks(arrayType, arrayLength);
1149         int nargs = type().parameterCount();
1150         if (pos < 0 || pos >= nargs) {
1151             throw newIllegalArgumentException("bad collect position");
1152         }
1153         if (nargs != 0) {
1154             Class<?> param = type().parameterType(pos);
1155             if (param == arrayType)  return true;
1156             if (param.isAssignableFrom(arrayType))  return false;
1157         }
1158         throw newIllegalArgumentException("array type not assignable to argument", this, arrayType);
1159     }
1160 
1161     /**
1162      * Makes a <em>variable arity</em> adapter which is able to accept
1163      * any number of trailing positional arguments and collect them
1164      * into an array argument.
1165      * <p>
1166      * The type and behavior of the adapter will be the same as
1167      * the type and behavior of the target, except that certain
1168      * {@code invoke} and {@code asType} requests can lead to
1169      * trailing positional arguments being collected into target's
1170      * trailing parameter.
1171      * Also, the last parameter type of the adapter will be
1172      * {@code arrayType}, even if the target has a different
1173      * last parameter type.
1174      * <p>
1175      * This transformation may return {@code this} if the method handle is
1176      * already of variable arity and its trailing parameter type
1177      * is identical to {@code arrayType}.
1178      * <p>
1179      * When called with {@link #invokeExact invokeExact}, the adapter invokes
1180      * the target with no argument changes.
1181      * (<em>Note:</em> This behavior is different from a
1182      * {@linkplain #asCollector fixed arity collector},
1183      * since it accepts a whole array of indeterminate length,
1184      * rather than a fixed number of arguments.)
1185      * <p>
1186      * When called with plain, inexact {@link #invoke invoke}, if the caller
1187      * type is the same as the adapter, the adapter invokes the target as with
1188      * {@code invokeExact}.
1189      * (This is the normal behavior for {@code invoke} when types match.)
1190      * <p>
1191      * Otherwise, if the caller and adapter arity are the same, and the
1192      * trailing parameter type of the caller is a reference type identical to
1193      * or assignable to the trailing parameter type of the adapter,
1194      * the arguments and return values are converted pairwise,
1195      * as if by {@link #asType asType} on a fixed arity
1196      * method handle.
1197      * <p>
1198      * Otherwise, the arities differ, or the adapter's trailing parameter
1199      * type is not assignable from the corresponding caller type.
1200      * In this case, the adapter replaces all trailing arguments from
1201      * the original trailing argument position onward, by
1202      * a new array of type {@code arrayType}, whose elements
1203      * comprise (in order) the replaced arguments.
1204      * <p>
1205      * The caller type must provides as least enough arguments,
1206      * and of the correct type, to satisfy the target's requirement for
1207      * positional arguments before the trailing array argument.
1208      * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1209      * where {@code N} is the arity of the target.
1210      * Also, there must exist conversions from the incoming arguments
1211      * to the target's arguments.
1212      * As with other uses of plain {@code invoke}, if these basic
1213      * requirements are not fulfilled, a {@code WrongMethodTypeException}
1214      * may be thrown.
1215      * <p>
1216      * In all cases, what the target eventually returns is returned unchanged by the adapter.
1217      * <p>
1218      * In the final case, it is exactly as if the target method handle were
1219      * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1220      * to the arity required by the caller type.
1221      * (As with {@code asCollector}, if the array length is zero,
1222      * a shared constant may be used instead of a new array.
1223      * If the implied call to {@code asCollector} would throw
1224      * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1225      * the call to the variable arity adapter must throw
1226      * {@code WrongMethodTypeException}.)
1227      * <p>
1228      * The behavior of {@link #asType asType} is also specialized for
1229      * variable arity adapters, to maintain the invariant that
1230      * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1231      * call to adjust the target type, followed by {@code invokeExact}.
1232      * Therefore, a variable arity adapter responds
1233      * to an {@code asType} request by building a fixed arity collector,
1234      * if and only if the adapter and requested type differ either
1235      * in arity or trailing argument type.
1236      * The resulting fixed arity collector has its type further adjusted
1237      * (if necessary) to the requested type by pairwise conversion,
1238      * as if by another application of {@code asType}.
1239      * <p>
1240      * When a method handle is obtained by executing an {@code ldc} instruction
1241      * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1242      * as a variable arity method (with the modifier bit {@code 0x0080}),
1243      * the method handle will accept multiple arities, as if the method handle
1244      * constant were created by means of a call to {@code asVarargsCollector}.
1245      * <p>
1246      * In order to create a collecting adapter which collects a predetermined
1247      * number of arguments, and whose type reflects this predetermined number,
1248      * use {@link #asCollector asCollector} instead.
1249      * <p>
1250      * No method handle transformations produce new method handles with
1251      * variable arity, unless they are documented as doing so.
1252      * Therefore, besides {@code asVarargsCollector} and {@code withVarargs},
1253      * all methods in {@code MethodHandle} and {@code MethodHandles}
1254      * will return a method handle with fixed arity,
1255      * except in the cases where they are specified to return their original
1256      * operand (e.g., {@code asType} of the method handle's own type).
1257      * <p>
1258      * Calling {@code asVarargsCollector} on a method handle which is already
1259      * of variable arity will produce a method handle with the same type and behavior.
1260      * It may (or may not) return the original variable arity method handle.
1261      * <p>
1262      * Here is an example, of a list-making variable arity method handle:
1263      * <blockquote><pre>{@code
1264 MethodHandle deepToString = publicLookup()
1265   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1266 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1267 assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
1268 assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
1269 assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
1270 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1271 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1272 MethodHandle asList = publicLookup()
1273   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1274 assertEquals(methodType(List.class, Object[].class), asList.type());
1275 assert(asList.isVarargsCollector());
1276 assertEquals("[]", asList.invoke().toString());
1277 assertEquals("[1]", asList.invoke(1).toString());
1278 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1279 String[] argv = { "three", "thee", "tee" };
1280 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1281 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1282 List ls = (List) asList.invoke((Object)argv);
1283 assertEquals(1, ls.size());
1284 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1285      * }</pre></blockquote>
1286      * <p style="font-size:smaller;">
1287      * <em>Discussion:</em>
1288      * These rules are designed as a dynamically-typed variation
1289      * of the Java rules for variable arity methods.
1290      * In both cases, callers to a variable arity method or method handle
1291      * can either pass zero or more positional arguments, or else pass
1292      * pre-collected arrays of any length.  Users should be aware of the
1293      * special role of the final argument, and of the effect of a
1294      * type match on that final argument, which determines whether
1295      * or not a single trailing argument is interpreted as a whole
1296      * array or a single element of an array to be collected.
1297      * Note that the dynamic type of the trailing argument has no
1298      * effect on this decision, only a comparison between the symbolic
1299      * type descriptor of the call site and the type descriptor of the method handle.)
1300      *
1301      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1302      * @return a new method handle which can collect any number of trailing arguments
1303      *         into an array, before calling the original method handle
1304      * @throws NullPointerException if {@code arrayType} is a null reference
1305      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1306      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type
1307      * @see #asCollector
1308      * @see #isVarargsCollector
1309      * @see #withVarargs
1310      * @see #asFixedArity
1311      */
1312     public MethodHandle asVarargsCollector(Class<?> arrayType) {
1313         Objects.requireNonNull(arrayType);
1314         boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0);
1315         if (isVarargsCollector() && lastMatch)
1316             return this;
1317         return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1318     }
1319 
1320     /**
1321      * Determines if this method handle
1322      * supports {@linkplain #asVarargsCollector variable arity} calls.
1323      * Such method handles arise from the following sources:
1324      * <ul>
1325      * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1326      * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1327      *     which resolves to a variable arity Java method or constructor
1328      * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1329      *     which resolves to a variable arity Java method or constructor
1330      * </ul>
1331      * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1332      * @see #asVarargsCollector
1333      * @see #asFixedArity
1334      */
1335     public boolean isVarargsCollector() {
1336         return false;
1337     }
1338 
1339     /**
1340      * Makes a <em>fixed arity</em> method handle which is otherwise
1341      * equivalent to the current method handle.
1342      * <p>
1343      * If the current method handle is not of
1344      * {@linkplain #asVarargsCollector variable arity},
1345      * the current method handle is returned.
1346      * This is true even if the current method handle
1347      * could not be a valid input to {@code asVarargsCollector}.
1348      * <p>
1349      * Otherwise, the resulting fixed-arity method handle has the same
1350      * type and behavior of the current method handle,
1351      * except that {@link #isVarargsCollector isVarargsCollector}
1352      * will be false.
1353      * The fixed-arity method handle may (or may not) be the
1354      * a previous argument to {@code asVarargsCollector}.
1355      * <p>
1356      * Here is an example, of a list-making variable arity method handle:
1357      * <blockquote><pre>{@code
1358 MethodHandle asListVar = publicLookup()
1359   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1360   .asVarargsCollector(Object[].class);
1361 MethodHandle asListFix = asListVar.asFixedArity();
1362 assertEquals("[1]", asListVar.invoke(1).toString());
1363 Exception caught = null;
1364 try { asListFix.invoke((Object)1); }
1365 catch (Exception ex) { caught = ex; }
1366 assert(caught instanceof ClassCastException);
1367 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1368 try { asListFix.invoke("two", "too"); }
1369 catch (Exception ex) { caught = ex; }
1370 assert(caught instanceof WrongMethodTypeException);
1371 Object[] argv = { "three", "thee", "tee" };
1372 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1373 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1374 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1375 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1376      * }</pre></blockquote>
1377      *
1378      * @return a new method handle which accepts only a fixed number of arguments
1379      * @see #asVarargsCollector
1380      * @see #isVarargsCollector
1381      * @see #withVarargs
1382      */
1383     public MethodHandle asFixedArity() {
1384         assert(!isVarargsCollector());
1385         return this;
1386     }
1387 
1388     /**
1389      * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1390      * The new method handle adapts, as its <i>target</i>,
1391      * the current method handle by binding it to the given argument.
1392      * The type of the bound handle will be
1393      * the same as the type of the target, except that a single leading
1394      * reference parameter will be omitted.
1395      * <p>
1396      * When called, the bound handle inserts the given value {@code x}
1397      * as a new leading argument to the target.  The other arguments are
1398      * also passed unchanged.
1399      * What the target eventually returns is returned unchanged by the bound handle.
1400      * <p>
1401      * The reference {@code x} must be convertible to the first parameter
1402      * type of the target.
1403      * <p>
1404      * <em>Note:</em>  Because method handles are immutable, the target method handle
1405      * retains its original type and behavior.
1406      * <p>
1407      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
1408      * variable-arity method handle}, even if the original target method handle was.
1409      * @param x  the value to bind to the first argument of the target
1410      * @return a new method handle which prepends the given value to the incoming
1411      *         argument list, before calling the original method handle
1412      * @throws IllegalArgumentException if the target does not have a
1413      *         leading parameter type that is a reference type
1414      * @throws ClassCastException if {@code x} cannot be converted
1415      *         to the leading parameter type of the target
1416      * @see MethodHandles#insertArguments
1417      */
1418     public MethodHandle bindTo(Object x) {
1419         x = type.leadingReferenceParameter().cast(x);  // throw CCE if needed
1420         return bindArgumentL(0, x);
1421     }
1422 
1423     /**
1424      * Returns a string representation of the method handle,
1425      * starting with the string {@code "MethodHandle"} and
1426      * ending with the string representation of the method handle's type.
1427      * In other words, this method returns a string equal to the value of:
1428      * <blockquote><pre>{@code
1429      * "MethodHandle" + type().toString()
1430      * }</pre></blockquote>
1431      * <p>
1432      * (<em>Note:</em>  Future releases of this API may add further information
1433      * to the string representation.
1434      * Therefore, the present syntax should not be parsed by applications.)
1435      *
1436      * @return a string representation of the method handle
1437      */
1438     @Override
1439     public String toString() {
1440         if (DEBUG_METHOD_HANDLE_NAMES)  return "MethodHandle"+debugString();
1441         return standardString();
1442     }
1443     String standardString() {
1444         return "MethodHandle"+type;
1445     }
1446     /** Return a string with a several lines describing the method handle structure.
1447      *  This string would be suitable for display in an IDE debugger.
1448      */
1449     String debugString() {
1450         return type+" : "+internalForm()+internalProperties();
1451     }
1452 
1453     //// Implementation methods.
1454     //// Sub-classes can override these default implementations.
1455     //// All these methods assume arguments are already validated.
1456 
1457     // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch
1458 
1459     BoundMethodHandle bindArgumentL(int pos, Object value) {
1460         return rebind().bindArgumentL(pos, value);
1461     }
1462 
1463     /*non-public*/
1464     MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1465         if (!member.isVarargs())  return this;
1466         try {
1467             return this.withVarargs(true);
1468         } catch (IllegalArgumentException ex) {
1469             throw member.makeAccessException("cannot make variable arity", null);
1470         }
1471     }
1472 
1473     /*non-public*/
1474     MethodHandle viewAsType(MethodType newType, boolean strict) {
1475         // No actual conversions, just a new view of the same method.
1476         // Note that this operation must not produce a DirectMethodHandle,
1477         // because retyped DMHs, like any transformed MHs,
1478         // cannot be cracked into MethodHandleInfo.
1479         assert viewAsTypeChecks(newType, strict);
1480         BoundMethodHandle mh = rebind();
1481         return mh.copyWith(newType, mh.form);
1482     }
1483 
1484     /*non-public*/
1485     boolean viewAsTypeChecks(MethodType newType, boolean strict) {
1486         if (strict) {
1487             assert(type().isViewableAs(newType, true))
1488                 : Arrays.asList(this, newType);
1489         } else {
1490             assert(type().basicType().isViewableAs(newType.basicType(), true))
1491                 : Arrays.asList(this, newType);
1492         }
1493         return true;
1494     }
1495 
1496     // Decoding
1497 
1498     /*non-public*/
1499     LambdaForm internalForm() {
1500         return form;
1501     }
1502 
1503     /*non-public*/
1504     MemberName internalMemberName() {
1505         return null;  // DMH returns DMH.member
1506     }
1507 
1508     /*non-public*/
1509     Class<?> internalCallerClass() {
1510         return null;  // caller-bound MH for @CallerSensitive method returns caller
1511     }
1512 
1513     /*non-public*/
1514     MethodHandleImpl.Intrinsic intrinsicName() {
1515         // no special intrinsic meaning to most MHs
1516         return MethodHandleImpl.Intrinsic.NONE;
1517     }
1518 
1519     /*non-public*/
1520     MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
1521         if (member != null) {
1522             return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
1523         } else if (internalMemberName() == null) {
1524             // The required internaMemberName is null, and this MH (like most) doesn't have one.
1525             return this;
1526         } else {
1527             // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1528             MethodHandle result = rebind();
1529             assert (result.internalMemberName() == null);
1530             return result;
1531         }
1532     }
1533 
1534     /*non-public*/
1535     boolean isInvokeSpecial() {
1536         return false;  // DMH.Special returns true
1537     }
1538 
1539     /*non-public*/
1540     Object internalValues() {
1541         return null;
1542     }
1543 
1544     /*non-public*/
1545     Object internalProperties() {
1546         // Override to something to follow this.form, like "\n& FOO=bar"
1547         return "";
1548     }
1549 
1550     //// Method handle implementation methods.
1551     //// Sub-classes can override these default implementations.
1552     //// All these methods assume arguments are already validated.
1553 
1554     /*non-public*/
1555     abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1556 
1557     /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
1558      *  Many transforms are implemented only for BMHs.
1559      *  @return a behaviorally equivalent BMH
1560      */
1561     abstract BoundMethodHandle rebind();
1562 
1563     /**
1564      * Replace the old lambda form of this method handle with a new one.
1565      * The new one must be functionally equivalent to the old one.
1566      * Threads may continue running the old form indefinitely,
1567      * but it is likely that the new one will be preferred for new executions.
1568      * Use with discretion.
1569      */
1570     /*non-public*/
1571     void updateForm(LambdaForm newForm) {
1572         assert(newForm.customized == null || newForm.customized == this);
1573         if (form == newForm)  return;
1574         newForm.prepare();  // as in MethodHandle.<init>
1575         UNSAFE.putObject(this, FORM_OFFSET, newForm);
1576         UNSAFE.fullFence();
1577     }
1578 
1579     /** Craft a LambdaForm customized for this particular MethodHandle */
1580     /*non-public*/
1581     void customize() {
1582         if (form.customized == null) {
1583             LambdaForm newForm = form.customize(this);
1584             updateForm(newForm);
1585         } else {
1586             assert(form.customized == this);
1587         }
1588     }
1589 
1590     private static final long FORM_OFFSET
1591             = UNSAFE.objectFieldOffset(MethodHandle.class, "form");
1592 }