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25 
26 package java.lang.invoke;
27 
28 
29 import dalvik.system.EmulatedStackFrame;
30 
31 import static java.lang.invoke.MethodHandleStatics.*;
32 
33 /**
34  * A method handle is a typed, directly executable reference to an underlying method,
35  * constructor, field, or similar low-level operation, with optional
36  * transformations of arguments or return values.
37  * These transformations are quite general, and include such patterns as
38  * {@linkplain #asType conversion},
39  * {@linkplain #bindTo insertion},
40  * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
41  * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
42  *
43  * <h1>Method handle contents</h1>
44  * Method handles are dynamically and strongly typed according to their parameter and return types.
45  * They are not distinguished by the name or the defining class of their underlying methods.
46  * A method handle must be invoked using a symbolic type descriptor which matches
47  * the method handle's own {@linkplain #type type descriptor}.
48  * <p>
49  * Every method handle reports its type descriptor via the {@link #type type} accessor.
50  * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
51  * whose structure is a series of classes, one of which is
52  * the return type of the method (or {@code void.class} if none).
53  * <p>
54  * A method handle's type controls the types of invocations it accepts,
55  * and the kinds of transformations that apply to it.
56  * <p>
57  * A method handle contains a pair of special invoker methods
58  * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
59  * Both invoker methods provide direct access to the method handle's
60  * underlying method, constructor, field, or other operation,
61  * as modified by transformations of arguments and return values.
62  * Both invokers accept calls which exactly match the method handle's own type.
63  * The plain, inexact invoker also accepts a range of other call types.
64  * <p>
65  * Method handles are immutable and have no visible state.
66  * Of course, they can be bound to underlying methods or data which exhibit state.
67  * With respect to the Java Memory Model, any method handle will behave
68  * as if all of its (internal) fields are final variables.  This means that any method
69  * handle made visible to the application will always be fully formed.
70  * This is true even if the method handle is published through a shared
71  * variable in a data race.
72  * <p>
73  * Method handles cannot be subclassed by the user.
74  * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
75  * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
76  * operation.  The programmer should not draw conclusions about a method handle
77  * from its specific class, as the method handle class hierarchy (if any)
78  * may change from time to time or across implementations from different vendors.
79  *
80  * <h1>Method handle compilation</h1>
81  * A Java method call expression naming {@code invokeExact} or {@code invoke}
82  * can invoke a method handle from Java source code.
83  * From the viewpoint of source code, these methods can take any arguments
84  * and their result can be cast to any return type.
85  * Formally this is accomplished by giving the invoker methods
86  * {@code Object} return types and variable arity {@code Object} arguments,
87  * but they have an additional quality called <em>signature polymorphism</em>
88  * which connects this freedom of invocation directly to the JVM execution stack.
89  * <p>
90  * As is usual with virtual methods, source-level calls to {@code invokeExact}
91  * and {@code invoke} compile to an {@code invokevirtual} instruction.
92  * More unusually, the compiler must record the actual argument types,
93  * and may not perform method invocation conversions on the arguments.
94  * Instead, it must push them on the stack according to their own unconverted types.
95  * The method handle object itself is pushed on the stack before the arguments.
96  * The compiler then calls the method handle with a symbolic type descriptor which
97  * describes the argument and return types.
98  * <p>
99  * To issue a complete symbolic type descriptor, the compiler must also determine
100  * the return type.  This is based on a cast on the method invocation expression,
101  * if there is one, or else {@code Object} if the invocation is an expression
102  * or else {@code void} if the invocation is a statement.
103  * The cast may be to a primitive type (but not {@code void}).
104  * <p>
105  * As a corner case, an uncasted {@code null} argument is given
106  * a symbolic type descriptor of {@code java.lang.Void}.
107  * The ambiguity with the type {@code Void} is harmless, since there are no references of type
108  * {@code Void} except the null reference.
109  *
110  * <h1>Method handle invocation</h1>
111  * The first time a {@code invokevirtual} instruction is executed
112  * it is linked, by symbolically resolving the names in the instruction
113  * and verifying that the method call is statically legal.
114  * This is true of calls to {@code invokeExact} and {@code invoke}.
115  * In this case, the symbolic type descriptor emitted by the compiler is checked for
116  * correct syntax and names it contains are resolved.
117  * Thus, an {@code invokevirtual} instruction which invokes
118  * a method handle will always link, as long
119  * as the symbolic type descriptor is syntactically well-formed
120  * and the types exist.
121  * <p>
122  * When the {@code invokevirtual} is executed after linking,
123  * the receiving method handle's type is first checked by the JVM
124  * to ensure that it matches the symbolic type descriptor.
125  * If the type match fails, it means that the method which the
126  * caller is invoking is not present on the individual
127  * method handle being invoked.
128  * <p>
129  * In the case of {@code invokeExact}, the type descriptor of the invocation
130  * (after resolving symbolic type names) must exactly match the method type
131  * of the receiving method handle.
132  * In the case of plain, inexact {@code invoke}, the resolved type descriptor
133  * must be a valid argument to the receiver's {@link #asType asType} method.
134  * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
135  * <p>
136  * After type matching, a call to {@code invokeExact} directly
137  * and immediately invoke the method handle's underlying method
138  * (or other behavior, as the case may be).
139  * <p>
140  * A call to plain {@code invoke} works the same as a call to
141  * {@code invokeExact}, if the symbolic type descriptor specified by the caller
142  * exactly matches the method handle's own type.
143  * If there is a type mismatch, {@code invoke} attempts
144  * to adjust the type of the receiving method handle,
145  * as if by a call to {@link #asType asType},
146  * to obtain an exactly invokable method handle {@code M2}.
147  * This allows a more powerful negotiation of method type
148  * between caller and callee.
149  * <p>
150  * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
151  * and implementations are therefore not required to materialize it.)
152  *
153  * <h1>Invocation checking</h1>
154  * In typical programs, method handle type matching will usually succeed.
155  * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
156  * either directly (in the case of {@code invokeExact}) or indirectly as if
157  * by a failed call to {@code asType} (in the case of {@code invoke}).
158  * <p>
159  * Thus, a method type mismatch which might show up as a linkage error
160  * in a statically typed program can show up as
161  * a dynamic {@code WrongMethodTypeException}
162  * in a program which uses method handles.
163  * <p>
164  * Because method types contain "live" {@code Class} objects,
165  * method type matching takes into account both types names and class loaders.
166  * Thus, even if a method handle {@code M} is created in one
167  * class loader {@code L1} and used in another {@code L2},
168  * method handle calls are type-safe, because the caller's symbolic type
169  * descriptor, as resolved in {@code L2},
170  * is matched against the original callee method's symbolic type descriptor,
171  * as resolved in {@code L1}.
172  * The resolution in {@code L1} happens when {@code M} is created
173  * and its type is assigned, while the resolution in {@code L2} happens
174  * when the {@code invokevirtual} instruction is linked.
175  * <p>
176  * Apart from the checking of type descriptors,
177  * a method handle's capability to call its underlying method is unrestricted.
178  * If a method handle is formed on a non-public method by a class
179  * that has access to that method, the resulting handle can be used
180  * in any place by any caller who receives a reference to it.
181  * <p>
182  * Unlike with the Core Reflection API, where access is checked every time
183  * a reflective method is invoked,
184  * method handle access checking is performed
185  * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
186  * In the case of {@code ldc} (see below), access checking is performed as part of linking
187  * the constant pool entry underlying the constant method handle.
188  * <p>
189  * Thus, handles to non-public methods, or to methods in non-public classes,
190  * should generally be kept secret.
191  * They should not be passed to untrusted code unless their use from
192  * the untrusted code would be harmless.
193  *
194  * <h1>Method handle creation</h1>
195  * Java code can create a method handle that directly accesses
196  * any method, constructor, or field that is accessible to that code.
197  * This is done via a reflective, capability-based API called
198  * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
199  * For example, a static method handle can be obtained
200  * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
201  * There are also conversion methods from Core Reflection API objects,
202  * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
203  * <p>
204  * Like classes and strings, method handles that correspond to accessible
205  * fields, methods, and constructors can also be represented directly
206  * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
207  * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
208  * refers directly to an associated {@code CONSTANT_Methodref},
209  * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
210  * constant pool entry.
211  * (For full details on method handle constants,
212  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
213  * <p>
214  * Method handles produced by lookups or constant loads from methods or
215  * constructors with the variable arity modifier bit ({@code 0x0080})
216  * have a corresponding variable arity, as if they were defined with
217  * the help of {@link #asVarargsCollector asVarargsCollector}.
218  * <p>
219  * A method reference may refer either to a static or non-static method.
220  * In the non-static case, the method handle type includes an explicit
221  * receiver argument, prepended before any other arguments.
222  * In the method handle's type, the initial receiver argument is typed
223  * according to the class under which the method was initially requested.
224  * (E.g., if a non-static method handle is obtained via {@code ldc},
225  * the type of the receiver is the class named in the constant pool entry.)
226  * <p>
227  * Method handle constants are subject to the same link-time access checks
228  * their corresponding bytecode instructions, and the {@code ldc} instruction
229  * will throw corresponding linkage errors if the bytecode behaviors would
230  * throw such errors.
231  * <p>
232  * As a corollary of this, access to protected members is restricted
233  * to receivers only of the accessing class, or one of its subclasses,
234  * and the accessing class must in turn be a subclass (or package sibling)
235  * of the protected member's defining class.
236  * If a method reference refers to a protected non-static method or field
237  * of a class outside the current package, the receiver argument will
238  * be narrowed to the type of the accessing class.
239  * <p>
240  * When a method handle to a virtual method is invoked, the method is
241  * always looked up in the receiver (that is, the first argument).
242  * <p>
243  * A non-virtual method handle to a specific virtual method implementation
244  * can also be created.  These do not perform virtual lookup based on
245  * receiver type.  Such a method handle simulates the effect of
246  * an {@code invokespecial} instruction to the same method.
247  *
248  * <h1>Usage examples</h1>
249  * Here are some examples of usage:
250  * <blockquote><pre>{@code
251 Object x, y; String s; int i;
252 MethodType mt; MethodHandle mh;
253 MethodHandles.Lookup lookup = MethodHandles.lookup();
254 // mt is (char,char)String
255 mt = MethodType.methodType(String.class, char.class, char.class);
256 mh = lookup.findVirtual(String.class, "replace", mt);
257 s = (String) mh.invokeExact("daddy",'d','n');
258 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
259 assertEquals(s, "nanny");
260 // weakly typed invocation (using MHs.invoke)
261 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
262 assertEquals(s, "savvy");
263 // mt is (Object[])List
264 mt = MethodType.methodType(java.util.List.class, Object[].class);
265 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
266 assert(mh.isVarargsCollector());
267 x = mh.invoke("one", "two");
268 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
269 assertEquals(x, java.util.Arrays.asList("one","two"));
270 // mt is (Object,Object,Object)Object
271 mt = MethodType.genericMethodType(3);
272 mh = mh.asType(mt);
273 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
274 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
275 assertEquals(x, java.util.Arrays.asList(1,2,3));
276 // mt is ()int
277 mt = MethodType.methodType(int.class);
278 mh = lookup.findVirtual(java.util.List.class, "size", mt);
279 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
280 // invokeExact(Ljava/util/List;)I
281 assert(i == 3);
282 mt = MethodType.methodType(void.class, String.class);
283 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
284 mh.invokeExact(System.out, "Hello, world.");
285 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
286  * }</pre></blockquote>
287  * Each of the above calls to {@code invokeExact} or plain {@code invoke}
288  * generates a single invokevirtual instruction with
289  * the symbolic type descriptor indicated in the following comment.
290  * In these examples, the helper method {@code assertEquals} is assumed to
291  * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
292  * on its arguments, and asserts that the result is true.
293  *
294  * <h1>Exceptions</h1>
295  * The methods {@code invokeExact} and {@code invoke} are declared
296  * to throw {@link java.lang.Throwable Throwable},
297  * which is to say that there is no static restriction on what a method handle
298  * can throw.  Since the JVM does not distinguish between checked
299  * and unchecked exceptions (other than by their class, of course),
300  * there is no particular effect on bytecode shape from ascribing
301  * checked exceptions to method handle invocations.  But in Java source
302  * code, methods which perform method handle calls must either explicitly
303  * throw {@code Throwable}, or else must catch all
304  * throwables locally, rethrowing only those which are legal in the context,
305  * and wrapping ones which are illegal.
306  *
307  * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
308  * The unusual compilation and linkage behavior of
309  * {@code invokeExact} and plain {@code invoke}
310  * is referenced by the term <em>signature polymorphism</em>.
311  * As defined in the Java Language Specification,
312  * a signature polymorphic method is one which can operate with
313  * any of a wide range of call signatures and return types.
314  * <p>
315  * In source code, a call to a signature polymorphic method will
316  * compile, regardless of the requested symbolic type descriptor.
317  * As usual, the Java compiler emits an {@code invokevirtual}
318  * instruction with the given symbolic type descriptor against the named method.
319  * The unusual part is that the symbolic type descriptor is derived from
320  * the actual argument and return types, not from the method declaration.
321  * <p>
322  * When the JVM processes bytecode containing signature polymorphic calls,
323  * it will successfully link any such call, regardless of its symbolic type descriptor.
324  * (In order to retain type safety, the JVM will guard such calls with suitable
325  * dynamic type checks, as described elsewhere.)
326  * <p>
327  * Bytecode generators, including the compiler back end, are required to emit
328  * untransformed symbolic type descriptors for these methods.
329  * Tools which determine symbolic linkage are required to accept such
330  * untransformed descriptors, without reporting linkage errors.
331  *
332  * <h1>Interoperation between method handles and the Core Reflection API</h1>
333  * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
334  * any class member represented by a Core Reflection API object
335  * can be converted to a behaviorally equivalent method handle.
336  * For example, a reflective {@link java.lang.reflect.Method Method} can
337  * be converted to a method handle using
338  * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
339  * The resulting method handles generally provide more direct and efficient
340  * access to the underlying class members.
341  * <p>
342  * As a special case,
343  * when the Core Reflection API is used to view the signature polymorphic
344  * methods {@code invokeExact} or plain {@code invoke} in this class,
345  * they appear as ordinary non-polymorphic methods.
346  * Their reflective appearance, as viewed by
347  * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
348  * is unaffected by their special status in this API.
349  * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
350  * will report exactly those modifier bits required for any similarly
351  * declared method, including in this case {@code native} and {@code varargs} bits.
352  * <p>
353  * As with any reflected method, these methods (when reflected) may be
354  * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
355  * However, such reflective calls do not result in method handle invocations.
356  * Such a call, if passed the required argument
357  * (a single one, of type {@code Object[]}), will ignore the argument and
358  * will throw an {@code UnsupportedOperationException}.
359  * <p>
360  * Since {@code invokevirtual} instructions can natively
361  * invoke method handles under any symbolic type descriptor, this reflective view conflicts
362  * with the normal presentation of these methods via bytecodes.
363  * Thus, these two native methods, when reflectively viewed by
364  * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
365  * <p>
366  * In order to obtain an invoker method for a particular type descriptor,
367  * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
368  * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
369  * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
370  * API is also able to return a method handle
371  * to call {@code invokeExact} or plain {@code invoke},
372  * for any specified type descriptor .
373  *
374  * <h1>Interoperation between method handles and Java generics</h1>
375  * A method handle can be obtained on a method, constructor, or field
376  * which is declared with Java generic types.
377  * As with the Core Reflection API, the type of the method handle
378  * will constructed from the erasure of the source-level type.
379  * When a method handle is invoked, the types of its arguments
380  * or the return value cast type may be generic types or type instances.
381  * If this occurs, the compiler will replace those
382  * types by their erasures when it constructs the symbolic type descriptor
383  * for the {@code invokevirtual} instruction.
384  * <p>
385  * Method handles do not represent
386  * their function-like types in terms of Java parameterized (generic) types,
387  * because there are three mismatches between function-like types and parameterized
388  * Java types.
389  * <ul>
390  * <li>Method types range over all possible arities,
391  * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
392  * Generics are not variadic, and so cannot represent this.</li>
393  * <li>Method types can specify arguments of primitive types,
394  * which Java generic types cannot range over.</li>
395  * <li>Higher order functions over method handles (combinators) are
396  * often generic across a wide range of function types, including
397  * those of multiple arities.  It is impossible to represent such
398  * genericity with a Java type parameter.</li>
399  * </ul>
400  *
401  * <h1><a name="maxarity"></a>Arity limits</h1>
402  * The JVM imposes on all methods and constructors of any kind an absolute
403  * limit of 255 stacked arguments.  This limit can appear more restrictive
404  * in certain cases:
405  * <ul>
406  * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
407  * <li>A non-static method consumes an extra argument for the object on which the method is called.
408  * <li>A constructor consumes an extra argument for the object which is being constructed.
409  * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
410  *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
411  * </ul>
412  * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
413  * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
414  * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
415  * In particular, a method handle&rsquo;s type must not have an arity of the exact maximum 255.
416  *
417  * @see MethodType
418  * @see MethodHandles
419  * @author John Rose, JSR 292 EG
420  */
421 public abstract class MethodHandle {
422     // Android-changed:
423     //
424     // static { MethodHandleImpl.initStatics(); }
425     //
426     // LambdaForm and customizationCount are currently unused in our implementation
427     // and will be substituted with appropriate implementation / delegate classes.
428     //
429     // /*private*/ final LambdaForm form;
430     // form is not private so that invokers can easily fetch it
431     // /*non-public*/ byte customizationCount;
432     // customizationCount should be accessible from invokers
433 
434 
435     /**
436      * Internal marker interface which distinguishes (to the Java compiler)
437      * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
438      *
439      * @hide
440      */
441     @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
442     @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
443     public @interface PolymorphicSignature { }
444 
445     /**
446      * The type of this method handle, this corresponds to the exact type of the method
447      * being invoked.
448      */
449     private final MethodType type;
450 
451     /**
452      * The nominal type of this method handle, will be non-null if a method handle declares
453      * a different type from its "real" type, which is either the type of the method being invoked
454      * or the type of the emulated stackframe expected by an underyling adapter.
455      */
456     private MethodType nominalType;
457 
458     /**
459      * The spread invoker associated with this type with zero trailing arguments.
460      * This is used to speed up invokeWithArguments.
461      */
462     private MethodHandle cachedSpreadInvoker;
463 
464     /**
465      * The INVOKE* constants and SGET/SPUT and IGET/IPUT constants specify the behaviour of this
466      * method handle with respect to the ArtField* or the ArtMethod* that it operates on. These
467      * behaviours are equivalent to the dex bytecode behaviour on the respective method_id or
468      * field_id in the equivalent instruction.
469      *
470      * INVOKE_TRANSFORM is a special type of handle which doesn't encode any dex bytecode behaviour,
471      * instead it transforms the list of input arguments or performs other higher order operations
472      * before (optionally) delegating to another method handle.
473      *
474      * INVOKE_CALLSITE_TRANSFORM is a variation on INVOKE_TRANSFORM where the method type of
475      * a MethodHandle dynamically varies based on the callsite. This is used by
476      * the VarargsCollector implementation which places any number of trailing arguments
477      * into an array before invoking an arity method. The "any number of trailing arguments" means
478      * it would otherwise generate WrongMethodTypeExceptions as the callsite method type and
479      * VarargsCollector method type appear incompatible.
480      */
481 
482     /** @hide */ public static final int INVOKE_VIRTUAL = 0;
483     /** @hide */ public static final int INVOKE_SUPER = 1;
484     /** @hide */ public static final int INVOKE_DIRECT = 2;
485     /** @hide */ public static final int INVOKE_STATIC = 3;
486     /** @hide */ public static final int INVOKE_INTERFACE = 4;
487     /** @hide */ public static final int INVOKE_TRANSFORM = 5;
488     /** @hide */ public static final int INVOKE_CALLSITE_TRANSFORM = 6;
489     /** @hide */ public static final int IGET = 7;
490     /** @hide */ public static final int IPUT = 8;
491     /** @hide */ public static final int SGET = 9;
492     /** @hide */ public static final int SPUT = 10;
493 
494     // The kind of this method handle (used by the runtime). This is one of the INVOKE_*
495     // constants or SGET/SPUT, IGET/IPUT.
496     /** @hide */ protected final int handleKind;
497 
498     // The ArtMethod* or ArtField* associated with this method handle (used by the runtime).
499     /** @hide */ protected final long artFieldOrMethod;
500 
501     /** @hide */
MethodHandle(long artFieldOrMethod, int handleKind, MethodType type)502     protected MethodHandle(long artFieldOrMethod, int handleKind, MethodType type) {
503         this.artFieldOrMethod = artFieldOrMethod;
504         this.handleKind = handleKind;
505         this.type = type;
506     }
507 
508     /**
509      * Reports the type of this method handle.
510      * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
511      * @return the method handle type
512      */
type()513     public MethodType type() {
514         if (nominalType != null) {
515             return nominalType;
516         }
517 
518         return type;
519     }
520 
521     /**
522      * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
523      * The symbolic type descriptor at the call site of {@code invokeExact} must
524      * exactly match this method handle's {@link #type type}.
525      * No conversions are allowed on arguments or return values.
526      * <p>
527      * When this method is observed via the Core Reflection API,
528      * it will appear as a single native method, taking an object array and returning an object.
529      * If this native method is invoked directly via
530      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
531      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
532      * it will throw an {@code UnsupportedOperationException}.
533      * @param args the signature-polymorphic parameter list, statically represented using varargs
534      * @return the signature-polymorphic result, statically represented using {@code Object}
535      * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
536      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
537      */
invokeExact(Object... args)538     public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
539 
540     /**
541      * Invokes the method handle, allowing any caller type descriptor,
542      * and optionally performing conversions on arguments and return values.
543      * <p>
544      * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
545      * the call proceeds as if by {@link #invokeExact invokeExact}.
546      * <p>
547      * Otherwise, the call proceeds as if this method handle were first
548      * adjusted by calling {@link #asType asType} to adjust this method handle
549      * to the required type, and then the call proceeds as if by
550      * {@link #invokeExact invokeExact} on the adjusted method handle.
551      * <p>
552      * There is no guarantee that the {@code asType} call is actually made.
553      * If the JVM can predict the results of making the call, it may perform
554      * adaptations directly on the caller's arguments,
555      * and call the target method handle according to its own exact type.
556      * <p>
557      * The resolved type descriptor at the call site of {@code invoke} must
558      * be a valid argument to the receivers {@code asType} method.
559      * In particular, the caller must specify the same argument arity
560      * as the callee's type,
561      * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
562      * <p>
563      * When this method is observed via the Core Reflection API,
564      * it will appear as a single native method, taking an object array and returning an object.
565      * If this native method is invoked directly via
566      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
567      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
568      * it will throw an {@code UnsupportedOperationException}.
569      * @param args the signature-polymorphic parameter list, statically represented using varargs
570      * @return the signature-polymorphic result, statically represented using {@code Object}
571      * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
572      * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
573      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
574      */
invoke(Object... args)575     public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
576 
577     // Android-changed: Removed implementation details.
578     //
579     // /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args)
580     // /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args)
581     // /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args)
582     // /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args)
583     // /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args)
584 
585     /**
586      * Performs a variable arity invocation, passing the arguments in the given list
587      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
588      * which mentions only the type {@code Object}, and whose arity is the length
589      * of the argument list.
590      * <p>
591      * Specifically, execution proceeds as if by the following steps,
592      * although the methods are not guaranteed to be called if the JVM
593      * can predict their effects.
594      * <ul>
595      * <li>Determine the length of the argument array as {@code N}.
596      *     For a null reference, {@code N=0}. </li>
597      * <li>Determine the general type {@code TN} of {@code N} arguments as
598      *     as {@code TN=MethodType.genericMethodType(N)}.</li>
599      * <li>Force the original target method handle {@code MH0} to the
600      *     required type, as {@code MH1 = MH0.asType(TN)}. </li>
601      * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
602      * <li>Invoke the type-adjusted method handle on the unpacked arguments:
603      *     MH1.invokeExact(A0, ...). </li>
604      * <li>Take the return value as an {@code Object} reference. </li>
605      * </ul>
606      * <p>
607      * Because of the action of the {@code asType} step, the following argument
608      * conversions are applied as necessary:
609      * <ul>
610      * <li>reference casting
611      * <li>unboxing
612      * <li>widening primitive conversions
613      * </ul>
614      * <p>
615      * The result returned by the call is boxed if it is a primitive,
616      * or forced to null if the return type is void.
617      * <p>
618      * This call is equivalent to the following code:
619      * <blockquote><pre>{@code
620      * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
621      * Object result = invoker.invokeExact(this, arguments);
622      * }</pre></blockquote>
623      * <p>
624      * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
625      * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
626      * It can therefore be used as a bridge between native or reflective code and method handles.
627      *
628      * @param arguments the arguments to pass to the target
629      * @return the result returned by the target
630      * @throws ClassCastException if an argument cannot be converted by reference casting
631      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
632      * @throws Throwable anything thrown by the target method invocation
633      * @see MethodHandles#spreadInvoker
634      */
invokeWithArguments(Object... arguments)635     public Object invokeWithArguments(Object... arguments) throws Throwable {
636         MethodHandle invoker = null;
637         synchronized (this) {
638             if (cachedSpreadInvoker == null) {
639                 cachedSpreadInvoker = MethodHandles.spreadInvoker(this.type(), 0);
640             }
641 
642             invoker = cachedSpreadInvoker;
643         }
644 
645         return invoker.invoke(this, arguments);
646     }
647 
648     /**
649      * Performs a variable arity invocation, passing the arguments in the given array
650      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
651      * which mentions only the type {@code Object}, and whose arity is the length
652      * of the argument array.
653      * <p>
654      * This method is also equivalent to the following code:
655      * <blockquote><pre>{@code
656      *   invokeWithArguments(arguments.toArray()
657      * }</pre></blockquote>
658      *
659      * @param arguments the arguments to pass to the target
660      * @return the result returned by the target
661      * @throws NullPointerException if {@code arguments} is a null reference
662      * @throws ClassCastException if an argument cannot be converted by reference casting
663      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
664      * @throws Throwable anything thrown by the target method invocation
665      */
invokeWithArguments(java.util.List<?> arguments)666     public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
667         return invokeWithArguments(arguments.toArray());
668     }
669 
670     /**
671      * Produces an adapter method handle which adapts the type of the
672      * current method handle to a new type.
673      * The resulting method handle is guaranteed to report a type
674      * which is equal to the desired new type.
675      * <p>
676      * If the original type and new type are equal, returns {@code this}.
677      * <p>
678      * The new method handle, when invoked, will perform the following
679      * steps:
680      * <ul>
681      * <li>Convert the incoming argument list to match the original
682      *     method handle's argument list.
683      * <li>Invoke the original method handle on the converted argument list.
684      * <li>Convert any result returned by the original method handle
685      *     to the return type of new method handle.
686      * </ul>
687      * <p>
688      * This method provides the crucial behavioral difference between
689      * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
690      * The two methods
691      * perform the same steps when the caller's type descriptor exactly m atches
692      * the callee's, but when the types differ, plain {@link #invoke invoke}
693      * also calls {@code asType} (or some internal equivalent) in order
694      * to match up the caller's and callee's types.
695      * <p>
696      * If the current method is a variable arity method handle
697      * argument list conversion may involve the conversion and collection
698      * of several arguments into an array, as
699      * {@linkplain #asVarargsCollector described elsewhere}.
700      * In every other case, all conversions are applied <em>pairwise</em>,
701      * which means that each argument or return value is converted to
702      * exactly one argument or return value (or no return value).
703      * The applied conversions are defined by consulting the
704      * the corresponding component types of the old and new
705      * method handle types.
706      * <p>
707      * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
708      * or old and new return types.  Specifically, for some valid index {@code i}, let
709      * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
710      * Or else, going the other way for return values, let
711      * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
712      * If the types are the same, the new method handle makes no change
713      * to the corresponding argument or return value (if any).
714      * Otherwise, one of the following conversions is applied
715      * if possible:
716      * <ul>
717      * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
718      *     (The types do not need to be related in any particular way.
719      *     This is because a dynamic value of null can convert to any reference type.)
720      * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
721      *     conversion (JLS 5.3) is applied, if one exists.
722      *     (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
723      * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
724      *     a Java casting conversion (JLS 5.5) is applied if one exists.
725      *     (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
726      *     which is then widened as needed to <em>T1</em>.)
727      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
728      *     conversion will be applied at runtime, possibly followed
729      *     by a Java method invocation conversion (JLS 5.3)
730      *     on the primitive value.  (These are the primitive widening conversions.)
731      *     <em>T0</em> must be a wrapper class or a supertype of one.
732      *     (In the case where <em>T0</em> is Object, these are the conversions
733      *     allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
734      *     The unboxing conversion must have a possibility of success, which means that
735      *     if <em>T0</em> is not itself a wrapper class, there must exist at least one
736      *     wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
737      *     primitive value can be widened to <em>T1</em>.
738      * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
739      * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
740      * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
741      *     a zero value is introduced.
742      * </ul>
743      * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
744      * because neither corresponds specifically to the <em>dynamic type</em> of any
745      * actual argument or return value.)
746      * <p>
747      * The method handle conversion cannot be made if any one of the required
748      * pairwise conversions cannot be made.
749      * <p>
750      * At runtime, the conversions applied to reference arguments
751      * or return values may require additional runtime checks which can fail.
752      * An unboxing operation may fail because the original reference is null,
753      * causing a {@link java.lang.NullPointerException NullPointerException}.
754      * An unboxing operation or a reference cast may also fail on a reference
755      * to an object of the wrong type,
756      * causing a {@link java.lang.ClassCastException ClassCastException}.
757      * Although an unboxing operation may accept several kinds of wrappers,
758      * if none are available, a {@code ClassCastException} will be thrown.
759      *
760      * @param newType the expected type of the new method handle
761      * @return a method handle which delegates to {@code this} after performing
762      *           any necessary argument conversions, and arranges for any
763      *           necessary return value conversions
764      * @throws NullPointerException if {@code newType} is a null reference
765      * @throws WrongMethodTypeException if the conversion cannot be made
766      * @see MethodHandles#explicitCastArguments
767      */
asType(MethodType newType)768     public MethodHandle asType(MethodType newType) {
769         // Fast path alternative to a heavyweight {@code asType} call.
770         // Return 'this' if the conversion will be a no-op.
771         if (newType == type) {
772             return this;
773         }
774 
775         if (!type.isConvertibleTo(newType)) {
776             throw new WrongMethodTypeException("cannot convert " + this + " to " + newType);
777         }
778 
779         MethodHandle mh = duplicate();
780         mh.nominalType = newType;
781         return mh;
782     }
783 
784     /**
785      * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
786      * and spreads its elements as positional arguments.
787      * The new method handle adapts, as its <i>target</i>,
788      * the current method handle.  The type of the adapter will be
789      * the same as the type of the target, except that the final
790      * {@code arrayLength} parameters of the target's type are replaced
791      * by a single array parameter of type {@code arrayType}.
792      * <p>
793      * If the array element type differs from any of the corresponding
794      * argument types on the original target,
795      * the original target is adapted to take the array elements directly,
796      * as if by a call to {@link #asType asType}.
797      * <p>
798      * When called, the adapter replaces a trailing array argument
799      * by the array's elements, each as its own argument to the target.
800      * (The order of the arguments is preserved.)
801      * They are converted pairwise by casting and/or unboxing
802      * to the types of the trailing parameters of the target.
803      * Finally the target is called.
804      * What the target eventually returns is returned unchanged by the adapter.
805      * <p>
806      * Before calling the target, the adapter verifies that the array
807      * contains exactly enough elements to provide a correct argument count
808      * to the target method handle.
809      * (The array may also be null when zero elements are required.)
810      * <p>
811      * If, when the adapter is called, the supplied array argument does
812      * not have the correct number of elements, the adapter will throw
813      * an {@link IllegalArgumentException} instead of invoking the target.
814      * <p>
815      * Here are some simple examples of array-spreading method handles:
816      * <blockquote><pre>{@code
817 MethodHandle equals = publicLookup()
818   .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
819 assert( (boolean) equals.invokeExact("me", (Object)"me"));
820 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
821 // spread both arguments from a 2-array:
822 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
823 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
824 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
825 // try to spread from anything but a 2-array:
826 for (int n = 0; n <= 10; n++) {
827   Object[] badArityArgs = (n == 2 ? null : new Object[n]);
828   try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
829   catch (IllegalArgumentException ex) { } // OK
830 }
831 // spread both arguments from a String array:
832 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
833 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
834 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
835 // spread second arguments from a 1-array:
836 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
837 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
838 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
839 // spread no arguments from a 0-array or null:
840 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
841 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
842 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
843 // asSpreader and asCollector are approximate inverses:
844 for (int n = 0; n <= 2; n++) {
845     for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
846         MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
847         assert( (boolean) equals2.invokeWithArguments("me", "me"));
848         assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
849     }
850 }
851 MethodHandle caToString = publicLookup()
852   .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
853 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
854 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
855 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
856 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
857 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
858      * }</pre></blockquote>
859      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
860      * @param arrayLength the number of arguments to spread from an incoming array argument
861      * @return a new method handle which spreads its final array argument,
862      *         before calling the original method handle
863      * @throws NullPointerException if {@code arrayType} is a null reference
864      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
865      *         or if target does not have at least
866      *         {@code arrayLength} parameter types,
867      *         or if {@code arrayLength} is negative,
868      *         or if the resulting method handle's type would have
869      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
870      * @throws WrongMethodTypeException if the implied {@code asType} call fails
871      * @see #asCollector
872      */
asSpreader(Class<?> arrayType, int arrayLength)873     public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
874         MethodType postSpreadType = asSpreaderChecks(arrayType, arrayLength);
875 
876         final int targetParamCount = postSpreadType.parameterCount();
877         MethodType dropArrayArgs = postSpreadType.dropParameterTypes(
878                 (targetParamCount - arrayLength), targetParamCount);
879         MethodType adapterType = dropArrayArgs.appendParameterTypes(arrayType);
880 
881         return new Transformers.Spreader(this, adapterType, arrayLength);
882     }
883 
884     /**
885      * See if {@code asSpreader} can be validly called with the given arguments.
886      * Return the type of the method handle call after spreading but before conversions.
887      */
asSpreaderChecks(Class<?> arrayType, int arrayLength)888     private MethodType asSpreaderChecks(Class<?> arrayType, int arrayLength) {
889         spreadArrayChecks(arrayType, arrayLength);
890         int nargs = type().parameterCount();
891         if (nargs < arrayLength || arrayLength < 0)
892             throw newIllegalArgumentException("bad spread array length");
893         Class<?> arrayElement = arrayType.getComponentType();
894         MethodType mtype = type();
895         boolean match = true, fail = false;
896         for (int i = nargs - arrayLength; i < nargs; i++) {
897             Class<?> ptype = mtype.parameterType(i);
898             if (ptype != arrayElement) {
899                 match = false;
900                 if (!MethodType.canConvert(arrayElement, ptype)) {
901                     fail = true;
902                     break;
903                 }
904             }
905         }
906         if (match)  return mtype;
907         MethodType needType = mtype.asSpreaderType(arrayType, arrayLength);
908         if (!fail)  return needType;
909         // elicit an error:
910         this.asType(needType);
911         throw newInternalError("should not return", null);
912     }
913 
spreadArrayChecks(Class<?> arrayType, int arrayLength)914     private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
915         Class<?> arrayElement = arrayType.getComponentType();
916         if (arrayElement == null)
917             throw newIllegalArgumentException("not an array type", arrayType);
918         if ((arrayLength & 0x7F) != arrayLength) {
919             if ((arrayLength & 0xFF) != arrayLength)
920                 throw newIllegalArgumentException("array length is not legal", arrayLength);
921             assert(arrayLength >= 128);
922             if (arrayElement == long.class ||
923                 arrayElement == double.class)
924                 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
925         }
926     }
927 
928     /**
929      * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
930      * positional arguments and collects them into an array argument.
931      * The new method handle adapts, as its <i>target</i>,
932      * the current method handle.  The type of the adapter will be
933      * the same as the type of the target, except that a single trailing
934      * parameter (usually of type {@code arrayType}) is replaced by
935      * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
936      * <p>
937      * If the array type differs from the final argument type on the original target,
938      * the original target is adapted to take the array type directly,
939      * as if by a call to {@link #asType asType}.
940      * <p>
941      * When called, the adapter replaces its trailing {@code arrayLength}
942      * arguments by a single new array of type {@code arrayType}, whose elements
943      * comprise (in order) the replaced arguments.
944      * Finally the target is called.
945      * What the target eventually returns is returned unchanged by the adapter.
946      * <p>
947      * (The array may also be a shared constant when {@code arrayLength} is zero.)
948      * <p>
949      * (<em>Note:</em> The {@code arrayType} is often identical to the last
950      * parameter type of the original target.
951      * It is an explicit argument for symmetry with {@code asSpreader}, and also
952      * to allow the target to use a simple {@code Object} as its last parameter type.)
953      * <p>
954      * In order to create a collecting adapter which is not restricted to a particular
955      * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
956      * <p>
957      * Here are some examples of array-collecting method handles:
958      * <blockquote><pre>{@code
959 MethodHandle deepToString = publicLookup()
960   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
961 assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
962 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
963 assertEquals(methodType(String.class, Object.class), ts1.type());
964 //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
965 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
966 // arrayType can be a subtype of Object[]
967 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
968 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
969 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
970 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
971 assertEquals("[]", (String) ts0.invokeExact());
972 // collectors can be nested, Lisp-style
973 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
974 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
975 // arrayType can be any primitive array type
976 MethodHandle bytesToString = publicLookup()
977   .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
978   .asCollector(byte[].class, 3);
979 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
980 MethodHandle longsToString = publicLookup()
981   .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
982   .asCollector(long[].class, 1);
983 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
984      * }</pre></blockquote>
985      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
986      * @param arrayLength the number of arguments to collect into a new array argument
987      * @return a new method handle which collects some trailing argument
988      *         into an array, before calling the original method handle
989      * @throws NullPointerException if {@code arrayType} is a null reference
990      * @throws IllegalArgumentException if {@code arrayType} is not an array type
991      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type,
992      *         or {@code arrayLength} is not a legal array size,
993      *         or the resulting method handle's type would have
994      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
995      * @throws WrongMethodTypeException if the implied {@code asType} call fails
996      * @see #asSpreader
997      * @see #asVarargsCollector
998      */
asCollector(Class<?> arrayType, int arrayLength)999     public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1000         asCollectorChecks(arrayType, arrayLength);
1001 
1002         return new Transformers.Collector(this, arrayType, arrayLength);
1003     }
1004 
1005     /**
1006      * See if {@code asCollector} can be validly called with the given arguments.
1007      * Return false if the last parameter is not an exact match to arrayType.
1008      */
asCollectorChecks(Class<?> arrayType, int arrayLength)1009     /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
1010         spreadArrayChecks(arrayType, arrayLength);
1011         int nargs = type().parameterCount();
1012         if (nargs != 0) {
1013             Class<?> lastParam = type().parameterType(nargs-1);
1014             if (lastParam == arrayType)  return true;
1015             if (lastParam.isAssignableFrom(arrayType))  return false;
1016         }
1017         throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1018     }
1019 
1020     /**
1021      * Makes a <em>variable arity</em> adapter which is able to accept
1022      * any number of trailing positional arguments and collect them
1023      * into an array argument.
1024      * <p>
1025      * The type and behavior of the adapter will be the same as
1026      * the type and behavior of the target, except that certain
1027      * {@code invoke} and {@code asType} requests can lead to
1028      * trailing positional arguments being collected into target's
1029      * trailing parameter.
1030      * Also, the last parameter type of the adapter will be
1031      * {@code arrayType}, even if the target has a different
1032      * last parameter type.
1033      * <p>
1034      * This transformation may return {@code this} if the method handle is
1035      * already of variable arity and its trailing parameter type
1036      * is identical to {@code arrayType}.
1037      * <p>
1038      * When called with {@link #invokeExact invokeExact}, the adapter invokes
1039      * the target with no argument changes.
1040      * (<em>Note:</em> This behavior is different from a
1041      * {@linkplain #asCollector fixed arity collector},
1042      * since it accepts a whole array of indeterminate length,
1043      * rather than a fixed number of arguments.)
1044      * <p>
1045      * When called with plain, inexact {@link #invoke invoke}, if the caller
1046      * type is the same as the adapter, the adapter invokes the target as with
1047      * {@code invokeExact}.
1048      * (This is the normal behavior for {@code invoke} when types match.)
1049      * <p>
1050      * Otherwise, if the caller and adapter arity are the same, and the
1051      * trailing parameter type of the caller is a reference type identical to
1052      * or assignable to the trailing parameter type of the adapter,
1053      * the arguments and return values are converted pairwise,
1054      * as if by {@link #asType asType} on a fixed arity
1055      * method handle.
1056      * <p>
1057      * Otherwise, the arities differ, or the adapter's trailing parameter
1058      * type is not assignable from the corresponding caller type.
1059      * In this case, the adapter replaces all trailing arguments from
1060      * the original trailing argument position onward, by
1061      * a new array of type {@code arrayType}, whose elements
1062      * comprise (in order) the replaced arguments.
1063      * <p>
1064      * The caller type must provides as least enough arguments,
1065      * and of the correct type, to satisfy the target's requirement for
1066      * positional arguments before the trailing array argument.
1067      * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1068      * where {@code N} is the arity of the target.
1069      * Also, there must exist conversions from the incoming arguments
1070      * to the target's arguments.
1071      * As with other uses of plain {@code invoke}, if these basic
1072      * requirements are not fulfilled, a {@code WrongMethodTypeException}
1073      * may be thrown.
1074      * <p>
1075      * In all cases, what the target eventually returns is returned unchanged by the adapter.
1076      * <p>
1077      * In the final case, it is exactly as if the target method handle were
1078      * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1079      * to the arity required by the caller type.
1080      * (As with {@code asCollector}, if the array length is zero,
1081      * a shared constant may be used instead of a new array.
1082      * If the implied call to {@code asCollector} would throw
1083      * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1084      * the call to the variable arity adapter must throw
1085      * {@code WrongMethodTypeException}.)
1086      * <p>
1087      * The behavior of {@link #asType asType} is also specialized for
1088      * variable arity adapters, to maintain the invariant that
1089      * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1090      * call to adjust the target type, followed by {@code invokeExact}.
1091      * Therefore, a variable arity adapter responds
1092      * to an {@code asType} request by building a fixed arity collector,
1093      * if and only if the adapter and requested type differ either
1094      * in arity or trailing argument type.
1095      * The resulting fixed arity collector has its type further adjusted
1096      * (if necessary) to the requested type by pairwise conversion,
1097      * as if by another application of {@code asType}.
1098      * <p>
1099      * When a method handle is obtained by executing an {@code ldc} instruction
1100      * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1101      * as a variable arity method (with the modifier bit {@code 0x0080}),
1102      * the method handle will accept multiple arities, as if the method handle
1103      * constant were created by means of a call to {@code asVarargsCollector}.
1104      * <p>
1105      * In order to create a collecting adapter which collects a predetermined
1106      * number of arguments, and whose type reflects this predetermined number,
1107      * use {@link #asCollector asCollector} instead.
1108      * <p>
1109      * No method handle transformations produce new method handles with
1110      * variable arity, unless they are documented as doing so.
1111      * Therefore, besides {@code asVarargsCollector},
1112      * all methods in {@code MethodHandle} and {@code MethodHandles}
1113      * will return a method handle with fixed arity,
1114      * except in the cases where they are specified to return their original
1115      * operand (e.g., {@code asType} of the method handle's own type).
1116      * <p>
1117      * Calling {@code asVarargsCollector} on a method handle which is already
1118      * of variable arity will produce a method handle with the same type and behavior.
1119      * It may (or may not) return the original variable arity method handle.
1120      * <p>
1121      * Here is an example, of a list-making variable arity method handle:
1122      * <blockquote><pre>{@code
1123 MethodHandle deepToString = publicLookup()
1124   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1125 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1126 assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
1127 assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
1128 assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
1129 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1130 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1131 MethodHandle asList = publicLookup()
1132   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1133 assertEquals(methodType(List.class, Object[].class), asList.type());
1134 assert(asList.isVarargsCollector());
1135 assertEquals("[]", asList.invoke().toString());
1136 assertEquals("[1]", asList.invoke(1).toString());
1137 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1138 String[] argv = { "three", "thee", "tee" };
1139 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1140 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1141 List ls = (List) asList.invoke((Object)argv);
1142 assertEquals(1, ls.size());
1143 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1144      * }</pre></blockquote>
1145      * <p style="font-size:smaller;">
1146      * <em>Discussion:</em>
1147      * These rules are designed as a dynamically-typed variation
1148      * of the Java rules for variable arity methods.
1149      * In both cases, callers to a variable arity method or method handle
1150      * can either pass zero or more positional arguments, or else pass
1151      * pre-collected arrays of any length.  Users should be aware of the
1152      * special role of the final argument, and of the effect of a
1153      * type match on that final argument, which determines whether
1154      * or not a single trailing argument is interpreted as a whole
1155      * array or a single element of an array to be collected.
1156      * Note that the dynamic type of the trailing argument has no
1157      * effect on this decision, only a comparison between the symbolic
1158      * type descriptor of the call site and the type descriptor of the method handle.)
1159      *
1160      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1161      * @return a new method handle which can collect any number of trailing arguments
1162      *         into an array, before calling the original method handle
1163      * @throws NullPointerException if {@code arrayType} is a null reference
1164      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1165      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type
1166      * @see #asCollector
1167      * @see #isVarargsCollector
1168      * @see #asFixedArity
1169      */
asVarargsCollector(Class<?> arrayType)1170     public MethodHandle asVarargsCollector(Class<?> arrayType) {
1171         arrayType.getClass(); // explicit NPE
1172         boolean lastMatch = asCollectorChecks(arrayType, 0);
1173         if (isVarargsCollector() && lastMatch)
1174             return this;
1175 
1176         return new Transformers.VarargsCollector(this);
1177     }
1178 
1179     /**
1180      * Determines if this method handle
1181      * supports {@linkplain #asVarargsCollector variable arity} calls.
1182      * Such method handles arise from the following sources:
1183      * <ul>
1184      * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1185      * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1186      *     which resolves to a variable arity Java method or constructor
1187      * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1188      *     which resolves to a variable arity Java method or constructor
1189      * </ul>
1190      * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1191      * @see #asVarargsCollector
1192      * @see #asFixedArity
1193      */
isVarargsCollector()1194     public boolean isVarargsCollector() {
1195         return false;
1196     }
1197 
1198     /**
1199      * Makes a <em>fixed arity</em> method handle which is otherwise
1200      * equivalent to the current method handle.
1201      * <p>
1202      * If the current method handle is not of
1203      * {@linkplain #asVarargsCollector variable arity},
1204      * the current method handle is returned.
1205      * This is true even if the current method handle
1206      * could not be a valid input to {@code asVarargsCollector}.
1207      * <p>
1208      * Otherwise, the resulting fixed-arity method handle has the same
1209      * type and behavior of the current method handle,
1210      * except that {@link #isVarargsCollector isVarargsCollector}
1211      * will be false.
1212      * The fixed-arity method handle may (or may not) be the
1213      * a previous argument to {@code asVarargsCollector}.
1214      * <p>
1215      * Here is an example, of a list-making variable arity method handle:
1216      * <blockquote><pre>{@code
1217 MethodHandle asListVar = publicLookup()
1218   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1219   .asVarargsCollector(Object[].class);
1220 MethodHandle asListFix = asListVar.asFixedArity();
1221 assertEquals("[1]", asListVar.invoke(1).toString());
1222 Exception caught = null;
1223 try { asListFix.invoke((Object)1); }
1224 catch (Exception ex) { caught = ex; }
1225 assert(caught instanceof ClassCastException);
1226 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1227 try { asListFix.invoke("two", "too"); }
1228 catch (Exception ex) { caught = ex; }
1229 assert(caught instanceof WrongMethodTypeException);
1230 Object[] argv = { "three", "thee", "tee" };
1231 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1232 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1233 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1234 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1235      * }</pre></blockquote>
1236      *
1237      * @return a new method handle which accepts only a fixed number of arguments
1238      * @see #asVarargsCollector
1239      * @see #isVarargsCollector
1240      */
asFixedArity()1241     public MethodHandle asFixedArity() {
1242         // Android-changed: implementation specific.
1243         MethodHandle mh = this;
1244         if (mh.isVarargsCollector()) {
1245             mh = ((Transformers.VarargsCollector) mh).asFixedArity();
1246         }
1247         assert(!mh.isVarargsCollector());
1248         return mh;
1249     }
1250 
1251     /**
1252      * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1253      * The new method handle adapts, as its <i>target</i>,
1254      * the current method handle by binding it to the given argument.
1255      * The type of the bound handle will be
1256      * the same as the type of the target, except that a single leading
1257      * reference parameter will be omitted.
1258      * <p>
1259      * When called, the bound handle inserts the given value {@code x}
1260      * as a new leading argument to the target.  The other arguments are
1261      * also passed unchanged.
1262      * What the target eventually returns is returned unchanged by the bound handle.
1263      * <p>
1264      * The reference {@code x} must be convertible to the first parameter
1265      * type of the target.
1266      * <p>
1267      * (<em>Note:</em>  Because method handles are immutable, the target method handle
1268      * retains its original type and behavior.)
1269      * @param x  the value to bind to the first argument of the target
1270      * @return a new method handle which prepends the given value to the incoming
1271      *         argument list, before calling the original method handle
1272      * @throws IllegalArgumentException if the target does not have a
1273      *         leading parameter type that is a reference type
1274      * @throws ClassCastException if {@code x} cannot be converted
1275      *         to the leading parameter type of the target
1276      * @see MethodHandles#insertArguments
1277      */
bindTo(Object x)1278     public MethodHandle bindTo(Object x) {
1279         x = type.leadingReferenceParameter().cast(x);  // throw CCE if needed
1280 
1281         return new Transformers.BindTo(this, x);
1282     }
1283 
1284     /**
1285      * Returns a string representation of the method handle,
1286      * starting with the string {@code "MethodHandle"} and
1287      * ending with the string representation of the method handle's type.
1288      * In other words, this method returns a string equal to the value of:
1289      * <blockquote><pre>{@code
1290      * "MethodHandle" + type().toString()
1291      * }</pre></blockquote>
1292      * <p>
1293      * (<em>Note:</em>  Future releases of this API may add further information
1294      * to the string representation.
1295      * Therefore, the present syntax should not be parsed by applications.)
1296      *
1297      * @return a string representation of the method handle
1298      */
1299     @Override
toString()1300     public String toString() {
1301         // Android-changed: Removed debugging support.
1302         return "MethodHandle"+type;
1303     }
1304 
1305     /** @hide */
getHandleKind()1306     public int getHandleKind() {
1307         return handleKind;
1308     }
1309 
1310     /** @hide */
transform(EmulatedStackFrame arguments)1311     protected void transform(EmulatedStackFrame arguments) throws Throwable {
1312         throw new AssertionError("MethodHandle.transform should never be called.");
1313     }
1314 
1315     /**
1316      * Creates a copy of this method handle, copying all relevant data.
1317      *
1318      * @hide
1319      */
duplicate()1320     protected MethodHandle duplicate() {
1321         try {
1322             return (MethodHandle) this.clone();
1323         } catch (CloneNotSupportedException cnse) {
1324             throw new AssertionError("Subclass of Transformer is not cloneable");
1325         }
1326     }
1327 
1328 
1329     /**
1330      * This is the entry point for all transform calls, and dispatches to the protected
1331      * transform method. This layer of indirection exists purely for convenience, because
1332      * we can invoke-direct on a fixed ArtMethod for all transform variants.
1333      *
1334      * NOTE: If this extra layer of indirection proves to be a problem, we can get rid
1335      * of this layer of indirection at the cost of some additional ugliness.
1336      */
transformInternal(EmulatedStackFrame arguments)1337     private void transformInternal(EmulatedStackFrame arguments) throws Throwable {
1338         transform(arguments);
1339     }
1340 
1341     // Android-changed: Removed implementation details :
1342     //
1343     // String standardString();
1344     // String debugString();
1345     //
1346     //// Implementation methods.
1347     //// Sub-classes can override these default implementations.
1348     //// All these methods assume arguments are already validated.
1349     //
1350     // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch
1351     //
1352     // BoundMethodHandle bindArgumentL(int pos, Object value);
1353     // /*non-public*/ MethodHandle setVarargs(MemberName member);
1354     // /*non-public*/ MethodHandle viewAsType(MethodType newType, boolean strict);
1355     // /*non-public*/ boolean viewAsTypeChecks(MethodType newType, boolean strict);
1356     //
1357     // Decoding
1358     //
1359     // /*non-public*/ LambdaForm internalForm();
1360     // /*non-public*/ MemberName internalMemberName();
1361     // /*non-public*/ Class<?> internalCallerClass();
1362     // /*non-public*/ MethodHandleImpl.Intrinsic intrinsicName();
1363     // /*non-public*/ MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial);
1364     // /*non-public*/ boolean isInvokeSpecial();
1365     // /*non-public*/ Object internalValues();
1366     // /*non-public*/ Object internalProperties();
1367     //
1368     //// Method handle implementation methods.
1369     //// Sub-classes can override these default implementations.
1370     //// All these methods assume arguments are already validated.
1371     //
1372     // /*non-public*/ abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);
1373     // abstract BoundMethodHandle rebind();
1374     // /*non-public*/ void updateForm(LambdaForm newForm);
1375     // /*non-public*/ void customize();
1376     // private static final long FORM_OFFSET;
1377 }
1378