1 /* 2 * Copyright (c) 2008, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package java.lang.invoke; 27 28 29 import java.util.*; 30 import java.lang.invoke.LambdaForm.BasicType; 31 import sun.invoke.util.*; 32 import sun.misc.Unsafe; 33 34 import static java.lang.invoke.MethodHandleStatics.*; 35 import static java.lang.invoke.LambdaForm.BasicType.*; 36 37 /** 38 * A method handle is a typed, directly executable reference to an underlying method, 39 * constructor, field, or similar low-level operation, with optional 40 * transformations of arguments or return values. 41 * These transformations are quite general, and include such patterns as 42 * {@linkplain #asType conversion}, 43 * {@linkplain #bindTo insertion}, 44 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion}, 45 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}. 46 * 47 * <h1>Method handle contents</h1> 48 * Method handles are dynamically and strongly typed according to their parameter and return types. 49 * They are not distinguished by the name or the defining class of their underlying methods. 50 * A method handle must be invoked using a symbolic type descriptor which matches 51 * the method handle's own {@linkplain #type type descriptor}. 52 * <p> 53 * Every method handle reports its type descriptor via the {@link #type type} accessor. 54 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object, 55 * whose structure is a series of classes, one of which is 56 * the return type of the method (or {@code void.class} if none). 57 * <p> 58 * A method handle's type controls the types of invocations it accepts, 59 * and the kinds of transformations that apply to it. 60 * <p> 61 * A method handle contains a pair of special invoker methods 62 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}. 63 * Both invoker methods provide direct access to the method handle's 64 * underlying method, constructor, field, or other operation, 65 * as modified by transformations of arguments and return values. 66 * Both invokers accept calls which exactly match the method handle's own type. 67 * The plain, inexact invoker also accepts a range of other call types. 68 * <p> 69 * Method handles are immutable and have no visible state. 70 * Of course, they can be bound to underlying methods or data which exhibit state. 71 * With respect to the Java Memory Model, any method handle will behave 72 * as if all of its (internal) fields are final variables. This means that any method 73 * handle made visible to the application will always be fully formed. 74 * This is true even if the method handle is published through a shared 75 * variable in a data race. 76 * <p> 77 * Method handles cannot be subclassed by the user. 78 * Implementations may (or may not) create internal subclasses of {@code MethodHandle} 79 * which may be visible via the {@link java.lang.Object#getClass Object.getClass} 80 * operation. The programmer should not draw conclusions about a method handle 81 * from its specific class, as the method handle class hierarchy (if any) 82 * may change from time to time or across implementations from different vendors. 83 * 84 * <h1>Method handle compilation</h1> 85 * A Java method call expression naming {@code invokeExact} or {@code invoke} 86 * can invoke a method handle from Java source code. 87 * From the viewpoint of source code, these methods can take any arguments 88 * and their result can be cast to any return type. 89 * Formally this is accomplished by giving the invoker methods 90 * {@code Object} return types and variable arity {@code Object} arguments, 91 * but they have an additional quality called <em>signature polymorphism</em> 92 * which connects this freedom of invocation directly to the JVM execution stack. 93 * <p> 94 * As is usual with virtual methods, source-level calls to {@code invokeExact} 95 * and {@code invoke} compile to an {@code invokevirtual} instruction. 96 * More unusually, the compiler must record the actual argument types, 97 * and may not perform method invocation conversions on the arguments. 98 * Instead, it must push them on the stack according to their own unconverted types. 99 * The method handle object itself is pushed on the stack before the arguments. 100 * The compiler then calls the method handle with a symbolic type descriptor which 101 * describes the argument and return types. 102 * <p> 103 * To issue a complete symbolic type descriptor, the compiler must also determine 104 * the return type. This is based on a cast on the method invocation expression, 105 * if there is one, or else {@code Object} if the invocation is an expression 106 * or else {@code void} if the invocation is a statement. 107 * The cast may be to a primitive type (but not {@code void}). 108 * <p> 109 * As a corner case, an uncasted {@code null} argument is given 110 * a symbolic type descriptor of {@code java.lang.Void}. 111 * The ambiguity with the type {@code Void} is harmless, since there are no references of type 112 * {@code Void} except the null reference. 113 * 114 * <h1>Method handle invocation</h1> 115 * The first time a {@code invokevirtual} instruction is executed 116 * it is linked, by symbolically resolving the names in the instruction 117 * and verifying that the method call is statically legal. 118 * This is true of calls to {@code invokeExact} and {@code invoke}. 119 * In this case, the symbolic type descriptor emitted by the compiler is checked for 120 * correct syntax and names it contains are resolved. 121 * Thus, an {@code invokevirtual} instruction which invokes 122 * a method handle will always link, as long 123 * as the symbolic type descriptor is syntactically well-formed 124 * and the types exist. 125 * <p> 126 * When the {@code invokevirtual} is executed after linking, 127 * the receiving method handle's type is first checked by the JVM 128 * to ensure that it matches the symbolic type descriptor. 129 * If the type match fails, it means that the method which the 130 * caller is invoking is not present on the individual 131 * method handle being invoked. 132 * <p> 133 * In the case of {@code invokeExact}, the type descriptor of the invocation 134 * (after resolving symbolic type names) must exactly match the method type 135 * of the receiving method handle. 136 * In the case of plain, inexact {@code invoke}, the resolved type descriptor 137 * must be a valid argument to the receiver's {@link #asType asType} method. 138 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}. 139 * <p> 140 * After type matching, a call to {@code invokeExact} directly 141 * and immediately invoke the method handle's underlying method 142 * (or other behavior, as the case may be). 143 * <p> 144 * A call to plain {@code invoke} works the same as a call to 145 * {@code invokeExact}, if the symbolic type descriptor specified by the caller 146 * exactly matches the method handle's own type. 147 * If there is a type mismatch, {@code invoke} attempts 148 * to adjust the type of the receiving method handle, 149 * as if by a call to {@link #asType asType}, 150 * to obtain an exactly invokable method handle {@code M2}. 151 * This allows a more powerful negotiation of method type 152 * between caller and callee. 153 * <p> 154 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable, 155 * and implementations are therefore not required to materialize it.) 156 * 157 * <h1>Invocation checking</h1> 158 * In typical programs, method handle type matching will usually succeed. 159 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, 160 * either directly (in the case of {@code invokeExact}) or indirectly as if 161 * by a failed call to {@code asType} (in the case of {@code invoke}). 162 * <p> 163 * Thus, a method type mismatch which might show up as a linkage error 164 * in a statically typed program can show up as 165 * a dynamic {@code WrongMethodTypeException} 166 * in a program which uses method handles. 167 * <p> 168 * Because method types contain "live" {@code Class} objects, 169 * method type matching takes into account both types names and class loaders. 170 * Thus, even if a method handle {@code M} is created in one 171 * class loader {@code L1} and used in another {@code L2}, 172 * method handle calls are type-safe, because the caller's symbolic type 173 * descriptor, as resolved in {@code L2}, 174 * is matched against the original callee method's symbolic type descriptor, 175 * as resolved in {@code L1}. 176 * The resolution in {@code L1} happens when {@code M} is created 177 * and its type is assigned, while the resolution in {@code L2} happens 178 * when the {@code invokevirtual} instruction is linked. 179 * <p> 180 * Apart from the checking of type descriptors, 181 * a method handle's capability to call its underlying method is unrestricted. 182 * If a method handle is formed on a non-public method by a class 183 * that has access to that method, the resulting handle can be used 184 * in any place by any caller who receives a reference to it. 185 * <p> 186 * Unlike with the Core Reflection API, where access is checked every time 187 * a reflective method is invoked, 188 * method handle access checking is performed 189 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>. 190 * In the case of {@code ldc} (see below), access checking is performed as part of linking 191 * the constant pool entry underlying the constant method handle. 192 * <p> 193 * Thus, handles to non-public methods, or to methods in non-public classes, 194 * should generally be kept secret. 195 * They should not be passed to untrusted code unless their use from 196 * the untrusted code would be harmless. 197 * 198 * <h1>Method handle creation</h1> 199 * Java code can create a method handle that directly accesses 200 * any method, constructor, or field that is accessible to that code. 201 * This is done via a reflective, capability-based API called 202 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup} 203 * For example, a static method handle can be obtained 204 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}. 205 * There are also conversion methods from Core Reflection API objects, 206 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 207 * <p> 208 * Like classes and strings, method handles that correspond to accessible 209 * fields, methods, and constructors can also be represented directly 210 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes. 211 * A new type of constant pool entry, {@code CONSTANT_MethodHandle}, 212 * refers directly to an associated {@code CONSTANT_Methodref}, 213 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref} 214 * constant pool entry. 215 * (For full details on method handle constants, 216 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.) 217 * <p> 218 * Method handles produced by lookups or constant loads from methods or 219 * constructors with the variable arity modifier bit ({@code 0x0080}) 220 * have a corresponding variable arity, as if they were defined with 221 * the help of {@link #asVarargsCollector asVarargsCollector}. 222 * <p> 223 * A method reference may refer either to a static or non-static method. 224 * In the non-static case, the method handle type includes an explicit 225 * receiver argument, prepended before any other arguments. 226 * In the method handle's type, the initial receiver argument is typed 227 * according to the class under which the method was initially requested. 228 * (E.g., if a non-static method handle is obtained via {@code ldc}, 229 * the type of the receiver is the class named in the constant pool entry.) 230 * <p> 231 * Method handle constants are subject to the same link-time access checks 232 * their corresponding bytecode instructions, and the {@code ldc} instruction 233 * will throw corresponding linkage errors if the bytecode behaviors would 234 * throw such errors. 235 * <p> 236 * As a corollary of this, access to protected members is restricted 237 * to receivers only of the accessing class, or one of its subclasses, 238 * and the accessing class must in turn be a subclass (or package sibling) 239 * of the protected member's defining class. 240 * If a method reference refers to a protected non-static method or field 241 * of a class outside the current package, the receiver argument will 242 * be narrowed to the type of the accessing class. 243 * <p> 244 * When a method handle to a virtual method is invoked, the method is 245 * always looked up in the receiver (that is, the first argument). 246 * <p> 247 * A non-virtual method handle to a specific virtual method implementation 248 * can also be created. These do not perform virtual lookup based on 249 * receiver type. Such a method handle simulates the effect of 250 * an {@code invokespecial} instruction to the same method. 251 * 252 * <h1>Usage examples</h1> 253 * Here are some examples of usage: 254 * <blockquote><pre>{@code 255 Object x, y; String s; int i; 256 MethodType mt; MethodHandle mh; 257 MethodHandles.Lookup lookup = MethodHandles.lookup(); 258 // mt is (char,char)String 259 mt = MethodType.methodType(String.class, char.class, char.class); 260 mh = lookup.findVirtual(String.class, "replace", mt); 261 s = (String) mh.invokeExact("daddy",'d','n'); 262 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String; 263 assertEquals(s, "nanny"); 264 // weakly typed invocation (using MHs.invoke) 265 s = (String) mh.invokeWithArguments("sappy", 'p', 'v'); 266 assertEquals(s, "savvy"); 267 // mt is (Object[])List 268 mt = MethodType.methodType(java.util.List.class, Object[].class); 269 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt); 270 assert(mh.isVarargsCollector()); 271 x = mh.invoke("one", "two"); 272 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object; 273 assertEquals(x, java.util.Arrays.asList("one","two")); 274 // mt is (Object,Object,Object)Object 275 mt = MethodType.genericMethodType(3); 276 mh = mh.asType(mt); 277 x = mh.invokeExact((Object)1, (Object)2, (Object)3); 278 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object; 279 assertEquals(x, java.util.Arrays.asList(1,2,3)); 280 // mt is ()int 281 mt = MethodType.methodType(int.class); 282 mh = lookup.findVirtual(java.util.List.class, "size", mt); 283 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3)); 284 // invokeExact(Ljava/util/List;)I 285 assert(i == 3); 286 mt = MethodType.methodType(void.class, String.class); 287 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt); 288 mh.invokeExact(System.out, "Hello, world."); 289 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V 290 * }</pre></blockquote> 291 * Each of the above calls to {@code invokeExact} or plain {@code invoke} 292 * generates a single invokevirtual instruction with 293 * the symbolic type descriptor indicated in the following comment. 294 * In these examples, the helper method {@code assertEquals} is assumed to 295 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals} 296 * on its arguments, and asserts that the result is true. 297 * 298 * <h1>Exceptions</h1> 299 * The methods {@code invokeExact} and {@code invoke} are declared 300 * to throw {@link java.lang.Throwable Throwable}, 301 * which is to say that there is no static restriction on what a method handle 302 * can throw. Since the JVM does not distinguish between checked 303 * and unchecked exceptions (other than by their class, of course), 304 * there is no particular effect on bytecode shape from ascribing 305 * checked exceptions to method handle invocations. But in Java source 306 * code, methods which perform method handle calls must either explicitly 307 * throw {@code Throwable}, or else must catch all 308 * throwables locally, rethrowing only those which are legal in the context, 309 * and wrapping ones which are illegal. 310 * 311 * <h1><a name="sigpoly"></a>Signature polymorphism</h1> 312 * The unusual compilation and linkage behavior of 313 * {@code invokeExact} and plain {@code invoke} 314 * is referenced by the term <em>signature polymorphism</em>. 315 * As defined in the Java Language Specification, 316 * a signature polymorphic method is one which can operate with 317 * any of a wide range of call signatures and return types. 318 * <p> 319 * In source code, a call to a signature polymorphic method will 320 * compile, regardless of the requested symbolic type descriptor. 321 * As usual, the Java compiler emits an {@code invokevirtual} 322 * instruction with the given symbolic type descriptor against the named method. 323 * The unusual part is that the symbolic type descriptor is derived from 324 * the actual argument and return types, not from the method declaration. 325 * <p> 326 * When the JVM processes bytecode containing signature polymorphic calls, 327 * it will successfully link any such call, regardless of its symbolic type descriptor. 328 * (In order to retain type safety, the JVM will guard such calls with suitable 329 * dynamic type checks, as described elsewhere.) 330 * <p> 331 * Bytecode generators, including the compiler back end, are required to emit 332 * untransformed symbolic type descriptors for these methods. 333 * Tools which determine symbolic linkage are required to accept such 334 * untransformed descriptors, without reporting linkage errors. 335 * 336 * <h1>Interoperation between method handles and the Core Reflection API</h1> 337 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, 338 * any class member represented by a Core Reflection API object 339 * can be converted to a behaviorally equivalent method handle. 340 * For example, a reflective {@link java.lang.reflect.Method Method} can 341 * be converted to a method handle using 342 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 343 * The resulting method handles generally provide more direct and efficient 344 * access to the underlying class members. 345 * <p> 346 * As a special case, 347 * when the Core Reflection API is used to view the signature polymorphic 348 * methods {@code invokeExact} or plain {@code invoke} in this class, 349 * they appear as ordinary non-polymorphic methods. 350 * Their reflective appearance, as viewed by 351 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, 352 * is unaffected by their special status in this API. 353 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} 354 * will report exactly those modifier bits required for any similarly 355 * declared method, including in this case {@code native} and {@code varargs} bits. 356 * <p> 357 * As with any reflected method, these methods (when reflected) may be 358 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}. 359 * However, such reflective calls do not result in method handle invocations. 360 * Such a call, if passed the required argument 361 * (a single one, of type {@code Object[]}), will ignore the argument and 362 * will throw an {@code UnsupportedOperationException}. 363 * <p> 364 * Since {@code invokevirtual} instructions can natively 365 * invoke method handles under any symbolic type descriptor, this reflective view conflicts 366 * with the normal presentation of these methods via bytecodes. 367 * Thus, these two native methods, when reflectively viewed by 368 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only. 369 * <p> 370 * In order to obtain an invoker method for a particular type descriptor, 371 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker}, 372 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}. 373 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual} 374 * API is also able to return a method handle 375 * to call {@code invokeExact} or plain {@code invoke}, 376 * for any specified type descriptor . 377 * 378 * <h1>Interoperation between method handles and Java generics</h1> 379 * A method handle can be obtained on a method, constructor, or field 380 * which is declared with Java generic types. 381 * As with the Core Reflection API, the type of the method handle 382 * will constructed from the erasure of the source-level type. 383 * When a method handle is invoked, the types of its arguments 384 * or the return value cast type may be generic types or type instances. 385 * If this occurs, the compiler will replace those 386 * types by their erasures when it constructs the symbolic type descriptor 387 * for the {@code invokevirtual} instruction. 388 * <p> 389 * Method handles do not represent 390 * their function-like types in terms of Java parameterized (generic) types, 391 * because there are three mismatches between function-like types and parameterized 392 * Java types. 393 * <ul> 394 * <li>Method types range over all possible arities, 395 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments. 396 * Generics are not variadic, and so cannot represent this.</li> 397 * <li>Method types can specify arguments of primitive types, 398 * which Java generic types cannot range over.</li> 399 * <li>Higher order functions over method handles (combinators) are 400 * often generic across a wide range of function types, including 401 * those of multiple arities. It is impossible to represent such 402 * genericity with a Java type parameter.</li> 403 * </ul> 404 * 405 * <h1><a name="maxarity"></a>Arity limits</h1> 406 * The JVM imposes on all methods and constructors of any kind an absolute 407 * limit of 255 stacked arguments. This limit can appear more restrictive 408 * in certain cases: 409 * <ul> 410 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots. 411 * <li>A non-static method consumes an extra argument for the object on which the method is called. 412 * <li>A constructor consumes an extra argument for the object which is being constructed. 413 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual, 414 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object. 415 * </ul> 416 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments. 417 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it. 418 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}. 419 * In particular, a method handle’s type must not have an arity of the exact maximum 255. 420 * 421 * @see MethodType 422 * @see MethodHandles 423 * @author John Rose, JSR 292 EG 424 */ 425 public abstract class MethodHandle { 426 static { MethodHandleImpl.initStatics(); } 427 428 /** 429 * Internal marker interface which distinguishes (to the Java compiler) 430 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>. 431 */ 432 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD}) 433 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME) 434 @interface PolymorphicSignature { } 435 436 private final MethodType type; 437 /*private*/ final LambdaForm form; 438 // form is not private so that invokers can easily fetch it 439 /*private*/ MethodHandle asTypeCache; 440 // asTypeCache is not private so that invokers can easily fetch it 441 442 /** 443 * Reports the type of this method handle. 444 * Every invocation of this method handle via {@code invokeExact} must exactly match this type. 445 * @return the method handle type 446 */ 447 public MethodType type() { 448 return type; 449 } 450 451 /** 452 * Package-private constructor for the method handle implementation hierarchy. 453 * Method handle inheritance will be contained completely within 454 * the {@code java.lang.invoke} package. 455 */ 456 // @param type type (permanently assigned) of the new method handle 457 /*non-public*/ MethodHandle(MethodType type, LambdaForm form) { 458 type.getClass(); // explicit NPE 459 form.getClass(); // explicit NPE 460 this.type = type; 461 this.form = form; 462 463 form.prepare(); // TO DO: Try to delay this step until just before invocation. 464 } 465 466 /** 467 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match. 468 * The symbolic type descriptor at the call site of {@code invokeExact} must 469 * exactly match this method handle's {@link #type type}. 470 * No conversions are allowed on arguments or return values. 471 * <p> 472 * When this method is observed via the Core Reflection API, 473 * it will appear as a single native method, taking an object array and returning an object. 474 * If this native method is invoked directly via 475 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 476 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 477 * it will throw an {@code UnsupportedOperationException}. 478 * @param args the signature-polymorphic parameter list, statically represented using varargs 479 * @return the signature-polymorphic result, statically represented using {@code Object} 480 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor 481 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 482 */ 483 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable; 484 485 /** 486 * Invokes the method handle, allowing any caller type descriptor, 487 * and optionally performing conversions on arguments and return values. 488 * <p> 489 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type}, 490 * the call proceeds as if by {@link #invokeExact invokeExact}. 491 * <p> 492 * Otherwise, the call proceeds as if this method handle were first 493 * adjusted by calling {@link #asType asType} to adjust this method handle 494 * to the required type, and then the call proceeds as if by 495 * {@link #invokeExact invokeExact} on the adjusted method handle. 496 * <p> 497 * There is no guarantee that the {@code asType} call is actually made. 498 * If the JVM can predict the results of making the call, it may perform 499 * adaptations directly on the caller's arguments, 500 * and call the target method handle according to its own exact type. 501 * <p> 502 * The resolved type descriptor at the call site of {@code invoke} must 503 * be a valid argument to the receivers {@code asType} method. 504 * In particular, the caller must specify the same argument arity 505 * as the callee's type, 506 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}. 507 * <p> 508 * When this method is observed via the Core Reflection API, 509 * it will appear as a single native method, taking an object array and returning an object. 510 * If this native method is invoked directly via 511 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 512 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 513 * it will throw an {@code UnsupportedOperationException}. 514 * @param args the signature-polymorphic parameter list, statically represented using varargs 515 * @return the signature-polymorphic result, statically represented using {@code Object} 516 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor 517 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails 518 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 519 */ 520 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable; 521 522 /** 523 * Private method for trusted invocation of a method handle respecting simplified signatures. 524 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM. 525 * <p> 526 * The caller signature is restricted to the following basic types: 527 * Object, int, long, float, double, and void return. 528 * <p> 529 * The caller is responsible for maintaining type correctness by ensuring 530 * that the each outgoing argument value is a member of the range of the corresponding 531 * callee argument type. 532 * (The caller should therefore issue appropriate casts and integer narrowing 533 * operations on outgoing argument values.) 534 * The caller can assume that the incoming result value is part of the range 535 * of the callee's return type. 536 * @param args the signature-polymorphic parameter list, statically represented using varargs 537 * @return the signature-polymorphic result, statically represented using {@code Object} 538 */ 539 /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; 540 541 /** 542 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}. 543 * The caller signature is restricted to basic types as with {@code invokeBasic}. 544 * The trailing (not leading) argument must be a MemberName. 545 * @param args the signature-polymorphic parameter list, statically represented using varargs 546 * @return the signature-polymorphic result, statically represented using {@code Object} 547 */ 548 /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable; 549 550 /** 551 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}. 552 * The caller signature is restricted to basic types as with {@code invokeBasic}. 553 * The trailing (not leading) argument must be a MemberName. 554 * @param args the signature-polymorphic parameter list, statically represented using varargs 555 * @return the signature-polymorphic result, statically represented using {@code Object} 556 */ 557 /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable; 558 559 /** 560 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}. 561 * The caller signature is restricted to basic types as with {@code invokeBasic}. 562 * The trailing (not leading) argument must be a MemberName. 563 * @param args the signature-polymorphic parameter list, statically represented using varargs 564 * @return the signature-polymorphic result, statically represented using {@code Object} 565 */ 566 /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable; 567 568 /** 569 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}. 570 * The caller signature is restricted to basic types as with {@code invokeBasic}. 571 * The trailing (not leading) argument must be a MemberName. 572 * @param args the signature-polymorphic parameter list, statically represented using varargs 573 * @return the signature-polymorphic result, statically represented using {@code Object} 574 */ 575 /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; 576 577 /** 578 * Performs a variable arity invocation, passing the arguments in the given list 579 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 580 * which mentions only the type {@code Object}, and whose arity is the length 581 * of the argument list. 582 * <p> 583 * Specifically, execution proceeds as if by the following steps, 584 * although the methods are not guaranteed to be called if the JVM 585 * can predict their effects. 586 * <ul> 587 * <li>Determine the length of the argument array as {@code N}. 588 * For a null reference, {@code N=0}. </li> 589 * <li>Determine the general type {@code TN} of {@code N} arguments as 590 * as {@code TN=MethodType.genericMethodType(N)}.</li> 591 * <li>Force the original target method handle {@code MH0} to the 592 * required type, as {@code MH1 = MH0.asType(TN)}. </li> 593 * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li> 594 * <li>Invoke the type-adjusted method handle on the unpacked arguments: 595 * MH1.invokeExact(A0, ...). </li> 596 * <li>Take the return value as an {@code Object} reference. </li> 597 * </ul> 598 * <p> 599 * Because of the action of the {@code asType} step, the following argument 600 * conversions are applied as necessary: 601 * <ul> 602 * <li>reference casting 603 * <li>unboxing 604 * <li>widening primitive conversions 605 * </ul> 606 * <p> 607 * The result returned by the call is boxed if it is a primitive, 608 * or forced to null if the return type is void. 609 * <p> 610 * This call is equivalent to the following code: 611 * <blockquote><pre>{@code 612 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0); 613 * Object result = invoker.invokeExact(this, arguments); 614 * }</pre></blockquote> 615 * <p> 616 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke}, 617 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI. 618 * It can therefore be used as a bridge between native or reflective code and method handles. 619 * 620 * @param arguments the arguments to pass to the target 621 * @return the result returned by the target 622 * @throws ClassCastException if an argument cannot be converted by reference casting 623 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 624 * @throws Throwable anything thrown by the target method invocation 625 * @see MethodHandles#spreadInvoker 626 */ 627 public Object invokeWithArguments(Object... arguments) throws Throwable { 628 int argc = arguments == null ? 0 : arguments.length; 629 @SuppressWarnings("LocalVariableHidesMemberVariable") 630 MethodType type = type(); 631 if (type.parameterCount() != argc || isVarargsCollector()) { 632 // simulate invoke 633 return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments); 634 } 635 MethodHandle invoker = type.invokers().varargsInvoker(); 636 return invoker.invokeExact(this, arguments); 637 } 638 639 /** 640 * Performs a variable arity invocation, passing the arguments in the given array 641 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 642 * which mentions only the type {@code Object}, and whose arity is the length 643 * of the argument array. 644 * <p> 645 * This method is also equivalent to the following code: 646 * <blockquote><pre>{@code 647 * invokeWithArguments(arguments.toArray() 648 * }</pre></blockquote> 649 * 650 * @param arguments the arguments to pass to the target 651 * @return the result returned by the target 652 * @throws NullPointerException if {@code arguments} is a null reference 653 * @throws ClassCastException if an argument cannot be converted by reference casting 654 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 655 * @throws Throwable anything thrown by the target method invocation 656 */ 657 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { 658 return invokeWithArguments(arguments.toArray()); 659 } 660 661 /** 662 * Produces an adapter method handle which adapts the type of the 663 * current method handle to a new type. 664 * The resulting method handle is guaranteed to report a type 665 * which is equal to the desired new type. 666 * <p> 667 * If the original type and new type are equal, returns {@code this}. 668 * <p> 669 * The new method handle, when invoked, will perform the following 670 * steps: 671 * <ul> 672 * <li>Convert the incoming argument list to match the original 673 * method handle's argument list. 674 * <li>Invoke the original method handle on the converted argument list. 675 * <li>Convert any result returned by the original method handle 676 * to the return type of new method handle. 677 * </ul> 678 * <p> 679 * This method provides the crucial behavioral difference between 680 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. 681 * The two methods 682 * perform the same steps when the caller's type descriptor exactly m atches 683 * the callee's, but when the types differ, plain {@link #invoke invoke} 684 * also calls {@code asType} (or some internal equivalent) in order 685 * to match up the caller's and callee's types. 686 * <p> 687 * If the current method is a variable arity method handle 688 * argument list conversion may involve the conversion and collection 689 * of several arguments into an array, as 690 * {@linkplain #asVarargsCollector described elsewhere}. 691 * In every other case, all conversions are applied <em>pairwise</em>, 692 * which means that each argument or return value is converted to 693 * exactly one argument or return value (or no return value). 694 * The applied conversions are defined by consulting the 695 * the corresponding component types of the old and new 696 * method handle types. 697 * <p> 698 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, 699 * or old and new return types. Specifically, for some valid index {@code i}, let 700 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. 701 * Or else, going the other way for return values, let 702 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. 703 * If the types are the same, the new method handle makes no change 704 * to the corresponding argument or return value (if any). 705 * Otherwise, one of the following conversions is applied 706 * if possible: 707 * <ul> 708 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. 709 * (The types do not need to be related in any particular way. 710 * This is because a dynamic value of null can convert to any reference type.) 711 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation 712 * conversion (JLS 5.3) is applied, if one exists. 713 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) 714 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, 715 * a Java casting conversion (JLS 5.5) is applied if one exists. 716 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, 717 * which is then widened as needed to <em>T1</em>.) 718 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing 719 * conversion will be applied at runtime, possibly followed 720 * by a Java method invocation conversion (JLS 5.3) 721 * on the primitive value. (These are the primitive widening conversions.) 722 * <em>T0</em> must be a wrapper class or a supertype of one. 723 * (In the case where <em>T0</em> is Object, these are the conversions 724 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) 725 * The unboxing conversion must have a possibility of success, which means that 726 * if <em>T0</em> is not itself a wrapper class, there must exist at least one 727 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed 728 * primitive value can be widened to <em>T1</em>. 729 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded 730 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. 731 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, 732 * a zero value is introduced. 733 * </ul> 734 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, 735 * because neither corresponds specifically to the <em>dynamic type</em> of any 736 * actual argument or return value.) 737 * <p> 738 * The method handle conversion cannot be made if any one of the required 739 * pairwise conversions cannot be made. 740 * <p> 741 * At runtime, the conversions applied to reference arguments 742 * or return values may require additional runtime checks which can fail. 743 * An unboxing operation may fail because the original reference is null, 744 * causing a {@link java.lang.NullPointerException NullPointerException}. 745 * An unboxing operation or a reference cast may also fail on a reference 746 * to an object of the wrong type, 747 * causing a {@link java.lang.ClassCastException ClassCastException}. 748 * Although an unboxing operation may accept several kinds of wrappers, 749 * if none are available, a {@code ClassCastException} will be thrown. 750 * 751 * @param newType the expected type of the new method handle 752 * @return a method handle which delegates to {@code this} after performing 753 * any necessary argument conversions, and arranges for any 754 * necessary return value conversions 755 * @throws NullPointerException if {@code newType} is a null reference 756 * @throws WrongMethodTypeException if the conversion cannot be made 757 * @see MethodHandles#explicitCastArguments 758 */ 759 public MethodHandle asType(MethodType newType) { 760 // Fast path alternative to a heavyweight {@code asType} call. 761 // Return 'this' if the conversion will be a no-op. 762 if (newType == type) { 763 return this; 764 } 765 // Return 'this.asTypeCache' if the conversion is already memoized. 766 MethodHandle atc = asTypeCache; 767 if (atc != null && newType == atc.type) { 768 return atc; 769 } 770 return asTypeUncached(newType); 771 } 772 773 /** Override this to change asType behavior. */ 774 /*non-public*/ MethodHandle asTypeUncached(MethodType newType) { 775 if (!type.isConvertibleTo(newType)) 776 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType); 777 return asTypeCache = convertArguments(newType); 778 } 779 780 /** 781 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument 782 * and spreads its elements as positional arguments. 783 * The new method handle adapts, as its <i>target</i>, 784 * the current method handle. The type of the adapter will be 785 * the same as the type of the target, except that the final 786 * {@code arrayLength} parameters of the target's type are replaced 787 * by a single array parameter of type {@code arrayType}. 788 * <p> 789 * If the array element type differs from any of the corresponding 790 * argument types on the original target, 791 * the original target is adapted to take the array elements directly, 792 * as if by a call to {@link #asType asType}. 793 * <p> 794 * When called, the adapter replaces a trailing array argument 795 * by the array's elements, each as its own argument to the target. 796 * (The order of the arguments is preserved.) 797 * They are converted pairwise by casting and/or unboxing 798 * to the types of the trailing parameters of the target. 799 * Finally the target is called. 800 * What the target eventually returns is returned unchanged by the adapter. 801 * <p> 802 * Before calling the target, the adapter verifies that the array 803 * contains exactly enough elements to provide a correct argument count 804 * to the target method handle. 805 * (The array may also be null when zero elements are required.) 806 * <p> 807 * If, when the adapter is called, the supplied array argument does 808 * not have the correct number of elements, the adapter will throw 809 * an {@link IllegalArgumentException} instead of invoking the target. 810 * <p> 811 * Here are some simple examples of array-spreading method handles: 812 * <blockquote><pre>{@code 813 MethodHandle equals = publicLookup() 814 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class)); 815 assert( (boolean) equals.invokeExact("me", (Object)"me")); 816 assert(!(boolean) equals.invokeExact("me", (Object)"thee")); 817 // spread both arguments from a 2-array: 818 MethodHandle eq2 = equals.asSpreader(Object[].class, 2); 819 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" })); 820 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" })); 821 // try to spread from anything but a 2-array: 822 for (int n = 0; n <= 10; n++) { 823 Object[] badArityArgs = (n == 2 ? null : new Object[n]); 824 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); } 825 catch (IllegalArgumentException ex) { } // OK 826 } 827 // spread both arguments from a String array: 828 MethodHandle eq2s = equals.asSpreader(String[].class, 2); 829 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" })); 830 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" })); 831 // spread second arguments from a 1-array: 832 MethodHandle eq1 = equals.asSpreader(Object[].class, 1); 833 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" })); 834 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" })); 835 // spread no arguments from a 0-array or null: 836 MethodHandle eq0 = equals.asSpreader(Object[].class, 0); 837 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0])); 838 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null)); 839 // asSpreader and asCollector are approximate inverses: 840 for (int n = 0; n <= 2; n++) { 841 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) { 842 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n); 843 assert( (boolean) equals2.invokeWithArguments("me", "me")); 844 assert(!(boolean) equals2.invokeWithArguments("me", "thee")); 845 } 846 } 847 MethodHandle caToString = publicLookup() 848 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class)); 849 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray())); 850 MethodHandle caString3 = caToString.asCollector(char[].class, 3); 851 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C')); 852 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2); 853 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray())); 854 * }</pre></blockquote> 855 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 856 * @param arrayLength the number of arguments to spread from an incoming array argument 857 * @return a new method handle which spreads its final array argument, 858 * before calling the original method handle 859 * @throws NullPointerException if {@code arrayType} is a null reference 860 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 861 * or if target does not have at least 862 * {@code arrayLength} parameter types, 863 * or if {@code arrayLength} is negative, 864 * or if the resulting method handle's type would have 865 * <a href="MethodHandle.html#maxarity">too many parameters</a> 866 * @throws WrongMethodTypeException if the implied {@code asType} call fails 867 * @see #asCollector 868 */ 869 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) { 870 asSpreaderChecks(arrayType, arrayLength); 871 int spreadArgPos = type.parameterCount() - arrayLength; 872 return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength); 873 } 874 875 private void asSpreaderChecks(Class<?> arrayType, int arrayLength) { 876 spreadArrayChecks(arrayType, arrayLength); 877 int nargs = type().parameterCount(); 878 if (nargs < arrayLength || arrayLength < 0) 879 throw newIllegalArgumentException("bad spread array length"); 880 if (arrayType != Object[].class && arrayLength != 0) { 881 boolean sawProblem = false; 882 Class<?> arrayElement = arrayType.getComponentType(); 883 for (int i = nargs - arrayLength; i < nargs; i++) { 884 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) { 885 sawProblem = true; 886 break; 887 } 888 } 889 if (sawProblem) { 890 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList()); 891 for (int i = nargs - arrayLength; i < nargs; i++) { 892 ptypes.set(i, arrayElement); 893 } 894 // elicit an error: 895 this.asType(MethodType.methodType(type().returnType(), ptypes)); 896 } 897 } 898 } 899 900 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) { 901 Class<?> arrayElement = arrayType.getComponentType(); 902 if (arrayElement == null) 903 throw newIllegalArgumentException("not an array type", arrayType); 904 if ((arrayLength & 0x7F) != arrayLength) { 905 if ((arrayLength & 0xFF) != arrayLength) 906 throw newIllegalArgumentException("array length is not legal", arrayLength); 907 assert(arrayLength >= 128); 908 if (arrayElement == long.class || 909 arrayElement == double.class) 910 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength); 911 } 912 } 913 914 /** 915 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing 916 * positional arguments and collects them into an array argument. 917 * The new method handle adapts, as its <i>target</i>, 918 * the current method handle. The type of the adapter will be 919 * the same as the type of the target, except that a single trailing 920 * parameter (usually of type {@code arrayType}) is replaced by 921 * {@code arrayLength} parameters whose type is element type of {@code arrayType}. 922 * <p> 923 * If the array type differs from the final argument type on the original target, 924 * the original target is adapted to take the array type directly, 925 * as if by a call to {@link #asType asType}. 926 * <p> 927 * When called, the adapter replaces its trailing {@code arrayLength} 928 * arguments by a single new array of type {@code arrayType}, whose elements 929 * comprise (in order) the replaced arguments. 930 * Finally the target is called. 931 * What the target eventually returns is returned unchanged by the adapter. 932 * <p> 933 * (The array may also be a shared constant when {@code arrayLength} is zero.) 934 * <p> 935 * (<em>Note:</em> The {@code arrayType} is often identical to the last 936 * parameter type of the original target. 937 * It is an explicit argument for symmetry with {@code asSpreader}, and also 938 * to allow the target to use a simple {@code Object} as its last parameter type.) 939 * <p> 940 * In order to create a collecting adapter which is not restricted to a particular 941 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead. 942 * <p> 943 * Here are some examples of array-collecting method handles: 944 * <blockquote><pre>{@code 945 MethodHandle deepToString = publicLookup() 946 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 947 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"})); 948 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1); 949 assertEquals(methodType(String.class, Object.class), ts1.type()); 950 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL 951 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"})); 952 // arrayType can be a subtype of Object[] 953 MethodHandle ts2 = deepToString.asCollector(String[].class, 2); 954 assertEquals(methodType(String.class, String.class, String.class), ts2.type()); 955 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too")); 956 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0); 957 assertEquals("[]", (String) ts0.invokeExact()); 958 // collectors can be nested, Lisp-style 959 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2); 960 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D"))); 961 // arrayType can be any primitive array type 962 MethodHandle bytesToString = publicLookup() 963 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class)) 964 .asCollector(byte[].class, 3); 965 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3)); 966 MethodHandle longsToString = publicLookup() 967 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class)) 968 .asCollector(long[].class, 1); 969 assertEquals("[123]", (String) longsToString.invokeExact((long)123)); 970 * }</pre></blockquote> 971 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 972 * @param arrayLength the number of arguments to collect into a new array argument 973 * @return a new method handle which collects some trailing argument 974 * into an array, before calling the original method handle 975 * @throws NullPointerException if {@code arrayType} is a null reference 976 * @throws IllegalArgumentException if {@code arrayType} is not an array type 977 * or {@code arrayType} is not assignable to this method handle's trailing parameter type, 978 * or {@code arrayLength} is not a legal array size, 979 * or the resulting method handle's type would have 980 * <a href="MethodHandle.html#maxarity">too many parameters</a> 981 * @throws WrongMethodTypeException if the implied {@code asType} call fails 982 * @see #asSpreader 983 * @see #asVarargsCollector 984 */ 985 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) { 986 asCollectorChecks(arrayType, arrayLength); 987 int collectArgPos = type().parameterCount()-1; 988 MethodHandle target = this; 989 if (arrayType != type().parameterType(collectArgPos)) 990 target = convertArguments(type().changeParameterType(collectArgPos, arrayType)); 991 MethodHandle collector = ValueConversions.varargsArray(arrayType, arrayLength); 992 return MethodHandles.collectArguments(target, collectArgPos, collector); 993 } 994 995 // private API: return true if last param exactly matches arrayType 996 private boolean asCollectorChecks(Class<?> arrayType, int arrayLength) { 997 spreadArrayChecks(arrayType, arrayLength); 998 int nargs = type().parameterCount(); 999 if (nargs != 0) { 1000 Class<?> lastParam = type().parameterType(nargs-1); 1001 if (lastParam == arrayType) return true; 1002 if (lastParam.isAssignableFrom(arrayType)) return false; 1003 } 1004 throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType); 1005 } 1006 1007 /** 1008 * Makes a <em>variable arity</em> adapter which is able to accept 1009 * any number of trailing positional arguments and collect them 1010 * into an array argument. 1011 * <p> 1012 * The type and behavior of the adapter will be the same as 1013 * the type and behavior of the target, except that certain 1014 * {@code invoke} and {@code asType} requests can lead to 1015 * trailing positional arguments being collected into target's 1016 * trailing parameter. 1017 * Also, the last parameter type of the adapter will be 1018 * {@code arrayType}, even if the target has a different 1019 * last parameter type. 1020 * <p> 1021 * This transformation may return {@code this} if the method handle is 1022 * already of variable arity and its trailing parameter type 1023 * is identical to {@code arrayType}. 1024 * <p> 1025 * When called with {@link #invokeExact invokeExact}, the adapter invokes 1026 * the target with no argument changes. 1027 * (<em>Note:</em> This behavior is different from a 1028 * {@linkplain #asCollector fixed arity collector}, 1029 * since it accepts a whole array of indeterminate length, 1030 * rather than a fixed number of arguments.) 1031 * <p> 1032 * When called with plain, inexact {@link #invoke invoke}, if the caller 1033 * type is the same as the adapter, the adapter invokes the target as with 1034 * {@code invokeExact}. 1035 * (This is the normal behavior for {@code invoke} when types match.) 1036 * <p> 1037 * Otherwise, if the caller and adapter arity are the same, and the 1038 * trailing parameter type of the caller is a reference type identical to 1039 * or assignable to the trailing parameter type of the adapter, 1040 * the arguments and return values are converted pairwise, 1041 * as if by {@link #asType asType} on a fixed arity 1042 * method handle. 1043 * <p> 1044 * Otherwise, the arities differ, or the adapter's trailing parameter 1045 * type is not assignable from the corresponding caller type. 1046 * In this case, the adapter replaces all trailing arguments from 1047 * the original trailing argument position onward, by 1048 * a new array of type {@code arrayType}, whose elements 1049 * comprise (in order) the replaced arguments. 1050 * <p> 1051 * The caller type must provides as least enough arguments, 1052 * and of the correct type, to satisfy the target's requirement for 1053 * positional arguments before the trailing array argument. 1054 * Thus, the caller must supply, at a minimum, {@code N-1} arguments, 1055 * where {@code N} is the arity of the target. 1056 * Also, there must exist conversions from the incoming arguments 1057 * to the target's arguments. 1058 * As with other uses of plain {@code invoke}, if these basic 1059 * requirements are not fulfilled, a {@code WrongMethodTypeException} 1060 * may be thrown. 1061 * <p> 1062 * In all cases, what the target eventually returns is returned unchanged by the adapter. 1063 * <p> 1064 * In the final case, it is exactly as if the target method handle were 1065 * temporarily adapted with a {@linkplain #asCollector fixed arity collector} 1066 * to the arity required by the caller type. 1067 * (As with {@code asCollector}, if the array length is zero, 1068 * a shared constant may be used instead of a new array. 1069 * If the implied call to {@code asCollector} would throw 1070 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, 1071 * the call to the variable arity adapter must throw 1072 * {@code WrongMethodTypeException}.) 1073 * <p> 1074 * The behavior of {@link #asType asType} is also specialized for 1075 * variable arity adapters, to maintain the invariant that 1076 * plain, inexact {@code invoke} is always equivalent to an {@code asType} 1077 * call to adjust the target type, followed by {@code invokeExact}. 1078 * Therefore, a variable arity adapter responds 1079 * to an {@code asType} request by building a fixed arity collector, 1080 * if and only if the adapter and requested type differ either 1081 * in arity or trailing argument type. 1082 * The resulting fixed arity collector has its type further adjusted 1083 * (if necessary) to the requested type by pairwise conversion, 1084 * as if by another application of {@code asType}. 1085 * <p> 1086 * When a method handle is obtained by executing an {@code ldc} instruction 1087 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked 1088 * as a variable arity method (with the modifier bit {@code 0x0080}), 1089 * the method handle will accept multiple arities, as if the method handle 1090 * constant were created by means of a call to {@code asVarargsCollector}. 1091 * <p> 1092 * In order to create a collecting adapter which collects a predetermined 1093 * number of arguments, and whose type reflects this predetermined number, 1094 * use {@link #asCollector asCollector} instead. 1095 * <p> 1096 * No method handle transformations produce new method handles with 1097 * variable arity, unless they are documented as doing so. 1098 * Therefore, besides {@code asVarargsCollector}, 1099 * all methods in {@code MethodHandle} and {@code MethodHandles} 1100 * will return a method handle with fixed arity, 1101 * except in the cases where they are specified to return their original 1102 * operand (e.g., {@code asType} of the method handle's own type). 1103 * <p> 1104 * Calling {@code asVarargsCollector} on a method handle which is already 1105 * of variable arity will produce a method handle with the same type and behavior. 1106 * It may (or may not) return the original variable arity method handle. 1107 * <p> 1108 * Here is an example, of a list-making variable arity method handle: 1109 * <blockquote><pre>{@code 1110 MethodHandle deepToString = publicLookup() 1111 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1112 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); 1113 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); 1114 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); 1115 assertEquals("[won]", (String) ts1.invoke( "won" )); 1116 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); 1117 // findStatic of Arrays.asList(...) produces a variable arity method handle: 1118 MethodHandle asList = publicLookup() 1119 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); 1120 assertEquals(methodType(List.class, Object[].class), asList.type()); 1121 assert(asList.isVarargsCollector()); 1122 assertEquals("[]", asList.invoke().toString()); 1123 assertEquals("[1]", asList.invoke(1).toString()); 1124 assertEquals("[two, too]", asList.invoke("two", "too").toString()); 1125 String[] argv = { "three", "thee", "tee" }; 1126 assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); 1127 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); 1128 List ls = (List) asList.invoke((Object)argv); 1129 assertEquals(1, ls.size()); 1130 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); 1131 * }</pre></blockquote> 1132 * <p style="font-size:smaller;"> 1133 * <em>Discussion:</em> 1134 * These rules are designed as a dynamically-typed variation 1135 * of the Java rules for variable arity methods. 1136 * In both cases, callers to a variable arity method or method handle 1137 * can either pass zero or more positional arguments, or else pass 1138 * pre-collected arrays of any length. Users should be aware of the 1139 * special role of the final argument, and of the effect of a 1140 * type match on that final argument, which determines whether 1141 * or not a single trailing argument is interpreted as a whole 1142 * array or a single element of an array to be collected. 1143 * Note that the dynamic type of the trailing argument has no 1144 * effect on this decision, only a comparison between the symbolic 1145 * type descriptor of the call site and the type descriptor of the method handle.) 1146 * 1147 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1148 * @return a new method handle which can collect any number of trailing arguments 1149 * into an array, before calling the original method handle 1150 * @throws NullPointerException if {@code arrayType} is a null reference 1151 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1152 * or {@code arrayType} is not assignable to this method handle's trailing parameter type 1153 * @see #asCollector 1154 * @see #isVarargsCollector 1155 * @see #asFixedArity 1156 */ 1157 public MethodHandle asVarargsCollector(Class<?> arrayType) { 1158 Class<?> arrayElement = arrayType.getComponentType(); 1159 boolean lastMatch = asCollectorChecks(arrayType, 0); 1160 if (isVarargsCollector() && lastMatch) 1161 return this; 1162 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1163 } 1164 1165 /** 1166 * Determines if this method handle 1167 * supports {@linkplain #asVarargsCollector variable arity} calls. 1168 * Such method handles arise from the following sources: 1169 * <ul> 1170 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} 1171 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} 1172 * which resolves to a variable arity Java method or constructor 1173 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} 1174 * which resolves to a variable arity Java method or constructor 1175 * </ul> 1176 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls 1177 * @see #asVarargsCollector 1178 * @see #asFixedArity 1179 */ 1180 public boolean isVarargsCollector() { 1181 return false; 1182 } 1183 1184 /** 1185 * Makes a <em>fixed arity</em> method handle which is otherwise 1186 * equivalent to the current method handle. 1187 * <p> 1188 * If the current method handle is not of 1189 * {@linkplain #asVarargsCollector variable arity}, 1190 * the current method handle is returned. 1191 * This is true even if the current method handle 1192 * could not be a valid input to {@code asVarargsCollector}. 1193 * <p> 1194 * Otherwise, the resulting fixed-arity method handle has the same 1195 * type and behavior of the current method handle, 1196 * except that {@link #isVarargsCollector isVarargsCollector} 1197 * will be false. 1198 * The fixed-arity method handle may (or may not) be the 1199 * a previous argument to {@code asVarargsCollector}. 1200 * <p> 1201 * Here is an example, of a list-making variable arity method handle: 1202 * <blockquote><pre>{@code 1203 MethodHandle asListVar = publicLookup() 1204 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) 1205 .asVarargsCollector(Object[].class); 1206 MethodHandle asListFix = asListVar.asFixedArity(); 1207 assertEquals("[1]", asListVar.invoke(1).toString()); 1208 Exception caught = null; 1209 try { asListFix.invoke((Object)1); } 1210 catch (Exception ex) { caught = ex; } 1211 assert(caught instanceof ClassCastException); 1212 assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); 1213 try { asListFix.invoke("two", "too"); } 1214 catch (Exception ex) { caught = ex; } 1215 assert(caught instanceof WrongMethodTypeException); 1216 Object[] argv = { "three", "thee", "tee" }; 1217 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); 1218 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); 1219 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); 1220 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); 1221 * }</pre></blockquote> 1222 * 1223 * @return a new method handle which accepts only a fixed number of arguments 1224 * @see #asVarargsCollector 1225 * @see #isVarargsCollector 1226 */ 1227 public MethodHandle asFixedArity() { 1228 assert(!isVarargsCollector()); 1229 return this; 1230 } 1231 1232 /** 1233 * Binds a value {@code x} to the first argument of a method handle, without invoking it. 1234 * The new method handle adapts, as its <i>target</i>, 1235 * the current method handle by binding it to the given argument. 1236 * The type of the bound handle will be 1237 * the same as the type of the target, except that a single leading 1238 * reference parameter will be omitted. 1239 * <p> 1240 * When called, the bound handle inserts the given value {@code x} 1241 * as a new leading argument to the target. The other arguments are 1242 * also passed unchanged. 1243 * What the target eventually returns is returned unchanged by the bound handle. 1244 * <p> 1245 * The reference {@code x} must be convertible to the first parameter 1246 * type of the target. 1247 * <p> 1248 * (<em>Note:</em> Because method handles are immutable, the target method handle 1249 * retains its original type and behavior.) 1250 * @param x the value to bind to the first argument of the target 1251 * @return a new method handle which prepends the given value to the incoming 1252 * argument list, before calling the original method handle 1253 * @throws IllegalArgumentException if the target does not have a 1254 * leading parameter type that is a reference type 1255 * @throws ClassCastException if {@code x} cannot be converted 1256 * to the leading parameter type of the target 1257 * @see MethodHandles#insertArguments 1258 */ 1259 public MethodHandle bindTo(Object x) { 1260 Class<?> ptype; 1261 @SuppressWarnings("LocalVariableHidesMemberVariable") 1262 MethodType type = type(); 1263 if (type.parameterCount() == 0 || 1264 (ptype = type.parameterType(0)).isPrimitive()) 1265 throw newIllegalArgumentException("no leading reference parameter", x); 1266 x = ptype.cast(x); // throw CCE if needed 1267 return bindReceiver(x); 1268 } 1269 1270 /** 1271 * Returns a string representation of the method handle, 1272 * starting with the string {@code "MethodHandle"} and 1273 * ending with the string representation of the method handle's type. 1274 * In other words, this method returns a string equal to the value of: 1275 * <blockquote><pre>{@code 1276 * "MethodHandle" + type().toString() 1277 * }</pre></blockquote> 1278 * <p> 1279 * (<em>Note:</em> Future releases of this API may add further information 1280 * to the string representation. 1281 * Therefore, the present syntax should not be parsed by applications.) 1282 * 1283 * @return a string representation of the method handle 1284 */ 1285 @Override 1286 public String toString() { 1287 if (DEBUG_METHOD_HANDLE_NAMES) return debugString(); 1288 return standardString(); 1289 } 1290 String standardString() { 1291 return "MethodHandle"+type; 1292 } 1293 String debugString() { 1294 return standardString()+"/LF="+internalForm()+internalProperties(); 1295 } 1296 1297 //// Implementation methods. 1298 //// Sub-classes can override these default implementations. 1299 //// All these methods assume arguments are already validated. 1300 1301 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch 1302 1303 /*non-public*/ 1304 MethodHandle setVarargs(MemberName member) throws IllegalAccessException { 1305 if (!member.isVarargs()) return this; 1306 int argc = type().parameterCount(); 1307 if (argc != 0) { 1308 Class<?> arrayType = type().parameterType(argc-1); 1309 if (arrayType.isArray()) { 1310 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1311 } 1312 } 1313 throw member.makeAccessException("cannot make variable arity", null); 1314 } 1315 /*non-public*/ 1316 MethodHandle viewAsType(MethodType newType) { 1317 // No actual conversions, just a new view of the same method. 1318 return MethodHandleImpl.makePairwiseConvert(this, newType, 0); 1319 } 1320 1321 // Decoding 1322 1323 /*non-public*/ 1324 LambdaForm internalForm() { 1325 return form; 1326 } 1327 1328 /*non-public*/ 1329 MemberName internalMemberName() { 1330 return null; // DMH returns DMH.member 1331 } 1332 1333 /*non-public*/ 1334 Class<?> internalCallerClass() { 1335 return null; // caller-bound MH for @CallerSensitive method returns caller 1336 } 1337 1338 /*non-public*/ 1339 MethodHandle withInternalMemberName(MemberName member) { 1340 if (member != null) { 1341 return MethodHandleImpl.makeWrappedMember(this, member); 1342 } else if (internalMemberName() == null) { 1343 // The required internaMemberName is null, and this MH (like most) doesn't have one. 1344 return this; 1345 } else { 1346 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. 1347 MethodHandle result = rebind(); 1348 assert (result.internalMemberName() == null); 1349 return result; 1350 } 1351 } 1352 1353 /*non-public*/ 1354 boolean isInvokeSpecial() { 1355 return false; // DMH.Special returns true 1356 } 1357 1358 /*non-public*/ 1359 Object internalValues() { 1360 return null; 1361 } 1362 1363 /*non-public*/ 1364 Object internalProperties() { 1365 // Override to something like "/FOO=bar" 1366 return ""; 1367 } 1368 1369 //// Method handle implementation methods. 1370 //// Sub-classes can override these default implementations. 1371 //// All these methods assume arguments are already validated. 1372 1373 /*non-public*/ MethodHandle convertArguments(MethodType newType) { 1374 // Override this if it can be improved. 1375 return MethodHandleImpl.makePairwiseConvert(this, newType, 1); 1376 } 1377 1378 /*non-public*/ 1379 MethodHandle bindArgument(int pos, BasicType basicType, Object value) { 1380 // Override this if it can be improved. 1381 return rebind().bindArgument(pos, basicType, value); 1382 } 1383 1384 /*non-public*/ 1385 MethodHandle bindReceiver(Object receiver) { 1386 // Override this if it can be improved. 1387 return bindArgument(0, L_TYPE, receiver); 1388 } 1389 1390 /*non-public*/ 1391 MethodHandle dropArguments(MethodType srcType, int pos, int drops) { 1392 // Override this if it can be improved. 1393 return rebind().dropArguments(srcType, pos, drops); 1394 } 1395 1396 /*non-public*/ 1397 MethodHandle permuteArguments(MethodType newType, int[] reorder) { 1398 // Override this if it can be improved. 1399 return rebind().permuteArguments(newType, reorder); 1400 } 1401 1402 /*non-public*/ 1403 MethodHandle rebind() { 1404 // Bind 'this' into a new invoker, of the known class BMH. 1405 MethodType type2 = type(); 1406 LambdaForm form2 = reinvokerForm(this); 1407 // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) } 1408 return BoundMethodHandle.bindSingle(type2, form2, this); 1409 } 1410 1411 /*non-public*/ 1412 MethodHandle reinvokerTarget() { 1413 throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this); 1414 } 1415 1416 /** Create a LF which simply reinvokes a target of the given basic type. 1417 * The target MH must override {@link #reinvokerTarget} to provide the target. 1418 */ 1419 static LambdaForm reinvokerForm(MethodHandle target) { 1420 MethodType mtype = target.type().basicType(); 1421 LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE); 1422 if (reinvoker != null) return reinvoker; 1423 if (mtype.parameterSlotCount() >= MethodType.MAX_MH_ARITY) 1424 return makeReinvokerForm(target.type(), target); // cannot cache this 1425 reinvoker = makeReinvokerForm(mtype, null); 1426 return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, reinvoker); 1427 } 1428 private static LambdaForm makeReinvokerForm(MethodType mtype, MethodHandle customTargetOrNull) { 1429 boolean customized = (customTargetOrNull != null); 1430 MethodHandle MH_invokeBasic = customized ? null : MethodHandles.basicInvoker(mtype); 1431 final int THIS_BMH = 0; 1432 final int ARG_BASE = 1; 1433 final int ARG_LIMIT = ARG_BASE + mtype.parameterCount(); 1434 int nameCursor = ARG_LIMIT; 1435 final int NEXT_MH = customized ? -1 : nameCursor++; 1436 final int REINVOKE = nameCursor++; 1437 LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType()); 1438 Object[] targetArgs; 1439 MethodHandle targetMH; 1440 if (customized) { 1441 targetArgs = Arrays.copyOfRange(names, ARG_BASE, ARG_LIMIT, Object[].class); 1442 targetMH = customTargetOrNull; 1443 } else { 1444 names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]); 1445 targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class); 1446 targetArgs[0] = names[NEXT_MH]; // overwrite this MH with next MH 1447 targetMH = MethodHandles.basicInvoker(mtype); 1448 } 1449 names[REINVOKE] = new LambdaForm.Name(targetMH, targetArgs); 1450 return new LambdaForm("BMH.reinvoke", ARG_LIMIT, names); 1451 } 1452 1453 private static final LambdaForm.NamedFunction NF_reinvokerTarget; 1454 static { 1455 try { 1456 NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class 1457 .getDeclaredMethod("reinvokerTarget")); 1458 } catch (ReflectiveOperationException ex) { 1459 throw newInternalError(ex); 1460 } 1461 } 1462 1463 /** 1464 * Replace the old lambda form of this method handle with a new one. 1465 * The new one must be functionally equivalent to the old one. 1466 * Threads may continue running the old form indefinitely, 1467 * but it is likely that the new one will be preferred for new executions. 1468 * Use with discretion. 1469 */ 1470 /*non-public*/ 1471 void updateForm(LambdaForm newForm) { 1472 if (form == newForm) return; 1473 // ISSUE: Should we have a memory fence here? 1474 UNSAFE.putObject(this, FORM_OFFSET, newForm); 1475 this.form.prepare(); // as in MethodHandle.<init> 1476 } 1477 1478 private static final long FORM_OFFSET; 1479 static { 1480 try { 1481 FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form")); 1482 } catch (ReflectiveOperationException ex) { 1483 throw newInternalError(ex); 1484 } 1485 } 1486 }