1 /* 2 * Copyright (c) 2008, 2020, 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 jdk.internal.HotSpotIntrinsicCandidate; 30 31 import java.lang.constant.ClassDesc; 32 import java.lang.constant.Constable; 33 import java.lang.constant.ConstantDesc; 34 import java.lang.constant.DirectMethodHandleDesc; 35 import java.lang.constant.MethodHandleDesc; 36 import java.lang.constant.MethodTypeDesc; 37 import java.util.Arrays; 38 import java.util.Objects; 39 import java.util.Optional; 40 41 import static java.lang.invoke.MethodHandleInfo.*; 42 import static java.lang.invoke.MethodHandleStatics.*; 43 import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP; 44 45 /** 46 * A method handle is a typed, directly executable reference to an underlying method, 47 * constructor, field, or similar low-level operation, with optional 48 * transformations of arguments or return values. 49 * These transformations are quite general, and include such patterns as 50 * {@linkplain #asType conversion}, 51 * {@linkplain #bindTo insertion}, 52 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion}, 53 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}. 54 * 55 * <h2>Method handle contents</h2> 56 * Method handles are dynamically and strongly typed according to their parameter and return types. 57 * They are not distinguished by the name or the defining class of their underlying methods. 58 * A method handle must be invoked using a symbolic type descriptor which matches 59 * the method handle's own {@linkplain #type() type descriptor}. 60 * <p> 61 * Every method handle reports its type descriptor via the {@link #type() type} accessor. 62 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object, 63 * whose structure is a series of classes, one of which is 64 * the return type of the method (or {@code void.class} if none). 65 * <p> 66 * A method handle's type controls the types of invocations it accepts, 67 * and the kinds of transformations that apply to it. 68 * <p> 69 * A method handle contains a pair of special invoker methods 70 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}. 71 * Both invoker methods provide direct access to the method handle's 72 * underlying method, constructor, field, or other operation, 73 * as modified by transformations of arguments and return values. 74 * Both invokers accept calls which exactly match the method handle's own type. 75 * The plain, inexact invoker also accepts a range of other call types. 76 * <p> 77 * Method handles are immutable and have no visible state. 78 * Of course, they can be bound to underlying methods or data which exhibit state. 79 * With respect to the Java Memory Model, any method handle will behave 80 * as if all of its (internal) fields are final variables. This means that any method 81 * handle made visible to the application will always be fully formed. 82 * This is true even if the method handle is published through a shared 83 * variable in a data race. 84 * <p> 85 * Method handles cannot be subclassed by the user. 86 * Implementations may (or may not) create internal subclasses of {@code MethodHandle} 87 * which may be visible via the {@link java.lang.Object#getClass Object.getClass} 88 * operation. The programmer should not draw conclusions about a method handle 89 * from its specific class, as the method handle class hierarchy (if any) 90 * may change from time to time or across implementations from different vendors. 91 * 92 * <h2>Method handle compilation</h2> 93 * A Java method call expression naming {@code invokeExact} or {@code invoke} 94 * can invoke a method handle from Java source code. 95 * From the viewpoint of source code, these methods can take any arguments 96 * and their result can be cast to any return type. 97 * Formally this is accomplished by giving the invoker methods 98 * {@code Object} return types and variable arity {@code Object} arguments, 99 * but they have an additional quality called <em>signature polymorphism</em> 100 * which connects this freedom of invocation directly to the JVM execution stack. 101 * <p> 102 * As is usual with virtual methods, source-level calls to {@code invokeExact} 103 * and {@code invoke} compile to an {@code invokevirtual} instruction. 104 * More unusually, the compiler must record the actual argument types, 105 * and may not perform method invocation conversions on the arguments. 106 * Instead, it must generate instructions that push them on the stack according 107 * to their own unconverted types. The method handle object itself is pushed on 108 * the stack before the arguments. 109 * The compiler then generates an {@code invokevirtual} instruction that invokes 110 * the method handle with a symbolic type descriptor which describes the argument 111 * and return types. 112 * <p> 113 * To issue a complete symbolic type descriptor, the compiler must also determine 114 * the return type. This is based on a cast on the method invocation expression, 115 * if there is one, or else {@code Object} if the invocation is an expression, 116 * or else {@code void} if the invocation is a statement. 117 * The cast may be to a primitive type (but not {@code void}). 118 * <p> 119 * As a corner case, an uncasted {@code null} argument is given 120 * a symbolic type descriptor of {@code java.lang.Void}. 121 * The ambiguity with the type {@code Void} is harmless, since there are no references of type 122 * {@code Void} except the null reference. 123 * 124 * <h2>Method handle invocation</h2> 125 * The first time an {@code invokevirtual} instruction is executed 126 * it is linked by symbolically resolving the names in the instruction 127 * and verifying that the method call is statically legal. 128 * This also holds for calls to {@code invokeExact} and {@code invoke}. 129 * In this case, the symbolic type descriptor emitted by the compiler is checked for 130 * correct syntax, and names it contains are resolved. 131 * Thus, an {@code invokevirtual} instruction which invokes 132 * a method handle will always link, as long 133 * as the symbolic type descriptor is syntactically well-formed 134 * and the types exist. 135 * <p> 136 * When the {@code invokevirtual} is executed after linking, 137 * the receiving method handle's type is first checked by the JVM 138 * to ensure that it matches the symbolic type descriptor. 139 * If the type match fails, it means that the method which the 140 * caller is invoking is not present on the individual 141 * method handle being invoked. 142 * <p> 143 * In the case of {@code invokeExact}, the type descriptor of the invocation 144 * (after resolving symbolic type names) must exactly match the method type 145 * of the receiving method handle. 146 * In the case of plain, inexact {@code invoke}, the resolved type descriptor 147 * must be a valid argument to the receiver's {@link #asType asType} method. 148 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}. 149 * <p> 150 * After type matching, a call to {@code invokeExact} directly 151 * and immediately invoke the method handle's underlying method 152 * (or other behavior, as the case may be). 153 * <p> 154 * A call to plain {@code invoke} works the same as a call to 155 * {@code invokeExact}, if the symbolic type descriptor specified by the caller 156 * exactly matches the method handle's own type. 157 * If there is a type mismatch, {@code invoke} attempts 158 * to adjust the type of the receiving method handle, 159 * as if by a call to {@link #asType asType}, 160 * to obtain an exactly invokable method handle {@code M2}. 161 * This allows a more powerful negotiation of method type 162 * between caller and callee. 163 * <p> 164 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable, 165 * and implementations are therefore not required to materialize it.) 166 * 167 * <h2>Invocation checking</h2> 168 * In typical programs, method handle type matching will usually succeed. 169 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, 170 * either directly (in the case of {@code invokeExact}) or indirectly as if 171 * by a failed call to {@code asType} (in the case of {@code invoke}). 172 * <p> 173 * Thus, a method type mismatch which might show up as a linkage error 174 * in a statically typed program can show up as 175 * a dynamic {@code WrongMethodTypeException} 176 * in a program which uses method handles. 177 * <p> 178 * Because method types contain "live" {@code Class} objects, 179 * method type matching takes into account both type names and class loaders. 180 * Thus, even if a method handle {@code M} is created in one 181 * class loader {@code L1} and used in another {@code L2}, 182 * method handle calls are type-safe, because the caller's symbolic type 183 * descriptor, as resolved in {@code L2}, 184 * is matched against the original callee method's symbolic type descriptor, 185 * as resolved in {@code L1}. 186 * The resolution in {@code L1} happens when {@code M} is created 187 * and its type is assigned, while the resolution in {@code L2} happens 188 * when the {@code invokevirtual} instruction is linked. 189 * <p> 190 * Apart from type descriptor checks, 191 * a method handle's capability to call its underlying method is unrestricted. 192 * If a method handle is formed on a non-public method by a class 193 * that has access to that method, the resulting handle can be used 194 * in any place by any caller who receives a reference to it. 195 * <p> 196 * Unlike with the Core Reflection API, where access is checked every time 197 * a reflective method is invoked, 198 * method handle access checking is performed 199 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>. 200 * In the case of {@code ldc} (see below), access checking is performed as part of linking 201 * the constant pool entry underlying the constant method handle. 202 * <p> 203 * Thus, handles to non-public methods, or to methods in non-public classes, 204 * should generally be kept secret. 205 * They should not be passed to untrusted code unless their use from 206 * the untrusted code would be harmless. 207 * 208 * <h2>Method handle creation</h2> 209 * Java code can create a method handle that directly accesses 210 * any method, constructor, or field that is accessible to that code. 211 * This is done via a reflective, capability-based API called 212 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}. 213 * For example, a static method handle can be obtained 214 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}. 215 * There are also conversion methods from Core Reflection API objects, 216 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 217 * <p> 218 * Like classes and strings, method handles that correspond to accessible 219 * fields, methods, and constructors can also be represented directly 220 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes. 221 * A new type of constant pool entry, {@code CONSTANT_MethodHandle}, 222 * refers directly to an associated {@code CONSTANT_Methodref}, 223 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref} 224 * constant pool entry. 225 * (For full details on method handle constants, see sections {@jvms 226 * 4.4.8} and {@jvms 5.4.3.5} of the Java Virtual Machine 227 * Specification.) 228 * <p> 229 * Method handles produced by lookups or constant loads from methods or 230 * constructors with the variable arity modifier bit ({@code 0x0080}) 231 * have a corresponding variable arity, as if they were defined with 232 * the help of {@link #asVarargsCollector asVarargsCollector} 233 * or {@link #withVarargs withVarargs}. 234 * <p> 235 * A method reference may refer either to a static or non-static method. 236 * In the non-static case, the method handle type includes an explicit 237 * receiver argument, prepended before any other arguments. 238 * In the method handle's type, the initial receiver argument is typed 239 * according to the class under which the method was initially requested. 240 * (E.g., if a non-static method handle is obtained via {@code ldc}, 241 * the type of the receiver is the class named in the constant pool entry.) 242 * <p> 243 * Method handle constants are subject to the same link-time access checks 244 * their corresponding bytecode instructions, and the {@code ldc} instruction 245 * will throw corresponding linkage errors if the bytecode behaviors would 246 * throw such errors. 247 * <p> 248 * As a corollary of this, access to protected members is restricted 249 * to receivers only of the accessing class, or one of its subclasses, 250 * and the accessing class must in turn be a subclass (or package sibling) 251 * of the protected member's defining class. 252 * If a method reference refers to a protected non-static method or field 253 * of a class outside the current package, the receiver argument will 254 * be narrowed to the type of the accessing class. 255 * <p> 256 * When a method handle to a virtual method is invoked, the method is 257 * always looked up in the receiver (that is, the first argument). 258 * <p> 259 * A non-virtual method handle to a specific virtual method implementation 260 * can also be created. These do not perform virtual lookup based on 261 * receiver type. Such a method handle simulates the effect of 262 * an {@code invokespecial} instruction to the same method. 263 * A non-virtual method handle can also be created to simulate the effect 264 * of an {@code invokevirtual} or {@code invokeinterface} instruction on 265 * a private method (as applicable). 266 * 267 * <h2>Usage examples</h2> 268 * Here are some examples of usage: 269 * <blockquote><pre>{@code 270 Object x, y; String s; int i; 271 MethodType mt; MethodHandle mh; 272 MethodHandles.Lookup lookup = MethodHandles.lookup(); 273 // mt is (char,char)String 274 mt = MethodType.methodType(String.class, char.class, char.class); 275 mh = lookup.findVirtual(String.class, "replace", mt); 276 s = (String) mh.invokeExact("daddy",'d','n'); 277 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String; 278 assertEquals(s, "nanny"); 279 // weakly typed invocation (using MHs.invoke) 280 s = (String) mh.invokeWithArguments("sappy", 'p', 'v'); 281 assertEquals(s, "savvy"); 282 // mt is (Object[])List 283 mt = MethodType.methodType(java.util.List.class, Object[].class); 284 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt); 285 assert(mh.isVarargsCollector()); 286 x = mh.invoke("one", "two"); 287 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object; 288 assertEquals(x, java.util.Arrays.asList("one","two")); 289 // mt is (Object,Object,Object)Object 290 mt = MethodType.genericMethodType(3); 291 mh = mh.asType(mt); 292 x = mh.invokeExact((Object)1, (Object)2, (Object)3); 293 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object; 294 assertEquals(x, java.util.Arrays.asList(1,2,3)); 295 // mt is ()int 296 mt = MethodType.methodType(int.class); 297 mh = lookup.findVirtual(java.util.List.class, "size", mt); 298 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3)); 299 // invokeExact(Ljava/util/List;)I 300 assert(i == 3); 301 mt = MethodType.methodType(void.class, String.class); 302 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt); 303 mh.invokeExact(System.out, "Hello, world."); 304 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V 305 * }</pre></blockquote> 306 * Each of the above calls to {@code invokeExact} or plain {@code invoke} 307 * generates a single invokevirtual instruction with 308 * the symbolic type descriptor indicated in the following comment. 309 * In these examples, the helper method {@code assertEquals} is assumed to 310 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals} 311 * on its arguments, and asserts that the result is true. 312 * 313 * <h2>Exceptions</h2> 314 * The methods {@code invokeExact} and {@code invoke} are declared 315 * to throw {@link java.lang.Throwable Throwable}, 316 * which is to say that there is no static restriction on what a method handle 317 * can throw. Since the JVM does not distinguish between checked 318 * and unchecked exceptions (other than by their class, of course), 319 * there is no particular effect on bytecode shape from ascribing 320 * checked exceptions to method handle invocations. But in Java source 321 * code, methods which perform method handle calls must either explicitly 322 * throw {@code Throwable}, or else must catch all 323 * throwables locally, rethrowing only those which are legal in the context, 324 * and wrapping ones which are illegal. 325 * 326 * <h2><a id="sigpoly"></a>Signature polymorphism</h2> 327 * The unusual compilation and linkage behavior of 328 * {@code invokeExact} and plain {@code invoke} 329 * is referenced by the term <em>signature polymorphism</em>. 330 * As defined in the Java Language Specification, 331 * a signature polymorphic method is one which can operate with 332 * any of a wide range of call signatures and return types. 333 * <p> 334 * In source code, a call to a signature polymorphic method will 335 * compile, regardless of the requested symbolic type descriptor. 336 * As usual, the Java compiler emits an {@code invokevirtual} 337 * instruction with the given symbolic type descriptor against the named method. 338 * The unusual part is that the symbolic type descriptor is derived from 339 * the actual argument and return types, not from the method declaration. 340 * <p> 341 * When the JVM processes bytecode containing signature polymorphic calls, 342 * it will successfully link any such call, regardless of its symbolic type descriptor. 343 * (In order to retain type safety, the JVM will guard such calls with suitable 344 * dynamic type checks, as described elsewhere.) 345 * <p> 346 * Bytecode generators, including the compiler back end, are required to emit 347 * untransformed symbolic type descriptors for these methods. 348 * Tools which determine symbolic linkage are required to accept such 349 * untransformed descriptors, without reporting linkage errors. 350 * 351 * <h2>Interoperation between method handles and the Core Reflection API</h2> 352 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, 353 * any class member represented by a Core Reflection API object 354 * can be converted to a behaviorally equivalent method handle. 355 * For example, a reflective {@link java.lang.reflect.Method Method} can 356 * be converted to a method handle using 357 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. 358 * The resulting method handles generally provide more direct and efficient 359 * access to the underlying class members. 360 * <p> 361 * As a special case, 362 * when the Core Reflection API is used to view the signature polymorphic 363 * methods {@code invokeExact} or plain {@code invoke} in this class, 364 * they appear as ordinary non-polymorphic methods. 365 * Their reflective appearance, as viewed by 366 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, 367 * is unaffected by their special status in this API. 368 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} 369 * will report exactly those modifier bits required for any similarly 370 * declared method, including in this case {@code native} and {@code varargs} bits. 371 * <p> 372 * As with any reflected method, these methods (when reflected) may be 373 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}. 374 * However, such reflective calls do not result in method handle invocations. 375 * Such a call, if passed the required argument 376 * (a single one, of type {@code Object[]}), will ignore the argument and 377 * will throw an {@code UnsupportedOperationException}. 378 * <p> 379 * Since {@code invokevirtual} instructions can natively 380 * invoke method handles under any symbolic type descriptor, this reflective view conflicts 381 * with the normal presentation of these methods via bytecodes. 382 * Thus, these two native methods, when reflectively viewed by 383 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only. 384 * <p> 385 * In order to obtain an invoker method for a particular type descriptor, 386 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker}, 387 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}. 388 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual} 389 * API is also able to return a method handle 390 * to call {@code invokeExact} or plain {@code invoke}, 391 * for any specified type descriptor . 392 * 393 * <h2>Interoperation between method handles and Java generics</h2> 394 * A method handle can be obtained on a method, constructor, or field 395 * which is declared with Java generic types. 396 * As with the Core Reflection API, the type of the method handle 397 * will be constructed from the erasure of the source-level type. 398 * When a method handle is invoked, the types of its arguments 399 * or the return value cast type may be generic types or type instances. 400 * If this occurs, the compiler will replace those 401 * types by their erasures when it constructs the symbolic type descriptor 402 * for the {@code invokevirtual} instruction. 403 * <p> 404 * Method handles do not represent 405 * their function-like types in terms of Java parameterized (generic) types, 406 * because there are three mismatches between function-like types and parameterized 407 * Java types. 408 * <ul> 409 * <li>Method types range over all possible arities, 410 * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments. 411 * Generics are not variadic, and so cannot represent this.</li> 412 * <li>Method types can specify arguments of primitive types, 413 * which Java generic types cannot range over.</li> 414 * <li>Higher order functions over method handles (combinators) are 415 * often generic across a wide range of function types, including 416 * those of multiple arities. It is impossible to represent such 417 * genericity with a Java type parameter.</li> 418 * </ul> 419 * 420 * <h2><a id="maxarity"></a>Arity limits</h2> 421 * The JVM imposes on all methods and constructors of any kind an absolute 422 * limit of 255 stacked arguments. This limit can appear more restrictive 423 * in certain cases: 424 * <ul> 425 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots. 426 * <li>A non-static method consumes an extra argument for the object on which the method is called. 427 * <li>A constructor consumes an extra argument for the object which is being constructed. 428 * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual, 429 * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object. 430 * </ul> 431 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments. 432 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it. 433 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}. 434 * In particular, a method handle’s type must not have an arity of the exact maximum 255. 435 * 436 * @see MethodType 437 * @see MethodHandles 438 * @author John Rose, JSR 292 EG 439 * @since 1.7 440 */ 441 public abstract class MethodHandle implements Constable { 442 443 /** 444 * Internal marker interface which distinguishes (to the Java compiler) 445 * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>. 446 */ 447 @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD}) 448 @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME) 449 @interface PolymorphicSignature { } 450 451 private final MethodType type; 452 /*private*/ 453 final LambdaForm form; 454 // form is not private so that invokers can easily fetch it 455 /*private*/ 456 MethodHandle asTypeCache; 457 // asTypeCache is not private so that invokers can easily fetch it 458 /*non-public*/ 459 byte customizationCount; 460 // customizationCount should be accessible from invokers 461 462 /** 463 * Reports the type of this method handle. 464 * Every invocation of this method handle via {@code invokeExact} must exactly match this type. 465 * @return the method handle type 466 */ 467 public MethodType type() { 468 return type; 469 } 470 471 /** 472 * Package-private constructor for the method handle implementation hierarchy. 473 * Method handle inheritance will be contained completely within 474 * the {@code java.lang.invoke} package. 475 */ 476 // @param type type (permanently assigned) of the new method handle 477 /*non-public*/ 478 MethodHandle(MethodType type, LambdaForm form) { 479 this.type = Objects.requireNonNull(type); 480 this.form = Objects.requireNonNull(form).uncustomize(); 481 482 this.form.prepare(); // TO DO: Try to delay this step until just before invocation. 483 } 484 485 /** 486 * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match. 487 * The symbolic type descriptor at the call site of {@code invokeExact} must 488 * exactly match this method handle's {@link #type() type}. 489 * No conversions are allowed on arguments or return values. 490 * <p> 491 * When this method is observed via the Core Reflection API, 492 * it will appear as a single native method, taking an object array and returning an object. 493 * If this native method is invoked directly via 494 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 495 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 496 * it will throw an {@code UnsupportedOperationException}. 497 * @param args the signature-polymorphic parameter list, statically represented using varargs 498 * @return the signature-polymorphic result, statically represented using {@code Object} 499 * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor 500 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 501 */ 502 @HotSpotIntrinsicCandidate 503 public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable; 504 505 /** 506 * Invokes the method handle, allowing any caller type descriptor, 507 * and optionally performing conversions on arguments and return values. 508 * <p> 509 * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type() type}, 510 * the call proceeds as if by {@link #invokeExact invokeExact}. 511 * <p> 512 * Otherwise, the call proceeds as if this method handle were first 513 * adjusted by calling {@link #asType asType} to adjust this method handle 514 * to the required type, and then the call proceeds as if by 515 * {@link #invokeExact invokeExact} on the adjusted method handle. 516 * <p> 517 * There is no guarantee that the {@code asType} call is actually made. 518 * If the JVM can predict the results of making the call, it may perform 519 * adaptations directly on the caller's arguments, 520 * and call the target method handle according to its own exact type. 521 * <p> 522 * The resolved type descriptor at the call site of {@code invoke} must 523 * be a valid argument to the receivers {@code asType} method. 524 * In particular, the caller must specify the same argument arity 525 * as the callee's type, 526 * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}. 527 * <p> 528 * When this method is observed via the Core Reflection API, 529 * it will appear as a single native method, taking an object array and returning an object. 530 * If this native method is invoked directly via 531 * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, 532 * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, 533 * it will throw an {@code UnsupportedOperationException}. 534 * @param args the signature-polymorphic parameter list, statically represented using varargs 535 * @return the signature-polymorphic result, statically represented using {@code Object} 536 * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor 537 * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails 538 * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call 539 */ 540 @HotSpotIntrinsicCandidate 541 public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable; 542 543 /** 544 * Private method for trusted invocation of a method handle respecting simplified signatures. 545 * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM. 546 * <p> 547 * The caller signature is restricted to the following basic types: 548 * Object, int, long, float, double, and void return. 549 * <p> 550 * The caller is responsible for maintaining type correctness by ensuring 551 * that the each outgoing argument value is a member of the range of the corresponding 552 * callee argument type. 553 * (The caller should therefore issue appropriate casts and integer narrowing 554 * operations on outgoing argument values.) 555 * The caller can assume that the incoming result value is part of the range 556 * of the callee's return type. 557 * @param args the signature-polymorphic parameter list, statically represented using varargs 558 * @return the signature-polymorphic result, statically represented using {@code Object} 559 */ 560 @HotSpotIntrinsicCandidate 561 /*non-public*/ 562 final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; 563 564 /** 565 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}. 566 * The caller signature is restricted to basic types as with {@code invokeBasic}. 567 * The trailing (not leading) argument must be a MemberName. 568 * @param args the signature-polymorphic parameter list, statically represented using varargs 569 * @return the signature-polymorphic result, statically represented using {@code Object} 570 */ 571 @HotSpotIntrinsicCandidate 572 /*non-public*/ 573 static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable; 574 575 /** 576 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}. 577 * The caller signature is restricted to basic types as with {@code invokeBasic}. 578 * The trailing (not leading) argument must be a MemberName. 579 * @param args the signature-polymorphic parameter list, statically represented using varargs 580 * @return the signature-polymorphic result, statically represented using {@code Object} 581 */ 582 @HotSpotIntrinsicCandidate 583 /*non-public*/ 584 static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable; 585 586 /** 587 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}. 588 * The caller signature is restricted to basic types as with {@code invokeBasic}. 589 * The trailing (not leading) argument must be a MemberName. 590 * @param args the signature-polymorphic parameter list, statically represented using varargs 591 * @return the signature-polymorphic result, statically represented using {@code Object} 592 */ 593 @HotSpotIntrinsicCandidate 594 /*non-public*/ 595 static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable; 596 597 /** 598 * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}. 599 * The caller signature is restricted to basic types as with {@code invokeBasic}. 600 * The trailing (not leading) argument must be a MemberName. 601 * @param args the signature-polymorphic parameter list, statically represented using varargs 602 * @return the signature-polymorphic result, statically represented using {@code Object} 603 */ 604 @HotSpotIntrinsicCandidate 605 /*non-public*/ 606 static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; 607 608 /** 609 * Performs a variable arity invocation, passing the arguments in the given array 610 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 611 * which mentions only the type {@code Object}, and whose actual argument count is the length 612 * of the argument array. 613 * <p> 614 * Specifically, execution proceeds as if by the following steps, 615 * although the methods are not guaranteed to be called if the JVM 616 * can predict their effects. 617 * <ul> 618 * <li>Determine the length of the argument array as {@code N}. 619 * For a null reference, {@code N=0}. </li> 620 * <li>Collect the {@code N} elements of the array as a logical 621 * argument list, each argument statically typed as an {@code Object}. </li> 622 * <li>Determine, as {@code M}, the parameter count of the type of this 623 * method handle. </li> 624 * <li>Determine the general type {@code TN} of {@code N} arguments or 625 * {@code M} arguments, if smaller than {@code N}, as 626 * {@code TN=MethodType.genericMethodType(Math.min(N, M))}.</li> 627 * <li>If {@code N} is greater than {@code M}, perform the following 628 * checks and actions to shorten the logical argument list: <ul> 629 * <li>Check that this method handle has variable arity with a 630 * {@linkplain MethodType#lastParameterType trailing parameter} 631 * of some array type {@code A[]}. If not, fail with a 632 * {@code WrongMethodTypeException}. </li> 633 * <li>Collect the trailing elements (there are {@code N-M+1} of them) 634 * from the logical argument list into a single array of 635 * type {@code A[]}, using {@code asType} conversions to 636 * convert each trailing argument to type {@code A}. </li> 637 * <li>If any of these conversions proves impossible, fail with either 638 * a {@code ClassCastException} if any trailing element cannot be 639 * cast to {@code A} or a {@code NullPointerException} if any 640 * trailing element is {@code null} and {@code A} is not a reference 641 * type. </li> 642 * <li>Replace the logical arguments gathered into the array of 643 * type {@code A[]} with the array itself, thus shortening 644 * the argument list to length {@code M}. This final argument 645 * retains the static type {@code A[]}.</li> 646 * <li>Adjust the type {@code TN} by changing the {@code N}th 647 * parameter type from {@code Object} to {@code A[]}. 648 * </ul> 649 * <li>Force the original target method handle {@code MH0} to the 650 * required type, as {@code MH1 = MH0.asType(TN)}. </li> 651 * <li>Spread the argument list into {@code N} separate arguments {@code A0, ...}. </li> 652 * <li>Invoke the type-adjusted method handle on the unpacked arguments: 653 * MH1.invokeExact(A0, ...). </li> 654 * <li>Take the return value as an {@code Object} reference. </li> 655 * </ul> 656 * <p> 657 * If the target method handle has variable arity, and the argument list is longer 658 * than that arity, the excess arguments, starting at the position of the trailing 659 * array argument, will be gathered (if possible, as if by {@code asType} conversions) 660 * into an array of the appropriate type, and invocation will proceed on the 661 * shortened argument list. 662 * In this way, <em>jumbo argument lists</em> which would spread into more 663 * than 254 slots can still be processed uniformly. 664 * <p> 665 * Unlike the {@link #invoke(Object...) generic} invocation mode, which can 666 * "recycle" an array argument, passing it directly to the target method, 667 * this invocation mode <em>always</em> creates a new array parameter, even 668 * if the original array passed to {@code invokeWithArguments} would have 669 * been acceptable as a direct argument to the target method. 670 * Even if the number {@code M} of actual arguments is the arity {@code N}, 671 * and the last argument is dynamically a suitable array of type {@code A[]}, 672 * it will still be boxed into a new one-element array, since the call 673 * site statically types the argument as {@code Object}, not an array type. 674 * This is not a special rule for this method, but rather a regular effect 675 * of the {@linkplain #asVarargsCollector rules for variable-arity invocation}. 676 * <p> 677 * Because of the action of the {@code asType} step, the following argument 678 * conversions are applied as necessary: 679 * <ul> 680 * <li>reference casting 681 * <li>unboxing 682 * <li>widening primitive conversions 683 * <li>variable arity conversion 684 * </ul> 685 * <p> 686 * The result returned by the call is boxed if it is a primitive, 687 * or forced to null if the return type is void. 688 * <p> 689 * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke}, 690 * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI. 691 * It can therefore be used as a bridge between native or reflective code and method handles. 692 * @apiNote 693 * This call is approximately equivalent to the following code: 694 * <blockquote><pre>{@code 695 * // for jumbo argument lists, adapt varargs explicitly: 696 * int N = (arguments == null? 0: arguments.length); 697 * int M = this.type.parameterCount(); 698 * int MAX_SAFE = 127; // 127 longs require 254 slots, which is OK 699 * if (N > MAX_SAFE && N > M && this.isVarargsCollector()) { 700 * Class<?> arrayType = this.type().lastParameterType(); 701 * Class<?> elemType = arrayType.getComponentType(); 702 * if (elemType != null) { 703 * Object args2 = Array.newInstance(elemType, M); 704 * MethodHandle arraySetter = MethodHandles.arrayElementSetter(arrayType); 705 * for (int i = 0; i < M; i++) { 706 * arraySetter.invoke(args2, i, arguments[M-1 + i]); 707 * } 708 * arguments = Arrays.copyOf(arguments, M); 709 * arguments[M-1] = args2; 710 * return this.asFixedArity().invokeWithArguments(arguments); 711 * } 712 * } // done with explicit varargs processing 713 * 714 * // Handle fixed arity and non-jumbo variable arity invocation. 715 * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0); 716 * Object result = invoker.invokeExact(this, arguments); 717 * }</pre></blockquote> 718 * 719 * @param arguments the arguments to pass to the target 720 * @return the result returned by the target 721 * @throws ClassCastException if an argument cannot be converted by reference casting 722 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 723 * @throws Throwable anything thrown by the target method invocation 724 * @see MethodHandles#spreadInvoker 725 */ 726 public Object invokeWithArguments(Object... arguments) throws Throwable { 727 // Note: Jumbo argument lists are handled in the variable-arity subclass. 728 MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length); 729 return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments); 730 } 731 732 /** 733 * Performs a variable arity invocation, passing the arguments in the given list 734 * to the method handle, as if via an inexact {@link #invoke invoke} from a call site 735 * which mentions only the type {@code Object}, and whose actual argument count is the length 736 * of the argument list. 737 * <p> 738 * This method is also equivalent to the following code: 739 * <blockquote><pre>{@code 740 * invokeWithArguments(arguments.toArray()) 741 * }</pre></blockquote> 742 * <p> 743 * Jumbo-sized lists are acceptable if this method handle has variable arity. 744 * See {@link #invokeWithArguments(Object[])} for details. 745 * 746 * @param arguments the arguments to pass to the target 747 * @return the result returned by the target 748 * @throws NullPointerException if {@code arguments} is a null reference 749 * @throws ClassCastException if an argument cannot be converted by reference casting 750 * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments 751 * @throws Throwable anything thrown by the target method invocation 752 */ 753 public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { 754 return invokeWithArguments(arguments.toArray()); 755 } 756 757 /** 758 * Produces an adapter method handle which adapts the type of the 759 * current method handle to a new type. 760 * The resulting method handle is guaranteed to report a type 761 * which is equal to the desired new type. 762 * <p> 763 * If the original type and new type are equal, returns {@code this}. 764 * <p> 765 * The new method handle, when invoked, will perform the following 766 * steps: 767 * <ul> 768 * <li>Convert the incoming argument list to match the original 769 * method handle's argument list. 770 * <li>Invoke the original method handle on the converted argument list. 771 * <li>Convert any result returned by the original method handle 772 * to the return type of new method handle. 773 * </ul> 774 * <p> 775 * This method provides the crucial behavioral difference between 776 * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. 777 * The two methods 778 * perform the same steps when the caller's type descriptor exactly matches 779 * the callee's, but when the types differ, plain {@link #invoke invoke} 780 * also calls {@code asType} (or some internal equivalent) in order 781 * to match up the caller's and callee's types. 782 * <p> 783 * If the current method is a variable arity method handle 784 * argument list conversion may involve the conversion and collection 785 * of several arguments into an array, as 786 * {@linkplain #asVarargsCollector described elsewhere}. 787 * In every other case, all conversions are applied <em>pairwise</em>, 788 * which means that each argument or return value is converted to 789 * exactly one argument or return value (or no return value). 790 * The applied conversions are defined by consulting 791 * the corresponding component types of the old and new 792 * method handle types. 793 * <p> 794 * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, 795 * or old and new return types. Specifically, for some valid index {@code i}, let 796 * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. 797 * Or else, going the other way for return values, let 798 * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. 799 * If the types are the same, the new method handle makes no change 800 * to the corresponding argument or return value (if any). 801 * Otherwise, one of the following conversions is applied 802 * if possible: 803 * <ul> 804 * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. 805 * (The types do not need to be related in any particular way. 806 * This is because a dynamic value of null can convert to any reference type.) 807 * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation 808 * conversion (JLS 5.3) is applied, if one exists. 809 * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) 810 * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, 811 * a Java casting conversion (JLS 5.5) is applied if one exists. 812 * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, 813 * which is then widened as needed to <em>T1</em>.) 814 * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing 815 * conversion will be applied at runtime, possibly followed 816 * by a Java method invocation conversion (JLS 5.3) 817 * on the primitive value. (These are the primitive widening conversions.) 818 * <em>T0</em> must be a wrapper class or a supertype of one. 819 * (In the case where <em>T0</em> is Object, these are the conversions 820 * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) 821 * The unboxing conversion must have a possibility of success, which means that 822 * if <em>T0</em> is not itself a wrapper class, there must exist at least one 823 * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed 824 * primitive value can be widened to <em>T1</em>. 825 * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded 826 * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. 827 * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, 828 * a zero value is introduced. 829 * </ul> 830 * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, 831 * because neither corresponds specifically to the <em>dynamic type</em> of any 832 * actual argument or return value.) 833 * <p> 834 * The method handle conversion cannot be made if any one of the required 835 * pairwise conversions cannot be made. 836 * <p> 837 * At runtime, the conversions applied to reference arguments 838 * or return values may require additional runtime checks which can fail. 839 * An unboxing operation may fail because the original reference is null, 840 * causing a {@link java.lang.NullPointerException NullPointerException}. 841 * An unboxing operation or a reference cast may also fail on a reference 842 * to an object of the wrong type, 843 * causing a {@link java.lang.ClassCastException ClassCastException}. 844 * Although an unboxing operation may accept several kinds of wrappers, 845 * if none are available, a {@code ClassCastException} will be thrown. 846 * 847 * @param newType the expected type of the new method handle 848 * @return a method handle which delegates to {@code this} after performing 849 * any necessary argument conversions, and arranges for any 850 * necessary return value conversions 851 * @throws NullPointerException if {@code newType} is a null reference 852 * @throws WrongMethodTypeException if the conversion cannot be made 853 * @see MethodHandles#explicitCastArguments 854 */ 855 public MethodHandle asType(MethodType newType) { 856 // Fast path alternative to a heavyweight {@code asType} call. 857 // Return 'this' if the conversion will be a no-op. 858 if (newType == type) { 859 return this; 860 } 861 // Return 'this.asTypeCache' if the conversion is already memoized. 862 MethodHandle atc = asTypeCached(newType); 863 if (atc != null) { 864 return atc; 865 } 866 return asTypeUncached(newType); 867 } 868 869 private MethodHandle asTypeCached(MethodType newType) { 870 MethodHandle atc = asTypeCache; 871 if (atc != null && newType == atc.type) { 872 return atc; 873 } 874 return null; 875 } 876 877 /** Override this to change asType behavior. */ 878 /*non-public*/ 879 MethodHandle asTypeUncached(MethodType newType) { 880 if (!type.isConvertibleTo(newType)) 881 throw new WrongMethodTypeException("cannot convert "+this+" to "+newType); 882 return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true); 883 } 884 885 /** 886 * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument 887 * and spreads its elements as positional arguments. 888 * The new method handle adapts, as its <i>target</i>, 889 * the current method handle. The type of the adapter will be 890 * the same as the type of the target, except that the final 891 * {@code arrayLength} parameters of the target's type are replaced 892 * by a single array parameter of type {@code arrayType}. 893 * <p> 894 * If the array element type differs from any of the corresponding 895 * argument types on the original target, 896 * the original target is adapted to take the array elements directly, 897 * as if by a call to {@link #asType asType}. 898 * <p> 899 * When called, the adapter replaces a trailing array argument 900 * by the array's elements, each as its own argument to the target. 901 * (The order of the arguments is preserved.) 902 * They are converted pairwise by casting and/or unboxing 903 * to the types of the trailing parameters of the target. 904 * Finally the target is called. 905 * What the target eventually returns is returned unchanged by the adapter. 906 * <p> 907 * Before calling the target, the adapter verifies that the array 908 * contains exactly enough elements to provide a correct argument count 909 * to the target method handle. 910 * (The array may also be null when zero elements are required.) 911 * <p> 912 * When the adapter is called, the length of the supplied {@code array} 913 * argument is queried as if by {@code array.length} or {@code arraylength} 914 * bytecode. If the adapter accepts a zero-length trailing array argument, 915 * the supplied {@code array} argument can either be a zero-length array or 916 * {@code null}; otherwise, the adapter will throw a {@code NullPointerException} 917 * if the array is {@code null} and throw an {@link IllegalArgumentException} 918 * if the array does not have the correct number of elements. 919 * <p> 920 * Here are some simple examples of array-spreading method handles: 921 * <blockquote><pre>{@code 922 MethodHandle equals = publicLookup() 923 .findVirtual(String.class, "equals", methodType(boolean.class, Object.class)); 924 assert( (boolean) equals.invokeExact("me", (Object)"me")); 925 assert(!(boolean) equals.invokeExact("me", (Object)"thee")); 926 // spread both arguments from a 2-array: 927 MethodHandle eq2 = equals.asSpreader(Object[].class, 2); 928 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" })); 929 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" })); 930 // try to spread from anything but a 2-array: 931 for (int n = 0; n <= 10; n++) { 932 Object[] badArityArgs = (n == 2 ? new Object[0] : new Object[n]); 933 try { assert((boolean) eq2.invokeExact(badArityArgs) && false); } 934 catch (IllegalArgumentException ex) { } // OK 935 } 936 // spread both arguments from a String array: 937 MethodHandle eq2s = equals.asSpreader(String[].class, 2); 938 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" })); 939 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" })); 940 // spread second arguments from a 1-array: 941 MethodHandle eq1 = equals.asSpreader(Object[].class, 1); 942 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" })); 943 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" })); 944 // spread no arguments from a 0-array or null: 945 MethodHandle eq0 = equals.asSpreader(Object[].class, 0); 946 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0])); 947 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null)); 948 // asSpreader and asCollector are approximate inverses: 949 for (int n = 0; n <= 2; n++) { 950 for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) { 951 MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n); 952 assert( (boolean) equals2.invokeWithArguments("me", "me")); 953 assert(!(boolean) equals2.invokeWithArguments("me", "thee")); 954 } 955 } 956 MethodHandle caToString = publicLookup() 957 .findStatic(Arrays.class, "toString", methodType(String.class, char[].class)); 958 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray())); 959 MethodHandle caString3 = caToString.asCollector(char[].class, 3); 960 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C')); 961 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2); 962 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray())); 963 * }</pre></blockquote> 964 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 965 * @param arrayLength the number of arguments to spread from an incoming array argument 966 * @return a new method handle which spreads its final array argument, 967 * before calling the original method handle 968 * @throws NullPointerException if {@code arrayType} is a null reference 969 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 970 * or if target does not have at least 971 * {@code arrayLength} parameter types, 972 * or if {@code arrayLength} is negative, 973 * or if the resulting method handle's type would have 974 * <a href="MethodHandle.html#maxarity">too many parameters</a> 975 * @throws WrongMethodTypeException if the implied {@code asType} call fails 976 * @see #asCollector 977 */ 978 public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) { 979 return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength); 980 } 981 982 /** 983 * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads 984 * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>, 985 * the current method handle. The type of the adapter will be the same as the type of the target, except that the 986 * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos}, 987 * are replaced by a single array parameter of type {@code arrayType}. 988 * <p> 989 * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos} 990 * argument to indicate at which position in the parameter list the spreading should take place. 991 * 992 * @apiNote Example: 993 * <blockquote><pre>{@code 994 MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class)); 995 MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2); 996 Object[] ints = new Object[]{3, 9, 7, 7}; 997 Comparator<Integer> cmp = (a, b) -> a - b; 998 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0); 999 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0); 1000 assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0); 1001 * }</pre></blockquote> 1002 * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start. 1003 * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments 1004 * @param arrayLength the number of arguments to spread from an incoming array argument 1005 * @return a new method handle which spreads an array argument at a given position, 1006 * before calling the original method handle 1007 * @throws NullPointerException if {@code arrayType} is a null reference 1008 * @throws IllegalArgumentException if {@code arrayType} is not an array type, 1009 * or if target does not have at least 1010 * {@code arrayLength} parameter types, 1011 * or if {@code arrayLength} is negative, 1012 * or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the 1013 * number of arguments), 1014 * or if the resulting method handle's type would have 1015 * <a href="MethodHandle.html#maxarity">too many parameters</a> 1016 * @throws WrongMethodTypeException if the implied {@code asType} call fails 1017 * 1018 * @see #asSpreader(Class, int) 1019 * @since 9 1020 */ 1021 public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) { 1022 MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength); 1023 MethodHandle afterSpread = this.asType(postSpreadType); 1024 BoundMethodHandle mh = afterSpread.rebind(); 1025 LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength); 1026 MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType); 1027 return mh.copyWith(preSpreadType, lform); 1028 } 1029 1030 /** 1031 * See if {@code asSpreader} can be validly called with the given arguments. 1032 * Return the type of the method handle call after spreading but before conversions. 1033 */ 1034 private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) { 1035 spreadArrayChecks(arrayType, arrayLength); 1036 int nargs = type().parameterCount(); 1037 if (nargs < arrayLength || arrayLength < 0) 1038 throw newIllegalArgumentException("bad spread array length"); 1039 if (pos < 0 || pos + arrayLength > nargs) { 1040 throw newIllegalArgumentException("bad spread position"); 1041 } 1042 Class<?> arrayElement = arrayType.getComponentType(); 1043 MethodType mtype = type(); 1044 boolean match = true, fail = false; 1045 for (int i = pos; i < pos + arrayLength; i++) { 1046 Class<?> ptype = mtype.parameterType(i); 1047 if (ptype != arrayElement) { 1048 match = false; 1049 if (!MethodType.canConvert(arrayElement, ptype)) { 1050 fail = true; 1051 break; 1052 } 1053 } 1054 } 1055 if (match) return mtype; 1056 MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength); 1057 if (!fail) return needType; 1058 // elicit an error: 1059 this.asType(needType); 1060 throw newInternalError("should not return"); 1061 } 1062 1063 private void spreadArrayChecks(Class<?> arrayType, int arrayLength) { 1064 Class<?> arrayElement = arrayType.getComponentType(); 1065 if (arrayElement == null) 1066 throw newIllegalArgumentException("not an array type", arrayType); 1067 if ((arrayLength & 0x7F) != arrayLength) { 1068 if ((arrayLength & 0xFF) != arrayLength) 1069 throw newIllegalArgumentException("array length is not legal", arrayLength); 1070 assert(arrayLength >= 128); 1071 if (arrayElement == long.class || 1072 arrayElement == double.class) 1073 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength); 1074 } 1075 } 1076 1077 /** 1078 * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity} 1079 * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}. 1080 * If the method handle is already of the proper arity mode, it is returned 1081 * unchanged. 1082 * @apiNote 1083 * <p>This method is sometimes useful when adapting a method handle that 1084 * may be variable arity, to ensure that the resulting adapter is also 1085 * variable arity if and only if the original handle was. For example, 1086 * this code changes the first argument of a handle {@code mh} to {@code int} without 1087 * disturbing its variable arity property: 1088 * {@code mh.asType(mh.type().changeParameterType(0,int.class)) 1089 * .withVarargs(mh.isVarargsCollector())} 1090 * <p> 1091 * This call is approximately equivalent to the following code: 1092 * <blockquote><pre>{@code 1093 * if (makeVarargs == isVarargsCollector()) 1094 * return this; 1095 * else if (makeVarargs) 1096 * return asVarargsCollector(type().lastParameterType()); 1097 * else 1098 * return asFixedArity(); 1099 * }</pre></blockquote> 1100 * @param makeVarargs true if the return method handle should have variable arity behavior 1101 * @return a method handle of the same type, with possibly adjusted variable arity behavior 1102 * @throws IllegalArgumentException if {@code makeVarargs} is true and 1103 * this method handle does not have a trailing array parameter 1104 * @since 9 1105 * @see #asVarargsCollector 1106 * @see #asFixedArity 1107 */ 1108 public MethodHandle withVarargs(boolean makeVarargs) { 1109 assert(!isVarargsCollector()); // subclass responsibility 1110 if (makeVarargs) { 1111 return asVarargsCollector(type().lastParameterType()); 1112 } else { 1113 return this; 1114 } 1115 } 1116 1117 /** 1118 * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing 1119 * positional arguments and collects them into an array argument. 1120 * The new method handle adapts, as its <i>target</i>, 1121 * the current method handle. The type of the adapter will be 1122 * the same as the type of the target, except that a single trailing 1123 * parameter (usually of type {@code arrayType}) is replaced by 1124 * {@code arrayLength} parameters whose type is element type of {@code arrayType}. 1125 * <p> 1126 * If the array type differs from the final argument type on the original target, 1127 * the original target is adapted to take the array type directly, 1128 * as if by a call to {@link #asType asType}. 1129 * <p> 1130 * When called, the adapter replaces its trailing {@code arrayLength} 1131 * arguments by a single new array of type {@code arrayType}, whose elements 1132 * comprise (in order) the replaced arguments. 1133 * Finally the target is called. 1134 * What the target eventually returns is returned unchanged by the adapter. 1135 * <p> 1136 * (The array may also be a shared constant when {@code arrayLength} is zero.) 1137 * <p> 1138 * (<em>Note:</em> The {@code arrayType} is often identical to the 1139 * {@linkplain MethodType#lastParameterType last parameter type} 1140 * of the original target. 1141 * It is an explicit argument for symmetry with {@code asSpreader}, and also 1142 * to allow the target to use a simple {@code Object} as its last parameter type.) 1143 * <p> 1144 * In order to create a collecting adapter which is not restricted to a particular 1145 * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} 1146 * or {@link #withVarargs withVarargs} instead. 1147 * <p> 1148 * Here are some examples of array-collecting method handles: 1149 * <blockquote><pre>{@code 1150 MethodHandle deepToString = publicLookup() 1151 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1152 assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"})); 1153 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1); 1154 assertEquals(methodType(String.class, Object.class), ts1.type()); 1155 //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL 1156 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"})); 1157 // arrayType can be a subtype of Object[] 1158 MethodHandle ts2 = deepToString.asCollector(String[].class, 2); 1159 assertEquals(methodType(String.class, String.class, String.class), ts2.type()); 1160 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too")); 1161 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0); 1162 assertEquals("[]", (String) ts0.invokeExact()); 1163 // collectors can be nested, Lisp-style 1164 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2); 1165 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D"))); 1166 // arrayType can be any primitive array type 1167 MethodHandle bytesToString = publicLookup() 1168 .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class)) 1169 .asCollector(byte[].class, 3); 1170 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3)); 1171 MethodHandle longsToString = publicLookup() 1172 .findStatic(Arrays.class, "toString", methodType(String.class, long[].class)) 1173 .asCollector(long[].class, 1); 1174 assertEquals("[123]", (String) longsToString.invokeExact((long)123)); 1175 * }</pre></blockquote> 1176 * <p> 1177 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1178 * variable-arity method handle}, even if the original target method handle was. 1179 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1180 * @param arrayLength the number of arguments to collect into a new array argument 1181 * @return a new method handle which collects some trailing argument 1182 * into an array, before calling the original method handle 1183 * @throws NullPointerException if {@code arrayType} is a null reference 1184 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1185 * or {@code arrayType} is not assignable to this method handle's trailing parameter type, 1186 * or {@code arrayLength} is not a legal array size, 1187 * or the resulting method handle's type would have 1188 * <a href="MethodHandle.html#maxarity">too many parameters</a> 1189 * @throws WrongMethodTypeException if the implied {@code asType} call fails 1190 * @see #asSpreader 1191 * @see #asVarargsCollector 1192 */ 1193 public MethodHandle asCollector(Class<?> arrayType, int arrayLength) { 1194 return asCollector(type().parameterCount() - 1, arrayType, arrayLength); 1195 } 1196 1197 /** 1198 * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting 1199 * at a given position, and collects them into an array argument. The new method handle adapts, as its 1200 * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target, 1201 * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType}) 1202 * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}. 1203 * <p> 1204 * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code 1205 * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This 1206 * index is zero-based. 1207 * 1208 * @apiNote Examples: 1209 * <blockquote><pre>{@code 1210 StringWriter swr = new StringWriter(); 1211 MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr); 1212 MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4); 1213 swWrite4.invoke('A', 'B', 'C', 'D', 1, 2); 1214 assertEquals("BC", swr.toString()); 1215 swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4); 1216 assertEquals("BCPQRS", swr.toString()); 1217 swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1); 1218 assertEquals("BCPQRSZ", swr.toString()); 1219 * }</pre></blockquote> 1220 * <p> 1221 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1222 * variable-arity method handle}, even if the original target method handle was. 1223 * @param collectArgPos the zero-based position in the parameter list at which to start collecting. 1224 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1225 * @param arrayLength the number of arguments to collect into a new array argument 1226 * @return a new method handle which collects some arguments 1227 * into an array, before calling the original method handle 1228 * @throws NullPointerException if {@code arrayType} is a null reference 1229 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1230 * or {@code arrayType} is not assignable to this method handle's array parameter type, 1231 * or {@code arrayLength} is not a legal array size, 1232 * or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments), 1233 * or the resulting method handle's type would have 1234 * <a href="MethodHandle.html#maxarity">too many parameters</a> 1235 * @throws WrongMethodTypeException if the implied {@code asType} call fails 1236 * 1237 * @see #asCollector(Class, int) 1238 * @since 9 1239 */ 1240 public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) { 1241 asCollectorChecks(arrayType, collectArgPos, arrayLength); 1242 BoundMethodHandle mh = rebind(); 1243 MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength); 1244 MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength); 1245 LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray); 1246 if (lform != null) { 1247 return mh.copyWith(resultType, lform); 1248 } 1249 lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType()); 1250 return mh.copyWithExtendL(resultType, lform, newArray); 1251 } 1252 1253 /** 1254 * See if {@code asCollector} can be validly called with the given arguments. 1255 * Return false if the last parameter is not an exact match to arrayType. 1256 */ 1257 /*non-public*/ 1258 boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) { 1259 spreadArrayChecks(arrayType, arrayLength); 1260 int nargs = type().parameterCount(); 1261 if (pos < 0 || pos >= nargs) { 1262 throw newIllegalArgumentException("bad collect position"); 1263 } 1264 if (nargs != 0) { 1265 Class<?> param = type().parameterType(pos); 1266 if (param == arrayType) return true; 1267 if (param.isAssignableFrom(arrayType)) return false; 1268 } 1269 throw newIllegalArgumentException("array type not assignable to argument", this, arrayType); 1270 } 1271 1272 /** 1273 * Makes a <em>variable arity</em> adapter which is able to accept 1274 * any number of trailing positional arguments and collect them 1275 * into an array argument. 1276 * <p> 1277 * The type and behavior of the adapter will be the same as 1278 * the type and behavior of the target, except that certain 1279 * {@code invoke} and {@code asType} requests can lead to 1280 * trailing positional arguments being collected into target's 1281 * trailing parameter. 1282 * Also, the 1283 * {@linkplain MethodType#lastParameterType last parameter type} 1284 * of the adapter will be 1285 * {@code arrayType}, even if the target has a different 1286 * last parameter type. 1287 * <p> 1288 * This transformation may return {@code this} if the method handle is 1289 * already of variable arity and its trailing parameter type 1290 * is identical to {@code arrayType}. 1291 * <p> 1292 * When called with {@link #invokeExact invokeExact}, the adapter invokes 1293 * the target with no argument changes. 1294 * (<em>Note:</em> This behavior is different from a 1295 * {@linkplain #asCollector fixed arity collector}, 1296 * since it accepts a whole array of indeterminate length, 1297 * rather than a fixed number of arguments.) 1298 * <p> 1299 * When called with plain, inexact {@link #invoke invoke}, if the caller 1300 * type is the same as the adapter, the adapter invokes the target as with 1301 * {@code invokeExact}. 1302 * (This is the normal behavior for {@code invoke} when types match.) 1303 * <p> 1304 * Otherwise, if the caller and adapter arity are the same, and the 1305 * trailing parameter type of the caller is a reference type identical to 1306 * or assignable to the trailing parameter type of the adapter, 1307 * the arguments and return values are converted pairwise, 1308 * as if by {@link #asType asType} on a fixed arity 1309 * method handle. 1310 * <p> 1311 * Otherwise, the arities differ, or the adapter's trailing parameter 1312 * type is not assignable from the corresponding caller type. 1313 * In this case, the adapter replaces all trailing arguments from 1314 * the original trailing argument position onward, by 1315 * a new array of type {@code arrayType}, whose elements 1316 * comprise (in order) the replaced arguments. 1317 * <p> 1318 * The caller type must provides as least enough arguments, 1319 * and of the correct type, to satisfy the target's requirement for 1320 * positional arguments before the trailing array argument. 1321 * Thus, the caller must supply, at a minimum, {@code N-1} arguments, 1322 * where {@code N} is the arity of the target. 1323 * Also, there must exist conversions from the incoming arguments 1324 * to the target's arguments. 1325 * As with other uses of plain {@code invoke}, if these basic 1326 * requirements are not fulfilled, a {@code WrongMethodTypeException} 1327 * may be thrown. 1328 * <p> 1329 * In all cases, what the target eventually returns is returned unchanged by the adapter. 1330 * <p> 1331 * In the final case, it is exactly as if the target method handle were 1332 * temporarily adapted with a {@linkplain #asCollector fixed arity collector} 1333 * to the arity required by the caller type. 1334 * (As with {@code asCollector}, if the array length is zero, 1335 * a shared constant may be used instead of a new array. 1336 * If the implied call to {@code asCollector} would throw 1337 * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, 1338 * the call to the variable arity adapter must throw 1339 * {@code WrongMethodTypeException}.) 1340 * <p> 1341 * The behavior of {@link #asType asType} is also specialized for 1342 * variable arity adapters, to maintain the invariant that 1343 * plain, inexact {@code invoke} is always equivalent to an {@code asType} 1344 * call to adjust the target type, followed by {@code invokeExact}. 1345 * Therefore, a variable arity adapter responds 1346 * to an {@code asType} request by building a fixed arity collector, 1347 * if and only if the adapter and requested type differ either 1348 * in arity or trailing argument type. 1349 * The resulting fixed arity collector has its type further adjusted 1350 * (if necessary) to the requested type by pairwise conversion, 1351 * as if by another application of {@code asType}. 1352 * <p> 1353 * When a method handle is obtained by executing an {@code ldc} instruction 1354 * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked 1355 * as a variable arity method (with the modifier bit {@code 0x0080}), 1356 * the method handle will accept multiple arities, as if the method handle 1357 * constant were created by means of a call to {@code asVarargsCollector}. 1358 * <p> 1359 * In order to create a collecting adapter which collects a predetermined 1360 * number of arguments, and whose type reflects this predetermined number, 1361 * use {@link #asCollector asCollector} instead. 1362 * <p> 1363 * No method handle transformations produce new method handles with 1364 * variable arity, unless they are documented as doing so. 1365 * Therefore, besides {@code asVarargsCollector} and {@code withVarargs}, 1366 * all methods in {@code MethodHandle} and {@code MethodHandles} 1367 * will return a method handle with fixed arity, 1368 * except in the cases where they are specified to return their original 1369 * operand (e.g., {@code asType} of the method handle's own type). 1370 * <p> 1371 * Calling {@code asVarargsCollector} on a method handle which is already 1372 * of variable arity will produce a method handle with the same type and behavior. 1373 * It may (or may not) return the original variable arity method handle. 1374 * <p> 1375 * Here is an example, of a list-making variable arity method handle: 1376 * <blockquote><pre>{@code 1377 MethodHandle deepToString = publicLookup() 1378 .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); 1379 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); 1380 assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); 1381 assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); 1382 assertEquals("[won]", (String) ts1.invoke( "won" )); 1383 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); 1384 // findStatic of Arrays.asList(...) produces a variable arity method handle: 1385 MethodHandle asList = publicLookup() 1386 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); 1387 assertEquals(methodType(List.class, Object[].class), asList.type()); 1388 assert(asList.isVarargsCollector()); 1389 assertEquals("[]", asList.invoke().toString()); 1390 assertEquals("[1]", asList.invoke(1).toString()); 1391 assertEquals("[two, too]", asList.invoke("two", "too").toString()); 1392 String[] argv = { "three", "thee", "tee" }; 1393 assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); 1394 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); 1395 List ls = (List) asList.invoke((Object)argv); 1396 assertEquals(1, ls.size()); 1397 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); 1398 * }</pre></blockquote> 1399 * <p style="font-size:smaller;"> 1400 * <em>Discussion:</em> 1401 * These rules are designed as a dynamically-typed variation 1402 * of the Java rules for variable arity methods. 1403 * In both cases, callers to a variable arity method or method handle 1404 * can either pass zero or more positional arguments, or else pass 1405 * pre-collected arrays of any length. Users should be aware of the 1406 * special role of the final argument, and of the effect of a 1407 * type match on that final argument, which determines whether 1408 * or not a single trailing argument is interpreted as a whole 1409 * array or a single element of an array to be collected. 1410 * Note that the dynamic type of the trailing argument has no 1411 * effect on this decision, only a comparison between the symbolic 1412 * type descriptor of the call site and the type descriptor of the method handle.) 1413 * 1414 * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments 1415 * @return a new method handle which can collect any number of trailing arguments 1416 * into an array, before calling the original method handle 1417 * @throws NullPointerException if {@code arrayType} is a null reference 1418 * @throws IllegalArgumentException if {@code arrayType} is not an array type 1419 * or {@code arrayType} is not assignable to this method handle's trailing parameter type 1420 * @see #asCollector 1421 * @see #isVarargsCollector 1422 * @see #withVarargs 1423 * @see #asFixedArity 1424 */ 1425 public MethodHandle asVarargsCollector(Class<?> arrayType) { 1426 Objects.requireNonNull(arrayType); 1427 boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0); 1428 if (isVarargsCollector() && lastMatch) 1429 return this; 1430 return MethodHandleImpl.makeVarargsCollector(this, arrayType); 1431 } 1432 1433 /** 1434 * Determines if this method handle 1435 * supports {@linkplain #asVarargsCollector variable arity} calls. 1436 * Such method handles arise from the following sources: 1437 * <ul> 1438 * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} 1439 * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} 1440 * which resolves to a variable arity Java method or constructor 1441 * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} 1442 * which resolves to a variable arity Java method or constructor 1443 * </ul> 1444 * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls 1445 * @see #asVarargsCollector 1446 * @see #asFixedArity 1447 */ 1448 public boolean isVarargsCollector() { 1449 return false; 1450 } 1451 1452 /** 1453 * Makes a <em>fixed arity</em> method handle which is otherwise 1454 * equivalent to the current method handle. 1455 * <p> 1456 * If the current method handle is not of 1457 * {@linkplain #asVarargsCollector variable arity}, 1458 * the current method handle is returned. 1459 * This is true even if the current method handle 1460 * could not be a valid input to {@code asVarargsCollector}. 1461 * <p> 1462 * Otherwise, the resulting fixed-arity method handle has the same 1463 * type and behavior of the current method handle, 1464 * except that {@link #isVarargsCollector isVarargsCollector} 1465 * will be false. 1466 * The fixed-arity method handle may (or may not) be the 1467 * a previous argument to {@code asVarargsCollector}. 1468 * <p> 1469 * Here is an example, of a list-making variable arity method handle: 1470 * <blockquote><pre>{@code 1471 MethodHandle asListVar = publicLookup() 1472 .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) 1473 .asVarargsCollector(Object[].class); 1474 MethodHandle asListFix = asListVar.asFixedArity(); 1475 assertEquals("[1]", asListVar.invoke(1).toString()); 1476 Exception caught = null; 1477 try { asListFix.invoke((Object)1); } 1478 catch (Exception ex) { caught = ex; } 1479 assert(caught instanceof ClassCastException); 1480 assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); 1481 try { asListFix.invoke("two", "too"); } 1482 catch (Exception ex) { caught = ex; } 1483 assert(caught instanceof WrongMethodTypeException); 1484 Object[] argv = { "three", "thee", "tee" }; 1485 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); 1486 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); 1487 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); 1488 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); 1489 * }</pre></blockquote> 1490 * 1491 * @return a new method handle which accepts only a fixed number of arguments 1492 * @see #asVarargsCollector 1493 * @see #isVarargsCollector 1494 * @see #withVarargs 1495 */ 1496 public MethodHandle asFixedArity() { 1497 assert(!isVarargsCollector()); 1498 return this; 1499 } 1500 1501 /** 1502 * Binds a value {@code x} to the first argument of a method handle, without invoking it. 1503 * The new method handle adapts, as its <i>target</i>, 1504 * the current method handle by binding it to the given argument. 1505 * The type of the bound handle will be 1506 * the same as the type of the target, except that a single leading 1507 * reference parameter will be omitted. 1508 * <p> 1509 * When called, the bound handle inserts the given value {@code x} 1510 * as a new leading argument to the target. The other arguments are 1511 * also passed unchanged. 1512 * What the target eventually returns is returned unchanged by the bound handle. 1513 * <p> 1514 * The reference {@code x} must be convertible to the first parameter 1515 * type of the target. 1516 * <p> 1517 * <em>Note:</em> Because method handles are immutable, the target method handle 1518 * retains its original type and behavior. 1519 * <p> 1520 * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector 1521 * variable-arity method handle}, even if the original target method handle was. 1522 * @param x the value to bind to the first argument of the target 1523 * @return a new method handle which prepends the given value to the incoming 1524 * argument list, before calling the original method handle 1525 * @throws IllegalArgumentException if the target does not have a 1526 * leading parameter type that is a reference type 1527 * @throws ClassCastException if {@code x} cannot be converted 1528 * to the leading parameter type of the target 1529 * @see MethodHandles#insertArguments 1530 */ 1531 public MethodHandle bindTo(Object x) { 1532 x = type.leadingReferenceParameter().cast(x); // throw CCE if needed 1533 return bindArgumentL(0, x); 1534 } 1535 1536 /** 1537 * Return a nominal descriptor for this instance, if one can be 1538 * constructed, or an empty {@link Optional} if one cannot be. 1539 * 1540 * @return An {@link Optional} containing the resulting nominal descriptor, 1541 * or an empty {@link Optional} if one cannot be constructed. 1542 * @since 12 1543 */ 1544 @Override 1545 public Optional<MethodHandleDesc> describeConstable() { 1546 MethodHandleInfo info; 1547 ClassDesc owner; 1548 String name; 1549 MethodTypeDesc type; 1550 boolean isInterface; 1551 try { 1552 info = IMPL_LOOKUP.revealDirect(this); 1553 isInterface = info.getDeclaringClass().isInterface(); 1554 owner = info.getDeclaringClass().describeConstable().orElseThrow(); 1555 type = info.getMethodType().describeConstable().orElseThrow(); 1556 name = info.getName(); 1557 } 1558 catch (Exception e) { 1559 return Optional.empty(); 1560 } 1561 1562 switch (info.getReferenceKind()) { 1563 case REF_getField: 1564 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.GETTER, owner, name, type.returnType())); 1565 case REF_putField: 1566 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.SETTER, owner, name, type.parameterType(0))); 1567 case REF_getStatic: 1568 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_GETTER, owner, name, type.returnType())); 1569 case REF_putStatic: 1570 return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_SETTER, owner, name, type.parameterType(0))); 1571 case REF_invokeVirtual: 1572 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.VIRTUAL, owner, name, type)); 1573 case REF_invokeStatic: 1574 return isInterface ? 1575 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_STATIC, owner, name, type)) : 1576 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.STATIC, owner, name, type)); 1577 case REF_invokeSpecial: 1578 return isInterface ? 1579 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_SPECIAL, owner, name, type)) : 1580 Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.SPECIAL, owner, name, type)); 1581 case REF_invokeInterface: 1582 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_VIRTUAL, owner, name, type)); 1583 case REF_newInvokeSpecial: 1584 return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.CONSTRUCTOR, owner, name, type)); 1585 default: 1586 return Optional.empty(); 1587 } 1588 } 1589 1590 /** 1591 * Returns a string representation of the method handle, 1592 * starting with the string {@code "MethodHandle"} and 1593 * ending with the string representation of the method handle's type. 1594 * In other words, this method returns a string equal to the value of: 1595 * <blockquote><pre>{@code 1596 * "MethodHandle" + type().toString() 1597 * }</pre></blockquote> 1598 * <p> 1599 * (<em>Note:</em> Future releases of this API may add further information 1600 * to the string representation. 1601 * Therefore, the present syntax should not be parsed by applications.) 1602 * 1603 * @return a string representation of the method handle 1604 */ 1605 @Override 1606 public String toString() { 1607 if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle"+debugString(); 1608 return standardString(); 1609 } 1610 String standardString() { 1611 return "MethodHandle"+type; 1612 } 1613 /** Return a string with a several lines describing the method handle structure. 1614 * This string would be suitable for display in an IDE debugger. 1615 */ 1616 String debugString() { 1617 return type+" : "+internalForm()+internalProperties(); 1618 } 1619 1620 //// Implementation methods. 1621 //// Sub-classes can override these default implementations. 1622 //// All these methods assume arguments are already validated. 1623 1624 // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch 1625 1626 BoundMethodHandle bindArgumentL(int pos, Object value) { 1627 return rebind().bindArgumentL(pos, value); 1628 } 1629 1630 /*non-public*/ 1631 MethodHandle setVarargs(MemberName member) throws IllegalAccessException { 1632 if (!member.isVarargs()) return this; 1633 try { 1634 return this.withVarargs(true); 1635 } catch (IllegalArgumentException ex) { 1636 throw member.makeAccessException("cannot make variable arity", null); 1637 } 1638 } 1639 1640 /*non-public*/ 1641 MethodHandle viewAsType(MethodType newType, boolean strict) { 1642 // No actual conversions, just a new view of the same method. 1643 // Overridden in DMH, which has special rules 1644 assert viewAsTypeChecks(newType, strict); 1645 return copyWith(newType, form); 1646 } 1647 1648 /*non-public*/ 1649 boolean viewAsTypeChecks(MethodType newType, boolean strict) { 1650 if (strict) { 1651 assert(type().isViewableAs(newType, true)) 1652 : Arrays.asList(this, newType); 1653 } else { 1654 assert(type().basicType().isViewableAs(newType.basicType(), true)) 1655 : Arrays.asList(this, newType); 1656 } 1657 return true; 1658 } 1659 1660 // Decoding 1661 1662 /*non-public*/ 1663 LambdaForm internalForm() { 1664 return form; 1665 } 1666 1667 /*non-public*/ 1668 MemberName internalMemberName() { 1669 return null; // DMH returns DMH.member 1670 } 1671 1672 /*non-public*/ 1673 Class<?> internalCallerClass() { 1674 return null; // caller-bound MH for @CallerSensitive method returns caller 1675 } 1676 1677 /*non-public*/ 1678 MethodHandleImpl.Intrinsic intrinsicName() { 1679 // no special intrinsic meaning to most MHs 1680 return MethodHandleImpl.Intrinsic.NONE; 1681 } 1682 1683 /*non-public*/ 1684 MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) { 1685 if (member != null) { 1686 return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial); 1687 } else if (internalMemberName() == null) { 1688 // The required internaMemberName is null, and this MH (like most) doesn't have one. 1689 return this; 1690 } else { 1691 // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. 1692 MethodHandle result = rebind(); 1693 assert (result.internalMemberName() == null); 1694 return result; 1695 } 1696 } 1697 1698 /*non-public*/ 1699 boolean isInvokeSpecial() { 1700 return false; // DMH.Special returns true 1701 } 1702 1703 /*non-public*/ 1704 boolean isCrackable() { 1705 return false; 1706 } 1707 1708 /*non-public*/ 1709 Object internalValues() { 1710 return null; 1711 } 1712 1713 /*non-public*/ 1714 Object internalProperties() { 1715 // Override to something to follow this.form, like "\n& FOO=bar" 1716 return ""; 1717 } 1718 1719 //// Method handle implementation methods. 1720 //// Sub-classes can override these default implementations. 1721 //// All these methods assume arguments are already validated. 1722 1723 /*non-public*/ 1724 abstract MethodHandle copyWith(MethodType mt, LambdaForm lf); 1725 1726 /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH. 1727 * Many transforms are implemented only for BMHs. 1728 * @return a behaviorally equivalent BMH 1729 */ 1730 abstract BoundMethodHandle rebind(); 1731 1732 /** 1733 * Replace the old lambda form of this method handle with a new one. 1734 * The new one must be functionally equivalent to the old one. 1735 * Threads may continue running the old form indefinitely, 1736 * but it is likely that the new one will be preferred for new executions. 1737 * Use with discretion. 1738 */ 1739 /*non-public*/ 1740 void updateForm(LambdaForm newForm) { 1741 assert(newForm.customized == null || newForm.customized == this); 1742 if (form == newForm) return; 1743 newForm.prepare(); // as in MethodHandle.<init> 1744 UNSAFE.putReference(this, FORM_OFFSET, newForm); 1745 UNSAFE.fullFence(); 1746 } 1747 1748 /** Craft a LambdaForm customized for this particular MethodHandle */ 1749 /*non-public*/ 1750 void customize() { 1751 final LambdaForm form = this.form; 1752 if (form.customized == null) { 1753 LambdaForm newForm = form.customize(this); 1754 updateForm(newForm); 1755 } else { 1756 assert(form.customized == this); 1757 } 1758 } 1759 1760 private static final long FORM_OFFSET 1761 = UNSAFE.objectFieldOffset(MethodHandle.class, "form"); 1762 }