1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/rt/emul/compact/src/main/java/java/lang/invoke/MethodHandle.java Tue Jan 17 07:04:06 2017 +0100
1.3 @@ -0,0 +1,1168 @@
1.4 +/*
1.5 + * Copyright (c) 2008, 2013, Oracle and/or its affiliates. All rights reserved.
1.6 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.7 + *
1.8 + * This code is free software; you can redistribute it and/or modify it
1.9 + * under the terms of the GNU General Public License version 2 only, as
1.10 + * published by the Free Software Foundation. Oracle designates this
1.11 + * particular file as subject to the "Classpath" exception as provided
1.12 + * by Oracle in the LICENSE file that accompanied this code.
1.13 + *
1.14 + * This code is distributed in the hope that it will be useful, but WITHOUT
1.15 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
1.16 + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
1.17 + * version 2 for more details (a copy is included in the LICENSE file that
1.18 + * accompanied this code).
1.19 + *
1.20 + * You should have received a copy of the GNU General Public License version
1.21 + * 2 along with this work; if not, write to the Free Software Foundation,
1.22 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
1.23 + *
1.24 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
1.25 + * or visit www.oracle.com if you need additional information or have any
1.26 + * questions.
1.27 + */
1.28 +
1.29 +package java.lang.invoke;
1.30 +
1.31 +
1.32 +import java.util.*;
1.33 +
1.34 +/**
1.35 + * A method handle is a typed, directly executable reference to an underlying method,
1.36 + * constructor, field, or similar low-level operation, with optional
1.37 + * transformations of arguments or return values.
1.38 + * These transformations are quite general, and include such patterns as
1.39 + * {@linkplain #asType conversion},
1.40 + * {@linkplain #bindTo insertion},
1.41 + * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
1.42 + * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
1.43 + *
1.44 + * <h1>Method handle contents</h1>
1.45 + * Method handles are dynamically and strongly typed according to their parameter and return types.
1.46 + * They are not distinguished by the name or the defining class of their underlying methods.
1.47 + * A method handle must be invoked using a symbolic type descriptor which matches
1.48 + * the method handle's own {@linkplain #type type descriptor}.
1.49 + * <p>
1.50 + * Every method handle reports its type descriptor via the {@link #type type} accessor.
1.51 + * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
1.52 + * whose structure is a series of classes, one of which is
1.53 + * the return type of the method (or {@code void.class} if none).
1.54 + * <p>
1.55 + * A method handle's type controls the types of invocations it accepts,
1.56 + * and the kinds of transformations that apply to it.
1.57 + * <p>
1.58 + * A method handle contains a pair of special invoker methods
1.59 + * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
1.60 + * Both invoker methods provide direct access to the method handle's
1.61 + * underlying method, constructor, field, or other operation,
1.62 + * as modified by transformations of arguments and return values.
1.63 + * Both invokers accept calls which exactly match the method handle's own type.
1.64 + * The plain, inexact invoker also accepts a range of other call types.
1.65 + * <p>
1.66 + * Method handles are immutable and have no visible state.
1.67 + * Of course, they can be bound to underlying methods or data which exhibit state.
1.68 + * With respect to the Java Memory Model, any method handle will behave
1.69 + * as if all of its (internal) fields are final variables. This means that any method
1.70 + * handle made visible to the application will always be fully formed.
1.71 + * This is true even if the method handle is published through a shared
1.72 + * variable in a data race.
1.73 + * <p>
1.74 + * Method handles cannot be subclassed by the user.
1.75 + * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
1.76 + * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
1.77 + * operation. The programmer should not draw conclusions about a method handle
1.78 + * from its specific class, as the method handle class hierarchy (if any)
1.79 + * may change from time to time or across implementations from different vendors.
1.80 + *
1.81 + * <h1>Method handle compilation</h1>
1.82 + * A Java method call expression naming {@code invokeExact} or {@code invoke}
1.83 + * can invoke a method handle from Java source code.
1.84 + * From the viewpoint of source code, these methods can take any arguments
1.85 + * and their result can be cast to any return type.
1.86 + * Formally this is accomplished by giving the invoker methods
1.87 + * {@code Object} return types and variable arity {@code Object} arguments,
1.88 + * but they have an additional quality called <em>signature polymorphism</em>
1.89 + * which connects this freedom of invocation directly to the JVM execution stack.
1.90 + * <p>
1.91 + * As is usual with virtual methods, source-level calls to {@code invokeExact}
1.92 + * and {@code invoke} compile to an {@code invokevirtual} instruction.
1.93 + * More unusually, the compiler must record the actual argument types,
1.94 + * and may not perform method invocation conversions on the arguments.
1.95 + * Instead, it must push them on the stack according to their own unconverted types.
1.96 + * The method handle object itself is pushed on the stack before the arguments.
1.97 + * The compiler then calls the method handle with a symbolic type descriptor which
1.98 + * describes the argument and return types.
1.99 + * <p>
1.100 + * To issue a complete symbolic type descriptor, the compiler must also determine
1.101 + * the return type. This is based on a cast on the method invocation expression,
1.102 + * if there is one, or else {@code Object} if the invocation is an expression
1.103 + * or else {@code void} if the invocation is a statement.
1.104 + * The cast may be to a primitive type (but not {@code void}).
1.105 + * <p>
1.106 + * As a corner case, an uncasted {@code null} argument is given
1.107 + * a symbolic type descriptor of {@code java.lang.Void}.
1.108 + * The ambiguity with the type {@code Void} is harmless, since there are no references of type
1.109 + * {@code Void} except the null reference.
1.110 + *
1.111 + * <h1>Method handle invocation</h1>
1.112 + * The first time a {@code invokevirtual} instruction is executed
1.113 + * it is linked, by symbolically resolving the names in the instruction
1.114 + * and verifying that the method call is statically legal.
1.115 + * This is true of calls to {@code invokeExact} and {@code invoke}.
1.116 + * In this case, the symbolic type descriptor emitted by the compiler is checked for
1.117 + * correct syntax and names it contains are resolved.
1.118 + * Thus, an {@code invokevirtual} instruction which invokes
1.119 + * a method handle will always link, as long
1.120 + * as the symbolic type descriptor is syntactically well-formed
1.121 + * and the types exist.
1.122 + * <p>
1.123 + * When the {@code invokevirtual} is executed after linking,
1.124 + * the receiving method handle's type is first checked by the JVM
1.125 + * to ensure that it matches the symbolic type descriptor.
1.126 + * If the type match fails, it means that the method which the
1.127 + * caller is invoking is not present on the individual
1.128 + * method handle being invoked.
1.129 + * <p>
1.130 + * In the case of {@code invokeExact}, the type descriptor of the invocation
1.131 + * (after resolving symbolic type names) must exactly match the method type
1.132 + * of the receiving method handle.
1.133 + * In the case of plain, inexact {@code invoke}, the resolved type descriptor
1.134 + * must be a valid argument to the receiver's {@link #asType asType} method.
1.135 + * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
1.136 + * <p>
1.137 + * After type matching, a call to {@code invokeExact} directly
1.138 + * and immediately invoke the method handle's underlying method
1.139 + * (or other behavior, as the case may be).
1.140 + * <p>
1.141 + * A call to plain {@code invoke} works the same as a call to
1.142 + * {@code invokeExact}, if the symbolic type descriptor specified by the caller
1.143 + * exactly matches the method handle's own type.
1.144 + * If there is a type mismatch, {@code invoke} attempts
1.145 + * to adjust the type of the receiving method handle,
1.146 + * as if by a call to {@link #asType asType},
1.147 + * to obtain an exactly invokable method handle {@code M2}.
1.148 + * This allows a more powerful negotiation of method type
1.149 + * between caller and callee.
1.150 + * <p>
1.151 + * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
1.152 + * and implementations are therefore not required to materialize it.)
1.153 + *
1.154 + * <h1>Invocation checking</h1>
1.155 + * In typical programs, method handle type matching will usually succeed.
1.156 + * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
1.157 + * either directly (in the case of {@code invokeExact}) or indirectly as if
1.158 + * by a failed call to {@code asType} (in the case of {@code invoke}).
1.159 + * <p>
1.160 + * Thus, a method type mismatch which might show up as a linkage error
1.161 + * in a statically typed program can show up as
1.162 + * a dynamic {@code WrongMethodTypeException}
1.163 + * in a program which uses method handles.
1.164 + * <p>
1.165 + * Because method types contain "live" {@code Class} objects,
1.166 + * method type matching takes into account both types names and class loaders.
1.167 + * Thus, even if a method handle {@code M} is created in one
1.168 + * class loader {@code L1} and used in another {@code L2},
1.169 + * method handle calls are type-safe, because the caller's symbolic type
1.170 + * descriptor, as resolved in {@code L2},
1.171 + * is matched against the original callee method's symbolic type descriptor,
1.172 + * as resolved in {@code L1}.
1.173 + * The resolution in {@code L1} happens when {@code M} is created
1.174 + * and its type is assigned, while the resolution in {@code L2} happens
1.175 + * when the {@code invokevirtual} instruction is linked.
1.176 + * <p>
1.177 + * Apart from the checking of type descriptors,
1.178 + * a method handle's capability to call its underlying method is unrestricted.
1.179 + * If a method handle is formed on a non-public method by a class
1.180 + * that has access to that method, the resulting handle can be used
1.181 + * in any place by any caller who receives a reference to it.
1.182 + * <p>
1.183 + * Unlike with the Core Reflection API, where access is checked every time
1.184 + * a reflective method is invoked,
1.185 + * method handle access checking is performed
1.186 + * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
1.187 + * In the case of {@code ldc} (see below), access checking is performed as part of linking
1.188 + * the constant pool entry underlying the constant method handle.
1.189 + * <p>
1.190 + * Thus, handles to non-public methods, or to methods in non-public classes,
1.191 + * should generally be kept secret.
1.192 + * They should not be passed to untrusted code unless their use from
1.193 + * the untrusted code would be harmless.
1.194 + *
1.195 + * <h1>Method handle creation</h1>
1.196 + * Java code can create a method handle that directly accesses
1.197 + * any method, constructor, or field that is accessible to that code.
1.198 + * This is done via a reflective, capability-based API called
1.199 + * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
1.200 + * For example, a static method handle can be obtained
1.201 + * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
1.202 + * There are also conversion methods from Core Reflection API objects,
1.203 + * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
1.204 + * <p>
1.205 + * Like classes and strings, method handles that correspond to accessible
1.206 + * fields, methods, and constructors can also be represented directly
1.207 + * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
1.208 + * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
1.209 + * refers directly to an associated {@code CONSTANT_Methodref},
1.210 + * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
1.211 + * constant pool entry.
1.212 + * (For full details on method handle constants,
1.213 + * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
1.214 + * <p>
1.215 + * Method handles produced by lookups or constant loads from methods or
1.216 + * constructors with the variable arity modifier bit ({@code 0x0080})
1.217 + * have a corresponding variable arity, as if they were defined with
1.218 + * the help of {@link #asVarargsCollector asVarargsCollector}.
1.219 + * <p>
1.220 + * A method reference may refer either to a static or non-static method.
1.221 + * In the non-static case, the method handle type includes an explicit
1.222 + * receiver argument, prepended before any other arguments.
1.223 + * In the method handle's type, the initial receiver argument is typed
1.224 + * according to the class under which the method was initially requested.
1.225 + * (E.g., if a non-static method handle is obtained via {@code ldc},
1.226 + * the type of the receiver is the class named in the constant pool entry.)
1.227 + * <p>
1.228 + * Method handle constants are subject to the same link-time access checks
1.229 + * their corresponding bytecode instructions, and the {@code ldc} instruction
1.230 + * will throw corresponding linkage errors if the bytecode behaviors would
1.231 + * throw such errors.
1.232 + * <p>
1.233 + * As a corollary of this, access to protected members is restricted
1.234 + * to receivers only of the accessing class, or one of its subclasses,
1.235 + * and the accessing class must in turn be a subclass (or package sibling)
1.236 + * of the protected member's defining class.
1.237 + * If a method reference refers to a protected non-static method or field
1.238 + * of a class outside the current package, the receiver argument will
1.239 + * be narrowed to the type of the accessing class.
1.240 + * <p>
1.241 + * When a method handle to a virtual method is invoked, the method is
1.242 + * always looked up in the receiver (that is, the first argument).
1.243 + * <p>
1.244 + * A non-virtual method handle to a specific virtual method implementation
1.245 + * can also be created. These do not perform virtual lookup based on
1.246 + * receiver type. Such a method handle simulates the effect of
1.247 + * an {@code invokespecial} instruction to the same method.
1.248 + *
1.249 + * <h1>Usage examples</h1>
1.250 + * Here are some examples of usage:
1.251 + * <blockquote><pre>{@code
1.252 +Object x, y; String s; int i;
1.253 +MethodType mt; MethodHandle mh;
1.254 +MethodHandles.Lookup lookup = MethodHandles.lookup();
1.255 +// mt is (char,char)String
1.256 +mt = MethodType.methodType(String.class, char.class, char.class);
1.257 +mh = lookup.findVirtual(String.class, "replace", mt);
1.258 +s = (String) mh.invokeExact("daddy",'d','n');
1.259 +// invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
1.260 +assertEquals(s, "nanny");
1.261 +// weakly typed invocation (using MHs.invoke)
1.262 +s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
1.263 +assertEquals(s, "savvy");
1.264 +// mt is (Object[])List
1.265 +mt = MethodType.methodType(java.util.List.class, Object[].class);
1.266 +mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
1.267 +assert(mh.isVarargsCollector());
1.268 +x = mh.invoke("one", "two");
1.269 +// invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
1.270 +assertEquals(x, java.util.Arrays.asList("one","two"));
1.271 +// mt is (Object,Object,Object)Object
1.272 +mt = MethodType.genericMethodType(3);
1.273 +mh = mh.asType(mt);
1.274 +x = mh.invokeExact((Object)1, (Object)2, (Object)3);
1.275 +// invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
1.276 +assertEquals(x, java.util.Arrays.asList(1,2,3));
1.277 +// mt is ()int
1.278 +mt = MethodType.methodType(int.class);
1.279 +mh = lookup.findVirtual(java.util.List.class, "size", mt);
1.280 +i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
1.281 +// invokeExact(Ljava/util/List;)I
1.282 +assert(i == 3);
1.283 +mt = MethodType.methodType(void.class, String.class);
1.284 +mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
1.285 +mh.invokeExact(System.out, "Hello, world.");
1.286 +// invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
1.287 + * }</pre></blockquote>
1.288 + * Each of the above calls to {@code invokeExact} or plain {@code invoke}
1.289 + * generates a single invokevirtual instruction with
1.290 + * the symbolic type descriptor indicated in the following comment.
1.291 + * In these examples, the helper method {@code assertEquals} is assumed to
1.292 + * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
1.293 + * on its arguments, and asserts that the result is true.
1.294 + *
1.295 + * <h1>Exceptions</h1>
1.296 + * The methods {@code invokeExact} and {@code invoke} are declared
1.297 + * to throw {@link java.lang.Throwable Throwable},
1.298 + * which is to say that there is no static restriction on what a method handle
1.299 + * can throw. Since the JVM does not distinguish between checked
1.300 + * and unchecked exceptions (other than by their class, of course),
1.301 + * there is no particular effect on bytecode shape from ascribing
1.302 + * checked exceptions to method handle invocations. But in Java source
1.303 + * code, methods which perform method handle calls must either explicitly
1.304 + * throw {@code Throwable}, or else must catch all
1.305 + * throwables locally, rethrowing only those which are legal in the context,
1.306 + * and wrapping ones which are illegal.
1.307 + *
1.308 + * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
1.309 + * The unusual compilation and linkage behavior of
1.310 + * {@code invokeExact} and plain {@code invoke}
1.311 + * is referenced by the term <em>signature polymorphism</em>.
1.312 + * As defined in the Java Language Specification,
1.313 + * a signature polymorphic method is one which can operate with
1.314 + * any of a wide range of call signatures and return types.
1.315 + * <p>
1.316 + * In source code, a call to a signature polymorphic method will
1.317 + * compile, regardless of the requested symbolic type descriptor.
1.318 + * As usual, the Java compiler emits an {@code invokevirtual}
1.319 + * instruction with the given symbolic type descriptor against the named method.
1.320 + * The unusual part is that the symbolic type descriptor is derived from
1.321 + * the actual argument and return types, not from the method declaration.
1.322 + * <p>
1.323 + * When the JVM processes bytecode containing signature polymorphic calls,
1.324 + * it will successfully link any such call, regardless of its symbolic type descriptor.
1.325 + * (In order to retain type safety, the JVM will guard such calls with suitable
1.326 + * dynamic type checks, as described elsewhere.)
1.327 + * <p>
1.328 + * Bytecode generators, including the compiler back end, are required to emit
1.329 + * untransformed symbolic type descriptors for these methods.
1.330 + * Tools which determine symbolic linkage are required to accept such
1.331 + * untransformed descriptors, without reporting linkage errors.
1.332 + *
1.333 + * <h1>Interoperation between method handles and the Core Reflection API</h1>
1.334 + * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
1.335 + * any class member represented by a Core Reflection API object
1.336 + * can be converted to a behaviorally equivalent method handle.
1.337 + * For example, a reflective {@link java.lang.reflect.Method Method} can
1.338 + * be converted to a method handle using
1.339 + * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
1.340 + * The resulting method handles generally provide more direct and efficient
1.341 + * access to the underlying class members.
1.342 + * <p>
1.343 + * As a special case,
1.344 + * when the Core Reflection API is used to view the signature polymorphic
1.345 + * methods {@code invokeExact} or plain {@code invoke} in this class,
1.346 + * they appear as ordinary non-polymorphic methods.
1.347 + * Their reflective appearance, as viewed by
1.348 + * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
1.349 + * is unaffected by their special status in this API.
1.350 + * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
1.351 + * will report exactly those modifier bits required for any similarly
1.352 + * declared method, including in this case {@code native} and {@code varargs} bits.
1.353 + * <p>
1.354 + * As with any reflected method, these methods (when reflected) may be
1.355 + * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
1.356 + * However, such reflective calls do not result in method handle invocations.
1.357 + * Such a call, if passed the required argument
1.358 + * (a single one, of type {@code Object[]}), will ignore the argument and
1.359 + * will throw an {@code UnsupportedOperationException}.
1.360 + * <p>
1.361 + * Since {@code invokevirtual} instructions can natively
1.362 + * invoke method handles under any symbolic type descriptor, this reflective view conflicts
1.363 + * with the normal presentation of these methods via bytecodes.
1.364 + * Thus, these two native methods, when reflectively viewed by
1.365 + * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
1.366 + * <p>
1.367 + * In order to obtain an invoker method for a particular type descriptor,
1.368 + * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
1.369 + * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
1.370 + * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
1.371 + * API is also able to return a method handle
1.372 + * to call {@code invokeExact} or plain {@code invoke},
1.373 + * for any specified type descriptor .
1.374 + *
1.375 + * <h1>Interoperation between method handles and Java generics</h1>
1.376 + * A method handle can be obtained on a method, constructor, or field
1.377 + * which is declared with Java generic types.
1.378 + * As with the Core Reflection API, the type of the method handle
1.379 + * will constructed from the erasure of the source-level type.
1.380 + * When a method handle is invoked, the types of its arguments
1.381 + * or the return value cast type may be generic types or type instances.
1.382 + * If this occurs, the compiler will replace those
1.383 + * types by their erasures when it constructs the symbolic type descriptor
1.384 + * for the {@code invokevirtual} instruction.
1.385 + * <p>
1.386 + * Method handles do not represent
1.387 + * their function-like types in terms of Java parameterized (generic) types,
1.388 + * because there are three mismatches between function-like types and parameterized
1.389 + * Java types.
1.390 + * <ul>
1.391 + * <li>Method types range over all possible arities,
1.392 + * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
1.393 + * Generics are not variadic, and so cannot represent this.</li>
1.394 + * <li>Method types can specify arguments of primitive types,
1.395 + * which Java generic types cannot range over.</li>
1.396 + * <li>Higher order functions over method handles (combinators) are
1.397 + * often generic across a wide range of function types, including
1.398 + * those of multiple arities. It is impossible to represent such
1.399 + * genericity with a Java type parameter.</li>
1.400 + * </ul>
1.401 + *
1.402 + * <h1><a name="maxarity"></a>Arity limits</h1>
1.403 + * The JVM imposes on all methods and constructors of any kind an absolute
1.404 + * limit of 255 stacked arguments. This limit can appear more restrictive
1.405 + * in certain cases:
1.406 + * <ul>
1.407 + * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
1.408 + * <li>A non-static method consumes an extra argument for the object on which the method is called.
1.409 + * <li>A constructor consumes an extra argument for the object which is being constructed.
1.410 + * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
1.411 + * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
1.412 + * </ul>
1.413 + * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
1.414 + * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
1.415 + * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
1.416 + * In particular, a method handle’s type must not have an arity of the exact maximum 255.
1.417 + *
1.418 + * @see MethodType
1.419 + * @see MethodHandles
1.420 + * @author John Rose, JSR 292 EG
1.421 + */
1.422 +public abstract class MethodHandle {
1.423 + /**
1.424 + * Internal marker interface which distinguishes (to the Java compiler)
1.425 + * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
1.426 + */
1.427 + @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
1.428 + @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
1.429 + @interface PolymorphicSignature { }
1.430 +
1.431 + /**
1.432 + * Reports the type of this method handle.
1.433 + * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
1.434 + * @return the method handle type
1.435 + */
1.436 + public MethodType type() {
1.437 + throw new IllegalStateException();
1.438 + }
1.439 +
1.440 + /**
1.441 + * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
1.442 + * The symbolic type descriptor at the call site of {@code invokeExact} must
1.443 + * exactly match this method handle's {@link #type type}.
1.444 + * No conversions are allowed on arguments or return values.
1.445 + * <p>
1.446 + * When this method is observed via the Core Reflection API,
1.447 + * it will appear as a single native method, taking an object array and returning an object.
1.448 + * If this native method is invoked directly via
1.449 + * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
1.450 + * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
1.451 + * it will throw an {@code UnsupportedOperationException}.
1.452 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.453 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.454 + * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
1.455 + * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
1.456 + */
1.457 + public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
1.458 +
1.459 + /**
1.460 + * Invokes the method handle, allowing any caller type descriptor,
1.461 + * and optionally performing conversions on arguments and return values.
1.462 + * <p>
1.463 + * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
1.464 + * the call proceeds as if by {@link #invokeExact invokeExact}.
1.465 + * <p>
1.466 + * Otherwise, the call proceeds as if this method handle were first
1.467 + * adjusted by calling {@link #asType asType} to adjust this method handle
1.468 + * to the required type, and then the call proceeds as if by
1.469 + * {@link #invokeExact invokeExact} on the adjusted method handle.
1.470 + * <p>
1.471 + * There is no guarantee that the {@code asType} call is actually made.
1.472 + * If the JVM can predict the results of making the call, it may perform
1.473 + * adaptations directly on the caller's arguments,
1.474 + * and call the target method handle according to its own exact type.
1.475 + * <p>
1.476 + * The resolved type descriptor at the call site of {@code invoke} must
1.477 + * be a valid argument to the receivers {@code asType} method.
1.478 + * In particular, the caller must specify the same argument arity
1.479 + * as the callee's type,
1.480 + * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
1.481 + * <p>
1.482 + * When this method is observed via the Core Reflection API,
1.483 + * it will appear as a single native method, taking an object array and returning an object.
1.484 + * If this native method is invoked directly via
1.485 + * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
1.486 + * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
1.487 + * it will throw an {@code UnsupportedOperationException}.
1.488 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.489 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.490 + * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
1.491 + * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
1.492 + * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
1.493 + */
1.494 + public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
1.495 +
1.496 + /**
1.497 + * Private method for trusted invocation of a method handle respecting simplified signatures.
1.498 + * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
1.499 + * <p>
1.500 + * The caller signature is restricted to the following basic types:
1.501 + * Object, int, long, float, double, and void return.
1.502 + * <p>
1.503 + * The caller is responsible for maintaining type correctness by ensuring
1.504 + * that the each outgoing argument value is a member of the range of the corresponding
1.505 + * callee argument type.
1.506 + * (The caller should therefore issue appropriate casts and integer narrowing
1.507 + * operations on outgoing argument values.)
1.508 + * The caller can assume that the incoming result value is part of the range
1.509 + * of the callee's return type.
1.510 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.511 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.512 + */
1.513 + /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
1.514 +
1.515 + /**
1.516 + * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
1.517 + * The caller signature is restricted to basic types as with {@code invokeBasic}.
1.518 + * The trailing (not leading) argument must be a MemberName.
1.519 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.520 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.521 + */
1.522 + /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
1.523 +
1.524 + /**
1.525 + * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
1.526 + * The caller signature is restricted to basic types as with {@code invokeBasic}.
1.527 + * The trailing (not leading) argument must be a MemberName.
1.528 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.529 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.530 + */
1.531 + /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
1.532 +
1.533 + /**
1.534 + * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
1.535 + * The caller signature is restricted to basic types as with {@code invokeBasic}.
1.536 + * The trailing (not leading) argument must be a MemberName.
1.537 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.538 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.539 + */
1.540 + /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
1.541 +
1.542 + /**
1.543 + * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
1.544 + * The caller signature is restricted to basic types as with {@code invokeBasic}.
1.545 + * The trailing (not leading) argument must be a MemberName.
1.546 + * @param args the signature-polymorphic parameter list, statically represented using varargs
1.547 + * @return the signature-polymorphic result, statically represented using {@code Object}
1.548 + */
1.549 + /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
1.550 +
1.551 + /**
1.552 + * Performs a variable arity invocation, passing the arguments in the given list
1.553 + * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
1.554 + * which mentions only the type {@code Object}, and whose arity is the length
1.555 + * of the argument list.
1.556 + * <p>
1.557 + * Specifically, execution proceeds as if by the following steps,
1.558 + * although the methods are not guaranteed to be called if the JVM
1.559 + * can predict their effects.
1.560 + * <ul>
1.561 + * <li>Determine the length of the argument array as {@code N}.
1.562 + * For a null reference, {@code N=0}. </li>
1.563 + * <li>Determine the general type {@code TN} of {@code N} arguments as
1.564 + * as {@code TN=MethodType.genericMethodType(N)}.</li>
1.565 + * <li>Force the original target method handle {@code MH0} to the
1.566 + * required type, as {@code MH1 = MH0.asType(TN)}. </li>
1.567 + * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
1.568 + * <li>Invoke the type-adjusted method handle on the unpacked arguments:
1.569 + * MH1.invokeExact(A0, ...). </li>
1.570 + * <li>Take the return value as an {@code Object} reference. </li>
1.571 + * </ul>
1.572 + * <p>
1.573 + * Because of the action of the {@code asType} step, the following argument
1.574 + * conversions are applied as necessary:
1.575 + * <ul>
1.576 + * <li>reference casting
1.577 + * <li>unboxing
1.578 + * <li>widening primitive conversions
1.579 + * </ul>
1.580 + * <p>
1.581 + * The result returned by the call is boxed if it is a primitive,
1.582 + * or forced to null if the return type is void.
1.583 + * <p>
1.584 + * This call is equivalent to the following code:
1.585 + * <blockquote><pre>{@code
1.586 + * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
1.587 + * Object result = invoker.invokeExact(this, arguments);
1.588 + * }</pre></blockquote>
1.589 + * <p>
1.590 + * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
1.591 + * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
1.592 + * It can therefore be used as a bridge between native or reflective code and method handles.
1.593 + *
1.594 + * @param arguments the arguments to pass to the target
1.595 + * @return the result returned by the target
1.596 + * @throws ClassCastException if an argument cannot be converted by reference casting
1.597 + * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
1.598 + * @throws Throwable anything thrown by the target method invocation
1.599 + * @see MethodHandles#spreadInvoker
1.600 + */
1.601 + public Object invokeWithArguments(Object... arguments) throws Throwable {
1.602 + throw new IllegalStateException();
1.603 + }
1.604 +
1.605 + /**
1.606 + * Performs a variable arity invocation, passing the arguments in the given array
1.607 + * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
1.608 + * which mentions only the type {@code Object}, and whose arity is the length
1.609 + * of the argument array.
1.610 + * <p>
1.611 + * This method is also equivalent to the following code:
1.612 + * <blockquote><pre>{@code
1.613 + * invokeWithArguments(arguments.toArray()
1.614 + * }</pre></blockquote>
1.615 + *
1.616 + * @param arguments the arguments to pass to the target
1.617 + * @return the result returned by the target
1.618 + * @throws NullPointerException if {@code arguments} is a null reference
1.619 + * @throws ClassCastException if an argument cannot be converted by reference casting
1.620 + * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
1.621 + * @throws Throwable anything thrown by the target method invocation
1.622 + */
1.623 + public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
1.624 + return invokeWithArguments(arguments.toArray());
1.625 + }
1.626 +
1.627 + /**
1.628 + * Produces an adapter method handle which adapts the type of the
1.629 + * current method handle to a new type.
1.630 + * The resulting method handle is guaranteed to report a type
1.631 + * which is equal to the desired new type.
1.632 + * <p>
1.633 + * If the original type and new type are equal, returns {@code this}.
1.634 + * <p>
1.635 + * The new method handle, when invoked, will perform the following
1.636 + * steps:
1.637 + * <ul>
1.638 + * <li>Convert the incoming argument list to match the original
1.639 + * method handle's argument list.
1.640 + * <li>Invoke the original method handle on the converted argument list.
1.641 + * <li>Convert any result returned by the original method handle
1.642 + * to the return type of new method handle.
1.643 + * </ul>
1.644 + * <p>
1.645 + * This method provides the crucial behavioral difference between
1.646 + * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
1.647 + * The two methods
1.648 + * perform the same steps when the caller's type descriptor exactly m atches
1.649 + * the callee's, but when the types differ, plain {@link #invoke invoke}
1.650 + * also calls {@code asType} (or some internal equivalent) in order
1.651 + * to match up the caller's and callee's types.
1.652 + * <p>
1.653 + * If the current method is a variable arity method handle
1.654 + * argument list conversion may involve the conversion and collection
1.655 + * of several arguments into an array, as
1.656 + * {@linkplain #asVarargsCollector described elsewhere}.
1.657 + * In every other case, all conversions are applied <em>pairwise</em>,
1.658 + * which means that each argument or return value is converted to
1.659 + * exactly one argument or return value (or no return value).
1.660 + * The applied conversions are defined by consulting the
1.661 + * the corresponding component types of the old and new
1.662 + * method handle types.
1.663 + * <p>
1.664 + * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
1.665 + * or old and new return types. Specifically, for some valid index {@code i}, let
1.666 + * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
1.667 + * Or else, going the other way for return values, let
1.668 + * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
1.669 + * If the types are the same, the new method handle makes no change
1.670 + * to the corresponding argument or return value (if any).
1.671 + * Otherwise, one of the following conversions is applied
1.672 + * if possible:
1.673 + * <ul>
1.674 + * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
1.675 + * (The types do not need to be related in any particular way.
1.676 + * This is because a dynamic value of null can convert to any reference type.)
1.677 + * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
1.678 + * conversion (JLS 5.3) is applied, if one exists.
1.679 + * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
1.680 + * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
1.681 + * a Java casting conversion (JLS 5.5) is applied if one exists.
1.682 + * (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
1.683 + * which is then widened as needed to <em>T1</em>.)
1.684 + * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
1.685 + * conversion will be applied at runtime, possibly followed
1.686 + * by a Java method invocation conversion (JLS 5.3)
1.687 + * on the primitive value. (These are the primitive widening conversions.)
1.688 + * <em>T0</em> must be a wrapper class or a supertype of one.
1.689 + * (In the case where <em>T0</em> is Object, these are the conversions
1.690 + * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
1.691 + * The unboxing conversion must have a possibility of success, which means that
1.692 + * if <em>T0</em> is not itself a wrapper class, there must exist at least one
1.693 + * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
1.694 + * primitive value can be widened to <em>T1</em>.
1.695 + * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
1.696 + * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
1.697 + * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
1.698 + * a zero value is introduced.
1.699 + * </ul>
1.700 + * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
1.701 + * because neither corresponds specifically to the <em>dynamic type</em> of any
1.702 + * actual argument or return value.)
1.703 + * <p>
1.704 + * The method handle conversion cannot be made if any one of the required
1.705 + * pairwise conversions cannot be made.
1.706 + * <p>
1.707 + * At runtime, the conversions applied to reference arguments
1.708 + * or return values may require additional runtime checks which can fail.
1.709 + * An unboxing operation may fail because the original reference is null,
1.710 + * causing a {@link java.lang.NullPointerException NullPointerException}.
1.711 + * An unboxing operation or a reference cast may also fail on a reference
1.712 + * to an object of the wrong type,
1.713 + * causing a {@link java.lang.ClassCastException ClassCastException}.
1.714 + * Although an unboxing operation may accept several kinds of wrappers,
1.715 + * if none are available, a {@code ClassCastException} will be thrown.
1.716 + *
1.717 + * @param newType the expected type of the new method handle
1.718 + * @return a method handle which delegates to {@code this} after performing
1.719 + * any necessary argument conversions, and arranges for any
1.720 + * necessary return value conversions
1.721 + * @throws NullPointerException if {@code newType} is a null reference
1.722 + * @throws WrongMethodTypeException if the conversion cannot be made
1.723 + * @see MethodHandles#explicitCastArguments
1.724 + */
1.725 + public MethodHandle asType(MethodType newType) {
1.726 + throw new IllegalStateException();
1.727 + }
1.728 +
1.729 + /**
1.730 + * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
1.731 + * and spreads its elements as positional arguments.
1.732 + * The new method handle adapts, as its <i>target</i>,
1.733 + * the current method handle. The type of the adapter will be
1.734 + * the same as the type of the target, except that the final
1.735 + * {@code arrayLength} parameters of the target's type are replaced
1.736 + * by a single array parameter of type {@code arrayType}.
1.737 + * <p>
1.738 + * If the array element type differs from any of the corresponding
1.739 + * argument types on the original target,
1.740 + * the original target is adapted to take the array elements directly,
1.741 + * as if by a call to {@link #asType asType}.
1.742 + * <p>
1.743 + * When called, the adapter replaces a trailing array argument
1.744 + * by the array's elements, each as its own argument to the target.
1.745 + * (The order of the arguments is preserved.)
1.746 + * They are converted pairwise by casting and/or unboxing
1.747 + * to the types of the trailing parameters of the target.
1.748 + * Finally the target is called.
1.749 + * What the target eventually returns is returned unchanged by the adapter.
1.750 + * <p>
1.751 + * Before calling the target, the adapter verifies that the array
1.752 + * contains exactly enough elements to provide a correct argument count
1.753 + * to the target method handle.
1.754 + * (The array may also be null when zero elements are required.)
1.755 + * <p>
1.756 + * If, when the adapter is called, the supplied array argument does
1.757 + * not have the correct number of elements, the adapter will throw
1.758 + * an {@link IllegalArgumentException} instead of invoking the target.
1.759 + * <p>
1.760 + * Here are some simple examples of array-spreading method handles:
1.761 + * <blockquote><pre>{@code
1.762 +MethodHandle equals = publicLookup()
1.763 + .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
1.764 +assert( (boolean) equals.invokeExact("me", (Object)"me"));
1.765 +assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
1.766 +// spread both arguments from a 2-array:
1.767 +MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
1.768 +assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
1.769 +assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
1.770 +// try to spread from anything but a 2-array:
1.771 +for (int n = 0; n <= 10; n++) {
1.772 + Object[] badArityArgs = (n == 2 ? null : new Object[n]);
1.773 + try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
1.774 + catch (IllegalArgumentException ex) { } // OK
1.775 +}
1.776 +// spread both arguments from a String array:
1.777 +MethodHandle eq2s = equals.asSpreader(String[].class, 2);
1.778 +assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
1.779 +assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
1.780 +// spread second arguments from a 1-array:
1.781 +MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
1.782 +assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
1.783 +assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
1.784 +// spread no arguments from a 0-array or null:
1.785 +MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
1.786 +assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
1.787 +assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
1.788 +// asSpreader and asCollector are approximate inverses:
1.789 +for (int n = 0; n <= 2; n++) {
1.790 + for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
1.791 + MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
1.792 + assert( (boolean) equals2.invokeWithArguments("me", "me"));
1.793 + assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
1.794 + }
1.795 +}
1.796 +MethodHandle caToString = publicLookup()
1.797 + .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
1.798 +assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
1.799 +MethodHandle caString3 = caToString.asCollector(char[].class, 3);
1.800 +assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
1.801 +MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
1.802 +assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
1.803 + * }</pre></blockquote>
1.804 + * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
1.805 + * @param arrayLength the number of arguments to spread from an incoming array argument
1.806 + * @return a new method handle which spreads its final array argument,
1.807 + * before calling the original method handle
1.808 + * @throws NullPointerException if {@code arrayType} is a null reference
1.809 + * @throws IllegalArgumentException if {@code arrayType} is not an array type,
1.810 + * or if target does not have at least
1.811 + * {@code arrayLength} parameter types,
1.812 + * or if {@code arrayLength} is negative,
1.813 + * or if the resulting method handle's type would have
1.814 + * <a href="MethodHandle.html#maxarity">too many parameters</a>
1.815 + * @throws WrongMethodTypeException if the implied {@code asType} call fails
1.816 + * @see #asCollector
1.817 + */
1.818 + public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
1.819 + throw new IllegalStateException();
1.820 + }
1.821 +
1.822 + /**
1.823 + * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
1.824 + * positional arguments and collects them into an array argument.
1.825 + * The new method handle adapts, as its <i>target</i>,
1.826 + * the current method handle. The type of the adapter will be
1.827 + * the same as the type of the target, except that a single trailing
1.828 + * parameter (usually of type {@code arrayType}) is replaced by
1.829 + * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
1.830 + * <p>
1.831 + * If the array type differs from the final argument type on the original target,
1.832 + * the original target is adapted to take the array type directly,
1.833 + * as if by a call to {@link #asType asType}.
1.834 + * <p>
1.835 + * When called, the adapter replaces its trailing {@code arrayLength}
1.836 + * arguments by a single new array of type {@code arrayType}, whose elements
1.837 + * comprise (in order) the replaced arguments.
1.838 + * Finally the target is called.
1.839 + * What the target eventually returns is returned unchanged by the adapter.
1.840 + * <p>
1.841 + * (The array may also be a shared constant when {@code arrayLength} is zero.)
1.842 + * <p>
1.843 + * (<em>Note:</em> The {@code arrayType} is often identical to the last
1.844 + * parameter type of the original target.
1.845 + * It is an explicit argument for symmetry with {@code asSpreader}, and also
1.846 + * to allow the target to use a simple {@code Object} as its last parameter type.)
1.847 + * <p>
1.848 + * In order to create a collecting adapter which is not restricted to a particular
1.849 + * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
1.850 + * <p>
1.851 + * Here are some examples of array-collecting method handles:
1.852 + * <blockquote><pre>{@code
1.853 +MethodHandle deepToString = publicLookup()
1.854 + .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1.855 +assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"}));
1.856 +MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
1.857 +assertEquals(methodType(String.class, Object.class), ts1.type());
1.858 +//assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL
1.859 +assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
1.860 +// arrayType can be a subtype of Object[]
1.861 +MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
1.862 +assertEquals(methodType(String.class, String.class, String.class), ts2.type());
1.863 +assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
1.864 +MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
1.865 +assertEquals("[]", (String) ts0.invokeExact());
1.866 +// collectors can be nested, Lisp-style
1.867 +MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
1.868 +assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
1.869 +// arrayType can be any primitive array type
1.870 +MethodHandle bytesToString = publicLookup()
1.871 + .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
1.872 + .asCollector(byte[].class, 3);
1.873 +assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
1.874 +MethodHandle longsToString = publicLookup()
1.875 + .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
1.876 + .asCollector(long[].class, 1);
1.877 +assertEquals("[123]", (String) longsToString.invokeExact((long)123));
1.878 + * }</pre></blockquote>
1.879 + * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1.880 + * @param arrayLength the number of arguments to collect into a new array argument
1.881 + * @return a new method handle which collects some trailing argument
1.882 + * into an array, before calling the original method handle
1.883 + * @throws NullPointerException if {@code arrayType} is a null reference
1.884 + * @throws IllegalArgumentException if {@code arrayType} is not an array type
1.885 + * or {@code arrayType} is not assignable to this method handle's trailing parameter type,
1.886 + * or {@code arrayLength} is not a legal array size,
1.887 + * or the resulting method handle's type would have
1.888 + * <a href="MethodHandle.html#maxarity">too many parameters</a>
1.889 + * @throws WrongMethodTypeException if the implied {@code asType} call fails
1.890 + * @see #asSpreader
1.891 + * @see #asVarargsCollector
1.892 + */
1.893 + public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
1.894 + throw new IllegalStateException();
1.895 + }
1.896 +
1.897 + /**
1.898 + * Makes a <em>variable arity</em> adapter which is able to accept
1.899 + * any number of trailing positional arguments and collect them
1.900 + * into an array argument.
1.901 + * <p>
1.902 + * The type and behavior of the adapter will be the same as
1.903 + * the type and behavior of the target, except that certain
1.904 + * {@code invoke} and {@code asType} requests can lead to
1.905 + * trailing positional arguments being collected into target's
1.906 + * trailing parameter.
1.907 + * Also, the last parameter type of the adapter will be
1.908 + * {@code arrayType}, even if the target has a different
1.909 + * last parameter type.
1.910 + * <p>
1.911 + * This transformation may return {@code this} if the method handle is
1.912 + * already of variable arity and its trailing parameter type
1.913 + * is identical to {@code arrayType}.
1.914 + * <p>
1.915 + * When called with {@link #invokeExact invokeExact}, the adapter invokes
1.916 + * the target with no argument changes.
1.917 + * (<em>Note:</em> This behavior is different from a
1.918 + * {@linkplain #asCollector fixed arity collector},
1.919 + * since it accepts a whole array of indeterminate length,
1.920 + * rather than a fixed number of arguments.)
1.921 + * <p>
1.922 + * When called with plain, inexact {@link #invoke invoke}, if the caller
1.923 + * type is the same as the adapter, the adapter invokes the target as with
1.924 + * {@code invokeExact}.
1.925 + * (This is the normal behavior for {@code invoke} when types match.)
1.926 + * <p>
1.927 + * Otherwise, if the caller and adapter arity are the same, and the
1.928 + * trailing parameter type of the caller is a reference type identical to
1.929 + * or assignable to the trailing parameter type of the adapter,
1.930 + * the arguments and return values are converted pairwise,
1.931 + * as if by {@link #asType asType} on a fixed arity
1.932 + * method handle.
1.933 + * <p>
1.934 + * Otherwise, the arities differ, or the adapter's trailing parameter
1.935 + * type is not assignable from the corresponding caller type.
1.936 + * In this case, the adapter replaces all trailing arguments from
1.937 + * the original trailing argument position onward, by
1.938 + * a new array of type {@code arrayType}, whose elements
1.939 + * comprise (in order) the replaced arguments.
1.940 + * <p>
1.941 + * The caller type must provides as least enough arguments,
1.942 + * and of the correct type, to satisfy the target's requirement for
1.943 + * positional arguments before the trailing array argument.
1.944 + * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1.945 + * where {@code N} is the arity of the target.
1.946 + * Also, there must exist conversions from the incoming arguments
1.947 + * to the target's arguments.
1.948 + * As with other uses of plain {@code invoke}, if these basic
1.949 + * requirements are not fulfilled, a {@code WrongMethodTypeException}
1.950 + * may be thrown.
1.951 + * <p>
1.952 + * In all cases, what the target eventually returns is returned unchanged by the adapter.
1.953 + * <p>
1.954 + * In the final case, it is exactly as if the target method handle were
1.955 + * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1.956 + * to the arity required by the caller type.
1.957 + * (As with {@code asCollector}, if the array length is zero,
1.958 + * a shared constant may be used instead of a new array.
1.959 + * If the implied call to {@code asCollector} would throw
1.960 + * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1.961 + * the call to the variable arity adapter must throw
1.962 + * {@code WrongMethodTypeException}.)
1.963 + * <p>
1.964 + * The behavior of {@link #asType asType} is also specialized for
1.965 + * variable arity adapters, to maintain the invariant that
1.966 + * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1.967 + * call to adjust the target type, followed by {@code invokeExact}.
1.968 + * Therefore, a variable arity adapter responds
1.969 + * to an {@code asType} request by building a fixed arity collector,
1.970 + * if and only if the adapter and requested type differ either
1.971 + * in arity or trailing argument type.
1.972 + * The resulting fixed arity collector has its type further adjusted
1.973 + * (if necessary) to the requested type by pairwise conversion,
1.974 + * as if by another application of {@code asType}.
1.975 + * <p>
1.976 + * When a method handle is obtained by executing an {@code ldc} instruction
1.977 + * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1.978 + * as a variable arity method (with the modifier bit {@code 0x0080}),
1.979 + * the method handle will accept multiple arities, as if the method handle
1.980 + * constant were created by means of a call to {@code asVarargsCollector}.
1.981 + * <p>
1.982 + * In order to create a collecting adapter which collects a predetermined
1.983 + * number of arguments, and whose type reflects this predetermined number,
1.984 + * use {@link #asCollector asCollector} instead.
1.985 + * <p>
1.986 + * No method handle transformations produce new method handles with
1.987 + * variable arity, unless they are documented as doing so.
1.988 + * Therefore, besides {@code asVarargsCollector},
1.989 + * all methods in {@code MethodHandle} and {@code MethodHandles}
1.990 + * will return a method handle with fixed arity,
1.991 + * except in the cases where they are specified to return their original
1.992 + * operand (e.g., {@code asType} of the method handle's own type).
1.993 + * <p>
1.994 + * Calling {@code asVarargsCollector} on a method handle which is already
1.995 + * of variable arity will produce a method handle with the same type and behavior.
1.996 + * It may (or may not) return the original variable arity method handle.
1.997 + * <p>
1.998 + * Here is an example, of a list-making variable arity method handle:
1.999 + * <blockquote><pre>{@code
1.1000 +MethodHandle deepToString = publicLookup()
1.1001 + .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1.1002 +MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1.1003 +assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"}));
1.1004 +assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"}));
1.1005 +assertEquals("[won]", (String) ts1.invoke( "won" ));
1.1006 +assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1.1007 +// findStatic of Arrays.asList(...) produces a variable arity method handle:
1.1008 +MethodHandle asList = publicLookup()
1.1009 + .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1.1010 +assertEquals(methodType(List.class, Object[].class), asList.type());
1.1011 +assert(asList.isVarargsCollector());
1.1012 +assertEquals("[]", asList.invoke().toString());
1.1013 +assertEquals("[1]", asList.invoke(1).toString());
1.1014 +assertEquals("[two, too]", asList.invoke("two", "too").toString());
1.1015 +String[] argv = { "three", "thee", "tee" };
1.1016 +assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1.1017 +assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1.1018 +List ls = (List) asList.invoke((Object)argv);
1.1019 +assertEquals(1, ls.size());
1.1020 +assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1.1021 + * }</pre></blockquote>
1.1022 + * <p style="font-size:smaller;">
1.1023 + * <em>Discussion:</em>
1.1024 + * These rules are designed as a dynamically-typed variation
1.1025 + * of the Java rules for variable arity methods.
1.1026 + * In both cases, callers to a variable arity method or method handle
1.1027 + * can either pass zero or more positional arguments, or else pass
1.1028 + * pre-collected arrays of any length. Users should be aware of the
1.1029 + * special role of the final argument, and of the effect of a
1.1030 + * type match on that final argument, which determines whether
1.1031 + * or not a single trailing argument is interpreted as a whole
1.1032 + * array or a single element of an array to be collected.
1.1033 + * Note that the dynamic type of the trailing argument has no
1.1034 + * effect on this decision, only a comparison between the symbolic
1.1035 + * type descriptor of the call site and the type descriptor of the method handle.)
1.1036 + *
1.1037 + * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1.1038 + * @return a new method handle which can collect any number of trailing arguments
1.1039 + * into an array, before calling the original method handle
1.1040 + * @throws NullPointerException if {@code arrayType} is a null reference
1.1041 + * @throws IllegalArgumentException if {@code arrayType} is not an array type
1.1042 + * or {@code arrayType} is not assignable to this method handle's trailing parameter type
1.1043 + * @see #asCollector
1.1044 + * @see #isVarargsCollector
1.1045 + * @see #asFixedArity
1.1046 + */
1.1047 + public MethodHandle asVarargsCollector(Class<?> arrayType) {
1.1048 + throw new IllegalStateException();
1.1049 + }
1.1050 +
1.1051 + /**
1.1052 + * Determines if this method handle
1.1053 + * supports {@linkplain #asVarargsCollector variable arity} calls.
1.1054 + * Such method handles arise from the following sources:
1.1055 + * <ul>
1.1056 + * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1.1057 + * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1.1058 + * which resolves to a variable arity Java method or constructor
1.1059 + * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1.1060 + * which resolves to a variable arity Java method or constructor
1.1061 + * </ul>
1.1062 + * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1.1063 + * @see #asVarargsCollector
1.1064 + * @see #asFixedArity
1.1065 + */
1.1066 + public boolean isVarargsCollector() {
1.1067 + return false;
1.1068 + }
1.1069 +
1.1070 + /**
1.1071 + * Makes a <em>fixed arity</em> method handle which is otherwise
1.1072 + * equivalent to the current method handle.
1.1073 + * <p>
1.1074 + * If the current method handle is not of
1.1075 + * {@linkplain #asVarargsCollector variable arity},
1.1076 + * the current method handle is returned.
1.1077 + * This is true even if the current method handle
1.1078 + * could not be a valid input to {@code asVarargsCollector}.
1.1079 + * <p>
1.1080 + * Otherwise, the resulting fixed-arity method handle has the same
1.1081 + * type and behavior of the current method handle,
1.1082 + * except that {@link #isVarargsCollector isVarargsCollector}
1.1083 + * will be false.
1.1084 + * The fixed-arity method handle may (or may not) be the
1.1085 + * a previous argument to {@code asVarargsCollector}.
1.1086 + * <p>
1.1087 + * Here is an example, of a list-making variable arity method handle:
1.1088 + * <blockquote><pre>{@code
1.1089 +MethodHandle asListVar = publicLookup()
1.1090 + .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1.1091 + .asVarargsCollector(Object[].class);
1.1092 +MethodHandle asListFix = asListVar.asFixedArity();
1.1093 +assertEquals("[1]", asListVar.invoke(1).toString());
1.1094 +Exception caught = null;
1.1095 +try { asListFix.invoke((Object)1); }
1.1096 +catch (Exception ex) { caught = ex; }
1.1097 +assert(caught instanceof ClassCastException);
1.1098 +assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1.1099 +try { asListFix.invoke("two", "too"); }
1.1100 +catch (Exception ex) { caught = ex; }
1.1101 +assert(caught instanceof WrongMethodTypeException);
1.1102 +Object[] argv = { "three", "thee", "tee" };
1.1103 +assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1.1104 +assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1.1105 +assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1.1106 +assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1.1107 + * }</pre></blockquote>
1.1108 + *
1.1109 + * @return a new method handle which accepts only a fixed number of arguments
1.1110 + * @see #asVarargsCollector
1.1111 + * @see #isVarargsCollector
1.1112 + */
1.1113 + public MethodHandle asFixedArity() {
1.1114 + assert(!isVarargsCollector());
1.1115 + return this;
1.1116 + }
1.1117 +
1.1118 + /**
1.1119 + * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1.1120 + * The new method handle adapts, as its <i>target</i>,
1.1121 + * the current method handle by binding it to the given argument.
1.1122 + * The type of the bound handle will be
1.1123 + * the same as the type of the target, except that a single leading
1.1124 + * reference parameter will be omitted.
1.1125 + * <p>
1.1126 + * When called, the bound handle inserts the given value {@code x}
1.1127 + * as a new leading argument to the target. The other arguments are
1.1128 + * also passed unchanged.
1.1129 + * What the target eventually returns is returned unchanged by the bound handle.
1.1130 + * <p>
1.1131 + * The reference {@code x} must be convertible to the first parameter
1.1132 + * type of the target.
1.1133 + * <p>
1.1134 + * (<em>Note:</em> Because method handles are immutable, the target method handle
1.1135 + * retains its original type and behavior.)
1.1136 + * @param x the value to bind to the first argument of the target
1.1137 + * @return a new method handle which prepends the given value to the incoming
1.1138 + * argument list, before calling the original method handle
1.1139 + * @throws IllegalArgumentException if the target does not have a
1.1140 + * leading parameter type that is a reference type
1.1141 + * @throws ClassCastException if {@code x} cannot be converted
1.1142 + * to the leading parameter type of the target
1.1143 + * @see MethodHandles#insertArguments
1.1144 + */
1.1145 + public MethodHandle bindTo(Object x) {
1.1146 + throw new IllegalStateException();
1.1147 + }
1.1148 +
1.1149 + /**
1.1150 + * Returns a string representation of the method handle,
1.1151 + * starting with the string {@code "MethodHandle"} and
1.1152 + * ending with the string representation of the method handle's type.
1.1153 + * In other words, this method returns a string equal to the value of:
1.1154 + * <blockquote><pre>{@code
1.1155 + * "MethodHandle" + type().toString()
1.1156 + * }</pre></blockquote>
1.1157 + * <p>
1.1158 + * (<em>Note:</em> Future releases of this API may add further information
1.1159 + * to the string representation.
1.1160 + * Therefore, the present syntax should not be parsed by applications.)
1.1161 + *
1.1162 + * @return a string representation of the method handle
1.1163 + */
1.1164 + @Override
1.1165 + public String toString() {
1.1166 + return standardString();
1.1167 + }
1.1168 + String standardString() {
1.1169 + throw new IllegalStateException();
1.1170 + }
1.1171 +}