1.1 --- a/rt/emul/compact/src/main/java/java/lang/invoke/package-info.java Sun Aug 17 20:09:05 2014 +0200
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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 -/**
1.30 - * The {@code java.lang.invoke} package contains dynamic language support provided directly by
1.31 - * the Java core class libraries and virtual machine.
1.32 - *
1.33 - * <p>
1.34 - * As described in the Java Virtual Machine Specification,
1.35 - * certain types in this package have special relations to dynamic
1.36 - * language support in the virtual machine:
1.37 - * <ul>
1.38 - * <li>The class {@link java.lang.invoke.MethodHandle MethodHandle} contains
1.39 - * <a href="MethodHandle.html#sigpoly">signature polymorphic methods</a>
1.40 - * which can be linked regardless of their type descriptor.
1.41 - * Normally, method linkage requires exact matching of type descriptors.
1.42 - * </li>
1.43 - *
1.44 - * <li>The JVM bytecode format supports immediate constants of
1.45 - * the classes {@link java.lang.invoke.MethodHandle MethodHandle} and {@link java.lang.invoke.MethodType MethodType}.
1.46 - * </li>
1.47 - * </ul>
1.48 - *
1.49 - * <h1><a name="jvm_mods"></a>Summary of relevant Java Virtual Machine changes</h1>
1.50 - * The following low-level information summarizes relevant parts of the
1.51 - * Java Virtual Machine specification. For full details, please see the
1.52 - * current version of that specification.
1.53 - *
1.54 - * Each occurrence of an {@code invokedynamic} instruction is called a <em>dynamic call site</em>.
1.55 - * <h2><a name="indyinsn"></a>{@code invokedynamic} instructions</h2>
1.56 - * A dynamic call site is originally in an unlinked state. In this state, there is
1.57 - * no target method for the call site to invoke.
1.58 - * <p>
1.59 - * Before the JVM can execute a dynamic call site (an {@code invokedynamic} instruction),
1.60 - * the call site must first be <em>linked</em>.
1.61 - * Linking is accomplished by calling a <em>bootstrap method</em>
1.62 - * which is given the static information content of the call site,
1.63 - * and which must produce a {@link java.lang.invoke.MethodHandle method handle}
1.64 - * that gives the behavior of the call site.
1.65 - * <p>
1.66 - * Each {@code invokedynamic} instruction statically specifies its own
1.67 - * bootstrap method as a constant pool reference.
1.68 - * The constant pool reference also specifies the call site's name and type descriptor,
1.69 - * just like {@code invokevirtual} and the other invoke instructions.
1.70 - * <p>
1.71 - * Linking starts with resolving the constant pool entry for the
1.72 - * bootstrap method, and resolving a {@link java.lang.invoke.MethodType MethodType} object for
1.73 - * the type descriptor of the dynamic call site.
1.74 - * This resolution process may trigger class loading.
1.75 - * It may therefore throw an error if a class fails to load.
1.76 - * This error becomes the abnormal termination of the dynamic
1.77 - * call site execution.
1.78 - * Linkage does not trigger class initialization.
1.79 - * <p>
1.80 - * The bootstrap method is invoked on at least three values:
1.81 - * <ul>
1.82 - * <li>a {@code MethodHandles.Lookup}, a lookup object on the <em>caller class</em> in which dynamic call site occurs </li>
1.83 - * <li>a {@code String}, the method name mentioned in the call site </li>
1.84 - * <li>a {@code MethodType}, the resolved type descriptor of the call </li>
1.85 - * <li>optionally, between 1 and 251 additional static arguments taken from the constant pool </li>
1.86 - * </ul>
1.87 - * Invocation is as if by
1.88 - * {@link java.lang.invoke.MethodHandle#invoke MethodHandle.invoke}.
1.89 - * The returned result must be a {@link java.lang.invoke.CallSite CallSite} (or a subclass).
1.90 - * The type of the call site's target must be exactly equal to the type
1.91 - * derived from the dynamic call site's type descriptor and passed to
1.92 - * the bootstrap method.
1.93 - * The call site then becomes permanently linked to the dynamic call site.
1.94 - * <p>
1.95 - * As documented in the JVM specification, all failures arising from
1.96 - * the linkage of a dynamic call site are reported
1.97 - * by a {@link java.lang.BootstrapMethodError BootstrapMethodError},
1.98 - * which is thrown as the abnormal termination of the dynamic call
1.99 - * site execution.
1.100 - * If this happens, the same error will the thrown for all subsequent
1.101 - * attempts to execute the dynamic call site.
1.102 - *
1.103 - * <h2>timing of linkage</h2>
1.104 - * A dynamic call site is linked just before its first execution.
1.105 - * The bootstrap method call implementing the linkage occurs within
1.106 - * a thread that is attempting a first execution.
1.107 - * <p>
1.108 - * If there are several such threads, the bootstrap method may be
1.109 - * invoked in several threads concurrently.
1.110 - * Therefore, bootstrap methods which access global application
1.111 - * data must take the usual precautions against race conditions.
1.112 - * In any case, every {@code invokedynamic} instruction is either
1.113 - * unlinked or linked to a unique {@code CallSite} object.
1.114 - * <p>
1.115 - * In an application which requires dynamic call sites with individually
1.116 - * mutable behaviors, their bootstrap methods should produce distinct
1.117 - * {@link java.lang.invoke.CallSite CallSite} objects, one for each linkage request.
1.118 - * Alternatively, an application can link a single {@code CallSite} object
1.119 - * to several {@code invokedynamic} instructions, in which case
1.120 - * a change to the target method will become visible at each of
1.121 - * the instructions.
1.122 - * <p>
1.123 - * If several threads simultaneously execute a bootstrap method for a single dynamic
1.124 - * call site, the JVM must choose one {@code CallSite} object and install it visibly to
1.125 - * all threads. Any other bootstrap method calls are allowed to complete, but their
1.126 - * results are ignored, and their dynamic call site invocations proceed with the originally
1.127 - * chosen target object.
1.128 -
1.129 - * <p style="font-size:smaller;">
1.130 - * <em>Discussion:</em>
1.131 - * These rules do not enable the JVM to duplicate dynamic call sites,
1.132 - * or to issue “causeless” bootstrap method calls.
1.133 - * Every dynamic call site transitions at most once from unlinked to linked,
1.134 - * just before its first invocation.
1.135 - * There is no way to undo the effect of a completed bootstrap method call.
1.136 - *
1.137 - * <h2>types of bootstrap methods</h2>
1.138 - * As long as each bootstrap method can be correctly invoked
1.139 - * by {@code MethodHandle.invoke}, its detailed type is arbitrary.
1.140 - * For example, the first argument could be {@code Object}
1.141 - * instead of {@code MethodHandles.Lookup}, and the return type
1.142 - * could also be {@code Object} instead of {@code CallSite}.
1.143 - * (Note that the types and number of the stacked arguments limit
1.144 - * the legal kinds of bootstrap methods to appropriately typed
1.145 - * static methods and constructors of {@code CallSite} subclasses.)
1.146 - * <p>
1.147 - * If a given {@code invokedynamic} instruction specifies no static arguments,
1.148 - * the instruction's bootstrap method will be invoked on three arguments,
1.149 - * conveying the instruction's caller class, name, and method type.
1.150 - * If the {@code invokedynamic} instruction specifies one or more static arguments,
1.151 - * those values will be passed as additional arguments to the method handle.
1.152 - * (Note that because there is a limit of 255 arguments to any method,
1.153 - * at most 251 extra arguments can be supplied, since the bootstrap method
1.154 - * handle itself and its first three arguments must also be stacked.)
1.155 - * The bootstrap method will be invoked as if by either {@code MethodHandle.invoke}
1.156 - * or {@code invokeWithArguments}. (There is no way to tell the difference.)
1.157 - * <p>
1.158 - * The normal argument conversion rules for {@code MethodHandle.invoke} apply to all stacked arguments.
1.159 - * For example, if a pushed value is a primitive type, it may be converted to a reference by boxing conversion.
1.160 - * If the bootstrap method is a variable arity method (its modifier bit {@code 0x0080} is set),
1.161 - * then some or all of the arguments specified here may be collected into a trailing array parameter.
1.162 - * (This is not a special rule, but rather a useful consequence of the interaction
1.163 - * between {@code CONSTANT_MethodHandle} constants, the modifier bit for variable arity methods,
1.164 - * and the {@link java.lang.invoke.MethodHandle#asVarargsCollector asVarargsCollector} transformation.)
1.165 - * <p>
1.166 - * Given these rules, here are examples of legal bootstrap method declarations,
1.167 - * given various numbers {@code N} of extra arguments.
1.168 - * The first rows (marked {@code *}) will work for any number of extra arguments.
1.169 - * <table border=1 cellpadding=5 summary="Static argument types">
1.170 - * <tr><th>N</th><th>sample bootstrap method</th></tr>
1.171 - * <tr><td>*</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
1.172 - * <tr><td>*</td><td><code>CallSite bootstrap(Object... args)</code></td></tr>
1.173 - * <tr><td>*</td><td><code>CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)</code></td></tr>
1.174 - * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type)</code></td></tr>
1.175 - * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, Object... nameAndType)</code></td></tr>
1.176 - * <tr><td>1</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)</code></td></tr>
1.177 - * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
1.178 - * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)</code></td></tr>
1.179 - * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)</code></td></tr>
1.180 - * </table>
1.181 - * The last example assumes that the extra arguments are of type
1.182 - * {@code CONSTANT_String} and {@code CONSTANT_Integer}, respectively.
1.183 - * The second-to-last example assumes that all extra arguments are of type
1.184 - * {@code CONSTANT_String}.
1.185 - * The other examples work with all types of extra arguments.
1.186 - * <p>
1.187 - * As noted above, the actual method type of the bootstrap method can vary.
1.188 - * For example, the fourth argument could be {@code MethodHandle},
1.189 - * if that is the type of the corresponding constant in
1.190 - * the {@code CONSTANT_InvokeDynamic} entry.
1.191 - * In that case, the {@code MethodHandle.invoke} call will pass the extra method handle
1.192 - * constant as an {@code Object}, but the type matching machinery of {@code MethodHandle.invoke}
1.193 - * will cast the reference back to {@code MethodHandle} before invoking the bootstrap method.
1.194 - * (If a string constant were passed instead, by badly generated code, that cast would then fail,
1.195 - * resulting in a {@code BootstrapMethodError}.)
1.196 - * <p>
1.197 - * Note that, as a consequence of the above rules, the bootstrap method may accept a primitive
1.198 - * argument, if it can be represented by a constant pool entry.
1.199 - * However, arguments of type {@code boolean}, {@code byte}, {@code short}, or {@code char}
1.200 - * cannot be created for bootstrap methods, since such constants cannot be directly
1.201 - * represented in the constant pool, and the invocation of the bootstrap method will
1.202 - * not perform the necessary narrowing primitive conversions.
1.203 - * <p>
1.204 - * Extra bootstrap method arguments are intended to allow language implementors
1.205 - * to safely and compactly encode metadata.
1.206 - * In principle, the name and extra arguments are redundant,
1.207 - * since each call site could be given its own unique bootstrap method.
1.208 - * Such a practice is likely to produce large class files and constant pools.
1.209 - *
1.210 - * @author John Rose, JSR 292 EG
1.211 - * @since 1.7
1.212 - */
1.213 -
1.214 -package java.lang.invoke;