273 lines
15 KiB
Java
273 lines
15 KiB
Java
/*
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* Copyright (c) 2008, 2020, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/**
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* The {@code java.lang.invoke} package provides low-level primitives for interacting
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* with the Java Virtual Machine.
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*
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* <p>
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* As described in the Java Virtual Machine Specification, certain types in this package
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* are given special treatment by the virtual machine:
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* <ul>
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* <li>The classes {@link java.lang.invoke.MethodHandle MethodHandle} and
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* {@link java.lang.invoke.VarHandle VarHandle} contain
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* <a href="MethodHandle.html#sigpoly">signature polymorphic methods</a>
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* which can be linked regardless of their type descriptor.
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* Normally, method linkage requires exact matching of type descriptors.
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* </li>
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*
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* <li>The JVM bytecode format supports immediate constants of
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* the classes {@link java.lang.invoke.MethodHandle MethodHandle} and
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* {@link java.lang.invoke.MethodType MethodType}.
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* </li>
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*
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* <li>The {@code invokedynamic} instruction makes use of bootstrap {@code MethodHandle}
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* constants to dynamically resolve {@code CallSite} objects for custom method invocation
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* behavior.
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* </li>
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*
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* <li>The {@code ldc} instruction makes use of bootstrap {@code MethodHandle} constants
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* to dynamically resolve custom constant values.
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* </li>
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* </ul>
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*
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* <h2><a id="jvm_mods"></a>Dynamic resolution of call sites and constants</h2>
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* The following low-level information summarizes relevant parts of the
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* Java Virtual Machine specification. For full details, please see the
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* current version of that specification.
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*
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* <h3><a id="indyinsn"></a>Dynamically-computed call sites</h3>
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* An {@code invokedynamic} instruction is originally in an unlinked state.
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* In this state, there is no target method for the instruction to invoke.
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* <p>
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* Before the JVM can execute an {@code invokedynamic} instruction,
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* the instruction must first be <em>linked</em>.
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* Linking is accomplished by calling a <em>bootstrap method</em>
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* which is given the static information content of the call,
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* and which must produce a {@link java.lang.invoke.CallSite}
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* that gives the behavior of the invocation.
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* <p>
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* Each {@code invokedynamic} instruction statically specifies its own
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* bootstrap method as a constant pool reference.
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* The constant pool reference also specifies the invocation's name and method type descriptor,
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* just like {@code invokestatic} and the other invoke instructions.
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*
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* <h3><a id="condycon"></a>Dynamically-computed constants</h3>
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* The constant pool may contain constants tagged {@code CONSTANT_Dynamic},
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* equipped with bootstrap methods which perform their resolution.
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* Such a <em>dynamic constant</em> is originally in an unresolved state.
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* Before the JVM can use a dynamically-computed constant, it must first be <em>resolved</em>.
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* Dynamically-computed constant resolution is accomplished by calling a <em>bootstrap method</em>
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* which is given the static information content of the constant,
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* and which must produce a value of the constant's statically declared type.
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* <p>
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* Each dynamically-computed constant statically specifies its own
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* bootstrap method as a constant pool reference.
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* The constant pool reference also specifies the constant's name and field type descriptor,
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* just like {@code getstatic} and the other field reference instructions.
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* (Roughly speaking, a dynamically-computed constant is to a dynamically-computed call site
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* as a {@code CONSTANT_Fieldref} is to a {@code CONSTANT_Methodref}.)
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*
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* <h3><a id="bsm"></a>Execution of bootstrap methods</h3>
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* Resolving a dynamically-computed call site or constant
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* starts with resolving constants from the constant pool for the
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* following items:
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* <ul>
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* <li>the bootstrap method, a {@code CONSTANT_MethodHandle}</li>
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* <li>the {@code Class} or {@code MethodType} derived from
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* type component of the {@code CONSTANT_NameAndType} descriptor</li>
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* <li>static arguments, if any (note that static arguments can themselves be
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* dynamically-computed constants)</li>
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* </ul>
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* <p>
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* The bootstrap method is then invoked, as if by
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* {@link java.lang.invoke.MethodHandle#invoke MethodHandle.invoke},
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* with the following arguments:
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* <ul>
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* <li>a {@code MethodHandles.Lookup}, which is a lookup object on the <em>caller class</em>
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* in which dynamically-computed constant or call site occurs</li>
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* <li>a {@code String}, the name mentioned in the {@code CONSTANT_NameAndType}</li>
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* <li>a {@code MethodType} or {@code Class}, the resolved type descriptor of the {@code CONSTANT_NameAndType}</li>
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* <li>a {@code Class}, the resolved type descriptor of the constant, if it is a dynamic constant </li>
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* <li>the additional resolved static arguments, if any</li>
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* </ul>
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* <p>
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* For a dynamically-computed call site, the returned result must be a non-null reference to a
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* {@link java.lang.invoke.CallSite CallSite}.
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* The type of the call site's target must be exactly equal to the type
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* derived from the invocation's type descriptor and passed to
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* the bootstrap method. If these conditions are not met, a {@code BootstrapMethodError} is thrown.
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* On success the call site then becomes permanently linked to the {@code invokedynamic}
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* instruction.
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* <p>
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* For a dynamically-computed constant, the first parameter of the bootstrap
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* method must be assignable to {@code MethodHandles.Lookup}. If this condition
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* is not met, a {@code BootstrapMethodError} is thrown.
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* On success the result of the bootstrap method is cached as the resolved
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* constant value.
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* <p>
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* If an exception, {@code E} say, occurs during execution of the bootstrap method, then
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* resolution fails and terminates abnormally. {@code E} is rethrown if the type of
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* {@code E} is {@code Error} or a subclass, otherwise a
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* {@code BootstrapMethodError} that wraps {@code E} is thrown.
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* If this happens, the same error will be thrown for all
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* subsequent attempts to execute the {@code invokedynamic} instruction or load the
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* dynamically-computed constant.
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*
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* <h3>Timing of resolution</h3>
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* An {@code invokedynamic} instruction is linked just before its first execution.
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* A dynamically-computed constant is resolved just before the first time it is used
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* (by pushing it on the stack or linking it as a bootstrap method parameter).
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* The bootstrap method call implementing the linkage occurs within
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* a thread that is attempting a first execution or first use.
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* <p>
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* If there are several such threads, the bootstrap method may be
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* invoked in several threads concurrently.
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* Therefore, bootstrap methods which access global application
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* data must take the usual precautions against race conditions.
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* In any case, every {@code invokedynamic} instruction is either
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* unlinked or linked to a unique {@code CallSite} object.
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* <p>
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* In an application which requires {@code invokedynamic} instructions with individually
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* mutable behaviors, their bootstrap methods should produce distinct
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* {@link java.lang.invoke.CallSite CallSite} objects, one for each linkage request.
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* Alternatively, an application can link a single {@code CallSite} object
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* to several {@code invokedynamic} instructions, in which case
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* a change to the target method will become visible at each of
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* the instructions.
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* <p>
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* If several threads simultaneously execute a bootstrap method for a single dynamically-computed
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* call site or constant, the JVM must choose one bootstrap method result and install it visibly to
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* all threads. Any other bootstrap method calls are allowed to complete, but their
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* results are ignored.
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* <p style="font-size:smaller;">
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* <em>Discussion:</em>
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* These rules do not enable the JVM to share call sites,
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* or to issue “causeless” bootstrap method calls.
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* Every {@code invokedynamic} instruction transitions at most once from unlinked to linked,
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* just before its first invocation.
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* There is no way to undo the effect of a completed bootstrap method call.
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*
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* <h3>Types of bootstrap methods</h3>
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* For a dynamically-computed call site, the bootstrap method is invoked with parameter
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* types {@code MethodHandles.Lookup}, {@code String}, {@code MethodType}, and the types
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* of any static arguments; the return type is {@code CallSite}.
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* <p>
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* For a dynamically-computed constant, the bootstrap method is invoked with parameter types
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* {@code MethodHandles.Lookup}, {@code String}, {@code Class}, and the types of any
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* static arguments; the return type is the type represented by the {@code Class}.
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* <p>
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* Because {@link java.lang.invoke.MethodHandle#invoke MethodHandle.invoke} allows for
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* adaptations between the invoked method type and the bootstrap method handle's method type,
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* there is flexibility in the declaration of the bootstrap method.
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* For a dynamically-computed constant the first parameter type of the bootstrap method handle
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* must be assignable to {@code MethodHandles.Lookup}, other than that constraint the same degree
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* of flexibility applies to bootstrap methods of dynamically-computed call sites and
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* dynamically-computed constants.
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* Note: this constraint allows for the future possibility where the bootstrap method is
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* invoked with just the parameter types of static arguments, thereby supporting a wider
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* range of methods compatible with the static arguments (such as methods that don't declare
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* or require the lookup, name, and type meta-data parameters).
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* <p> For example, for dynamically-computed call site, the first argument
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* could be {@code Object} instead of {@code MethodHandles.Lookup}, and the return type
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* could also be {@code Object} instead of {@code CallSite}.
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* (Note that the types and number of the stacked arguments limit
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* the legal kinds of bootstrap methods to appropriately typed
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* static methods and constructors.)
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* <p>
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* If a pushed value is a primitive type, it may be converted to a reference by boxing conversion.
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* If the bootstrap method is a variable arity method (its modifier bit {@code 0x0080} is set),
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* then some or all of the arguments specified here may be collected into a trailing array parameter.
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* (This is not a special rule, but rather a useful consequence of the interaction
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* between {@code CONSTANT_MethodHandle} constants, the modifier bit for variable arity methods,
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* and the {@link java.lang.invoke.MethodHandle#asVarargsCollector asVarargsCollector} transformation.)
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* <p>
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* Given these rules, here are examples of legal bootstrap method declarations for
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* dynamically-computed call sites, given various numbers {@code N} of extra arguments.
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* The first row (marked {@code *}) will work for any number of extra arguments.
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* <table class="plain" style="vertical-align:top">
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* <caption style="display:none">Static argument types</caption>
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* <thead>
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* <tr><th scope="col">N</th><th scope="col">Sample bootstrap method</th></tr>
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* </thead>
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* <tbody>
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* <tr><th scope="row" style="font-weight:normal; vertical-align:top">*</th><td>
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* <ul style="list-style:none; padding-left: 0; margin:0">
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* <li>{@code CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)}
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* <li>{@code CallSite bootstrap(Object... args)}
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* <li>{@code CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)}
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* </ul></td></tr>
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* <tr><th scope="row" style="font-weight:normal; vertical-align:top">0</th><td>
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* <ul style="list-style:none; padding-left: 0; margin:0">
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* <li>{@code CallSite bootstrap(Lookup caller, String name, MethodType type)}
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* <li>{@code CallSite bootstrap(Lookup caller, Object... nameAndType)}
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* </ul></td></tr>
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* <tr><th scope="row" style="font-weight:normal; vertical-align:top">1</th><td>
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* {@code CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)}</td></tr>
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* <tr><th scope="row" style="font-weight:normal; vertical-align:top">2</th><td>
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* <ul style="list-style:none; padding-left: 0; margin:0">
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* <li>{@code CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)}
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* <li>{@code CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)}
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* <li>{@code CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)}
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* </ul></td></tr>
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* </tbody>
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* </table>
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* The last example assumes that the extra arguments are of type
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* {@code String} and {@code Integer} (or {@code int}), respectively.
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* The second-to-last example assumes that all extra arguments are of type
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* {@code String}.
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* The other examples work with all types of extra arguments. Note that all
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* the examples except the second and third also work with dynamically-computed
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* constants if the return type is changed to be compatible with the
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* constant's declared type (such as {@code Object}, which is always compatible).
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* <p>
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* Since dynamically-computed constants can be provided as static arguments to bootstrap
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* methods, there are no limitations on the types of bootstrap arguments.
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* However, arguments of type {@code boolean}, {@code byte}, {@code short}, or {@code char}
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* cannot be <em>directly</em> supplied by {@code CONSTANT_Integer}
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* constant pool entries, since the {@code asType} conversions do
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* not perform the necessary narrowing primitive conversions.
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* <p>
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* In the above examples, the return type is always {@code CallSite},
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* but that is not a necessary feature of bootstrap methods.
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* In the case of a dynamically-computed call site, the only requirement is that
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* the return type of the bootstrap method must be convertible
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* (using the {@code asType} conversions) to {@code CallSite}, which
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* means the bootstrap method return type might be {@code Object} or
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* {@code ConstantCallSite}.
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* In the case of a dynamically-resolved constant, the return type of the bootstrap
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* method must be convertible to the type of the constant, as
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* represented by its field type descriptor. For example, if the
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* dynamic constant has a field type descriptor of {@code "C"}
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* ({@code char}) then the bootstrap method return type could be
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* {@code Object}, {@code Character}, or {@code char}, but not
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* {@code int} or {@code Integer}.
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*
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* @author John Rose, JSR 292 EG
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* @since 1.7
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*/
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package java.lang.invoke;
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