/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/**
* The {@code java.lang.invoke} package contains dynamic language support provided directly by
* the Java core class libraries and virtual machine.
*
* <p>
* As described in the Java Virtual Machine Specification,
* certain types in this package have special relations to dynamic
* language support in the virtual machine:
* <ul>
* <li>The class {@link java.lang.invoke.MethodHandle MethodHandle} contains
* <a href="MethodHandle.html#sigpoly">signature polymorphic methods</a>
* which can be linked regardless of their type descriptor.
* Normally, method linkage requires exact matching of type descriptors.
* </li>
*
* <li>The JVM bytecode format supports immediate constants of
* the classes {@link java.lang.invoke.MethodHandle MethodHandle} and {@link java.lang.invoke.MethodType MethodType}.
* </li>
* </ul>
*
* <h2><a name="jvm_mods"></a>Summary of relevant Java Virtual Machine changes</h2>
* The following low-level information summarizes relevant parts of the
* Java Virtual Machine specification. For full details, please see the
* current version of that specification.
*
* Each occurrence of an {@code invokedynamic} instruction is called a <em>dynamic call site</em>.
* <h3><a name="indyinsn"></a>{@code invokedynamic} instructions</h3>
* A dynamic call site is originally in an unlinked state. In this state, there is
* no target method for the call site to invoke.
* <p>
* Before the JVM can execute a dynamic call site (an {@code invokedynamic} instruction),
* the call site must first be <em>linked</em>.
* Linking is accomplished by calling a <em>bootstrap method</em>
* which is given the static information content of the call site,
* and which must produce a {@link java.lang.invoke.MethodHandle method handle}
* that gives the behavior of the call site.
* <p>
* Each {@code invokedynamic} instruction statically specifies its own
* bootstrap method as a constant pool reference.
* The constant pool reference also specifies the call site's name and type descriptor,
* just like {@code invokevirtual} and the other invoke instructions.
* <p>
* Linking starts with resolving the constant pool entry for the
* bootstrap method, and resolving a {@link java.lang.invoke.MethodType MethodType} object for
* the type descriptor of the dynamic call site.
* This resolution process may trigger class loading.
* It may therefore throw an error if a class fails to load.
* This error becomes the abnormal termination of the dynamic
* call site execution.
* Linkage does not trigger class initialization.
* <p>
* The bootstrap method is invoked on at least three values:
* <ul>
* <li>a {@code MethodHandles.Lookup}, a lookup object on the <em>caller class</em> in which dynamic call site occurs </li>
* <li>a {@code String}, the method name mentioned in the call site </li>
* <li>a {@code MethodType}, the resolved type descriptor of the call </li>
* <li>optionally, between 1 and 251 additional static arguments taken from the constant pool </li>
* </ul>
* Invocation is as if by
* {@link java.lang.invoke.MethodHandle#invoke MethodHandle.invoke}.
* The returned result must be a {@link java.lang.invoke.CallSite CallSite} (or a subclass).
* The type of the call site's target must be exactly equal to the type
* derived from the dynamic call site's type descriptor and passed to
* the bootstrap method.
* The call site then becomes permanently linked to the dynamic call site.
* <p>
* As documented in the JVM specification, all failures arising from
* the linkage of a dynamic call site are reported
* by a {@link java.lang.BootstrapMethodError BootstrapMethodError},
* which is thrown as the abnormal termination of the dynamic call
* site execution.
* If this happens, the same error will the thrown for all subsequent
* attempts to execute the dynamic call site.
*
* <h3>timing of linkage</h3>
* A dynamic call site is linked just before its first execution.
* The bootstrap method call implementing the linkage occurs within
* a thread that is attempting a first execution.
* <p>
* If there are several such threads, the bootstrap method may be
* invoked in several threads concurrently.
* Therefore, bootstrap methods which access global application
* data must take the usual precautions against race conditions.
* In any case, every {@code invokedynamic} instruction is either
* unlinked or linked to a unique {@code CallSite} object.
* <p>
* In an application which requires dynamic call sites with individually
* mutable behaviors, their bootstrap methods should produce distinct
* {@link java.lang.invoke.CallSite CallSite} objects, one for each linkage request.
* Alternatively, an application can link a single {@code CallSite} object
* to several {@code invokedynamic} instructions, in which case
* a change to the target method will become visible at each of
* the instructions.
* <p>
* If several threads simultaneously execute a bootstrap method for a single dynamic
* call site, the JVM must choose one {@code CallSite} object and install it visibly to
* all threads. Any other bootstrap method calls are allowed to complete, but their
* results are ignored, and their dynamic call site invocations proceed with the originally
* chosen target object.
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* These rules do not enable the JVM to duplicate dynamic call sites,
* or to issue “causeless” bootstrap method calls.
* Every dynamic call site transitions at most once from unlinked to linked,
* just before its first invocation.
* There is no way to undo the effect of a completed bootstrap method call.
*
* <h3>types of bootstrap methods</h3>
* As long as each bootstrap method can be correctly invoked
* by {@code MethodHandle.invoke}, its detailed type is arbitrary.
* For example, the first argument could be {@code Object}
* instead of {@code MethodHandles.Lookup}, and the return type
* could also be {@code Object} instead of {@code CallSite}.
* (Note that the types and number of the stacked arguments limit
* the legal kinds of bootstrap methods to appropriately typed
* static methods and constructors of {@code CallSite} subclasses.)
* <p>
* If a given {@code invokedynamic} instruction specifies no static arguments,
* the instruction's bootstrap method will be invoked on three arguments,
* conveying the instruction's caller class, name, and method type.
* If the {@code invokedynamic} instruction specifies one or more static arguments,
* those values will be passed as additional arguments to the method handle.
* (Note that because there is a limit of 255 arguments to any method,
* at most 251 extra arguments can be supplied, since the bootstrap method
* handle itself and its first three arguments must also be stacked.)
* The bootstrap method will be invoked as if by either {@code MethodHandle.invoke}
* or {@code invokeWithArguments}. (There is no way to tell the difference.)
* <p>
* The normal argument conversion rules for {@code MethodHandle.invoke} apply to all stacked arguments.
* For example, if a pushed value is a primitive type, it may be converted to a reference by boxing conversion.
* If the bootstrap method is a variable arity method (its modifier bit {@code 0x0080} is set),
* then some or all of the arguments specified here may be collected into a trailing array parameter.
* (This is not a special rule, but rather a useful consequence of the interaction
* between {@code CONSTANT_MethodHandle} constants, the modifier bit for variable arity methods,
* and the {@link java.lang.invoke.MethodHandle#asVarargsCollector asVarargsCollector} transformation.)
* <p>
* Given these rules, here are examples of legal bootstrap method declarations,
* given various numbers {@code N} of extra arguments.
* The first rows (marked {@code *}) will work for any number of extra arguments.
* <table border=1 cellpadding=5 summary="Static argument types">
* <tr><th>N</th><th>sample bootstrap method</th></tr>
* <tr><td>*</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
* <tr><td>*</td><td><code>CallSite bootstrap(Object... args)</code></td></tr>
* <tr><td>*</td><td><code>CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)</code></td></tr>
* <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type)</code></td></tr>
* <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, Object... nameAndType)</code></td></tr>
* <tr><td>1</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)</code></td></tr>
* <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
* <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)</code></td></tr>
* <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)</code></td></tr>
* </table>
* The last example assumes that the extra arguments are of type
* {@code CONSTANT_String} and {@code CONSTANT_Integer}, respectively.
* The second-to-last example assumes that all extra arguments are of type
* {@code CONSTANT_String}.
* The other examples work with all types of extra arguments.
* <p>
* As noted above, the actual method type of the bootstrap method can vary.
* For example, the fourth argument could be {@code MethodHandle},
* if that is the type of the corresponding constant in
* the {@code CONSTANT_InvokeDynamic} entry.
* In that case, the {@code MethodHandle.invoke} call will pass the extra method handle
* constant as an {@code Object}, but the type matching machinery of {@code MethodHandle.invoke}
* will cast the reference back to {@code MethodHandle} before invoking the bootstrap method.
* (If a string constant were passed instead, by badly generated code, that cast would then fail,
* resulting in a {@code BootstrapMethodError}.)
* <p>
* Note that, as a consequence of the above rules, the bootstrap method may accept a primitive
* argument, if it can be represented by a constant pool entry.
* However, arguments of type {@code boolean}, {@code byte}, {@code short}, or {@code char}
* cannot be created for bootstrap methods, since such constants cannot be directly
* represented in the constant pool, and the invocation of the bootstrap method will
* not perform the necessary narrowing primitive conversions.
* <p>
* Extra bootstrap method arguments are intended to allow language implementors
* to safely and compactly encode metadata.
* In principle, the name and extra arguments are redundant,
* since each call site could be given its own unique bootstrap method.
* Such a practice is likely to produce large class files and constant pools.
*
* @author John Rose, JSR 292 EG
* @since 1.7
*/