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/**

 * 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>

 *

 * <h1><a name="jvm_mods"></a>Summary of relevant Java Virtual Machine changes</h1>

 * 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>.

 * <h2><a name="indyinsn"></a>{@code invokedynamic} instructions</h2>

 * 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.

 *

 * <h2>timing of linkage</h2>

 * 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.

 *

 * <h2>types of bootstrap methods</h2>

 * 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

 */

package java.lang.invoke;