* As described in the Java Virtual Machine Specification, * certain types in this package have special relations to dynamic * language support in the virtual machine: *
* Before the JVM can execute a dynamic call site (an {@code invokedynamic} instruction), * the call site must first be linked. * Linking is accomplished by calling a bootstrap method * 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. *
* 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. *
* 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. *
* The bootstrap method is invoked on at least three values: *
* If an exception, {@code E} say, occurs when linking the call site then the * linkage fails and terminates abnormally. {@code E} is rethrown if the type of * {@code E} is {@code Error} or a subclass, otherwise a * {@code BootstrapMethodError} that wraps {@code E} is thrown. * If this happens, the same {@code Error} or subclass will the thrown for all * subsequent attempts to execute the dynamic call site. *
* 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. *
* 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. *
* 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. *
* Discussion: * 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. * *
* 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.) *
* 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.) *
* Given these rules, here are examples of legal bootstrap method declarations, * given various numbers {@code N} of extra arguments. * The first row (marked {@code *}) will work for any number of extra arguments. *
| N | Sample bootstrap method |
|---|---|
| * |
*
|
| 0 |
*
|
| 1 |
* CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg) |
| 2 |
*
|
* 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}.) *
* 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. *
* 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;