java.lang.invokepackage contains dynamic language support provided directly by the Java core class libraries and virtual machine.
As described in the Java Virtual Machine Specification, certain types in this package have special relations to dynamic language support in the virtual machine:
containsVarHandlecontain signature polymorphic methods which can be linked regardless of their type descriptor. Normally, method linkage requires exact matching of type descriptors.
invokedynamicinstruction is called a dynamic call site .
Before the JVM can execute a dynamic call site (an
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
method handle that gives the behavior of the call site.
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
invokevirtual and the other invoke instructions.
Linking starts with resolving the constant pool entry for the bootstrap method, and resolving a
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:
MethodHandles.Lookup, a lookup object on the caller class in which dynamic call site occurs
String, the method name mentioned in the call site
MethodType, the resolved type descriptor of the call
MethodHandle.invoke. The returned result must be a
CallSite(or a subclass), otherwise a
BootstrapMethodErroris thrown. 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, otherwise a
BootstrapMethodErroris thrown. On success the call site then becomes permanently linked to the dynamic call site.
As documented inIf an exception,
E say, occurs when linking the JVM specification, all failures arising fromcall site then the linkage fails and terminates abnormally.
E is rethrown if the type of
Error or a dynamic call site are reported bysubclass, otherwise a
BootstrapMethodError , whichthat wraps
E is thrown as the abnormal termination of the dynamic call site execution. If this happens, the same
errorError 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
invokedynamic instruction is either unlinked or linked to a unique
In an application which requires dynamic call sites with individually mutable behaviors, their bootstrap methods should produce distinct
CallSite objects, one for each linkage request. Alternatively, an application can link a single
CallSite object to several
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
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.
MethodHandle.invoke, its detailed type is arbitrary. For example, the first argument could be
MethodHandles.Lookup, and the return type could also be
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
If a given
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
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
invokeWithArguments. (There is no way to tell the difference.)
The normal argument conversion rules for
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
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
CONSTANT_MethodHandle constants, the modifier bit for variable arity methods, and the
Given these rules, here are examples of legal bootstrap method declarations, given various numbers
N of extra arguments. The first rows (marked
*) will work for any number of extra arguments.
|N||sampleSample bootstrap method|
CONSTANT_Integer, respectively. The second-to-last example assumes that all extra arguments are of type
CONSTANT_String. The other examples work with all types of extra arguments.
As noted above, the actual method type of the bootstrap method can vary. For example, the fourth argument could be
MethodHandle, if that is the type of the corresponding constant in the
CONSTANT_InvokeDynamic entry. In that case, the
MethodHandle.invoke call will pass the extra method handle constant as an
Object, but the type matching machinery of
MethodHandle.invoke will cast the reference back to
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
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
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.
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