CallSite (Java Platform SE 8 ) (original) (raw)

A CallSite is a holder for a variable MethodHandle, which is called its target. An invokedynamic instruction linked to a CallSite delegates all calls to the site's current target. A CallSite may be associated with several invokedynamic instructions, or it may be "free floating", associated with none. In any case, it may be invoked through an associated method handle called its dynamic invoker.

CallSite is an abstract class which does not allow direct subclassing by users. It has three immediate, concrete subclasses that may be either instantiated or subclassed.

• If a mutable target is not required, an invokedynamic instruction may be permanently bound by means of a constant call site.
• If a mutable target is required which has volatile variable semantics, because updates to the target must be immediately and reliably witnessed by other threads, a volatile call site may be used.
• Otherwise, if a mutable target is required, a mutable call site may be used.

A non-constant call site may be relinked by changing its target. The new target must have the same type as the previous target. Thus, though a call site can be relinked to a series of successive targets, it cannot change its type.

Here is a sample use of call sites and bootstrap methods which links every dynamic call site to print its arguments:

static void test() throws Throwable {
   // THE FOLLOWING LINE IS PSEUDOCODE FOR A JVM INSTRUCTION
   InvokeDynamic[#bootstrapDynamic].baz("baz arg", 2, 3.14);
}
private static void printArgs(Object... args) {
 System.out.println(java.util.Arrays.deepToString(args));
}
private static final MethodHandle printArgs;
static {
 MethodHandles.Lookup lookup = MethodHandles.lookup();
 Class thisClass = lookup.lookupClass();  // (who am I?)
 printArgs = lookup.findStatic(thisClass,
     "printArgs", MethodType.methodType(void.class, Object[].class));
}
private static CallSite bootstrapDynamic(MethodHandles.Lookup caller, String name, MethodType type) {
 // ignore caller and name, but match the type:
 return new ConstantCallSite(printArgs.asType(type));
}