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Showing content from https://docs.oracle.com/javase/8/docs/api/java/lang/invoke/CallSite.html below:

CallSite (Java Platform SE 8 )

A

is a holder for a variable

, which is called its

. An

instruction linked to a

delegates all calls to the site's current target. A

may be associated with several

instructions, or it may be "free floating", associated with none. In any case, it may be invoked through an associated method handle called its

.

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));
}

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