Module java.base
Package java.lang.invoke

Class CallSite

  • Direct Known Subclasses:
    ConstantCallSite,MutableCallSite,VolatileCallSite
    public abstract classCallSiteextendsObject
    ACallSite is a holder for a variableMethodHandle, which is called itstarget. Aninvokedynamic instruction linked to aCallSite delegates all calls to the site's current target. ACallSite may be associated with severalinvokedynamic instructions, or it may be "free floating", associated with none. In any case, it may be invoked through an associated method handle called itsdynamic 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, aninvokedynamic instruction may be permanently bound by means of aconstant 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, avolatile call site may be used.
    • Otherwise, if a mutable target is required, amutable call site may be used.

    A non-constant call site may berelinked by changing its target. The new target must have the sametype 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));}

    Since:
    1.7

    • Method Detail

      • type

        public MethodType type()
        Returns the type of this call site's target. Although targets may change, any call site's type is permanent, and can never change to an unequal type. ThesetTarget method enforces this invariant by refusing any new target that does not have the previous target's type.
        Returns:
        the type of the current target, which is also the type of any future target

      • dynamicInvoker

        public abstract MethodHandle dynamicInvoker()
        Produces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.

        This method is equivalent to the following code:

         MethodHandle getTarget, invoker, result; getTarget = MethodHandles.publicLookup().bind(this, "getTarget", MethodType.methodType(MethodHandle.class)); invoker = MethodHandles.exactInvoker(this.type()); result = MethodHandles.foldArguments(invoker, getTarget)

        Returns:
        a method handle which always invokes this call site's current target