Class CallSite
java.lang.Object
java.lang.invoke.CallSite
- Direct Known Subclasses:
ConstantCallSite,MutableCallSite,VolatileCallSite
public abstract sealed classCallSiteextendsObjectpermitsConstantCallSite,MutableCallSite,VolatileCallSite
A
CallSite 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 sealed class which does not allow direct subclassing by users. It has three immediate, concrete non-sealed subclasses that may be either instantiated or subclassed.
- If a mutable target is not required, an
invokedynamicinstruction 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));}
- Sealed Class Hierarchy Graph:
- Since:
- 1.7
Method Summary
Modifier and TypeMethodDescriptionabstractMethodHandleProduces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.abstractMethodHandleReturns the target method of the call site, according to the behavior defined by this call site's specific class.abstract voidsetTarget(MethodHandle newTarget) Updates the target method of this call site, according to the behavior defined by this call site's specific class.type()Returns the type of this call site's target.
Method Details
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. ThesetTargetmethod 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
getTarget
Returns the target method of the call site, according to the behavior defined by this call site's specific class. The immediate subclasses ofCallSitedocument the class-specific behaviors of this method.- Returns:
- the current linkage state of the call site, its target method handle
- See Also:
setTarget
Updates the target method of this call site, according to the behavior defined by this call site's specific class. The immediate subclasses ofCallSitedocument the class-specific behaviors of this method.The type of the new target must beequal to the type of the old target.
- Parameters:
newTarget- the new target- Throws:
NullPointerException- if the proposed new target is nullWrongMethodTypeException- if the proposed new target has a method type that differs from the previous target- See Also:
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