META-INF.modules.java.base.classes.java.lang.invoke.ConstantCallSite Maven / Gradle / Ivy
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package java.lang.invoke;
/**
* A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed.
* An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently
* bound to the call site's target.
* @author John Rose, JSR 292 EG
* @since 1.7
*/
public class ConstantCallSite extends CallSite {
private final boolean isFrozen;
/**
* Creates a call site with a permanent target.
* @param target the target to be permanently associated with this call site
* @throws NullPointerException if the proposed target is null
*/
public ConstantCallSite(MethodHandle target) {
super(target);
isFrozen = true;
}
/**
* Creates a call site with a permanent target, possibly bound to the call site itself.
*
* During construction of the call site, the {@code createTargetHook} is invoked to
* produce the actual target, as if by a call of the form
* {@code (MethodHandle) createTargetHook.invoke(this)}.
*
* Note that user code cannot perform such an action directly in a subclass constructor,
* since the target must be fixed before the {@code ConstantCallSite} constructor returns.
*
* The hook is said to bind the call site to a target method handle,
* and a typical action would be {@code someTarget.bindTo(this)}.
* However, the hook is free to take any action whatever,
* including ignoring the call site and returning a constant target.
*
* The result returned by the hook must be a method handle of exactly
* the same type as the call site.
*
* While the hook is being called, the new {@code ConstantCallSite}
* object is in a partially constructed state.
* In this state,
* a call to {@code getTarget}, or any other attempt to use the target,
* will result in an {@code IllegalStateException}.
* It is legal at all times to obtain the call site's type using the {@code type} method.
*
* @param targetType the type of the method handle to be permanently associated with this call site
* @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target
* @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,
* or if the target returned by the hook is not of the given {@code targetType}
* @throws NullPointerException if the hook returns a null value
* @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
* @throws Throwable anything else thrown by the hook function
*/
protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
super(targetType, createTargetHook);
isFrozen = true;
}
/**
* Returns the target method of the call site, which behaves
* like a {@code final} field of the {@code ConstantCallSite}.
* That is, the target is always the original value passed
* to the constructor call which created this instance.
*
* @return the immutable linkage state of this call site, a constant method handle
* @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
*/
@Override public final MethodHandle getTarget() {
if (!isFrozen) throw new IllegalStateException();
return target;
}
/**
* Always throws an {@link UnsupportedOperationException}.
* This kind of call site cannot change its target.
* @param ignore a new target proposed for the call site, which is ignored
* @throws UnsupportedOperationException because this kind of call site cannot change its target
*/
@Override public final void setTarget(MethodHandle ignore) {
throw new UnsupportedOperationException();
}
/**
* Returns this call site's permanent target.
* Since that target will never change, this is a correct implementation
* of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.
* @return the immutable linkage state of this call site, a constant method handle
* @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
*/
@Override
public final MethodHandle dynamicInvoker() {
return getTarget();
}
}