Class ConstantCallSite


public class ConstantCallSite
extends CallSite

A ConstantCallSite is a CallSite whose target is permanent, and can never be changed. An invokedynamic instruction linked to a ConstantCallSite is permanently bound to the call site's target.

Constructors

Modifier Constructor and Description
ConstantCallSite(MethodHandle target)

Creates a call site with a permanent target.
protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook)

Creates a call site with a permanent target, possibly bound to the call site itself.

Methods

Modifier and Type Method and Description
MethodHandle dynamicInvoker()

Returns this call site's permanent target.
MethodHandle getTarget()

Returns the target method of the call site, which behaves like a final field of the ConstantCallSite.
void setTarget(MethodHandle ignore)

Always throws an UnsupportedOperationException.

Methods inherited from class java.lang.invoke.CallSite

type

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructors

ConstantCallSite

public ConstantCallSite(MethodHandle target)

Creates a call site with a permanent target.

Parameters:
target - the target to be permanently associated with this call site
Throws:
NullPointerException - if the proposed target is null

ConstantCallSite

protected ConstantCallSite(MethodType targetType,
                           MethodHandle createTargetHook)
                    throws Throwable

Creates a call site with a permanent target, possibly bound to the call site itself.

During construction of the call site, the createTargetHook is invoked to produce the actual target, as if by a call of the form (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 ConstantCallSite constructor returns.

The hook is said to bind the call site to a target method handle, and a typical action would be 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 ConstantCallSite object is in a partially constructed state. In this state, a call to getTarget, or any other attempt to use the target, will result in an IllegalStateException. It is legal at all times to obtain the call site's type using the type method.

Parameters:
targetType - the type of the method handle to be permanently associated with this call site
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 targetType
NullPointerException - if the hook returns a null value
ClassCastException - if the hook returns something other than a MethodHandle
Throwable - anything else thrown by the hook function

Methods

getTarget

public final MethodHandle getTarget()

Returns the target method of the call site, which behaves like a final field of the ConstantCallSite. That is, the target is always the original value passed to the constructor call which created this instance.

Specified by:
getTarget in class CallSite
Returns:
the immutable linkage state of this call site, a constant method handle
Throws:
IllegalStateException - if the ConstantCallSite constructor has not completed
See Also:
ConstantCallSite, VolatileCallSite, CallSite.setTarget(java.lang.invoke.MethodHandle), getTarget(), MutableCallSite.getTarget(), VolatileCallSite.getTarget()

setTarget

public final void setTarget(MethodHandle ignore)

Always throws an UnsupportedOperationException. This kind of call site cannot change its target.

Specified by:
setTarget in class CallSite
Parameters:
ignore - a new target proposed for the call site, which is ignored
Throws:
UnsupportedOperationException - because this kind of call site cannot change its target
See Also:
CallSite.getTarget(), setTarget(java.lang.invoke.MethodHandle), MutableCallSite.setTarget(java.lang.invoke.MethodHandle), VolatileCallSite.setTarget(java.lang.invoke.MethodHandle)

dynamicInvoker

public final MethodHandle dynamicInvoker()

Returns this call site's permanent target. Since that target will never change, this is a correct implementation of CallSite.dynamicInvoker.

Specified by:
dynamicInvoker in class CallSite
Returns:
the immutable linkage state of this call site, a constant method handle
Throws:
IllegalStateException - if the ConstantCallSite constructor has not completed

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https://docs.oracle.com/javase/8/docs/api/java/lang/invoke/ConstantCallSite.html