java.lang.invoke.CallSite Maven / Gradle / Ivy
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package java.lang.invoke;
/**
* A {@code CallSite} is a holder for a variable {@link MethodHandle},
* which is called its {@code target}.
* An {@code invokedynamic} instruction linked to a {@code CallSite} delegates
* all calls to the site's current target.
* A {@code CallSite} may be associated with several {@code invokedynamic}
* instructions, or it may be "free floating", associated with none.
* In any case, it may be invoked through an associated method handle
* called its {@linkplain #dynamicInvoker dynamic invoker}.
*
* {@code 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 {@code invokedynamic} instruction
* may be permanently bound by means of a {@linkplain ConstantCallSite 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 {@linkplain VolatileCallSite volatile call site} may be used.
*
- Otherwise, if a mutable target is required,
* a {@linkplain MutableCallSite 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 {@linkplain MethodHandle#type() 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:
{@code
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));
}
}
* @author John Rose, JSR 292 EG
*/
abstract
public class CallSite {
// The actual payload of this call site:
/*package-private*/
MethodHandle target; // Note: This field is known to the JVM. Do not change.
/**
* Make a blank call site object with the given method type.
* An initial target method is supplied which will throw
* an {@link IllegalStateException} if called.
*
* Before this {@code CallSite} object is returned from a bootstrap method,
* it is usually provided with a more useful target method,
* via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
* @throws NullPointerException if the proposed type is null
*/
/*package-private*/
CallSite(MethodType type) {
throw new IllegalStateException();
}
/**
* Make a call site object equipped with an initial target method handle.
* @param target the method handle which will be the initial target of the call site
* @throws NullPointerException if the proposed target is null
*/
/*package-private*/
CallSite(MethodHandle target) {
target.type(); // null check
this.target = target;
}
/**
* Make a call site object equipped with an initial target method handle.
* @param targetType the desired type of the call site
* @param createTargetHook a hook which will bind the call site to the target method handle
* @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
*/
/*package-private*/
CallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
throw new IllegalStateException();
}
/**
* 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.
* The {@code setTarget} method enforces this invariant by refusing any new target that does
* not have the previous target's type.
* @return the type of the current target, which is also the type of any future target
*/
public MethodType type() {
// warning: do not call getTarget here, because CCS.getTarget can throw IllegalStateException
return target.type();
}
/**
* Returns the target method of the call site, according to the
* behavior defined by this call site's specific class.
* The immediate subclasses of {@code CallSite} document the
* class-specific behaviors of this method.
*
* @return the current linkage state of the call site, its target method handle
* @see ConstantCallSite
* @see VolatileCallSite
* @see #setTarget
* @see ConstantCallSite#getTarget
* @see MutableCallSite#getTarget
* @see VolatileCallSite#getTarget
*/
public abstract MethodHandle getTarget();
/**
* Updates the target method of this call site, according to the
* behavior defined by this call site's specific class.
* The immediate subclasses of {@code CallSite} document the
* class-specific behaviors of this method.
*
* The type of the new target must be {@linkplain MethodType#equals equal to}
* the type of the old target.
*
* @param newTarget the new target
* @throws NullPointerException if the proposed new target is null
* @throws WrongMethodTypeException if the proposed new target
* has a method type that differs from the previous target
* @see CallSite#getTarget
* @see ConstantCallSite#setTarget
* @see MutableCallSite#setTarget
* @see VolatileCallSite#setTarget
*/
public abstract void setTarget(MethodHandle newTarget);
/**
* 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:
*
{@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)
* }
*
* @return a method handle which always invokes this call site's current target
*/
public abstract MethodHandle dynamicInvoker();
}