- * {@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. - *
- * 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));
-}
-}- * 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) { - target = type.invokers().uninitializedCallSite(); - } - - /** - * 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 { - this(targetType); - ConstantCallSite selfCCS = (ConstantCallSite) this; - MethodHandle boundTarget = (MethodHandle) createTargetHook.invokeWithArguments(selfCCS); - checkTargetChange(this.target, boundTarget); - this.target = boundTarget; - } - - /** - * 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); - - void checkTargetChange(MethodHandle oldTarget, MethodHandle newTarget) { - MethodType oldType = oldTarget.type(); - MethodType newType = newTarget.type(); // null check! - if (!newType.equals(oldType)) - throw wrongTargetType(newTarget, oldType); - } - - private static WrongMethodTypeException wrongTargetType(MethodHandle target, MethodType type) { - return new WrongMethodTypeException(String.valueOf(target)+" should be of type "+type); - } - - /** - * 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)
- * }