+ * {@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: +
+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 blank call site object, possibly 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; + } + + /** + * 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() { + return target.type(); + } + + /** Called from JVM (or low-level Java code) after the BSM returns the newly created CallSite. + * The parameters are JVM-specific. + */ + void initializeFromJVM(String name, + MethodType type, + MemberName callerMethod, + int callerBCI) { + if (this.vmmethod != null) { + // FIXME + throw new BootstrapMethodError("call site has already been linked to an invokedynamic instruction"); + } + if (!this.type().equals(type)) { + throw wrongTargetType(target, type); + } + this.vmindex = callerBCI; + this.vmmethod = callerMethod; + } + + /** + * 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: + *
+ * + * @return a method handle which always invokes this call site's current target + */ + public abstract MethodHandle dynamicInvoker(); + + /*non-public*/ MethodHandle makeDynamicInvoker() { + MethodHandle getTarget = MethodHandleImpl.bindReceiver(GET_TARGET, this); + MethodHandle invoker = MethodHandles.exactInvoker(this.type()); + return MethodHandles.foldArguments(invoker, getTarget); + } + + private static final MethodHandle GET_TARGET; + static { + try { + GET_TARGET = IMPL_LOOKUP. + findVirtual(CallSite.class, "getTarget", MethodType.methodType(MethodHandle.class)); + } catch (ReflectiveOperationException ignore) { + throw new InternalError(); + } + } + + /** This guy is rolled into the default target if a MethodType is supplied to the constructor. */ + /*package-private*/ + static Empty uninitializedCallSite() { + throw new IllegalStateException("uninitialized call site"); + } + + // unsafe stuff: + private static final Unsafe unsafe = Unsafe.getUnsafe(); + private static final long TARGET_OFFSET; + + static { + try { + TARGET_OFFSET = unsafe.objectFieldOffset(CallSite.class.getDeclaredField("target")); + } catch (Exception ex) { throw new Error(ex); } + } + + /*package-private*/ + void setTargetNormal(MethodHandle newTarget) { + target = newTarget; + } + /*package-private*/ + MethodHandle getTargetVolatile() { + return (MethodHandle) unsafe.getObjectVolatile(this, TARGET_OFFSET); + } + /*package-private*/ + void setTargetVolatile(MethodHandle newTarget) { + unsafe.putObjectVolatile(this, TARGET_OFFSET, newTarget); + } + + // this implements the upcall from the JVM, MethodHandleNatives.makeDynamicCallSite: + static CallSite makeSite(MethodHandle bootstrapMethod, + // Callee information: + String name, MethodType type, + // Extra arguments for BSM, if any: + Object info, + // Caller information: + MemberName callerMethod, int callerBCI) { + Class callerClass = callerMethod.getDeclaringClass(); + Object caller = IMPL_LOOKUP.in(callerClass); + CallSite site; + try { + Object binding; + info = maybeReBox(info); + if (info == null) { + binding = bootstrapMethod.invokeGeneric(caller, name, type); + } else if (!info.getClass().isArray()) { + binding = bootstrapMethod.invokeGeneric(caller, name, type, info); + } else { + Object[] argv = (Object[]) info; + maybeReBoxElements(argv); + if (3 + argv.length > 255) + throw new BootstrapMethodError("too many bootstrap method arguments"); + MethodType bsmType = bootstrapMethod.type(); + if (bsmType.parameterCount() == 4 && bsmType.parameterType(3) == Object[].class) + binding = bootstrapMethod.invokeGeneric(caller, name, type, argv); + else + binding = MethodHandles.spreadInvoker(bsmType, 3) + .invokeGeneric(bootstrapMethod, caller, name, type, argv); + } + //System.out.println("BSM for "+name+type+" => "+binding); + if (binding instanceof CallSite) { + site = (CallSite) binding; + } else { + throw new ClassCastException("bootstrap method failed to produce a CallSite"); + } + assert(site.getTarget() != null); + assert(site.getTarget().type().equals(type)); + } catch (Throwable ex) { + BootstrapMethodError bex; + if (ex instanceof BootstrapMethodError) + bex = (BootstrapMethodError) ex; + else + bex = new BootstrapMethodError("call site initialization exception", ex); + throw bex; + } + return site; + } + + private static Object maybeReBox(Object x) { + if (x instanceof Integer) { + int xi = (int) x; + if (xi == (byte) xi) + x = xi; // must rebox; see JLS 5.1.7 + } + return x; + } + private static void maybeReBoxElements(Object[] xa) { + for (int i = 0; i < xa.length; i++) { + xa[i] = maybeReBox(xa[i]); + } + } +}+ * MethodHandle getTarget, invoker, result; + * getTarget = MethodHandles.publicLookup().bind(this, "getTarget", MethodType.methodType(MethodHandle.class)); + * invoker = MethodHandles.exactInvoker(this.type()); + * result = MethodHandles.foldArguments(invoker, getTarget) + *