The CallSite.java Java example source code
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package java.lang.invoke;
import sun.invoke.empty.Empty;
import static java.lang.invoke.MethodHandleStatics.*;
import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
/**
* 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}.
* <p>
* {@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.
* <ul>
* <li>If a mutable target is not required, an {@code invokedynamic} instruction
* may be permanently bound by means of a {@linkplain ConstantCallSite constant call site}.
* <li>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.
* <li>Otherwise, if a mutable target is required,
* a {@linkplain MutableCallSite mutable call site} may be used.
* </ul>
* <p>
* A non-constant call site may be <em>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.
* <p>
* Here is a sample use of call sites and bootstrap methods which links every
* dynamic call site to print its arguments:
<blockquote>{@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));
}
}</pre>
* @author John Rose, JSR 292 EG
*/
abstract
public class CallSite {
static { MethodHandleImpl.initStatics(); }
// 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.
* <p>
* 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.
* <p>
* 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.
* <p>
* This method is equivalent to the following code:
* <blockquote>{@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)
* }</pre>
*
* @return a method handle which always invokes this call site's current target
*/
public abstract MethodHandle dynamicInvoker();
/*non-public*/ MethodHandle makeDynamicInvoker() {
MethodHandle getTarget = GET_TARGET.bindReceiver(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 e) {
throw newInternalError(e);
}
}
/** 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 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) {
MethodHandleNatives.setCallSiteTargetNormal(this, newTarget);
}
/*package-private*/
MethodHandle getTargetVolatile() {
return (MethodHandle) UNSAFE.getObjectVolatile(this, TARGET_OFFSET);
}
/*package-private*/
void setTargetVolatile(MethodHandle newTarget) {
MethodHandleNatives.setCallSiteTargetVolatile(this, 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:
Class<?> callerClass) {
MethodHandles.Lookup caller = IMPL_LOOKUP.in(callerClass);
CallSite site;
try {
Object binding;
info = maybeReBox(info);
if (info == null) {
binding = bootstrapMethod.invoke(caller, name, type);
} else if (!info.getClass().isArray()) {
binding = bootstrapMethod.invoke(caller, name, type, info);
} else {
Object[] argv = (Object[]) info;
maybeReBoxElements(argv);
switch (argv.length) {
case 0:
binding = bootstrapMethod.invoke(caller, name, type);
break;
case 1:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0]);
break;
case 2:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0], argv[1]);
break;
case 3:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0], argv[1], argv[2]);
break;
case 4:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0], argv[1], argv[2], argv[3]);
break;
case 5:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0], argv[1], argv[2], argv[3], argv[4]);
break;
case 6:
binding = bootstrapMethod.invoke(caller, name, type,
argv[0], argv[1], argv[2], argv[3], argv[4], argv[5]);
break;
default:
final int NON_SPREAD_ARG_COUNT = 3; // (caller, name, type)
if (NON_SPREAD_ARG_COUNT + argv.length > MethodType.MAX_MH_ARITY)
throw new BootstrapMethodError("too many bootstrap method arguments");
MethodType bsmType = bootstrapMethod.type();
MethodType invocationType = MethodType.genericMethodType(NON_SPREAD_ARG_COUNT + argv.length);
MethodHandle typedBSM = bootstrapMethod.asType(invocationType);
MethodHandle spreader = invocationType.invokers().spreadInvoker(NON_SPREAD_ARG_COUNT);
binding = spreader.invokeExact(typedBSM, (Object)caller, (Object)name, (Object)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");
}
if (!site.getTarget().type().equals(type))
throw new WrongMethodTypeException("wrong type: "+site.getTarget());
} 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]);
}
}
}
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