CallSite.java

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package java.lang.invoke;

import sun.invoke.empty.Empty;
import sun.misc.Unsafe;
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</em> 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><pre><!-- see indy-demo/src/PrintArgsDemo.java -->
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></blockquote>
 * @author John Rose, JSR 292 EG
 */
abstract
public class CallSite {
    static { MethodHandleImpl.initStatics(); }

    // Fields used only by the JVM.  Do not use or change.
    private MemberName vmmethod; // supplied by the JVM (ref. to calling method)
    private int        vmindex;  // supplied by the JVM (BCI within calling method)

    // The actual payload of this call site:
    /*package-private*/
    MethodHandle target;

    /**
     * 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 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.
     * <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><pre>
     * 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></blockquote>
     *
     * @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]);
        }
    }
}