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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

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 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

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package java.lang.invoke;

import jdk.internal.misc.Unsafe;

import jdk.internal.vm.annotation.Stable;

/**

 * A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed.

 * An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently

 * bound to the call site's target.

 * @author John Rose, JSR 292 EG

 * @since 1.7

 */

public class ConstantCallSite extends CallSite {

    private static final Unsafe UNSAFE = Unsafe.getUnsafe();

    @Stable // should NOT be constant folded during instance initialization (isFrozen == false)

    /*final*/ private boolean isFrozen; // Note: This field is known to the JVM.

    /**

     * Creates a call site with a permanent target.

     * @param target the target to be permanently associated with this call site

     * @throws NullPointerException if the proposed target is null

     */

    public ConstantCallSite(MethodHandle target) {

        super(target);

        isFrozen = true;

        UNSAFE.storeStoreFence(); // properly publish isFrozen update

    }

    /**

     * Creates a call site with a permanent target, possibly bound to the call site itself.

     * <p>

     * During construction of the call site, the {@code createTargetHook} is invoked to

     * produce the actual target, as if by a call of the form

     * {@code (MethodHandle) createTargetHook.invoke(this)}.

     * <p>

     * Note that user code cannot perform such an action directly in a subclass constructor,

     * since the target must be fixed before the {@code ConstantCallSite} constructor returns.

     * <p>

     * The hook is said to bind the call site to a target method handle,

     * and a typical action would be {@code someTarget.bindTo(this)}.

     * However, the hook is free to take any action whatever,

     * including ignoring the call site and returning a constant target.

     * <p>

     * The result returned by the hook must be a method handle of exactly

     * the same type as the call site.

     * <p>

     * While the hook is being called, the new {@code ConstantCallSite}

     * object is in a partially constructed state.

     * In this state,

     * a call to {@code getTarget}, or any other attempt to use the target,

     * will result in an {@code IllegalStateException}.

     * It is legal at all times to obtain the call site's type using the {@code type} method.

     *

     * @param targetType the type of the method handle to be permanently associated with this call site

     * @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target

     * @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

     */

    protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {

        super(targetType, createTargetHook); // "this" instance leaks into createTargetHook

        isFrozen = true;

        UNSAFE.storeStoreFence(); // properly publish isFrozen

    }

    /**

     * Returns the target method of the call site, which behaves

     * like a {@code final} field of the {@code ConstantCallSite}.

     * That is, the target is always the original value passed

     * to the constructor call which created this instance.

     *

     * @return the immutable linkage state of this call site, a constant method handle

     * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed

     */

    @Override public final MethodHandle getTarget() {

        if (!isFrozen)  throw new IllegalStateException();

        return target;

    }

    /**

     * Always throws an {@link UnsupportedOperationException}.

     * This kind of call site cannot change its target.

     * @param ignore a new target proposed for the call site, which is ignored

     * @throws UnsupportedOperationException because this kind of call site cannot change its target

     */

    @Override public final void setTarget(MethodHandle ignore) {

        throw new UnsupportedOperationException();

    }

    /**

     * Returns this call site's permanent target.

     * Since that target will never change, this is a correct implementation

     * of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.

     * @return the immutable linkage state of this call site, a constant method handle

     * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed

     */

    @Override

    public final MethodHandle dynamicInvoker() {

        return getTarget();

    }

}