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

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

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 * accompanied this code).

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package java.lang.ref;

import jdk.internal.vm.annotation.IntrinsicCandidate;

/**

 * Phantom reference objects, which are enqueued after the collector

 * determines that their referents may otherwise be reclaimed.  Phantom

 * references are most often used to schedule post-mortem cleanup actions.

 *

 * <p> Suppose the garbage collector determines at a certain point in time

 * that an object is <a href="package-summary.html#reachability">

 * phantom reachable</a>.  At that time it will atomically clear

 * all phantom references to that object and all phantom references to

 * any other phantom-reachable objects from which that object is reachable.

 * At the same time or at some later time it will enqueue those newly-cleared

 * phantom references that are registered with reference queues.

 *

 * <p> In order to ensure that a reclaimable object remains so, the referent of

 * a phantom reference may not be retrieved: The {@code get} method of a

 * phantom reference always returns {@code null}.

 * The {@link #refersTo(Object) refersTo} method can be used to test

 * whether some object is the referent of a phantom reference.

 *

 * @author   Mark Reinhold

 * @since    1.2

 */

public class PhantomReference<T> extends Reference<T> {

    /**

     * Returns this reference object's referent.  Because the referent of a

     * phantom reference is always inaccessible, this method always returns

     * {@code null}.

     *

     * @return {@code null}

     */

    public T get() {

        return null;

    }

    /* Override the implementation of Reference.refersTo.

     * Phantom references are weaker than finalization, so the referent

     * access needs to be handled differently for garbage collectors that

     * do reference processing concurrently.

     */

    @Override

    @IntrinsicCandidate

    native final boolean refersTo0(Object o);

    /**

     * Creates a new phantom reference that refers to the given object and

     * is registered with the given queue.

     *

     * <p> It is possible to create a phantom reference with a {@code null}

     * queue.  Such a reference will never be enqueued.

     *

     * @param referent the object the new phantom reference will refer to

     * @param q the queue with which the reference is to be registered,

     *          or {@code null} if registration is not required

     */

    public PhantomReference(T referent, ReferenceQueue<? super T> q) {

        super(referent, q);

    }

}