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/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
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 * published by the Free Software Foundation.  Oracle designates this
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 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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.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.
 *
 *  Suppose the garbage collector determines at a certain point in time
 * that an object is 
 * phantom reachable.  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.
 *
 * 
 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 extends Reference {

    /**
     * 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
    boolean refersToImpl(T obj) {
        return refersTo0(obj);
    }

    @IntrinsicCandidate
    private native boolean refersTo0(Object o);

    /**
     * Creates a new phantom reference that refers to the given object and
     * is registered with the given queue.
     *
     *  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 q) {
        super(referent, q);
    }

}