1 /*
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  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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  5  * This code is free software; you can redistribute it and/or modify it
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  7  * published by the Free Software Foundation.  Oracle designates this
  8  * particular file as subject to the "Classpath" exception as provided
  9  * by Oracle in the LICENSE file that accompanied this code.
 10  *
 11  * This code is distributed in the hope that it will be useful, but WITHOUT
 12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 14  * version 2 for more details (a copy is included in the LICENSE file that
 15  * accompanied this code).
 16  *
 17  * You should have received a copy of the GNU General Public License version
 18  * 2 along with this work; if not, write to the Free Software Foundation,
 19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 20  *
 21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 22  * or visit www.oracle.com if you need additional information or have any
 23  * questions.
 24  */
 25 
 26 package java.lang.ref;
 27 
 28 import jdk.internal.vm.annotation.IntrinsicCandidate;
 29 
 30 /**
 31  * Phantom reference objects, which are enqueued after the collector
 32  * determines that their referents may otherwise be reclaimed.  Phantom
 33  * references are most often used to schedule post-mortem cleanup actions.
 34  *
 35  * <p> Suppose the garbage collector determines at a certain point in time
 36  * that an object is <a href="package-summary.html#reachability">
 37  * phantom reachable</a>.  At that time it will atomically clear
 38  * all phantom references to that object and all phantom references to
 39  * any other phantom-reachable objects from which that object is reachable.
 40  * At the same time or at some later time it will enqueue those newly-cleared
 41  * phantom references that are registered with reference queues.
 42  *
 43  * <p> In order to ensure that a reclaimable object remains so, the referent of
 44  * a phantom reference may not be retrieved: The {@code get} method of a
 45  * phantom reference always returns {@code null}.
 46  * The {@link #refersTo(Object) refersTo} method can be used to test
 47  * whether some object is the referent of a phantom reference.
 48  * @param <T> the type of the referent
 49  *
 50  * @author   Mark Reinhold
 51  * @since    1.2
 52  */
 53 
 54 public non-sealed class PhantomReference<@jdk.internal.RequiresIdentity T> extends Reference<T> {
 55 
 56     /**
 57      * Returns this reference object's referent.  Because the referent of a
 58      * phantom reference is always inaccessible, this method always returns
 59      * {@code null}.
 60      *
 61      * @return {@code null}
 62      */
 63     public T get() {
 64         return null;
 65     }
 66 
 67     /* Override the implementation of Reference.refersTo.
 68      * Phantom references are weaker than finalization, so the referent
 69      * access needs to be handled differently for garbage collectors that
 70      * do reference processing concurrently.
 71      */
 72     @Override
 73     boolean refersToImpl(T obj) {
 74         return refersTo0(obj);
 75     }
 76 
 77     @IntrinsicCandidate
 78     private native boolean refersTo0(Object o);
 79 
 80     /* Override the implementation of Reference.clear.
 81      * Phantom references are weaker than finalization, so the referent
 82      * access needs to be handled differently for garbage collectors that
 83      * do reference processing concurrently.
 84      */
 85     @Override
 86     void clearImpl() {
 87         clear0();
 88     }
 89 
 90     @IntrinsicCandidate
 91     private native void clear0();
 92 
 93     /**
 94      * Creates a new phantom reference that refers to the given object and
 95      * is registered with the given queue.
 96      *
 97      * <p> It is possible to create a phantom reference with a {@code null}
 98      * queue.  Such a reference will never be enqueued.
 99      *
100      * @param referent the object the new phantom reference will refer to
101      * @param q the queue with which the reference is to be registered,
102      *          or {@code null} if registration is not required
103      */
104     public PhantomReference(@jdk.internal.RequiresIdentity T referent, ReferenceQueue<? super T> q) {
105         super(referent, q);
106     }
107 
108 }