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).
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19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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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<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     /**
81      * Creates a new phantom reference that refers to the given object and
82      * is registered with the given queue.
83      *
84      * <p> It is possible to create a phantom reference with a {@code null}
85      * queue.  Such a reference will never be enqueued.
86      *
87      * @param referent the object the new phantom reference will refer to
88      * @param q the queue with which the reference is to be registered,
89      *          or {@code null} if registration is not required
90      */
91     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
92         super(referent, q);
93     }
94 
95 }