1 /*
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13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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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  *
49  * @author   Mark Reinhold
50  * @since    1.2
51  */
52 
53 public class PhantomReference<T> extends Reference<T> {
54 
55     /**
56      * Returns this reference object's referent.  Because the referent of a
57      * phantom reference is always inaccessible, this method always returns
58      * {@code null}.
59      *
60      * @return {@code null}
61      */
62     public T get() {
63         return null;
64     }
65 
66     /* Override the implementation of Reference.refersTo.
67      * Phantom references are weaker than finalization, so the referent
68      * access needs to be handled differently for garbage collectors that
69      * do reference processing concurrently.
70      */
71     @Override
72     boolean refersToImpl(T obj) {
73         return refersTo0(obj);
74     }
75 
76     @IntrinsicCandidate
77     private native boolean refersTo0(Object o);
78 
79     /**
80      * Creates a new phantom reference that refers to the given object and
81      * is registered with the given queue.
82      *
83      * <p> It is possible to create a phantom reference with a {@code null}
84      * queue.  Such a reference will never be enqueued.
85      *
86      * @param referent the object the new phantom reference will refer to
87      * @param q the queue with which the reference is to be registered,
88      *          or {@code null} if registration is not required
89      */
90     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
91         super(referent, q);
92     }
93 
94 }