1 /*
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 9  * by Oracle in the LICENSE file that accompanied this code.
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12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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15  * accompanied this code).
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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  * <p>
35  * The referent must not be an instance of a primitive class; such a value
36  * can never have another reference to it and cannot be held in a reference type.
37  *
38  * <p> Suppose the garbage collector determines at a certain point in time
39  * that an object is <a href="package-summary.html#reachability">
40  * phantom reachable</a>.  At that time it will atomically clear
41  * all phantom references to that object and all phantom references to
42  * any other phantom-reachable objects from which that object is reachable.
43  * At the same time or at some later time it will enqueue those newly-cleared
44  * phantom references that are registered with reference queues.
45  *
46  * <p> In order to ensure that a reclaimable object remains so, the referent of
47  * a phantom reference may not be retrieved: The {@code get} method of a
48  * phantom reference always returns {@code null}.
49  * The {@link #refersTo(Object) refersTo} method can be used to test
50  * whether some object is the referent of a phantom reference.
51  *
52  * @author   Mark Reinhold
53  * @since    1.2
54  */
55 
56 public class PhantomReference<T> extends Reference<T> {
57 
58     /**
59      * Returns this reference object's referent.  Because the referent of a
60      * phantom reference is always inaccessible, this method always returns
61      * {@code null}.
62      *
63      * @return {@code null}
64      */
65     public T get() {
66         return null;
67     }
68 
69     /* Override the implementation of Reference.refersTo.
70      * Phantom references are weaker than finalization, so the referent
71      * access needs to be handled differently for garbage collectors that
72      * do reference processing concurrently.
73      */
74     @Override
75     boolean refersToImpl(T obj) {
76         return refersTo0(obj);
77     }
78 
79     @IntrinsicCandidate
80     private native boolean refersTo0(Object o);
81 
82     /**
83      * Creates a new phantom reference that refers to the given object and
84      * is registered with the given queue.
85      *
86      * <p> It is possible to create a phantom reference with a {@code null}
87      * queue.  Such a reference will never be enqueued.
88      *
89      * @param referent the object the new phantom reference will refer to
90      * @param q the queue with which the reference is to be registered,
91      *          or {@code null} if registration is not required
92      * @throws IllegalArgumentException if the referent is an instance of an
93      *         {@link Class#isPrimitiveClass() primitive class}
94      */
95     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
96         super(referent, q);
97     }
98 
99 }
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