1 /*
  2  * Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  4  *
  5  * This code is free software; you can redistribute it and/or modify it
  6  * under the terms of the GNU General Public License version 2 only, as
  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  * <p>
 35  * The referent must not be an instance of a {@linkplain Class#isValue()
 36  * value class}; such a value can never have another reference to it
 37  * and cannot be held in a reference type.
 38  *
 39  * <p> Suppose the garbage collector determines at a certain point in time
 40  * that an object is <a href="package-summary.html#reachability">
 41  * phantom reachable</a>.  At that time it will atomically clear
 42  * all phantom references to that object and all phantom references to
 43  * any other phantom-reachable objects from which that object is reachable.
 44  * At the same time or at some later time it will enqueue those newly-cleared
 45  * phantom references that are registered with reference queues.
 46  *
 47  * <p> In order to ensure that a reclaimable object remains so, the referent of
 48  * a phantom reference may not be retrieved: The {@code get} method of a
 49  * phantom reference always returns {@code null}.
 50  * The {@link #refersTo(Object) refersTo} method can be used to test
 51  * whether some object is the referent of a phantom reference.
 52  * @param <T> the type of the referent
 53  *
 54  * @author   Mark Reinhold
 55  * @since    1.2
 56  */
 57 
 58 public non-sealed class PhantomReference<T> extends Reference<T> {
 59 
 60     /**
 61      * Returns this reference object's referent.  Because the referent of a
 62      * phantom reference is always inaccessible, this method always returns
 63      * {@code null}.
 64      *
 65      * @return {@code null}
 66      */
 67     public T get() {
 68         return null;
 69     }
 70 
 71     /* Override the implementation of Reference.refersTo.
 72      * Phantom references are weaker than finalization, so the referent
 73      * access needs to be handled differently for garbage collectors that
 74      * do reference processing concurrently.
 75      */
 76     @Override
 77     boolean refersToImpl(T obj) {
 78         return refersTo0(obj);
 79     }
 80 
 81     @IntrinsicCandidate
 82     private native boolean refersTo0(Object o);
 83 
 84     /**
 85      * Creates a new phantom reference that refers to the given object and
 86      * is registered with the given queue.
 87      *
 88      * <p> It is possible to create a phantom reference with a {@code null}
 89      * queue.  Such a reference will never be enqueued.
 90      *
 91      * @param referent the object the new phantom reference will refer to
 92      * @param q the queue with which the reference is to be registered,
 93      *          or {@code null} if registration is not required
 94      * @throws IdentityException if the referent is not an
 95      *         {@link java.util.Objects#isIdentityObject(Object) identity object}
 96      */
 97     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
 98         super(referent, q);
 99     }
100 
101 }