1 /*
   2  * Copyright (c) 1997, 2015, 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
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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 
  29 /**
  30  * Phantom reference objects, which are enqueued after the collector
  31  * determines that their referents may otherwise be reclaimed.  Phantom
  32  * references are most often used to schedule post-mortem cleanup actions.
  33  *
  34  * <p> Suppose the garbage collector determines at a certain point in time
  35  * that an object is <a href="package-summary.html#reachability">
  36  * phantom reachable</a>.  At that time it will atomically clear
  37  * all phantom references to that object and all phantom references to
  38  * any other phantom-reachable objects from which that object is reachable.
  39  * At the same time or at some later time it will enqueue those newly-cleared
  40  * phantom references that are registered with reference queues.
  41  *
  42  * <p> In order to ensure that a reclaimable object remains so, the referent of
  43  * a phantom reference may not be retrieved: The {@code get} method of a
  44  * phantom reference always returns {@code null}.
  45  *
  46  * @author   Mark Reinhold
  47  * @since    1.2
  48  */
  49 
  50 public class PhantomReference<T> extends Reference<T> {
  51 
  52     /**
  53      * Returns this reference object's referent.  Because the referent of a
  54      * phantom reference is always inaccessible, this method always returns
  55      * {@code null}.
  56      *
  57      * @return {@code null}
  58      */
  59     public T get() {
  60         return null;
  61     }
  62 
  63     /**
  64      * Creates a new phantom reference that refers to the given object and
  65      * is registered with the given queue.
  66      *
  67      * <p> It is possible to create a phantom reference with a {@code null}
  68      * queue, but such a reference is completely useless: Its {@code get}
  69      * method will always return {@code null} and, since it does not have a queue,
  70      * it will never be enqueued.
  71      *
  72      * @param referent the object the new phantom reference will refer to
  73      * @param q the queue with which the reference is to be registered,
  74      *          or {@code null} if registration is not required
  75      */
  76     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
  77         super(referent, q);
  78     }
  79 
  80 }