1 /*
   2  * Copyright (c) 1997, 2019, 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 
  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  * <p>
  34  * The referent must not be an instance of an inline class; such a value
  35  * can never have another reference to it and cannot be held in a reference type.
  36  *
  37  * <p> Suppose the garbage collector determines at a certain point in time
  38  * that an object is <a href="package-summary.html#reachability">
  39  * phantom reachable</a>.  At that time it will atomically clear
  40  * all phantom references to that object and all phantom references to
  41  * any other phantom-reachable objects from which that object is reachable.
  42  * At the same time or at some later time it will enqueue those newly-cleared
  43  * phantom references that are registered with reference queues.
  44  *
  45  * <p> In order to ensure that a reclaimable object remains so, the referent of
  46  * a phantom reference may not be retrieved: The {@code get} method of a
  47  * phantom reference always returns {@code null}.
  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     /**
  67      * Creates a new phantom reference that refers to the given object and
  68      * is registered with the given queue.
  69      *
  70      * <p> It is possible to create a phantom reference with a {@code null}
  71      * queue, but such a reference is completely useless: Its {@code get}
  72      * method will always return {@code null} and, since it does not have a queue,
  73      * it will never be enqueued.
  74      *
  75      * @param referent the object the new phantom reference will refer to
  76      * @param q the queue with which the reference is to be registered,
  77      *          or {@code null} if registration is not required
  78      * @throws IllegalArgumentException if the referent is an instance of an
  79      *         {@link Class#isInlineClass() inlineClass}
  80      */
  81     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
  82         super(referent, q);
  83     }
  84 
  85 }