1   /*
    2    * Copyright (c) 1997, 2003, 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 for scheduling pre-mortem cleanup actions in
   33    * a more flexible way than is possible with the Java finalization mechanism.
   34    *
   35    * <p> If the garbage collector determines at a certain point in time that the
   36    * referent of a phantom reference is <a
   37    * href="package-summary.html#reachability">phantom reachable</a>, then at that
   38    * time or at some later time it will enqueue the reference.
   39    *
   40    * <p> In order to ensure that a reclaimable object remains so, the referent of
   41    * a phantom reference may not be retrieved: The <code>get</code> method of a
   42    * phantom reference always returns <code>null</code>.
   43    *
   44    * <p> Unlike soft and weak references, phantom references are not
   45    * automatically cleared by the garbage collector as they are enqueued.  An
   46    * object that is reachable via phantom references will remain so until all
   47    * such references are cleared or themselves become unreachable.
   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</code>.
   59        *
   60        * @return  <code>null</code>
   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 <tt>null</tt>
   71        * queue, but such a reference is completely useless: Its <tt>get</tt>
   72        * method will always return null and, since it does not have a queue, it
   73        * 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 <tt>null</tt> if registration is not required
   78        */
   79       public PhantomReference(T referent, ReferenceQueue<? super T> q) {
   80           super(referent, q);
   81       }
   82   
   83   }