001/* java.lang.ref.ReferenceQueue
002   Copyright (C) 1999, 2004, 2006 Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038
039package java.lang.ref;
040
041/**
042 * This is the queue, where references can enqueue themselve on.  Each
043 * reference may be registered to a queue at initialization time and
044 * will be appended to the queue, when the enqueue method is called.
045 *
046 * The enqueue method may be automatically called by the garbage
047 * collector if it detects, that the object is only reachable through
048 * the Reference objects.
049 *
050 * @author Jochen Hoenicke
051 * @see Reference#enqueue()
052 */
053public class ReferenceQueue<T>
054{
055  /**
056   * This is a linked list of references.  If this is null, the list is
057   * empty.  Otherwise this points to the first reference on the queue.
058   * The first reference will point to the next reference via the
059   * <code>nextOnQueue</code> field.  The last reference will point to
060   * itself (not to null, since <code>nextOnQueue</code> is used to
061   * determine if a reference is enqueued).
062   */
063  private Reference<? extends T> first;
064
065  /**
066   * This is the lock that protects our linked list and is used to signal
067   * a thread waiting in remove().
068   */
069  private final Object lock = new Object();
070
071  /**
072   * Creates a new empty reference queue.
073   */
074  public ReferenceQueue()
075  {
076  }
077
078  /**
079   * Checks if there is a reference on the queue, returning it
080   * immediately.  The reference will be dequeued.
081   *
082   * @return a reference on the queue, if there is one,
083   * <code>null</code> otherwise.
084   */
085  public Reference<? extends T> poll()
086  {
087    return dequeue();
088  }
089
090  /**
091   * This is called by reference to enqueue itself on this queue.
092   * @param ref the reference that should be enqueued.
093   * @return true if successful, false if not.
094   */
095  final boolean enqueue(Reference<? extends T> ref)
096    {
097    synchronized (lock)
098      {
099        if (ref.queue != this)
100          return false;
101
102        /* last reference will point to itself */
103        ref.nextOnQueue = first == null ? ref : first;
104        ref.queue = null;
105        first = ref;
106        /* this wakes only one remove thread. */
107        lock.notify();
108        return true;
109      }
110  }
111
112  /**
113   * Remove a reference from the queue, if there is one.
114   * @return the first element of the queue, or null if there isn't any.
115   */
116  private Reference<? extends T> dequeue()
117  {
118    synchronized (lock)
119      {
120        if (first == null)
121          return null;
122
123        Reference<? extends T> result = first;
124        first = (first == first.nextOnQueue) ? null : first.nextOnQueue;
125        result.nextOnQueue = null;
126        return result;
127      }
128  }
129
130  /**
131   * Removes a reference from the queue, blocking for <code>timeout</code>
132   * until a reference is enqueued.
133   * @param timeout the timeout period in milliseconds, <code>0</code> means
134   * wait forever.
135   * @return the reference removed from the queue, or
136   * <code>null</code> if timeout period expired.
137   * @exception InterruptedException if the wait was interrupted.
138   */
139  public Reference<? extends T> remove(long timeout)
140    throws InterruptedException
141  {
142    synchronized (lock)
143      {
144        if (first == null)
145          lock.wait(timeout);
146      }
147
148    return dequeue();
149  }
150
151
152  /**
153   * Removes a reference from the queue, blocking until a reference is
154   * enqueued.
155   *
156   * @return the reference removed from the queue.
157   * @exception InterruptedException if the wait was interrupted.
158   */
159  public Reference<? extends T> remove()
160    throws InterruptedException
161  {
162    return remove(0L);
163  }
164}