001    /* java.lang.ref.ReferenceQueue
002       Copyright (C) 1999, 2004, 2006 Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010    
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    
039    package 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     */
053    public 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    }