Source for java.lang.ref.ReferenceQueue

 1:  /* java.lang.ref.ReferenceQueue
 2:  Copyright (C) 1999, 2004, 2006 Free Software Foundation, Inc.
 3: 
 4: This file is part of GNU Classpath.
 5: 
 6: GNU Classpath is free software; you can redistribute it and/or modify
 7: it under the terms of the GNU General Public License as published by
 8: the Free Software Foundation; either version 2, or (at your option)
 9: any later version.
 10:  
 11: GNU Classpath is distributed in the hope that it will be useful, but
 12: WITHOUT ANY WARRANTY; without even the implied warranty of
 13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 14: General Public License for more details.
 15: 
 16: You should have received a copy of the GNU General Public License
 17: along with GNU Classpath; see the file COPYING. If not, write to the
 18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 19: 02110-1301 USA.
 20: 
 21: Linking this library statically or dynamically with other modules is
 22: making a combined work based on this library. Thus, the terms and
 23: conditions of the GNU General Public License cover the whole
 24: combination.
 25: 
 26: As a special exception, the copyright holders of this library give you
 27: permission to link this library with independent modules to produce an
 28: executable, regardless of the license terms of these independent
 29: modules, and to copy and distribute the resulting executable under
 30: terms of your choice, provided that you also meet, for each linked
 31: independent module, the terms and conditions of the license of that
 32: module. An independent module is a module which is not derived from
 33: or based on this library. If you modify this library, you may extend
 34: this exception to your version of the library, but you are not
 35: obligated to do so. If you do not wish to do so, delete this
 36: exception statement from your version. */
 37: 
 38: 
 39:  package java.lang.ref;
 40: 
 41:  /**
 42:  * This is the queue, where references can enqueue themselve on. Each
 43:  * reference may be registered to a queue at initialization time and
 44:  * will be appended to the queue, when the enqueue method is called.
 45:  *
 46:  * The enqueue method may be automatically called by the garbage
 47:  * collector if it detects, that the object is only reachable through
 48:  * the Reference objects.
 49:  *
 50:  * @author Jochen Hoenicke
 51:  * @see Reference#enqueue()
 52:  */
 53:  public class ReferenceQueue<T>
 54: {
 55:  /**
 56:  * This is a linked list of references. If this is null, the list is
 57:  * empty. Otherwise this points to the first reference on the queue.
 58:  * The first reference will point to the next reference via the 
 59:  * <code>nextOnQueue</code> field. The last reference will point to
 60:  * itself (not to null, since <code>nextOnQueue</code> is used to 
 61:  * determine if a reference is enqueued).
 62:  */
 63:  private Reference<? extends T> first;
 64: 
 65:  /**
 66:  * This is the lock that protects our linked list and is used to signal
 67:  * a thread waiting in remove().
 68:  */
 69:  private final Object lock = new Object();
 70: 
 71:  /**
 72:  * Creates a new empty reference queue.
 73:  */
 74:  public ReferenceQueue()
 75:  {
 76:  }
 77: 
 78:  /**
 79:  * Checks if there is a reference on the queue, returning it
 80:  * immediately. The reference will be dequeued.
 81:  *
 82:  * @return a reference on the queue, if there is one,
 83:  * <code>null</code> otherwise. 
 84:  */
 85:  public Reference<? extends T> poll()
 86:  { 
 87:  return dequeue();
 88:  }
 89: 
 90:  /**
 91:  * This is called by reference to enqueue itself on this queue. 
 92:  * @param ref the reference that should be enqueued.
 93:  * @return true if successful, false if not.
 94:  */
 95:  final boolean enqueue(Reference<? extends T> ref)
 96:  {
 97:  synchronized (lock)
 98:  {
 99:  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: }