001/* java.lang.ref.Reference
002   Copyright (C) 1999, 2002, 2003, 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 base class of all references.  A reference allows
043 * refering to an object without preventing the garbage collector to
044 * collect it.  The only way to get the referred object is via the
045 * <code>get()</code>-method.  This method will return
046 * <code>null</code> if the object was collected. <br>
047 *
048 * A reference may be registered with a queue.  When a referred
049 * element gets collected the reference will be put on the queue, so
050 * that you will be notified. <br>
051 *
052 * There are currently three types of references:  soft reference,
053 * weak reference and phantom reference. <br>
054 *
055 * Soft references will be cleared if the garbage collector is told
056 * to free some memory and there are no unreferenced or weakly referenced
057 * objects.  It is useful for caches. <br>
058 *
059 * Weak references will be cleared as soon as the garbage collector
060 * determines that the refered object is only weakly reachable.  They
061 * are useful as keys in hashtables (see <code>WeakHashtable</code>) as
062 * you get notified when nobody has the key anymore.
063 *
064 * Phantom references don't prevent finalization.  If an object is only
065 * phantom reachable, it will be finalized, and the reference will be
066 * enqueued, but not cleared.  Since you mustn't access an finalized
067 * object, the <code>get</code> method of a phantom reference will never
068 * work.  It is useful to keep track, when an object is finalized.
069 *
070 * @author Jochen Hoenicke
071 * @see java.util.WeakHashMap
072 */
073public abstract class Reference<T>
074{
075  /**
076   * The underlying object.  This field is handled in a special way by
077   * the garbage collector.
078   */
079  T referent;
080
081  /**
082   * The queue this reference is registered on. This is null, if this
083   * wasn't registered to any queue or reference was already enqueued.
084   */
085  volatile ReferenceQueue<? super T> queue;
086
087  /**
088   * Link to the next entry on the queue.  If this is null, this
089   * reference is not enqueued.  Otherwise it points to the next
090   * reference.  The last reference on a queue will point to itself
091   * (not to null, that value is used to mark a not enqueued
092   * reference).
093   */
094  volatile Reference nextOnQueue;
095
096  /**
097   * This lock should be taken by the garbage collector, before
098   * determining reachability.  It will prevent the get()-method to
099   * return the reference so that reachability doesn't change.
100   */
101  static Object lock = new Object();
102
103  /**
104   * Creates a new reference that is not registered to any queue.
105   * Since it is package private, it is not possible to overload this
106   * class in a different package.
107   * @param ref the object we refer to.
108   */
109  Reference(T ref)
110  {
111    referent = ref;
112  }
113
114  /**
115   * Creates a reference that is registered to a queue.  Since this is
116   * package private, it is not possible to overload this class in a
117   * different package.
118   * @param ref the object we refer to.
119   * @param q the reference queue to register on.
120   * @exception NullPointerException if q is null.
121   */
122  Reference(T ref, ReferenceQueue<? super T> q)
123  {
124    if (q == null)
125      throw new NullPointerException();
126    referent = ref;
127    queue = q;
128  }
129
130  /**
131   * Returns the object, this reference refers to.
132   * @return the object, this reference refers to, or null if the
133   * reference was cleared.
134   */
135  public T get()
136  {
137    synchronized (lock)
138      {
139        return referent;
140      }
141  }
142
143  /**
144   * Clears the reference, so that it doesn't refer to its object
145   * anymore.  For soft and weak references this is called by the
146   * garbage collector.  For phantom references you should call
147   * this when enqueuing the reference.
148   */
149  public void clear()
150  {
151    referent = null;
152  }
153
154  /**
155   * Tells if the object is enqueued on a reference queue.
156   * @return true if it is enqueued, false otherwise.
157   */
158  public boolean isEnqueued()
159  {
160    return nextOnQueue != null;
161  }
162
163  /**
164   * Enqueue an object on a reference queue.  This is normally executed
165   * by the garbage collector.
166   */
167  public boolean enqueue()
168  {
169    ReferenceQueue q = queue;
170    if (q != null)
171      {
172        return q.enqueue(this);
173      }
174    return false;
175  }
176}