001    /* java.lang.ref.Reference
002       Copyright (C) 1999, 2002, 2003, 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 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.WeakHashtable
072     */
073    public abstract class Reference<T>
074    {
075      /**
076       * The underlying object.  This field is handled in a special way by
077       * the garbage collector.
078       * GCJ LOCAL:
079       * This is a RawData because it must be disguised from the GC.
080       * END GCJ LOCAL
081       */
082      gnu.gcj.RawData referent;
083    
084      /**
085       * This is like REFERENT but is not scanned by the GC.  We keep a
086       * copy around so that we can clean up our internal data structure
087       * even after clear() is called.
088       * GCJ LOCAL:
089       * This field doesn't exist in Classpath.
090       * END GCJ LOCAL
091       */
092      gnu.gcj.RawData copy;
093    
094      /**
095       * Set to true if {@link #clear()} is called.
096       * GCJ LOCAL:
097       * This field doesn't exist in Classpath.  It is used internally in
098       * natReference.cc, which enqueues the reference unless it is true
099       * (has been cleared).
100       * END GCJ LOCAL
101       */
102      boolean cleared = false;
103    
104      /**
105       * The queue this reference is registered on. This is null, if this
106       * wasn't registered to any queue or reference was already enqueued.
107       */
108      ReferenceQueue<? super T> queue;
109    
110      /**
111       * Link to the next entry on the queue.  If this is null, this
112       * reference is not enqueued.  Otherwise it points to the next
113       * reference.  The last reference on a queue will point to itself
114       * (not to null, that value is used to mark a not enqueued
115       * reference).  
116       */
117      Reference nextOnQueue;
118    
119      /**
120       * This lock should be taken by the garbage collector, before
121       * determining reachability.  It will prevent the get()-method to
122       * return the reference so that reachability doesn't change.
123       */
124      static Object lock = new Object();
125    
126      /**
127       * Creates a new reference that is not registered to any queue.
128       * Since it is package private, it is not possible to overload this
129       * class in a different package.  
130       * @param referent the object we refer to.
131       */
132      Reference(T ref)
133      {
134        create (ref);
135      }
136    
137      /**
138       * Creates a reference that is registered to a queue.  Since this is
139       * package private, it is not possible to overload this class in a
140       * different package.  
141       * @param referent the object we refer to.
142       * @param q the reference queue to register on.
143       * @exception NullPointerException if q is null.
144       */
145      Reference(T ref, ReferenceQueue<? super T> q)
146      {
147        if (q == null)
148          throw new NullPointerException();
149        queue = q;
150        create (ref);
151      }
152    
153      /**
154       * Notifies the VM that a new Reference has been created.
155       */
156      private native void create (T o);
157    
158      /**
159       * Returns the object, this reference refers to.
160       * @return the object, this reference refers to, or null if the 
161       * reference was cleared.
162       */
163      public native T get();
164    
165      /**
166       * Clears the reference, so that it doesn't refer to its object
167       * anymore.  For soft and weak references this is called by the
168       * garbage collector.  For phantom references you should call 
169       * this when enqueuing the reference.
170       */
171      public void clear()
172      {
173        // Must synchronize so changes are visible in finalizer thread.
174        synchronized (lock)
175          {
176            referent = null;
177            cleared = true;
178          }
179      }
180    
181      /**
182       * Tells if the object is enqueued on a reference queue.
183       * @return true if it is enqueued, false otherwise.
184       */
185      public boolean isEnqueued()
186      {
187        return nextOnQueue != null;
188      }
189    
190      /**
191       * Enqueue an object on a reference queue.  This is normally executed
192       * by the garbage collector.
193       */
194      public boolean enqueue() 
195      {
196        if (queue != null && nextOnQueue == null)
197          {
198            queue.enqueue(this);
199            queue = null;
200            return true;
201          }
202        return false;
203      }
204    }