libjava/java/lang/ref/Reference.java

   1 /* java.lang.ref.Reference
   2    Copyright (C) 1999, 2002, 2003 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., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 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 base class of all references.  A reference allows
  43  * refering to an object without preventing the garbage collector to
  44  * collect it.  The only way to get the referred object is via the
  45  * <code>get()</code>-method.  This method will return
  46  * <code>null</code> if the object was collected. <br>
  47  *
  48  * A reference may be registered with a queue.  When a referred
  49  * element gets collected the reference will be put on the queue, so
  50  * that you will be notified. <br>
  51  *
  52  * There are currently three types of references:  soft reference,
  53  * weak reference and phantom reference. <br>
  54  *
  55  * Soft references will be cleared if the garbage collector is told
  56  * to free some memory and there are no unreferenced or weakly referenced
  57  * objects.  It is useful for caches. <br>
  58  *
  59  * Weak references will be cleared as soon as the garbage collector
  60  * determines that the refered object is only weakly reachable.  They
  61  * are useful as keys in hashtables (see <code>WeakHashtable</code>) as
  62  * you get notified when nobody has the key anymore.
  63  *
  64  * Phantom references don't prevent finalization.  If an object is only
  65  * phantom reachable, it will be finalized, and the reference will be
  66  * enqueued, but not cleared.  Since you mustn't access an finalized
  67  * object, the <code>get</code> method of a phantom reference will never
  68  * work.  It is useful to keep track, when an object is finalized.
  69  *
  70  * @author Jochen Hoenicke
  71  * @see java.util.WeakHashtable
  72  */
  73 public abstract class Reference
  74 {
  75   /**
  76    * The underlying object.  This field is handled in a special way by
  77    * the garbage collector.
  78    * GCJ LOCAL:
  79    * This is a RawData because it must be disguised from the GC.
  80    * END GCJ LOCAL
  81    */
  82   gnu.gcj.RawData referent;
  83
  84   /**
  85    * This is like REFERENT but is not scanned by the GC.  We keep a
  86    * copy around so that we can clean up our internal data structure
  87    * even after clear() is called.
  88    * GCJ LOCAL:
  89    * This field doesn't exist in Classpath.
  90    * END GCJ LOCAL
  91    */
  92   gnu.gcj.RawData copy;
  93
  94   /**
  95    * Set to true if {@link #clear()} is called.
  96    * GCJ LOCAL:
  97    * This field doesn't exist in Classpath.  It is used internally in
  98    * natReference.cc, which enqueues the reference unless it is true
  99    * (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 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(Object 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(Object ref, ReferenceQueue 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 (Object 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 Object get()
 164   {
 165     synchronized (lock)
 166       {
 167         return referent;
 168       }
 169   }
 170
 171   /**
 172    * Clears the reference, so that it doesn't refer to its object
 173    * anymore.  For soft and weak references this is called by the
 174    * garbage collector.  For phantom references you should call
 175    * this when enqueuing the reference.
 176    */
 177   public void clear()
 178   {
 179     // Must synchronize so changes are visible in finalizer thread.
 180     synchronized (lock)
 181       {
 182         referent = null;
 183         cleared = true;
 184       }
 185   }
 186
 187   /**
 188    * Tells if the object is enqueued on a reference queue.
 189    * @return true if it is enqueued, false otherwise.
 190    */
 191   public boolean isEnqueued()
 192   {
 193     return nextOnQueue != null;
 194   }
 195
 196   /**
 197    * Enqueue an object on a reference queue.  This is normally executed
 198    * by the garbage collector.
 199    */
 200   public boolean enqueue()
 201   {
 202     if (queue != null && nextOnQueue == null)
 203       {
 204         queue.enqueue(this);
 205         queue = null;
 206         return true;
 207       }
 208     return false;
 209   }
 210 }