| /* java.lang.ref.Reference |
| Copyright (C) 1999, 2002, 2003 Free Software Foundation, Inc. |
| |
| This file is part of GNU Classpath. |
| |
| GNU Classpath is free software; you can redistribute it and/or modify |
| it under the terms of the GNU General Public License as published by |
| the Free Software Foundation; either version 2, or (at your option) |
| any later version. |
| |
| GNU Classpath is distributed in the hope that it will be useful, but |
| WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| General Public License for more details. |
| |
| You should have received a copy of the GNU General Public License |
| along with GNU Classpath; see the file COPYING. If not, write to the |
| Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA |
| 02110-1301 USA. |
| |
| Linking this library statically or dynamically with other modules is |
| making a combined work based on this library. Thus, the terms and |
| conditions of the GNU General Public License cover the whole |
| combination. |
| |
| As a special exception, the copyright holders of this library give you |
| permission to link this library with independent modules to produce an |
| executable, regardless of the license terms of these independent |
| modules, and to copy and distribute the resulting executable under |
| terms of your choice, provided that you also meet, for each linked |
| independent module, the terms and conditions of the license of that |
| module. An independent module is a module which is not derived from |
| or based on this library. If you modify this library, you may extend |
| this exception to your version of the library, but you are not |
| obligated to do so. If you do not wish to do so, delete this |
| exception statement from your version. */ |
| |
| |
| package java.lang.ref; |
| |
| /** |
| * This is the base class of all references. A reference allows |
| * refering to an object without preventing the garbage collector to |
| * collect it. The only way to get the referred object is via the |
| * <code>get()</code>-method. This method will return |
| * <code>null</code> if the object was collected. <br> |
| * |
| * A reference may be registered with a queue. When a referred |
| * element gets collected the reference will be put on the queue, so |
| * that you will be notified. <br> |
| * |
| * There are currently three types of references: soft reference, |
| * weak reference and phantom reference. <br> |
| * |
| * Soft references will be cleared if the garbage collector is told |
| * to free some memory and there are no unreferenced or weakly referenced |
| * objects. It is useful for caches. <br> |
| * |
| * Weak references will be cleared as soon as the garbage collector |
| * determines that the refered object is only weakly reachable. They |
| * are useful as keys in hashtables (see <code>WeakHashtable</code>) as |
| * you get notified when nobody has the key anymore. |
| * |
| * Phantom references don't prevent finalization. If an object is only |
| * phantom reachable, it will be finalized, and the reference will be |
| * enqueued, but not cleared. Since you mustn't access an finalized |
| * object, the <code>get</code> method of a phantom reference will never |
| * work. It is useful to keep track, when an object is finalized. |
| * |
| * @author Jochen Hoenicke |
| * @see java.util.WeakHashMap |
| */ |
| public abstract class Reference |
| { |
| /** |
| * The underlying object. This field is handled in a special way by |
| * the garbage collector. |
| */ |
| Object referent; |
| |
| /** |
| * The queue this reference is registered on. This is null, if this |
| * wasn't registered to any queue or reference was already enqueued. |
| */ |
| ReferenceQueue queue; |
| |
| /** |
| * Link to the next entry on the queue. If this is null, this |
| * reference is not enqueued. Otherwise it points to the next |
| * reference. The last reference on a queue will point to itself |
| * (not to null, that value is used to mark a not enqueued |
| * reference). |
| */ |
| Reference nextOnQueue; |
| |
| /** |
| * This lock should be taken by the garbage collector, before |
| * determining reachability. It will prevent the get()-method to |
| * return the reference so that reachability doesn't change. |
| */ |
| static Object lock = new Object(); |
| |
| /** |
| * Creates a new reference that is not registered to any queue. |
| * Since it is package private, it is not possible to overload this |
| * class in a different package. |
| * @param ref the object we refer to. |
| */ |
| Reference(Object ref) |
| { |
| referent = ref; |
| } |
| |
| /** |
| * Creates a reference that is registered to a queue. Since this is |
| * package private, it is not possible to overload this class in a |
| * different package. |
| * @param ref the object we refer to. |
| * @param q the reference queue to register on. |
| * @exception NullPointerException if q is null. |
| */ |
| Reference(Object ref, ReferenceQueue q) |
| { |
| if (q == null) |
| throw new NullPointerException(); |
| referent = ref; |
| queue = q; |
| } |
| |
| /** |
| * Returns the object, this reference refers to. |
| * @return the object, this reference refers to, or null if the |
| * reference was cleared. |
| */ |
| public Object get() |
| { |
| synchronized (lock) |
| { |
| return referent; |
| } |
| } |
| |
| /** |
| * Clears the reference, so that it doesn't refer to its object |
| * anymore. For soft and weak references this is called by the |
| * garbage collector. For phantom references you should call |
| * this when enqueuing the reference. |
| */ |
| public void clear() |
| { |
| referent = null; |
| } |
| |
| /** |
| * Tells if the object is enqueued on a reference queue. |
| * @return true if it is enqueued, false otherwise. |
| */ |
| public boolean isEnqueued() |
| { |
| return nextOnQueue != null; |
| } |
| |
| /** |
| * Enqueue an object on a reference queue. This is normally executed |
| * by the garbage collector. |
| */ |
| public boolean enqueue() |
| { |
| if (queue != null && nextOnQueue == null) |
| { |
| queue.enqueue(this); |
| queue = null; |
| return true; |
| } |
| return false; |
| } |
| } |