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author | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
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committer | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
commit | 554fd8c5195424bdbcabf5de30fdc183aba391bd (patch) | |
tree | 976dc5ab7fddf506dadce60ae936f43f58787092 /libjava/classpath/java/lang/ref/Reference.java | |
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Diffstat (limited to 'libjava/classpath/java/lang/ref/Reference.java')
-rw-r--r-- | libjava/classpath/java/lang/ref/Reference.java | 176 |
1 files changed, 176 insertions, 0 deletions
diff --git a/libjava/classpath/java/lang/ref/Reference.java b/libjava/classpath/java/lang/ref/Reference.java new file mode 100644 index 000000000..37cda6f02 --- /dev/null +++ b/libjava/classpath/java/lang/ref/Reference.java @@ -0,0 +1,176 @@ +/* java.lang.ref.Reference + Copyright (C) 1999, 2002, 2003, 2004, 2006 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<T> +{ + /** + * The underlying object. This field is handled in a special way by + * the garbage collector. + */ + T referent; + + /** + * The queue this reference is registered on. This is null, if this + * wasn't registered to any queue or reference was already enqueued. + */ + volatile ReferenceQueue<? super T> 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). + */ + volatile 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(T 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(T ref, ReferenceQueue<? super T> 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 T 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() + { + ReferenceQueue q = queue; + if (q != null) + { + return q.enqueue(this); + } + return false; + } +} |