--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/java/lang/ref/SoftReference.java	Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,115 @@
+/*
+ * Copyright 1997-2003 Sun Microsystems, Inc.  All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun in the LICENSE file that accompanied this code.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+package java.lang.ref;
+
+
+/**
+ * Soft reference objects, which are cleared at the discretion of the garbage
+ * collector in response to memory demand.  Soft references are most often used
+ * to implement memory-sensitive caches.
+ *
+ * <p> Suppose that the garbage collector determines at a certain point in time
+ * that an object is <a href="package-summary.html#reachability">softly
+ * reachable</a>.  At that time it may choose to clear atomically all soft
+ * references to that object and all soft references to any other
+ * softly-reachable objects from which that object is reachable through a chain
+ * of strong references.  At the same time or at some later time it will
+ * enqueue those newly-cleared soft references that are registered with
+ * reference queues.
+ *
+ * <p> All soft references to softly-reachable objects are guaranteed to have
+ * been cleared before the virtual machine throws an
+ * <code>OutOfMemoryError</code>.  Otherwise no constraints are placed upon the
+ * time at which a soft reference will be cleared or the order in which a set
+ * of such references to different objects will be cleared.  Virtual machine
+ * implementations are, however, encouraged to bias against clearing
+ * recently-created or recently-used soft references.
+ *
+ * <p> Direct instances of this class may be used to implement simple caches;
+ * this class or derived subclasses may also be used in larger data structures
+ * to implement more sophisticated caches.  As long as the referent of a soft
+ * reference is strongly reachable, that is, is actually in use, the soft
+ * reference will not be cleared.  Thus a sophisticated cache can, for example,
+ * prevent its most recently used entries from being discarded by keeping
+ * strong referents to those entries, leaving the remaining entries to be
+ * discarded at the discretion of the garbage collector.
+ *
+ * @author   Mark Reinhold
+ * @since    1.2
+ */
+
+public class SoftReference<T> extends Reference<T> {
+
+    /* Timestamp clock, updated by the garbage collector
+     */
+    static private long clock;
+
+    /* Timestamp updated by each invocation of the get method.  The VM may use
+     * this field when selecting soft references to be cleared, but it is not
+     * required to do so.
+     */
+    private long timestamp;
+
+    /**
+     * Creates a new soft reference that refers to the given object.  The new
+     * reference is not registered with any queue.
+     *
+     * @param referent object the new soft reference will refer to
+     */
+    public SoftReference(T referent) {
+        super(referent);
+        this.timestamp = clock;
+    }
+
+    /**
+     * Creates a new soft reference that refers to the given object and is
+     * registered with the given queue.
+     *
+     * @param referent object the new soft reference will refer to
+     * @param q the queue with which the reference is to be registered,
+     *          or <tt>null</tt> if registration is not required
+     *
+     */
+    public SoftReference(T referent, ReferenceQueue<? super T> q) {
+        super(referent, q);
+        this.timestamp = clock;
+    }
+
+    /**
+     * Returns this reference object's referent.  If this reference object has
+     * been cleared, either by the program or by the garbage collector, then
+     * this method returns <code>null</code>.
+     *
+     * @return   The object to which this reference refers, or
+     *           <code>null</code> if this reference object has been cleared
+     */
+    public T get() {
+        T o = super.get();
+        if (o != null) this.timestamp = clock;
+        return o;
+    }
+
+}