diff -r 5198affdb915 -r d382dacfd73f rt/emul/compact/src/main/java/java/lang/ref/SoftReference.java --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/rt/emul/compact/src/main/java/java/lang/ref/SoftReference.java Tue Feb 26 16:54:16 2013 +0100 @@ -0,0 +1,118 @@ +/* + * Copyright (c) 1997, 2003, Oracle and/or its affiliates. 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. Oracle designates this + * particular file as subject to the "Classpath" exception as provided + * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA + * or visit www.oracle.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. + * + *

Suppose that the garbage collector determines at a certain point in time + * that an object is softly + * reachable. 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. + * + *

All soft references to softly-reachable objects are guaranteed to have + * been cleared before the virtual machine throws an + * OutOfMemoryError. 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. + * + *

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 extends Reference { + + /** + * 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 null if registration is not required + * + */ + public SoftReference(T referent, ReferenceQueue 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 null. + * + * @return The object to which this reference refers, or + * null if this reference object has been cleared + */ + public T get() { + T o = super.get(); + if (o != null && this.timestamp != clock) + this.timestamp = clock; + return o; + } + +}