jaroslav@601: /*
jaroslav@601: * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
jaroslav@601: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@601: *
jaroslav@601: * This code is free software; you can redistribute it and/or modify it
jaroslav@601: * under the terms of the GNU General Public License version 2 only, as
jaroslav@601: * published by the Free Software Foundation. Oracle designates this
jaroslav@601: * particular file as subject to the "Classpath" exception as provided
jaroslav@601: * by Oracle in the LICENSE file that accompanied this code.
jaroslav@601: *
jaroslav@601: * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@601: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@601: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
jaroslav@601: * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@601: * accompanied this code).
jaroslav@601: *
jaroslav@601: * You should have received a copy of the GNU General Public License version
jaroslav@601: * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@601: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@601: *
jaroslav@601: * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@601: * or visit www.oracle.com if you need additional information or have any
jaroslav@601: * questions.
jaroslav@601: */
jaroslav@601:
jaroslav@601: package java.lang.ref;
jaroslav@601:
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Soft reference objects, which are cleared at the discretion of the garbage
jaroslav@601: * collector in response to memory demand. Soft references are most often used
jaroslav@601: * to implement memory-sensitive caches.
jaroslav@601: *
jaroslav@601: * Suppose that the garbage collector determines at a certain point in time
jaroslav@601: * that an object is softly
jaroslav@601: * reachable. At that time it may choose to clear atomically all soft
jaroslav@601: * references to that object and all soft references to any other
jaroslav@601: * softly-reachable objects from which that object is reachable through a chain
jaroslav@601: * of strong references. At the same time or at some later time it will
jaroslav@601: * enqueue those newly-cleared soft references that are registered with
jaroslav@601: * reference queues.
jaroslav@601: *
jaroslav@601: *
All soft references to softly-reachable objects are guaranteed to have
jaroslav@601: * been cleared before the virtual machine throws an
jaroslav@601: * OutOfMemoryError. Otherwise no constraints are placed upon the
jaroslav@601: * time at which a soft reference will be cleared or the order in which a set
jaroslav@601: * of such references to different objects will be cleared. Virtual machine
jaroslav@601: * implementations are, however, encouraged to bias against clearing
jaroslav@601: * recently-created or recently-used soft references.
jaroslav@601: *
jaroslav@601: *
Direct instances of this class may be used to implement simple caches;
jaroslav@601: * this class or derived subclasses may also be used in larger data structures
jaroslav@601: * to implement more sophisticated caches. As long as the referent of a soft
jaroslav@601: * reference is strongly reachable, that is, is actually in use, the soft
jaroslav@601: * reference will not be cleared. Thus a sophisticated cache can, for example,
jaroslav@601: * prevent its most recently used entries from being discarded by keeping
jaroslav@601: * strong referents to those entries, leaving the remaining entries to be
jaroslav@601: * discarded at the discretion of the garbage collector.
jaroslav@601: *
jaroslav@601: * @author Mark Reinhold
jaroslav@601: * @since 1.2
jaroslav@601: */
jaroslav@601:
jaroslav@601: public class SoftReference extends Reference {
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Timestamp clock, updated by the garbage collector
jaroslav@601: */
jaroslav@601: static private long clock;
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Timestamp updated by each invocation of the get method. The VM may use
jaroslav@601: * this field when selecting soft references to be cleared, but it is not
jaroslav@601: * required to do so.
jaroslav@601: */
jaroslav@601: private long timestamp;
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Creates a new soft reference that refers to the given object. The new
jaroslav@601: * reference is not registered with any queue.
jaroslav@601: *
jaroslav@601: * @param referent object the new soft reference will refer to
jaroslav@601: */
jaroslav@601: public SoftReference(T referent) {
jaroslav@601: super(referent);
jaroslav@601: this.timestamp = clock;
jaroslav@601: }
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Creates a new soft reference that refers to the given object and is
jaroslav@601: * registered with the given queue.
jaroslav@601: *
jaroslav@601: * @param referent object the new soft reference will refer to
jaroslav@601: * @param q the queue with which the reference is to be registered,
jaroslav@601: * or null if registration is not required
jaroslav@601: *
jaroslav@601: */
jaroslav@601: public SoftReference(T referent, ReferenceQueue q) {
jaroslav@601: super(referent, q);
jaroslav@601: this.timestamp = clock;
jaroslav@601: }
jaroslav@601:
jaroslav@601: /**
jaroslav@601: * Returns this reference object's referent. If this reference object has
jaroslav@601: * been cleared, either by the program or by the garbage collector, then
jaroslav@601: * this method returns null.
jaroslav@601: *
jaroslav@601: * @return The object to which this reference refers, or
jaroslav@601: * null if this reference object has been cleared
jaroslav@601: */
jaroslav@601: public T get() {
jaroslav@601: T o = super.get();
jaroslav@601: if (o != null && this.timestamp != clock)
jaroslav@601: this.timestamp = clock;
jaroslav@601: return o;
jaroslav@601: }
jaroslav@601:
jaroslav@601: }