Java tutorial
/* * 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. * * <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}. 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 */ private static 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 {@code null} 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}. * * @return The object to which this reference refers, or * {@code 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; } }