1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/rt/emul/compact/src/main/java/java/lang/ref/SoftReference.java Tue Feb 26 16:54:16 2013 +0100
1.3 @@ -0,0 +1,118 @@
1.4 +/*
1.5 + * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
1.6 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.7 + *
1.8 + * This code is free software; you can redistribute it and/or modify it
1.9 + * under the terms of the GNU General Public License version 2 only, as
1.10 + * published by the Free Software Foundation. Oracle designates this
1.11 + * particular file as subject to the "Classpath" exception as provided
1.12 + * by Oracle in the LICENSE file that accompanied this code.
1.13 + *
1.14 + * This code is distributed in the hope that it will be useful, but WITHOUT
1.15 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
1.16 + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
1.17 + * version 2 for more details (a copy is included in the LICENSE file that
1.18 + * accompanied this code).
1.19 + *
1.20 + * You should have received a copy of the GNU General Public License version
1.21 + * 2 along with this work; if not, write to the Free Software Foundation,
1.22 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
1.23 + *
1.24 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
1.25 + * or visit www.oracle.com if you need additional information or have any
1.26 + * questions.
1.27 + */
1.28 +
1.29 +package java.lang.ref;
1.30 +
1.31 +
1.32 +/**
1.33 + * Soft reference objects, which are cleared at the discretion of the garbage
1.34 + * collector in response to memory demand. Soft references are most often used
1.35 + * to implement memory-sensitive caches.
1.36 + *
1.37 + * <p> Suppose that the garbage collector determines at a certain point in time
1.38 + * that an object is <a href="package-summary.html#reachability">softly
1.39 + * reachable</a>. At that time it may choose to clear atomically all soft
1.40 + * references to that object and all soft references to any other
1.41 + * softly-reachable objects from which that object is reachable through a chain
1.42 + * of strong references. At the same time or at some later time it will
1.43 + * enqueue those newly-cleared soft references that are registered with
1.44 + * reference queues.
1.45 + *
1.46 + * <p> All soft references to softly-reachable objects are guaranteed to have
1.47 + * been cleared before the virtual machine throws an
1.48 + * <code>OutOfMemoryError</code>. Otherwise no constraints are placed upon the
1.49 + * time at which a soft reference will be cleared or the order in which a set
1.50 + * of such references to different objects will be cleared. Virtual machine
1.51 + * implementations are, however, encouraged to bias against clearing
1.52 + * recently-created or recently-used soft references.
1.53 + *
1.54 + * <p> Direct instances of this class may be used to implement simple caches;
1.55 + * this class or derived subclasses may also be used in larger data structures
1.56 + * to implement more sophisticated caches. As long as the referent of a soft
1.57 + * reference is strongly reachable, that is, is actually in use, the soft
1.58 + * reference will not be cleared. Thus a sophisticated cache can, for example,
1.59 + * prevent its most recently used entries from being discarded by keeping
1.60 + * strong referents to those entries, leaving the remaining entries to be
1.61 + * discarded at the discretion of the garbage collector.
1.62 + *
1.63 + * @author Mark Reinhold
1.64 + * @since 1.2
1.65 + */
1.66 +
1.67 +public class SoftReference<T> extends Reference<T> {
1.68 +
1.69 + /**
1.70 + * Timestamp clock, updated by the garbage collector
1.71 + */
1.72 + static private long clock;
1.73 +
1.74 + /**
1.75 + * Timestamp updated by each invocation of the get method. The VM may use
1.76 + * this field when selecting soft references to be cleared, but it is not
1.77 + * required to do so.
1.78 + */
1.79 + private long timestamp;
1.80 +
1.81 + /**
1.82 + * Creates a new soft reference that refers to the given object. The new
1.83 + * reference is not registered with any queue.
1.84 + *
1.85 + * @param referent object the new soft reference will refer to
1.86 + */
1.87 + public SoftReference(T referent) {
1.88 + super(referent);
1.89 + this.timestamp = clock;
1.90 + }
1.91 +
1.92 + /**
1.93 + * Creates a new soft reference that refers to the given object and is
1.94 + * registered with the given queue.
1.95 + *
1.96 + * @param referent object the new soft reference will refer to
1.97 + * @param q the queue with which the reference is to be registered,
1.98 + * or <tt>null</tt> if registration is not required
1.99 + *
1.100 + */
1.101 + public SoftReference(T referent, ReferenceQueue<? super T> q) {
1.102 + super(referent, q);
1.103 + this.timestamp = clock;
1.104 + }
1.105 +
1.106 + /**
1.107 + * Returns this reference object's referent. If this reference object has
1.108 + * been cleared, either by the program or by the garbage collector, then
1.109 + * this method returns <code>null</code>.
1.110 + *
1.111 + * @return The object to which this reference refers, or
1.112 + * <code>null</code> if this reference object has been cleared
1.113 + */
1.114 + public T get() {
1.115 + T o = super.get();
1.116 + if (o != null && this.timestamp != clock)
1.117 + this.timestamp = clock;
1.118 + return o;
1.119 + }
1.120 +
1.121 +}