1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/rt/emul/compact/src/main/java/java/lang/ref/Reference.java Tue Feb 26 16:54:16 2013 +0100
1.3 @@ -0,0 +1,185 @@
1.4 +/*
1.5 + * Copyright (c) 1997, 2006, 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 + * Abstract base class for reference objects. This class defines the
1.34 + * operations common to all reference objects. Because reference objects are
1.35 + * implemented in close cooperation with the garbage collector, this class may
1.36 + * not be subclassed directly.
1.37 + *
1.38 + * @author Mark Reinhold
1.39 + * @since 1.2
1.40 + */
1.41 +
1.42 +public abstract class Reference<T> {
1.43 +
1.44 + /* A Reference instance is in one of four possible internal states:
1.45 + *
1.46 + * Active: Subject to special treatment by the garbage collector. Some
1.47 + * time after the collector detects that the reachability of the
1.48 + * referent has changed to the appropriate state, it changes the
1.49 + * instance's state to either Pending or Inactive, depending upon
1.50 + * whether or not the instance was registered with a queue when it was
1.51 + * created. In the former case it also adds the instance to the
1.52 + * pending-Reference list. Newly-created instances are Active.
1.53 + *
1.54 + * Pending: An element of the pending-Reference list, waiting to be
1.55 + * enqueued by the Reference-handler thread. Unregistered instances
1.56 + * are never in this state.
1.57 + *
1.58 + * Enqueued: An element of the queue with which the instance was
1.59 + * registered when it was created. When an instance is removed from
1.60 + * its ReferenceQueue, it is made Inactive. Unregistered instances are
1.61 + * never in this state.
1.62 + *
1.63 + * Inactive: Nothing more to do. Once an instance becomes Inactive its
1.64 + * state will never change again.
1.65 + *
1.66 + * The state is encoded in the queue and next fields as follows:
1.67 + *
1.68 + * Active: queue = ReferenceQueue with which instance is registered, or
1.69 + * ReferenceQueue.NULL if it was not registered with a queue; next =
1.70 + * null.
1.71 + *
1.72 + * Pending: queue = ReferenceQueue with which instance is registered;
1.73 + * next = Following instance in queue, or this if at end of list.
1.74 + *
1.75 + * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
1.76 + * in queue, or this if at end of list.
1.77 + *
1.78 + * Inactive: queue = ReferenceQueue.NULL; next = this.
1.79 + *
1.80 + * With this scheme the collector need only examine the next field in order
1.81 + * to determine whether a Reference instance requires special treatment: If
1.82 + * the next field is null then the instance is active; if it is non-null,
1.83 + * then the collector should treat the instance normally.
1.84 + *
1.85 + * To ensure that concurrent collector can discover active Reference
1.86 + * objects without interfering with application threads that may apply
1.87 + * the enqueue() method to those objects, collectors should link
1.88 + * discovered objects through the discovered field.
1.89 + */
1.90 +
1.91 + private T referent; /* Treated specially by GC */
1.92 +
1.93 + ReferenceQueue<? super T> queue;
1.94 +
1.95 + Reference next;
1.96 + transient private Reference<T> discovered; /* used by VM */
1.97 +
1.98 +
1.99 + /* Object used to synchronize with the garbage collector. The collector
1.100 + * must acquire this lock at the beginning of each collection cycle. It is
1.101 + * therefore critical that any code holding this lock complete as quickly
1.102 + * as possible, allocate no new objects, and avoid calling user code.
1.103 + */
1.104 + static private class Lock { };
1.105 + private static Lock lock = new Lock();
1.106 +
1.107 +
1.108 + /* List of References waiting to be enqueued. The collector adds
1.109 + * References to this list, while the Reference-handler thread removes
1.110 + * them. This list is protected by the above lock object.
1.111 + */
1.112 + private static Reference pending = null;
1.113 +
1.114 +
1.115 +
1.116 + /* -- Referent accessor and setters -- */
1.117 +
1.118 + /**
1.119 + * Returns this reference object's referent. If this reference object has
1.120 + * been cleared, either by the program or by the garbage collector, then
1.121 + * this method returns <code>null</code>.
1.122 + *
1.123 + * @return The object to which this reference refers, or
1.124 + * <code>null</code> if this reference object has been cleared
1.125 + */
1.126 + public T get() {
1.127 + return this.referent;
1.128 + }
1.129 +
1.130 + /**
1.131 + * Clears this reference object. Invoking this method will not cause this
1.132 + * object to be enqueued.
1.133 + *
1.134 + * <p> This method is invoked only by Java code; when the garbage collector
1.135 + * clears references it does so directly, without invoking this method.
1.136 + */
1.137 + public void clear() {
1.138 + this.referent = null;
1.139 + }
1.140 +
1.141 +
1.142 + /* -- Queue operations -- */
1.143 +
1.144 + /**
1.145 + * Tells whether or not this reference object has been enqueued, either by
1.146 + * the program or by the garbage collector. If this reference object was
1.147 + * not registered with a queue when it was created, then this method will
1.148 + * always return <code>false</code>.
1.149 + *
1.150 + * @return <code>true</code> if and only if this reference object has
1.151 + * been enqueued
1.152 + */
1.153 + public boolean isEnqueued() {
1.154 + /* In terms of the internal states, this predicate actually tests
1.155 + whether the instance is either Pending or Enqueued */
1.156 + synchronized (this) {
1.157 + return (this.queue != ReferenceQueue.NULL) && (this.next != null);
1.158 + }
1.159 + }
1.160 +
1.161 + /**
1.162 + * Adds this reference object to the queue with which it is registered,
1.163 + * if any.
1.164 + *
1.165 + * <p> This method is invoked only by Java code; when the garbage collector
1.166 + * enqueues references it does so directly, without invoking this method.
1.167 + *
1.168 + * @return <code>true</code> if this reference object was successfully
1.169 + * enqueued; <code>false</code> if it was already enqueued or if
1.170 + * it was not registered with a queue when it was created
1.171 + */
1.172 + public boolean enqueue() {
1.173 + return this.queue.enqueue(this);
1.174 + }
1.175 +
1.176 +
1.177 + /* -- Constructors -- */
1.178 +
1.179 + Reference(T referent) {
1.180 + this(referent, null);
1.181 + }
1.182 +
1.183 + Reference(T referent, ReferenceQueue<? super T> queue) {
1.184 + this.referent = referent;
1.185 + this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
1.186 + }
1.187 +
1.188 +}