1.1 --- a/emul/compact/src/main/java/java/lang/ref/Reference.java Mon Feb 25 19:00:08 2013 +0100
1.2 +++ /dev/null Thu Jan 01 00:00:00 1970 +0000
1.3 @@ -1,185 +0,0 @@
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 -}