1 /*
2 * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang.ref;
27
28 import sun.misc.Cleaner;
29
30
31 /**
32 * Abstract base class for reference objects. This class defines the
33 * operations common to all reference objects. Because reference objects are
34 * implemented in close cooperation with the garbage collector, this class may
35 * not be subclassed directly.
36 *
37 * @author Mark Reinhold
38 * @since 1.2
39 */
40
41 public abstract class Reference<T> {
42
43 /* A Reference instance is in one of four possible internal states:
44 *
45 * Active: Subject to special treatment by the garbage collector. Some
46 * time after the collector detects that the reachability of the
47 * referent has changed to the appropriate state, it changes the
48 * instance's state to either Pending or Inactive, depending upon
49 * whether or not the instance was registered with a queue when it was
50 * created. In the former case it also adds the instance to the
51 * pending-Reference list. Newly-created instances are Active.
52 *
53 * Pending: An element of the pending-Reference list, waiting to be
54 * enqueued by the Reference-handler thread. Unregistered instances
55 * are never in this state.
56 *
57 * Enqueued: An element of the queue with which the instance was
58 * registered when it was created. When an instance is removed from
59 * its ReferenceQueue, it is made Inactive. Unregistered instances are
60 * never in this state.
61 *
62 * Inactive: Nothing more to do. Once an instance becomes Inactive its
63 * state will never change again.
64 *
65 * The state is encoded in the queue and next fields as follows:
66 *
67 * Active: queue = ReferenceQueue with which instance is registered, or
68 * ReferenceQueue.NULL if it was not registered with a queue; next =
69 * null.
70 *
71 * Pending: queue = ReferenceQueue with which instance is registered;
72 * next = Following instance in queue, or this if at end of list.
73 *
74 * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
75 * in queue, or this if at end of list.
76 *
77 * Inactive: queue = ReferenceQueue.NULL; next = this.
78 *
79 * With this scheme the collector need only examine the next field in order
80 * to determine whether a Reference instance requires special treatment: If
81 * the next field is null then the instance is active; if it is non-null,
82 * then the collector should treat the instance normally.
83 *
84 * To ensure that concurrent collector can discover active Reference
85 * objects without interfering with application threads that may apply
86 * the enqueue() method to those objects, collectors should link
87 * discovered objects through the discovered field.
88 */
89
90 private T referent; /* Treated specially by GC */
91
92 ReferenceQueue<? super T> queue;
93
94 Reference next;
95 transient private Reference<T> discovered; /* used by VM */
96
97
98 /* Object used to synchronize with the garbage collector. The collector
99 * must acquire this lock at the beginning of each collection cycle. It is
100 * therefore critical that any code holding this lock complete as quickly
101 * as possible, allocate no new objects, and avoid calling user code.
102 */
103 static private class Lock { };
104 private static Lock lock = new Lock();
105
106
107 /* List of References waiting to be enqueued. The collector adds
108 * References to this list, while the Reference-handler thread removes
109 * them. This list is protected by the above lock object.
110 */
111 private static Reference pending = null;
112
113 /* High-priority thread to enqueue pending References
114 */
115 private static class ReferenceHandler extends Thread {
116
117 ReferenceHandler(ThreadGroup g, String name) {
118 super(g, name);
119 }
120
121 public void run() {
122 for (;;) {
123
124 Reference r;
125 synchronized (lock) {
126 if (pending != null) {
127 r = pending;
128 Reference rn = r.next;
129 pending = (rn == r) ? null : rn;
130 r.next = r;
131 } else {
132 try {
133 lock.wait();
134 } catch (InterruptedException x) { }
135 continue;
136 }
137 }
138
139 // Fast path for cleaners
140 if (r instanceof Cleaner) {
141 ((Cleaner)r).clean();
142 continue;
143 }
144
145 ReferenceQueue q = r.queue;
146 if (q != ReferenceQueue.NULL) q.enqueue(r);
147 }
148 }
149 }
150
151 static {
152 ThreadGroup tg = Thread.currentThread().getThreadGroup();
153 for (ThreadGroup tgn = tg;
154 tgn != null;
155 tg = tgn, tgn = tg.getParent());
156 Thread handler = new ReferenceHandler(tg, "Reference Handler");
157 /* If there were a special system-only priority greater than
158 * MAX_PRIORITY, it would be used here
159 */
160 handler.setPriority(Thread.MAX_PRIORITY);
161 handler.setDaemon(true);
162 handler.start();
163 }
164
165
166 /* -- Referent accessor and setters -- */
167
168 /**
169 * Returns this reference object's referent. If this reference object has
170 * been cleared, either by the program or by the garbage collector, then
171 * this method returns <code>null</code>.
172 *
173 * @return The object to which this reference refers, or
174 * <code>null</code> if this reference object has been cleared
175 */
176 public T get() {
177 return this.referent;
178 }
179
180 /**
181 * Clears this reference object. Invoking this method will not cause this
182 * object to be enqueued.
183 *
184 * <p> This method is invoked only by Java code; when the garbage collector
185 * clears references it does so directly, without invoking this method.
186 */
187 public void clear() {
188 this.referent = null;
189 }
190
191
192 /* -- Queue operations -- */
193
194 /**
195 * Tells whether or not this reference object has been enqueued, either by
196 * the program or by the garbage collector. If this reference object was
197 * not registered with a queue when it was created, then this method will
198 * always return <code>false</code>.
199 *
200 * @return <code>true</code> if and only if this reference object has
201 * been enqueued
202 */
203 public boolean isEnqueued() {
204 /* In terms of the internal states, this predicate actually tests
205 whether the instance is either Pending or Enqueued */
206 synchronized (this) {
207 return (this.queue != ReferenceQueue.NULL) && (this.next != null);
208 }
209 }
210
211 /**
212 * Adds this reference object to the queue with which it is registered,
213 * if any.
214 *
215 * <p> This method is invoked only by Java code; when the garbage collector
216 * enqueues references it does so directly, without invoking this method.
217 *
218 * @return <code>true</code> if this reference object was successfully
219 * enqueued; <code>false</code> if it was already enqueued or if
220 * it was not registered with a queue when it was created
221 */
222 public boolean enqueue() {
223 return this.queue.enqueue(this);
224 }
225
226
227 /* -- Constructors -- */
228
229 Reference(T referent) {
230 this(referent, null);
231 }
232
233 Reference(T referent, ReferenceQueue<? super T> queue) {
234 this.referent = referent;
235 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
236 }
237
238 }