1 /*
2 * Copyright (c) 1997, 2013, 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 /**
29 * Reference queues, to which registered reference objects are appended by the
30 * garbage collector after the appropriate reachability changes are detected.
31 *
32 * @author Mark Reinhold
33 * @since 1.2
34 */
35
36 public class ReferenceQueue<T> {
37
38 /**
39 * Constructs a new reference-object queue.
40 */
41 public ReferenceQueue() { }
42
43 private static class Null<S> extends ReferenceQueue<S> {
44 boolean enqueue(Reference<? extends S> r) {
45 return false;
46 }
47 }
48
49 static final ReferenceQueue<Object> NULL = new Null<>();
50 static final ReferenceQueue<Object> ENQUEUED = new Null<>();
51
52 static private class Lock { }
53 private final Lock lock = new Lock();
54 private volatile int waiters;
55
56 @SuppressWarnings("unused")
57 private volatile Reference<? extends T> head; // we assign using Unsafe CAS
58
59 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
60 if (markEnqueued(r)) { // markEnqueued is atomic
61 addChunk(r, r);
62 return true;
63 } else {
64 return false;
65 }
66 }
67
68 boolean markEnqueued(Reference<?> r) {
69 ReferenceQueue<?> queue;
70 do {
71 // Check that this reference hasn't already been
72 // enqueued (and even then removed)
73 queue = r.queue;
74 if ((queue == NULL) || (queue == ENQUEUED)) {
75 return false;
76 }
77 } while (!r.casQueue(queue, ENQUEUED));
78 assert queue == this;
79 return true;
80 }
81
82 @SuppressWarnings("unchecked")
83 void addChunk(Reference<?> chunkHead, Reference<?> chunkTail) {
84 Reference<? extends T> h;
85 do {
86 h = head;
87 chunkTail.next = (h == null) ? chunkTail : h;
88 } while (!casHead(h, (Reference) chunkHead));
89 // notify waiters
90 if (waiters > 0) {
91 synchronized (lock) {
92 if (waiters > 0) {
93 lock.notifyAll();
94 }
95 }
96 }
97 }
98
99 private Reference<? extends T> reallyPoll() {
100 Reference<? extends T> r;
101 while ((r = head) != null) {
102 @SuppressWarnings("unchecked") // due to cast to raw type
103 Reference<? extends T> nh = (r.next == r)
104 ? null : (Reference) r.next;
105 if (casHead(r, nh)) {
106 r.lazySetQueue(NULL);
107 UNSAFE.storeFence();
108 r.next = r;
109 break;
110 }
111 }
112 return r;
113 }
114
115 /**
116 * Polls this queue to see if a reference object is available. If one is
117 * available without further delay then it is removed from the queue and
118 * returned. Otherwise this method immediately returns <tt>null</tt>.
119 *
120 * @return A reference object, if one was immediately available,
121 * otherwise <code>null</code>
122 */
123 public Reference<? extends T> poll() {
124 return reallyPoll();
125 }
126
127 /**
128 * Removes the next reference object in this queue, blocking until either
129 * one becomes available or the given timeout period expires.
130 *
131 * <p> This method does not offer real-time guarantees: It schedules the
132 * timeout as if by invoking the {@link Object#wait(long)} method.
133 *
134 * @param timeout If positive, block for up to <code>timeout</code>
135 * milliseconds while waiting for a reference to be
136 * added to this queue. If zero, block indefinitely.
137 *
138 * @return A reference object, if one was available within the specified
139 * timeout period, otherwise <code>null</code>
140 *
141 * @throws IllegalArgumentException
142 * If the value of the timeout argument is negative
143 *
144 * @throws InterruptedException
145 * If the timeout wait is interrupted
146 */
147 public Reference<? extends T> remove(long timeout)
148 throws IllegalArgumentException, InterruptedException
149 {
150 if (timeout < 0) {
151 throw new IllegalArgumentException("Negative timeout value");
152 }
153 Reference<? extends T> r = reallyPoll();
154 if (r != null) return r;
155 return reallyRemove(timeout);
156 }
157
158 private Reference<? extends T> reallyRemove(long timeout) throws InterruptedException {
159 long deadline = (timeout == 0)
160 ? 0 : System.nanoTime() + timeout * 1000_000L;
161
162 synchronized (lock) {
163 ++waiters;
164 try {
165 for (; ; ) {
166 Reference<? extends T> r = reallyPoll();
167 if (r != null) return r;
168 if (timeout == 0) {
169 lock.wait(0);
170 } else {
171 long timeoutNanos = deadline - System.nanoTime();
172 if (timeoutNanos <= 0) return null;
173 lock.wait(timeoutNanos / 1000_000L, (int) (timeoutNanos % 1000_000L));
174 }
175 }
176 } finally {
177 --waiters;
178 }
179 }
180 }
181
182 /**
183 * Removes the next reference object in this queue, blocking until one
184 * becomes available.
185 *
186 * @return A reference object, blocking until one becomes available
187 * @throws InterruptedException If the wait is interrupted
188 */
189 public Reference<? extends T> remove() throws InterruptedException {
190 return remove(0);
191 }
192
193 // Unsafe machinery
194
195 private boolean casHead(Reference<?> cmp, Reference<? extends T> val) {
196 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
197 }
198
199 private static final sun.misc.Unsafe UNSAFE;
200 private static final long headOffset;
201
202 static {
203 try {
204 UNSAFE = sun.misc.Unsafe.getUnsafe();
205 Class<ReferenceQueue> rqc = ReferenceQueue.class;
206 headOffset = UNSAFE.objectFieldOffset(rqc.getDeclaredField("head"));
207 } catch (Exception e) {
208 throw new Error(e);
209 }
210 }
211 }