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