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 import static java.util.concurrent.TimeUnit.*;
29
30 /**
31 * Reference queues, to which registered reference objects are appended by the
32 * garbage collector after the appropriate reachability changes are detected.
33 *
34 * @author Mark Reinhold
35 * @since 1.2
36 */
37
38 public class ReferenceQueue<T> {
39
40 /**
41 * Constructs a new reference-object queue.
42 */
43 public ReferenceQueue() { }
44
45 private static class Null<S> extends ReferenceQueue<S> {
46 boolean enqueue(Reference<? extends S> r) {
47 return false;
48 }
49 }
50
51 static ReferenceQueue<Object> NULL = new Null<>();
52 static ReferenceQueue<Object> ENQUEUED = new Null<>();
53
54 static private class Lock { };
55 private Lock lock = new Lock();
56 private volatile Reference<? extends T> head = null;
57 private long queueLength = 0;
58
59 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
60 synchronized (lock) {
61 // Check that since getting the lock this reference hasn't already been
62 // enqueued (and even then removed)
63 ReferenceQueue<?> queue = r.queue;
64 if ((queue == NULL) || (queue == ENQUEUED)) {
65 return false;
66 }
67 assert queue == this;
68 r.queue = ENQUEUED;
69 r.next = (head == null) ? r : head;
70 head = r;
71 queueLength++;
72 if (r instanceof FinalReference) {
73 sun.misc.VM.addFinalRefCount(1);
74 }
75 lock.notifyAll();
76 return true;
77 }
78 }
79
80 @SuppressWarnings("unchecked")
81 private Reference<? extends T> reallyPoll() { /* Must hold lock */
82 Reference<? extends T> r = head;
83 if (r != null) {
84 head = (r.next == r) ?
85 null :
86 r.next; // Unchecked due to the next field having a raw type in Reference
87 r.queue = NULL;
88 r.next = r;
89 queueLength--;
90 if (r instanceof FinalReference) {
91 sun.misc.VM.addFinalRefCount(-1);
92 }
93 return r;
94 }
95 return null;
96 }
97
98 /**
99 * Polls this queue to see if a reference object is available. If one is
100 * available without further delay then it is removed from the queue and
101 * returned. Otherwise this method immediately returns <tt>null</tt>.
102 *
103 * @return A reference object, if one was immediately available,
104 * otherwise <code>null</code>
105 */
106 public Reference<? extends T> poll() {
107 if (head == null)
108 return null;
109 synchronized (lock) {
110 return reallyPoll();
111 }
112 }
113
114 /**
115 * Removes the next reference object in this queue, blocking until either
116 * one becomes available or the given timeout period expires.
117 *
118 * <p> This method does not offer real-time guarantees: It schedules the
119 * timeout as if by invoking the {@link Object#wait(long)} method.
120 *
121 * @param timeout If positive, block for up to <code>timeout</code>
122 * milliseconds while waiting for a reference to be
123 * added to this queue. If zero, block indefinitely.
124 *
125 * @return A reference object, if one was available within the specified
126 * timeout period, otherwise <code>null</code>
127 *
128 * @throws IllegalArgumentException
129 * If the value of the timeout argument is negative
130 *
131 * @throws InterruptedException
132 * If the timeout wait is interrupted
133 */
134 public Reference<? extends T> remove(long timeout)
135 throws IllegalArgumentException, InterruptedException
136 {
137 if (timeout < 0) {
138 throw new IllegalArgumentException("Negative timeout value");
139 }
140 synchronized (lock) {
141 Reference<? extends T> r = reallyPoll();
142 if (r != null) return r;
143 long start = (timeout == 0) ? 0 : System.nanoTime();
144 for (;;) {
145 lock.wait(timeout);
146 r = reallyPoll();
147 if (r != null) return r;
148 if (timeout != 0) {
149 long end = System.nanoTime();
150 timeout -= NANOSECONDS.toMillis(end - start);
151 if (timeout <= 0) return null;
152 start = end;
153 }
154 }
155 }
156 }
157
158 /**
159 * Removes the next reference object in this queue, blocking until one
160 * becomes available.
161 *
162 * @return A reference object, blocking until one becomes available
163 * @throws InterruptedException If the wait is interrupted
164 */
165 public Reference<? extends T> remove() throws InterruptedException {
166 return remove(0);
167 }
168
169 }