1 /*
2 * Copyright (c) 1997, 2005, 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 extends ReferenceQueue {
44 boolean enqueue(Reference r) {
45 return false;
46 }
47 }
48
49 static ReferenceQueue NULL = new Null();
50 static ReferenceQueue ENQUEUED = new Null();
51
52 static private class Lock { };
53 private Lock lock = new Lock();
54 private volatile Reference<? extends T> head = null;
55 private long queueLength = 0;
56
57 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
58 synchronized (r) {
59 if (r.queue == ENQUEUED) return false;
60 synchronized (lock) {
61 // Check that since getting the lock this reference hasn't already been
62 // enqueued (and even then removed)
63 if (r.queue != this) {
64 return false;
65 }
66 r.queue = ENQUEUED;
67 r.next = (head == null) ? r : head;
68 head = r;
69 queueLength++;
70 if (r instanceof FinalReference) {
71 sun.misc.VM.addFinalRefCount(1);
72 }
73 lock.notifyAll();
74 return true;
75 }
76 }
77 }
78
79 private Reference<? extends T> reallyPoll() { /* Must hold lock */
80 if (head != null) {
81 Reference<? extends T> r = head;
82 head = (r.next == r) ? null : r.next;
83 r.queue = NULL;
84 r.next = r;
85 queueLength--;
86 if (r instanceof FinalReference) {
87 sun.misc.VM.addFinalRefCount(-1);
88 }
89 return r;
90 }
91 return null;
92 }
93
94 /**
95 * Polls this queue to see if a reference object is available. If one is
96 * available without further delay then it is removed from the queue and
97 * returned. Otherwise this method immediately returns <tt>null</tt>.
98 *
99 * @return A reference object, if one was immediately available,
100 * otherwise <code>null</code>
101 */
102 public Reference<? extends T> poll() {
103 if (head == null)
104 return null;
105 synchronized (lock) {
106 return reallyPoll();
107 }
108 }
109
110 /**
111 * Removes the next reference object in this queue, blocking until either
112 * one becomes available or the given timeout period expires.
113 *
114 * <p> This method does not offer real-time guarantees: It schedules the
115 * timeout as if by invoking the {@link Object#wait(long)} method.
116 *
117 * @param timeout If positive, block for up to <code>timeout</code>
118 * milliseconds while waiting for a reference to be
119 * added to this queue. If zero, block indefinitely.
120 *
121 * @return A reference object, if one was available within the specified
122 * timeout period, otherwise <code>null</code>
123 *
124 * @throws IllegalArgumentException
125 * If the value of the timeout argument is negative
126 *
127 * @throws InterruptedException
128 * If the timeout wait is interrupted
129 */
130 public Reference<? extends T> remove(long timeout)
131 throws IllegalArgumentException, InterruptedException
132 {
133 if (timeout < 0) {
134 throw new IllegalArgumentException("Negative timeout value");
135 }
136 synchronized (lock) {
137 Reference<? extends T> r = reallyPoll();
138 if (r != null) return r;
139 for (;;) {
140 lock.wait(timeout);
141 r = reallyPoll();
142 if (r != null) return r;
143 if (timeout != 0) return null;
144 }
145 }
146 }
147
148 /**
149 * Removes the next reference object in this queue, blocking until one
150 * becomes available.
151 *
152 * @return A reference object, blocking until one becomes available
153 * @throws InterruptedException If the wait is interrupted
154 */
155 public Reference<? extends T> remove() throws InterruptedException {
156 return remove(0);
157 }
158
159 }