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