1 /*
2 * Copyright (c) 1997, 2015, 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 ReferenceQueue<Object> NULL = new Null<>();
50 static ReferenceQueue<Object> 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.next = (head == null) ? r : head;
67 head = r;
68 queueLength++;
69 // Assignment of queue to special ENQUEUED queue must be
70 // after the reference is added to the queue's list.
71 // Otherwise, there is a race condition where the
72 // reference appears to be enqueued (isEnqueue returns
73 // true, another enqueue returns false), but polling the
74 // queue could find it still empty. The q.head and
75 // r.queue fields are volatile, ensuring that order.
76 r.queue = ENQUEUED;
77 if (r instanceof FinalReference) {
78 sun.misc.VM.addFinalRefCount(1);
79 }
80 lock.notifyAll();
81 return true;
82 }
83 }
84
85 @SuppressWarnings("unchecked")
86 private Reference<? extends T> reallyPoll() { /* Must hold lock */
87 Reference<? extends T> r = head;
88 if (r != null) {
89 head = (r.next == r) ?
90 null :
91 r.next; // Unchecked due to the next field having a raw type in Reference
92 r.queue = NULL;
93 r.next = r;
94 queueLength--;
95 if (r instanceof FinalReference) {
96 sun.misc.VM.addFinalRefCount(-1);
97 }
98 return r;
99 }
100 return null;
101 }
102
103 /**
104 * Polls this queue to see if a reference object is available. If one is
105 * available without further delay then it is removed from the queue and
106 * returned. Otherwise this method immediately returns <tt>null</tt>.
107 *
108 * @return A reference object, if one was immediately available,
109 * otherwise <code>null</code>
110 */
111 public Reference<? extends T> poll() {
112 if (head == null)
113 return null;
114 synchronized (lock) {
115 return reallyPoll();
116 }
117 }
118
119 /**
120 * Removes the next reference object in this queue, blocking until either
121 * one becomes available or the given timeout period expires.
122 *
123 * <p> This method does not offer real-time guarantees: It schedules the
124 * timeout as if by invoking the {@link Object#wait(long)} method.
125 *
126 * @param timeout If positive, block for up to <code>timeout</code>
127 * milliseconds while waiting for a reference to be
128 * added to this queue. If zero, block indefinitely.
129 *
130 * @return A reference object, if one was available within the specified
131 * timeout period, otherwise <code>null</code>
132 *
133 * @throws IllegalArgumentException
134 * If the value of the timeout argument is negative
135 *
136 * @throws InterruptedException
137 * If the timeout wait is interrupted
138 */
139 public Reference<? extends T> remove(long timeout)
140 throws IllegalArgumentException, InterruptedException
141 {
142 if (timeout < 0) {
143 throw new IllegalArgumentException("Negative timeout value");
144 }
145 synchronized (lock) {
146 Reference<? extends T> r = reallyPoll();
147 if (r != null) return r;
148 long start = (timeout == 0) ? 0 : System.nanoTime();
149 for (;;) {
150 lock.wait(timeout);
151 r = reallyPoll();
152 if (r != null) return r;
153 if (timeout != 0) {
154 long end = System.nanoTime();
155 timeout -= (end - start) / 1000_000;
156 if (timeout <= 0) return null;
157 start = end;
158 }
159 }
160 }
161 }
162
163 /**
164 * Removes the next reference object in this queue, blocking until one
165 * becomes available.
166 *
167 * @return A reference object, blocking until one becomes available
168 * @throws InterruptedException If the wait is interrupted
169 */
170 public Reference<? extends T> remove() throws InterruptedException {
171 return remove(0);
172 }
173
174 }