1 /*
2 * Copyright (c) 1997, 2018, 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 java.util.function.Consumer;
29 import jdk.internal.misc.VM;
30
31 /**
32 * Reference queues, to which registered reference objects are appended by the
33 * garbage collector after the appropriate reachability changes are detected.
34 *
35 * @author Mark Reinhold
36 * @since 1.2
37 */
38
39 public class ReferenceQueue<T> {
40
41 /**
42 * Constructs a new reference-object queue.
43 */
44 public ReferenceQueue() { }
45
46 private static class Null<S> extends ReferenceQueue<S> {
47 boolean enqueue(Reference<? extends S> r) {
48 return false;
49 }
50 }
51
52 static ReferenceQueue<Object> NULL = new Null<>();
53 static ReferenceQueue<Object> ENQUEUED = new Null<>();
54
55 private static class Lock { };
56 private Lock lock = new Lock();
57 private volatile Reference<? extends T> head;
58 private long queueLength = 0;
59
60 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
61 synchronized (lock) {
62 // Check that since getting the lock this reference hasn't already been
63 // enqueued (and even then removed)
64 ReferenceQueue<?> queue = r.queue;
65 if ((queue == NULL) || (queue == ENQUEUED)) {
66 return false;
67 }
68 assert queue == this;
69 // Self-loop end, so if a FinalReference it remains inactive.
70 r.next = (head == null) ? r : head;
71 head = r;
72 queueLength++;
73 // Update r.queue *after* adding to list, to avoid race
74 // with concurrent enqueued checks and fast-path poll().
75 // Volatiles ensure ordering.
76 r.queue = ENQUEUED;
77 if (r instanceof FinalReference) {
78 VM.addFinalRefCount(1);
79 }
80 lock.notifyAll();
81 return true;
82 }
83 }
84
85 private Reference<? extends T> reallyPoll() { /* Must hold lock */
86 Reference<? extends T> r = head;
87 if (r != null) {
88 r.queue = NULL;
89 // Update r.queue *before* removing from list, to avoid
90 // race with concurrent enqueued checks and fast-path
91 // poll(). Volatiles ensure ordering.
92 @SuppressWarnings("unchecked")
93 Reference<? extends T> rn = r.next;
94 // Handle self-looped next as end of list designator.
95 head = (rn == r) ? null : rn;
96 // Self-loop next rather than setting to null, so if a
97 // FinalReference it remains inactive.
98 r.next = r;
99 queueLength--;
100 if (r instanceof FinalReference) {
101 VM.addFinalRefCount(-1);
102 }
103 return r;
104 }
105 return null;
106 }
107
108 /**
109 * Polls this queue to see if a reference object is available. If one is
110 * available without further delay then it is removed from the queue and
111 * returned. Otherwise this method immediately returns {@code null}.
112 *
113 * @return A reference object, if one was immediately available,
114 * otherwise {@code null}
115 */
116 public Reference<? extends T> poll() {
117 if (head == null)
118 return null;
119 synchronized (lock) {
120 return reallyPoll();
121 }
122 }
123
124 /**
125 * Removes the next reference object in this queue, blocking until either
126 * one becomes available or the given timeout period expires.
127 *
128 * <p> This method does not offer real-time guarantees: It schedules the
129 * timeout as if by invoking the {@link Object#wait(long)} method.
130 *
131 * @param timeout If positive, block for up to {@code timeout}
132 * milliseconds while waiting for a reference to be
133 * added to this queue. If zero, block indefinitely.
134 *
135 * @return A reference object, if one was available within the specified
136 * timeout period, otherwise {@code null}
137 *
138 * @throws IllegalArgumentException
139 * If the value of the timeout argument is negative
140 *
141 * @throws InterruptedException
142 * If the timeout wait is interrupted
143 */
144 public Reference<? extends T> remove(long timeout)
145 throws IllegalArgumentException, InterruptedException
146 {
147 if (timeout < 0) {
148 throw new IllegalArgumentException("Negative timeout value");
149 }
150 synchronized (lock) {
151 Reference<? extends T> r = reallyPoll();
152 if (r != null) return r;
153 long start = (timeout == 0) ? 0 : System.nanoTime();
154 for (;;) {
155 lock.wait(timeout);
156 r = reallyPoll();
157 if (r != null) return r;
158 if (timeout != 0) {
159 long end = System.nanoTime();
160 timeout -= (end - start) / 1000_000;
161 if (timeout <= 0) return null;
162 start = end;
163 }
164 }
165 }
166 }
167
168 /**
169 * Removes the next reference object in this queue, blocking until one
170 * becomes available.
171 *
172 * @return A reference object, blocking until one becomes available
173 * @throws InterruptedException If the wait is interrupted
174 */
175 public Reference<? extends T> remove() throws InterruptedException {
176 return remove(0);
177 }
178
179 /**
180 * Iterate queue and invoke given action with each Reference.
181 * Suitable for diagnostic purposes.
182 * WARNING: any use of this method should make sure to not
183 * retain the referents of iterated references (in case of
184 * FinalReference(s)) so that their life is not prolonged more
185 * than necessary.
186 */
187 void forEach(Consumer<? super Reference<? extends T>> action) {
188 for (Reference<? extends T> r = head; r != null;) {
189 action.accept(r);
190 @SuppressWarnings("unchecked")
191 Reference<? extends T> rn = r.next;
192 if (rn == r) {
193 if (r.queue == ENQUEUED) {
194 // still enqueued -> we reached end of chain
195 r = null;
196 } else {
197 // already dequeued: r.queue == NULL; ->
198 // restart from head when overtaken by queue poller(s)
199 r = head;
200 }
201 } else {
202 // next in chain
203 r = rn;
204 }
205 }
206 }
207 }