ReferenceQueue.java revision 12745:f068a4ffddd2 
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
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24 */
25
26package java.lang.ref;
27
28import java.util.function.Consumer;
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
38public 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    private static 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.next = (head == null) ? r : head;
69            head = r;
70            queueLength++;
71            // Update r.queue *after* adding to list, to avoid race
72            // with concurrent enqueued checks and fast-path poll().
73            // Volatiles ensure ordering.
74            r.queue = ENQUEUED;
75            if (r instanceof FinalReference) {
76                sun.misc.VM.addFinalRefCount(1);
77            }
78            lock.notifyAll();
79            return true;
80        }
81    }
82
83    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
84        Reference<? extends T> r = head;
85        if (r != null) {
86            r.queue = NULL;
87            // Update r.queue *before* removing from list, to avoid
88            // race with concurrent enqueued checks and fast-path
89            // poll().  Volatiles ensure ordering.
90            @SuppressWarnings("unchecked")
91            Reference<? extends T> rn = r.next;
92            head = (rn == r) ? null : rn;
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 {@code null}.
107     *
108     * @return  A reference object, if one was immediately available,
109     *          otherwise {@code null}
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}
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}
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    /**
175     * Iterate queue and invoke given action with each Reference.
176     * Suitable for diagnostic purposes.
177     * WARNING: any use of this method should make sure to not
178     * retain the referents of iterated references (in case of
179     * FinalReference(s)) so that their life is not prolonged more
180     * than necessary.
181     */
182    void forEach(Consumer<? super Reference<? extends T>> action) {
183        for (Reference<? extends T> r = head; r != null;) {
184            action.accept(r);
185            @SuppressWarnings("unchecked")
186            Reference<? extends T> rn = r.next;
187            if (rn == r) {
188                if (r.queue == ENQUEUED) {
189                    // still enqueued -> we reached end of chain
190                    r = null;
191                } else {
192                    // already dequeued: r.queue == NULL; ->
193                    // restart from head when overtaken by queue poller(s)
194                    r = head;
195                }
196            } else {
197                // next in chain
198                r = rn;
199            }
200        }
201    }
202}
203