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
26package java.lang.ref;
27
28import java.util.function.Consumer;
29import 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
39public 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            r.next = (head == null) ? r : head;
70            head = r;
71            queueLength++;
72            // Update r.queue *after* adding to list, to avoid race
73            // with concurrent enqueued checks and fast-path poll().
74            // Volatiles ensure ordering.
75            r.queue = ENQUEUED;
76            if (r instanceof FinalReference) {
77                VM.addFinalRefCount(1);
78            }
79            lock.notifyAll();
80            return true;
81        }
82    }
83
84    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
85        Reference<? extends T> r = head;
86        if (r != null) {
87            r.queue = NULL;
88            // Update r.queue *before* removing from list, to avoid
89            // race with concurrent enqueued checks and fast-path
90            // poll().  Volatiles ensure ordering.
91            @SuppressWarnings("unchecked")
92            Reference<? extends T> rn = r.next;
93            head = (rn == r) ? null : rn;
94            r.next = r;
95            queueLength--;
96            if (r instanceof FinalReference) {
97                VM.addFinalRefCount(-1);
98            }
99            return r;
100        }
101        return null;
102    }
103
104    /**
105     * Polls this queue to see if a reference object is available.  If one is
106     * available without further delay then it is removed from the queue and
107     * returned.  Otherwise this method immediately returns {@code null}.
108     *
109     * @return  A reference object, if one was immediately available,
110     *          otherwise {@code null}
111     */
112    public Reference<? extends T> poll() {
113        if (head == null)
114            return null;
115        synchronized (lock) {
116            return reallyPoll();
117        }
118    }
119
120    /**
121     * Removes the next reference object in this queue, blocking until either
122     * one becomes available or the given timeout period expires.
123     *
124     * <p> This method does not offer real-time guarantees: It schedules the
125     * timeout as if by invoking the {@link Object#wait(long)} method.
126     *
127     * @param  timeout  If positive, block for up to {@code timeout}
128     *                  milliseconds while waiting for a reference to be
129     *                  added to this queue.  If zero, block indefinitely.
130     *
131     * @return  A reference object, if one was available within the specified
132     *          timeout period, otherwise {@code null}
133     *
134     * @throws  IllegalArgumentException
135     *          If the value of the timeout argument is negative
136     *
137     * @throws  InterruptedException
138     *          If the timeout wait is interrupted
139     */
140    public Reference<? extends T> remove(long timeout)
141        throws IllegalArgumentException, InterruptedException
142    {
143        if (timeout < 0) {
144            throw new IllegalArgumentException("Negative timeout value");
145        }
146        synchronized (lock) {
147            Reference<? extends T> r = reallyPoll();
148            if (r != null) return r;
149            long start = (timeout == 0) ? 0 : System.nanoTime();
150            for (;;) {
151                lock.wait(timeout);
152                r = reallyPoll();
153                if (r != null) return r;
154                if (timeout != 0) {
155                    long end = System.nanoTime();
156                    timeout -= (end - start) / 1000_000;
157                    if (timeout <= 0) return null;
158                    start = end;
159                }
160            }
161        }
162    }
163
164    /**
165     * Removes the next reference object in this queue, blocking until one
166     * becomes available.
167     *
168     * @return A reference object, blocking until one becomes available
169     * @throws  InterruptedException  If the wait is interrupted
170     */
171    public Reference<? extends T> remove() throws InterruptedException {
172        return remove(0);
173    }
174
175    /**
176     * Iterate queue and invoke given action with each Reference.
177     * Suitable for diagnostic purposes.
178     * WARNING: any use of this method should make sure to not
179     * retain the referents of iterated references (in case of
180     * FinalReference(s)) so that their life is not prolonged more
181     * than necessary.
182     */
183    void forEach(Consumer<? super Reference<? extends T>> action) {
184        for (Reference<? extends T> r = head; r != null;) {
185            action.accept(r);
186            @SuppressWarnings("unchecked")
187            Reference<? extends T> rn = r.next;
188            if (rn == r) {
189                if (r.queue == ENQUEUED) {
190                    // still enqueued -> we reached end of chain
191                    r = null;
192                } else {
193                    // already dequeued: r.queue == NULL; ->
194                    // restart from head when overtaken by queue poller(s)
195                    r = head;
196                }
197            } else {
198                // next in chain
199                r = rn;
200            }
201        }
202    }
203}
204