1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent;
37
38import java.util.concurrent.locks.AbstractQueuedSynchronizer;
39
40/**
41 * A synchronization aid that allows one or more threads to wait until
42 * a set of operations being performed in other threads completes.
43 *
44 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
45 * The {@link #await await} methods block until the current count reaches
46 * zero due to invocations of the {@link #countDown} method, after which
47 * all waiting threads are released and any subsequent invocations of
48 * {@link #await await} return immediately.  This is a one-shot phenomenon
49 * -- the count cannot be reset.  If you need a version that resets the
50 * count, consider using a {@link CyclicBarrier}.
51 *
52 * <p>A {@code CountDownLatch} is a versatile synchronization tool
53 * and can be used for a number of purposes.  A
54 * {@code CountDownLatch} initialized with a count of one serves as a
55 * simple on/off latch, or gate: all threads invoking {@link #await await}
56 * wait at the gate until it is opened by a thread invoking {@link
57 * #countDown}.  A {@code CountDownLatch} initialized to <em>N</em>
58 * can be used to make one thread wait until <em>N</em> threads have
59 * completed some action, or some action has been completed N times.
60 *
61 * <p>A useful property of a {@code CountDownLatch} is that it
62 * doesn't require that threads calling {@code countDown} wait for
63 * the count to reach zero before proceeding, it simply prevents any
64 * thread from proceeding past an {@link #await await} until all
65 * threads could pass.
66 *
67 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
68 * of worker threads use two countdown latches:
69 * <ul>
70 * <li>The first is a start signal that prevents any worker from proceeding
71 * until the driver is ready for them to proceed;
72 * <li>The second is a completion signal that allows the driver to wait
73 * until all workers have completed.
74 * </ul>
75 *
76 * <pre> {@code
77 * class Driver { // ...
78 *   void main() throws InterruptedException {
79 *     CountDownLatch startSignal = new CountDownLatch(1);
80 *     CountDownLatch doneSignal = new CountDownLatch(N);
81 *
82 *     for (int i = 0; i < N; ++i) // create and start threads
83 *       new Thread(new Worker(startSignal, doneSignal)).start();
84 *
85 *     doSomethingElse();            // don't let run yet
86 *     startSignal.countDown();      // let all threads proceed
87 *     doSomethingElse();
88 *     doneSignal.await();           // wait for all to finish
89 *   }
90 * }
91 *
92 * class Worker implements Runnable {
93 *   private final CountDownLatch startSignal;
94 *   private final CountDownLatch doneSignal;
95 *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
96 *     this.startSignal = startSignal;
97 *     this.doneSignal = doneSignal;
98 *   }
99 *   public void run() {
100 *     try {
101 *       startSignal.await();
102 *       doWork();
103 *       doneSignal.countDown();
104 *     } catch (InterruptedException ex) {} // return;
105 *   }
106 *
107 *   void doWork() { ... }
108 * }}</pre>
109 *
110 * <p>Another typical usage would be to divide a problem into N parts,
111 * describe each part with a Runnable that executes that portion and
112 * counts down on the latch, and queue all the Runnables to an
113 * Executor.  When all sub-parts are complete, the coordinating thread
114 * will be able to pass through await. (When threads must repeatedly
115 * count down in this way, instead use a {@link CyclicBarrier}.)
116 *
117 * <pre> {@code
118 * class Driver2 { // ...
119 *   void main() throws InterruptedException {
120 *     CountDownLatch doneSignal = new CountDownLatch(N);
121 *     Executor e = ...
122 *
123 *     for (int i = 0; i < N; ++i) // create and start threads
124 *       e.execute(new WorkerRunnable(doneSignal, i));
125 *
126 *     doneSignal.await();           // wait for all to finish
127 *   }
128 * }
129 *
130 * class WorkerRunnable implements Runnable {
131 *   private final CountDownLatch doneSignal;
132 *   private final int i;
133 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
134 *     this.doneSignal = doneSignal;
135 *     this.i = i;
136 *   }
137 *   public void run() {
138 *     try {
139 *       doWork(i);
140 *       doneSignal.countDown();
141 *     } catch (InterruptedException ex) {} // return;
142 *   }
143 *
144 *   void doWork() { ... }
145 * }}</pre>
146 *
147 * <p>Memory consistency effects: Until the count reaches
148 * zero, actions in a thread prior to calling
149 * {@code countDown()}
150 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
151 * actions following a successful return from a corresponding
152 * {@code await()} in another thread.
153 *
154 * @since 1.5
155 * @author Doug Lea
156 */
157public class CountDownLatch {
158    /**
159     * Synchronization control For CountDownLatch.
160     * Uses AQS state to represent count.
161     */
162    private static final class Sync extends AbstractQueuedSynchronizer {
163        private static final long serialVersionUID = 4982264981922014374L;
164
165        Sync(int count) {
166            setState(count);
167        }
168
169        int getCount() {
170            return getState();
171        }
172
173        protected int tryAcquireShared(int acquires) {
174            return (getState() == 0) ? 1 : -1;
175        }
176
177        protected boolean tryReleaseShared(int releases) {
178            // Decrement count; signal when transition to zero
179            for (;;) {
180                int c = getState();
181                if (c == 0)
182                    return false;
183                int nextc = c - 1;
184                if (compareAndSetState(c, nextc))
185                    return nextc == 0;
186            }
187        }
188    }
189
190    private final Sync sync;
191
192    /**
193     * Constructs a {@code CountDownLatch} initialized with the given count.
194     *
195     * @param count the number of times {@link #countDown} must be invoked
196     *        before threads can pass through {@link #await}
197     * @throws IllegalArgumentException if {@code count} is negative
198     */
199    public CountDownLatch(int count) {
200        if (count < 0) throw new IllegalArgumentException("count < 0");
201        this.sync = new Sync(count);
202    }
203
204    /**
205     * Causes the current thread to wait until the latch has counted down to
206     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
207     *
208     * <p>If the current count is zero then this method returns immediately.
209     *
210     * <p>If the current count is greater than zero then the current
211     * thread becomes disabled for thread scheduling purposes and lies
212     * dormant until one of two things happen:
213     * <ul>
214     * <li>The count reaches zero due to invocations of the
215     * {@link #countDown} method; or
216     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
217     * the current thread.
218     * </ul>
219     *
220     * <p>If the current thread:
221     * <ul>
222     * <li>has its interrupted status set on entry to this method; or
223     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
224     * </ul>
225     * then {@link InterruptedException} is thrown and the current thread's
226     * interrupted status is cleared.
227     *
228     * @throws InterruptedException if the current thread is interrupted
229     *         while waiting
230     */
231    public void await() throws InterruptedException {
232        sync.acquireSharedInterruptibly(1);
233    }
234
235    /**
236     * Causes the current thread to wait until the latch has counted down to
237     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
238     * or the specified waiting time elapses.
239     *
240     * <p>If the current count is zero then this method returns immediately
241     * with the value {@code true}.
242     *
243     * <p>If the current count is greater than zero then the current
244     * thread becomes disabled for thread scheduling purposes and lies
245     * dormant until one of three things happen:
246     * <ul>
247     * <li>The count reaches zero due to invocations of the
248     * {@link #countDown} method; or
249     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
250     * the current thread; or
251     * <li>The specified waiting time elapses.
252     * </ul>
253     *
254     * <p>If the count reaches zero then the method returns with the
255     * value {@code true}.
256     *
257     * <p>If the current thread:
258     * <ul>
259     * <li>has its interrupted status set on entry to this method; or
260     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
261     * </ul>
262     * then {@link InterruptedException} is thrown and the current thread's
263     * interrupted status is cleared.
264     *
265     * <p>If the specified waiting time elapses then the value {@code false}
266     * is returned.  If the time is less than or equal to zero, the method
267     * will not wait at all.
268     *
269     * @param timeout the maximum time to wait
270     * @param unit the time unit of the {@code timeout} argument
271     * @return {@code true} if the count reached zero and {@code false}
272     *         if the waiting time elapsed before the count reached zero
273     * @throws InterruptedException if the current thread is interrupted
274     *         while waiting
275     */
276    public boolean await(long timeout, TimeUnit unit)
277        throws InterruptedException {
278        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
279    }
280
281    /**
282     * Decrements the count of the latch, releasing all waiting threads if
283     * the count reaches zero.
284     *
285     * <p>If the current count is greater than zero then it is decremented.
286     * If the new count is zero then all waiting threads are re-enabled for
287     * thread scheduling purposes.
288     *
289     * <p>If the current count equals zero then nothing happens.
290     */
291    public void countDown() {
292        sync.releaseShared(1);
293    }
294
295    /**
296     * Returns the current count.
297     *
298     * <p>This method is typically used for debugging and testing purposes.
299     *
300     * @return the current count
301     */
302    public long getCount() {
303        return sync.getCount();
304    }
305
306    /**
307     * Returns a string identifying this latch, as well as its state.
308     * The state, in brackets, includes the String {@code "Count ="}
309     * followed by the current count.
310     *
311     * @return a string identifying this latch, as well as its state
312     */
313    public String toString() {
314        return super.toString() + "[Count = " + sync.getCount() + "]";
315    }
316}
317