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 
36 package java.util.concurrent;
37 
38 import 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  */
157 public 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 
Sync(int count)165         Sync(int count) {
166             setState(count);
167         }
168 
getCount()169         int getCount() {
170             return getState();
171         }
172 
tryAcquireShared(int acquires)173         protected int tryAcquireShared(int acquires) {
174             return (getState() == 0) ? 1 : -1;
175         }
176 
tryReleaseShared(int releases)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      */
CountDownLatch(int count)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      */
await()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      */
await(long timeout, TimeUnit unit)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      */
countDown()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      */
getCount()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      */
toString()313     public String toString() {
314         return super.toString() + "[Count = " + sync.getCount() + "]";
315     }
316 }
317