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
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