util/concurrent/CountDownLatch.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 *
 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
 * The {@link #await await} methods block until the current count reaches
 * zero due to invocations of the {@link #countDown} method, after which
 * all waiting threads are released and any subsequent invocations of
 * {@link #await await} return immediately.  This is a one-shot phenomenon
 * -- the count cannot be reset.  If you need a version that resets the
 * count, consider using a {@link CyclicBarrier}.
 *
 * <p>A {@code CountDownLatch} is a versatile synchronization tool
 * and can be used for a number of purposes.  A
 * {@code CountDownLatch} initialized with a count of one serves as a
 * simple on/off latch, or gate: all threads invoking {@link #await await}
 * wait at the gate until it is opened by a thread invoking {@link
 * #countDown}.  A {@code CountDownLatch} initialized to <em>N</em>
 * can be used to make one thread wait until <em>N</em> threads have
 * completed some action, or some action has been completed N times.
 *
 * <p>A useful property of a {@code CountDownLatch} is that it
 * doesn't require that threads calling {@code countDown} wait for
 * the count to reach zero before proceeding, it simply prevents any
 * thread from proceeding past an {@link #await await} until all
 * threads could pass.
 *
 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
 * of worker threads use two countdown latches:
 * <ul>
 * <li>The first is a start signal that prevents any worker from proceeding
 * until the driver is ready for them to proceed;
 * <li>The second is a completion signal that allows the driver to wait
 * until all workers have completed.
 * </ul>
 *
 * <pre> {@code
 * class Driver { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch startSignal = new CountDownLatch(1);
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       new Thread(new Worker(startSignal, doneSignal)).start();
 *
 *     doSomethingElse();            // don't let run yet
 *     startSignal.countDown();      // let all threads proceed
 *     doSomethingElse();
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class Worker implements Runnable {
 *   private final CountDownLatch startSignal;
 *   private final CountDownLatch doneSignal;
 *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
 *     this.startSignal = startSignal;
 *     this.doneSignal = doneSignal;
 *   }
 *   public void run() {
 *     try {
 *       startSignal.await();
 *       doWork();
 *       doneSignal.countDown();
 *     } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   void doWork() { ... }
 * }}</pre>
 *
 * <p>Another typical usage would be to divide a problem into N parts,
 * describe each part with a Runnable that executes that portion and
 * counts down on the latch, and queue all the Runnables to an
 * Executor.  When all sub-parts are complete, the coordinating thread
 * will be able to pass through await. (When threads must repeatedly
 * count down in this way, instead use a {@link CyclicBarrier}.)
 *
 * <pre> {@code
 * class Driver2 { // ...
 *   void main() throws InterruptedException {
 *     CountDownLatch doneSignal = new CountDownLatch(N);
 *     Executor e = ...;
 *
 *     for (int i = 0; i < N; ++i) // create and start threads
 *       e.execute(new WorkerRunnable(doneSignal, i));
 *
 *     doneSignal.await();           // wait for all to finish
 *   }
 * }
 *
 * class WorkerRunnable implements Runnable {
 *   private final CountDownLatch doneSignal;
 *   private final int i;
 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
 *     this.doneSignal = doneSignal;
 *     this.i = i;
 *   }
 *   public void run() {
 *     doWork();
 *     doneSignal.countDown();
 *   }
 *
 *   void doWork() { ... }
 * }}</pre>
 *
 * <p>Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * {@code await()} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c - 1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * Constructs a {@code CountDownLatch} initialized with the given count.
     *
     * @param count the number of times {@link #countDown} must be invoked
     *        before threads can pass through {@link #await}
     * @throws IllegalArgumentException if {@code count} is negative
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>If the current count is zero then this method returns immediately.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of two things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of three things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     *
     * <p>If the count reaches zero then the method returns with the
     * value {@code true}.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         if the waiting time elapsed before the count reached zero
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * <p>If the current count is greater than zero then it is decremented.
     * If the new count is zero then all waiting threads are re-enabled for
     * thread scheduling purposes.
     *
     * <p>If the current count equals zero then nothing happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String {@code "Count ="}
     * followed by the current count.
     *
     * @return a string identifying this latch, as well as its state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}
Revision:	1.45
Committed:	Fri Nov 27 17:41:59 2020 UTC (3 years, 6 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.44:	+3 -5 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.16	* Expert Group and released to the public domain, as explained at
4	jsr166	1.37	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	jsr166	1.43
9	jsr166	1.40	import java.util.concurrent.locks.AbstractQueuedSynchronizer;
10	tim	1.1
11			/**
12	brian	1.4	* A synchronization aid that allows one or more threads to wait until
13			* a set of operations being performed in other threads completes.
14	dl	1.3	*
15	jsr166	1.33	* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
16			* The {@link #await await} methods block until the current count reaches
17			* zero due to invocations of the {@link #countDown} method, after which
18			* all waiting threads are released and any subsequent invocations of
19			* {@link #await await} return immediately. This is a one-shot phenomenon
20			* -- the count cannot be reset. If you need a version that resets the
21			* count, consider using a {@link CyclicBarrier}.
22	tim	1.1	*
23	jsr166	1.33	* <p>A {@code CountDownLatch} is a versatile synchronization tool
24	dl	1.5	* and can be used for a number of purposes. A
25	jsr166	1.33	* {@code CountDownLatch} initialized with a count of one serves as a
26	dholmes	1.9	* simple on/off latch, or gate: all threads invoking {@link #await await}
27	dl	1.5	* wait at the gate until it is opened by a thread invoking {@link
28	jsr166	1.33	* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
29	dl	1.5	* can be used to make one thread wait until <em>N</em> threads have
30			* completed some action, or some action has been completed N times.
31	jsr166	1.33	*
32			* <p>A useful property of a {@code CountDownLatch} is that it
33			* doesn't require that threads calling {@code countDown} wait for
34	dl	1.5	* the count to reach zero before proceeding, it simply prevents any
35	dholmes	1.9	* thread from proceeding past an {@link #await await} until all
36	dl	1.5	* threads could pass.
37	tim	1.1	*
38			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
39			* of worker threads use two countdown latches:
40			* <ul>
41	dholmes	1.9	* <li>The first is a start signal that prevents any worker from proceeding
42	tim	1.1	* until the driver is ready for them to proceed;
43	dholmes	1.9	* <li>The second is a completion signal that allows the driver to wait
44	tim	1.1	* until all workers have completed.
45			* </ul>
46			*
47	jsr166	1.44	* <pre> {@code
48	tim	1.1	* class Driver { // ...
49			* void main() throws InterruptedException {
50			* CountDownLatch startSignal = new CountDownLatch(1);
51			* CountDownLatch doneSignal = new CountDownLatch(N);
52			*
53			* for (int i = 0; i < N; ++i) // create and start threads
54			* new Thread(new Worker(startSignal, doneSignal)).start();
55			*
56			* doSomethingElse(); // don't let run yet
57			* startSignal.countDown(); // let all threads proceed
58			* doSomethingElse();
59			* doneSignal.await(); // wait for all to finish
60			* }
61			* }
62			*
63			* class Worker implements Runnable {
64			* private final CountDownLatch startSignal;
65			* private final CountDownLatch doneSignal;
66			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
67	jsr166	1.41	* this.startSignal = startSignal;
68			* this.doneSignal = doneSignal;
69	tim	1.1	* }
70			* public void run() {
71	jsr166	1.41	* try {
72			* startSignal.await();
73			* doWork();
74			* doneSignal.countDown();
75			* } catch (InterruptedException ex) {} // return;
76	tim	1.1	* }
77			*
78			* void doWork() { ... }
79	jsr166	1.39	* }}</pre>
80	tim	1.1	*
81	dl	1.5	* <p>Another typical usage would be to divide a problem into N parts,
82			* describe each part with a Runnable that executes that portion and
83			* counts down on the latch, and queue all the Runnables to an
84			* Executor. When all sub-parts are complete, the coordinating thread
85	dl	1.13	* will be able to pass through await. (When threads must repeatedly
86			* count down in this way, instead use a {@link CyclicBarrier}.)
87	brian	1.4	*
88	jsr166	1.44	* <pre> {@code
89	brian	1.4	* class Driver2 { // ...
90			* void main() throws InterruptedException {
91			* CountDownLatch doneSignal = new CountDownLatch(N);
92	dl	1.45	* Executor e = ...;
93	brian	1.4	*
94			* for (int i = 0; i < N; ++i) // create and start threads
95			* e.execute(new WorkerRunnable(doneSignal, i));
96			*
97			* doneSignal.await(); // wait for all to finish
98			* }
99			* }
100			*
101			* class WorkerRunnable implements Runnable {
102			* private final CountDownLatch doneSignal;
103			* private final int i;
104	dl	1.13	* WorkerRunnable(CountDownLatch doneSignal, int i) {
105	jsr166	1.41	* this.doneSignal = doneSignal;
106			* this.i = i;
107	brian	1.4	* }
108			* public void run() {
109	dl	1.45	* doWork();
110			* doneSignal.countDown();
111	brian	1.4	* }
112			*
113			* void doWork() { ... }
114	jsr166	1.39	* }}</pre>
115	brian	1.4	*
116	dl	1.35	* <p>Memory consistency effects: Until the count reaches
117			* zero, actions in a thread prior to calling
118	jsr166	1.31	* {@code countDown()}
119			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
120	brian	1.29	* actions following a successful return from a corresponding
121	jsr166	1.31	* {@code await()} in another thread.
122	brian	1.29	*
123	tim	1.1	* @since 1.5
124	dl	1.5	* @author Doug Lea
125	tim	1.1	*/
126			public class CountDownLatch {
127	dl	1.16	/**
128			* Synchronization control For CountDownLatch.
129			* Uses AQS state to represent count.
130			*/
131			private static final class Sync extends AbstractQueuedSynchronizer {
132	dl	1.27	private static final long serialVersionUID = 4982264981922014374L;
133
134	dl	1.16	Sync(int count) {
135	jsr166	1.25	setState(count);
136	dl	1.16	}
137	jsr166	1.25
138	dl	1.17	int getCount() {
139			return getState();
140			}
141
142	jsr166	1.34	protected int tryAcquireShared(int acquires) {
143	jsr166	1.36	return (getState() == 0) ? 1 : -1;
144	dl	1.16	}
145	jsr166	1.25
146	jsr166	1.34	protected boolean tryReleaseShared(int releases) {
147	dl	1.16	// Decrement count; signal when transition to zero
148	dl	1.17	for (;;) {
149			int c = getState();
150			if (c == 0)
151			return false;
152	jsr166	1.42	int nextc = c - 1;
153	jsr166	1.25	if (compareAndSetState(c, nextc))
154	dl	1.19	return nextc == 0;
155	dl	1.17	}
156	dl	1.16	}
157			}
158	tim	1.1
159	dl	1.16	private final Sync sync;
160	jsr166	1.33
161	tim	1.1	/**
162	jsr166	1.33	* Constructs a {@code CountDownLatch} initialized with the given count.
163	jsr166	1.25	*
164	tim	1.1	* @param count the number of times {@link #countDown} must be invoked
165	jsr166	1.33	* before threads can pass through {@link #await}
166			* @throws IllegalArgumentException if {@code count} is negative
167	tim	1.1	*/
168	jsr166	1.25	public CountDownLatch(int count) {
169	dl	1.2	if (count < 0) throw new IllegalArgumentException("count < 0");
170	dl	1.16	this.sync = new Sync(count);
171	dl	1.2	}
172	tim	1.1
173			/**
174	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
175	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
176			*
177			* <p>If the current count is zero then this method returns immediately.
178	tim	1.1	*
179	jsr166	1.33	* <p>If the current count is greater than zero then the current
180			* thread becomes disabled for thread scheduling purposes and lies
181			* dormant until one of two things happen:
182	tim	1.1	* <ul>
183	dholmes	1.9	* <li>The count reaches zero due to invocations of the
184	tim	1.1	* {@link #countDown} method; or
185	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
186			* the current thread.
187	tim	1.1	* </ul>
188	jsr166	1.33	*
189	tim	1.1	* <p>If the current thread:
190			* <ul>
191	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
192	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
193	tim	1.1	* </ul>
194	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
195			* interrupted status is cleared.
196	tim	1.1	*
197			* @throws InterruptedException if the current thread is interrupted
198	jsr166	1.33	* while waiting
199	tim	1.1	*/
200	dl	1.2	public void await() throws InterruptedException {
201	dl	1.16	sync.acquireSharedInterruptibly(1);
202	dl	1.2	}
203
204	tim	1.1	/**
205	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
206	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
207	tim	1.1	* or the specified waiting time elapses.
208			*
209	jsr166	1.33	* <p>If the current count is zero then this method returns immediately
210			* with the value {@code true}.
211	tim	1.1	*
212	jsr166	1.33	* <p>If the current count is greater than zero then the current
213			* thread becomes disabled for thread scheduling purposes and lies
214			* dormant until one of three things happen:
215	tim	1.1	* <ul>
216			* <li>The count reaches zero due to invocations of the
217			* {@link #countDown} method; or
218	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
219			* the current thread; or
220	tim	1.1	* <li>The specified waiting time elapses.
221			* </ul>
222	jsr166	1.33	*
223	tim	1.1	* <p>If the count reaches zero then the method returns with the
224	jsr166	1.33	* value {@code true}.
225			*
226	tim	1.1	* <p>If the current thread:
227			* <ul>
228	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
229	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
230	tim	1.1	* </ul>
231	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
232			* interrupted status is cleared.
233	tim	1.1	*
234	jsr166	1.33	* <p>If the specified waiting time elapses then the value {@code false}
235			* is returned. If the time is less than or equal to zero, the method
236			* will not wait at all.
237	tim	1.1	*
238			* @param timeout the maximum time to wait
239	jsr166	1.33	* @param unit the time unit of the {@code timeout} argument
240			* @return {@code true} if the count reached zero and {@code false}
241			* if the waiting time elapsed before the count reached zero
242	tim	1.1	* @throws InterruptedException if the current thread is interrupted
243	jsr166	1.33	* while waiting
244	tim	1.1	*/
245	jsr166	1.25	public boolean await(long timeout, TimeUnit unit)
246	tim	1.1	throws InterruptedException {
247	dl	1.23	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
248	tim	1.1	}
249
250			/**
251			* Decrements the count of the latch, releasing all waiting threads if
252			* the count reaches zero.
253	jsr166	1.33	*
254			* <p>If the current count is greater than zero then it is decremented.
255			* If the new count is zero then all waiting threads are re-enabled for
256			* thread scheduling purposes.
257			*
258			* <p>If the current count equals zero then nothing happens.
259	tim	1.1	*/
260	dl	1.2	public void countDown() {
261	dl	1.16	sync.releaseShared(1);
262	dl	1.2	}
263	tim	1.1
264			/**
265			* Returns the current count.
266	jsr166	1.33	*
267	tim	1.1	* <p>This method is typically used for debugging and testing purposes.
268	jsr166	1.33	*
269			* @return the current count
270	tim	1.1	*/
271			public long getCount() {
272	dl	1.17	return sync.getCount();
273	tim	1.1	}
274	dl	1.21
275			/**
276	dl	1.24	* Returns a string identifying this latch, as well as its state.
277	jsr166	1.33	* The state, in brackets, includes the String {@code "Count ="}
278			* followed by the current count.
279			*
280			* @return a string identifying this latch, as well as its state
281	dl	1.21	*/
282			public String toString() {
283			return super.toString() + "[Count = " + sync.getCount() + "]";
284			}
285	tim	1.1	}