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.*;

/**
 * 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>
 * 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>
 * 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() {
 *      try {
 *        doWork(i);
 *        doneSignal.countDown();
 *      } catch (InterruptedException ex) {} // return;
 *   }
 *
 *   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:	1.38
Committed:	Wed Jun 8 00:50:35 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.37:	+0 -1 lines
Log Message:	clean up imports
#	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	dl	1.6	import java.util.concurrent.locks.*;
9	tim	1.1
10			/**
11	brian	1.4	* A synchronization aid that allows one or more threads to wait until
12			* a set of operations being performed in other threads completes.
13	dl	1.3	*
14	jsr166	1.33	* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
15			* The {@link #await await} methods block until the current count reaches
16			* zero due to invocations of the {@link #countDown} method, after which
17			* all waiting threads are released and any subsequent invocations of
18			* {@link #await await} return immediately. This is a one-shot phenomenon
19			* -- the count cannot be reset. If you need a version that resets the
20			* count, consider using a {@link CyclicBarrier}.
21	tim	1.1	*
22	jsr166	1.33	* <p>A {@code CountDownLatch} is a versatile synchronization tool
23	dl	1.5	* and can be used for a number of purposes. A
24	jsr166	1.33	* {@code CountDownLatch} initialized with a count of one serves as a
25	dholmes	1.9	* simple on/off latch, or gate: all threads invoking {@link #await await}
26	dl	1.5	* wait at the gate until it is opened by a thread invoking {@link
27	jsr166	1.33	* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
28	dl	1.5	* can be used to make one thread wait until <em>N</em> threads have
29			* completed some action, or some action has been completed N times.
30	jsr166	1.33	*
31			* <p>A useful property of a {@code CountDownLatch} is that it
32			* doesn't require that threads calling {@code countDown} wait for
33	dl	1.5	* the count to reach zero before proceeding, it simply prevents any
34	dholmes	1.9	* thread from proceeding past an {@link #await await} until all
35	dl	1.5	* threads could pass.
36	tim	1.1	*
37			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
38			* of worker threads use two countdown latches:
39			* <ul>
40	dholmes	1.9	* <li>The first is a start signal that prevents any worker from proceeding
41	tim	1.1	* until the driver is ready for them to proceed;
42	dholmes	1.9	* <li>The second is a completion signal that allows the driver to wait
43	tim	1.1	* until all workers have completed.
44			* </ul>
45			*
46			* <pre>
47			* class Driver { // ...
48			* void main() throws InterruptedException {
49			* CountDownLatch startSignal = new CountDownLatch(1);
50			* CountDownLatch doneSignal = new CountDownLatch(N);
51			*
52			* for (int i = 0; i < N; ++i) // create and start threads
53			* new Thread(new Worker(startSignal, doneSignal)).start();
54			*
55			* doSomethingElse(); // don't let run yet
56			* startSignal.countDown(); // let all threads proceed
57			* doSomethingElse();
58			* doneSignal.await(); // wait for all to finish
59			* }
60			* }
61			*
62			* class Worker implements Runnable {
63			* private final CountDownLatch startSignal;
64			* private final CountDownLatch doneSignal;
65			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
66			* this.startSignal = startSignal;
67			* this.doneSignal = doneSignal;
68			* }
69			* public void run() {
70			* try {
71			* startSignal.await();
72			* doWork();
73			* doneSignal.countDown();
74	tim	1.7	* } catch (InterruptedException ex) {} // return;
75	tim	1.1	* }
76			*
77			* void doWork() { ... }
78			* }
79			*
80			* </pre>
81			*
82	dl	1.5	* <p>Another typical usage would be to divide a problem into N parts,
83			* describe each part with a Runnable that executes that portion and
84			* counts down on the latch, and queue all the Runnables to an
85			* Executor. When all sub-parts are complete, the coordinating thread
86	dl	1.13	* will be able to pass through await. (When threads must repeatedly
87			* count down in this way, instead use a {@link CyclicBarrier}.)
88	brian	1.4	*
89			* <pre>
90			* class Driver2 { // ...
91			* void main() throws InterruptedException {
92			* CountDownLatch doneSignal = new CountDownLatch(N);
93			* Executor e = ...
94			*
95			* for (int i = 0; i < N; ++i) // create and start threads
96			* e.execute(new WorkerRunnable(doneSignal, i));
97			*
98			* doneSignal.await(); // wait for all to finish
99			* }
100			* }
101			*
102			* class WorkerRunnable implements Runnable {
103			* private final CountDownLatch doneSignal;
104			* private final int i;
105	dl	1.13	* WorkerRunnable(CountDownLatch doneSignal, int i) {
106	brian	1.4	* this.doneSignal = doneSignal;
107			* this.i = i;
108			* }
109			* public void run() {
110			* try {
111			* doWork(i);
112			* doneSignal.countDown();
113	tim	1.7	* } catch (InterruptedException ex) {} // return;
114	brian	1.4	* }
115			*
116			* void doWork() { ... }
117			* }
118			*
119			* </pre>
120			*
121	dl	1.35	* <p>Memory consistency effects: Until the count reaches
122			* zero, actions in a thread prior to calling
123	jsr166	1.31	* {@code countDown()}
124			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
125	brian	1.29	* actions following a successful return from a corresponding
126	jsr166	1.31	* {@code await()} in another thread.
127	brian	1.29	*
128	tim	1.1	* @since 1.5
129	dl	1.5	* @author Doug Lea
130	tim	1.1	*/
131			public class CountDownLatch {
132	dl	1.16	/**
133			* Synchronization control For CountDownLatch.
134			* Uses AQS state to represent count.
135			*/
136			private static final class Sync extends AbstractQueuedSynchronizer {
137	dl	1.27	private static final long serialVersionUID = 4982264981922014374L;
138
139	dl	1.16	Sync(int count) {
140	jsr166	1.25	setState(count);
141	dl	1.16	}
142	jsr166	1.25
143	dl	1.17	int getCount() {
144			return getState();
145			}
146
147	jsr166	1.34	protected int tryAcquireShared(int acquires) {
148	jsr166	1.36	return (getState() == 0) ? 1 : -1;
149	dl	1.16	}
150	jsr166	1.25
151	jsr166	1.34	protected boolean tryReleaseShared(int releases) {
152	dl	1.16	// Decrement count; signal when transition to zero
153	dl	1.17	for (;;) {
154			int c = getState();
155			if (c == 0)
156			return false;
157	dl	1.19	int nextc = c-1;
158	jsr166	1.25	if (compareAndSetState(c, nextc))
159	dl	1.19	return nextc == 0;
160	dl	1.17	}
161	dl	1.16	}
162			}
163	tim	1.1
164	dl	1.16	private final Sync sync;
165	jsr166	1.33
166	tim	1.1	/**
167	jsr166	1.33	* Constructs a {@code CountDownLatch} initialized with the given count.
168	jsr166	1.25	*
169	tim	1.1	* @param count the number of times {@link #countDown} must be invoked
170	jsr166	1.33	* before threads can pass through {@link #await}
171			* @throws IllegalArgumentException if {@code count} is negative
172	tim	1.1	*/
173	jsr166	1.25	public CountDownLatch(int count) {
174	dl	1.2	if (count < 0) throw new IllegalArgumentException("count < 0");
175	dl	1.16	this.sync = new Sync(count);
176	dl	1.2	}
177	tim	1.1
178			/**
179	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
180	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
181			*
182			* <p>If the current count is zero then this method returns immediately.
183	tim	1.1	*
184	jsr166	1.33	* <p>If the current count is greater than zero then the current
185			* thread becomes disabled for thread scheduling purposes and lies
186			* dormant until one of two things happen:
187	tim	1.1	* <ul>
188	dholmes	1.9	* <li>The count reaches zero due to invocations of the
189	tim	1.1	* {@link #countDown} method; or
190	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
191			* the current thread.
192	tim	1.1	* </ul>
193	jsr166	1.33	*
194	tim	1.1	* <p>If the current thread:
195			* <ul>
196	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
197	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
198	tim	1.1	* </ul>
199	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
200			* interrupted status is cleared.
201	tim	1.1	*
202			* @throws InterruptedException if the current thread is interrupted
203	jsr166	1.33	* while waiting
204	tim	1.1	*/
205	dl	1.2	public void await() throws InterruptedException {
206	dl	1.16	sync.acquireSharedInterruptibly(1);
207	dl	1.2	}
208
209	tim	1.1	/**
210	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
211	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
212	tim	1.1	* or the specified waiting time elapses.
213			*
214	jsr166	1.33	* <p>If the current count is zero then this method returns immediately
215			* with the value {@code true}.
216	tim	1.1	*
217	jsr166	1.33	* <p>If the current count is greater than zero then the current
218			* thread becomes disabled for thread scheduling purposes and lies
219			* dormant until one of three things happen:
220	tim	1.1	* <ul>
221			* <li>The count reaches zero due to invocations of the
222			* {@link #countDown} method; or
223	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
224			* the current thread; or
225	tim	1.1	* <li>The specified waiting time elapses.
226			* </ul>
227	jsr166	1.33	*
228	tim	1.1	* <p>If the count reaches zero then the method returns with the
229	jsr166	1.33	* value {@code true}.
230			*
231	tim	1.1	* <p>If the current thread:
232			* <ul>
233	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
234	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
235	tim	1.1	* </ul>
236	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
237			* interrupted status is cleared.
238	tim	1.1	*
239	jsr166	1.33	* <p>If the specified waiting time elapses then the value {@code false}
240			* is returned. If the time is less than or equal to zero, the method
241			* will not wait at all.
242	tim	1.1	*
243			* @param timeout the maximum time to wait
244	jsr166	1.33	* @param unit the time unit of the {@code timeout} argument
245			* @return {@code true} if the count reached zero and {@code false}
246			* if the waiting time elapsed before the count reached zero
247	tim	1.1	* @throws InterruptedException if the current thread is interrupted
248	jsr166	1.33	* while waiting
249	tim	1.1	*/
250	jsr166	1.25	public boolean await(long timeout, TimeUnit unit)
251	tim	1.1	throws InterruptedException {
252	dl	1.23	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
253	tim	1.1	}
254
255			/**
256			* Decrements the count of the latch, releasing all waiting threads if
257			* the count reaches zero.
258	jsr166	1.33	*
259			* <p>If the current count is greater than zero then it is decremented.
260			* If the new count is zero then all waiting threads are re-enabled for
261			* thread scheduling purposes.
262			*
263			* <p>If the current count equals zero then nothing happens.
264	tim	1.1	*/
265	dl	1.2	public void countDown() {
266	dl	1.16	sync.releaseShared(1);
267	dl	1.2	}
268	tim	1.1
269			/**
270			* Returns the current count.
271	jsr166	1.33	*
272	tim	1.1	* <p>This method is typically used for debugging and testing purposes.
273	jsr166	1.33	*
274			* @return the current count
275	tim	1.1	*/
276			public long getCount() {
277	dl	1.17	return sync.getCount();
278	tim	1.1	}
279	dl	1.21
280			/**
281	dl	1.24	* Returns a string identifying this latch, as well as its state.
282	jsr166	1.33	* The state, in brackets, includes the String {@code "Count ="}
283			* followed by the current count.
284			*
285			* @return a string identifying this latch, as well as its state
286	dl	1.21	*/
287			public String toString() {
288			return super.toString() + "[Count = " + sync.getCount() + "]";
289			}
290	tim	1.1	}