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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;

/**
 * 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:	1.36
Committed:	Mon Nov 29 20:58:06 2010 UTC (13 years, 6 months ago) by jsr166
Branch:	MAIN
Changes since 1.35:	+1 -1 lines
Log Message:	consistent ternary operator style
#	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			* http://creativecommons.org/licenses/publicdomain
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	dl	1.6	import java.util.concurrent.locks.*;
9	dl	1.16	import java.util.concurrent.atomic.*;
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			* <pre>
48			* 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			* this.startSignal = startSignal;
68			* this.doneSignal = doneSignal;
69			* }
70			* public void run() {
71			* try {
72			* startSignal.await();
73			* doWork();
74			* doneSignal.countDown();
75	tim	1.7	* } catch (InterruptedException ex) {} // return;
76	tim	1.1	* }
77			*
78			* void doWork() { ... }
79			* }
80			*
81			* </pre>
82			*
83	dl	1.5	* <p>Another typical usage would be to divide a problem into N parts,
84			* describe each part with a Runnable that executes that portion and
85			* counts down on the latch, and queue all the Runnables to an
86			* Executor. When all sub-parts are complete, the coordinating thread
87	dl	1.13	* will be able to pass through await. (When threads must repeatedly
88			* count down in this way, instead use a {@link CyclicBarrier}.)
89	brian	1.4	*
90			* <pre>
91			* class Driver2 { // ...
92			* void main() throws InterruptedException {
93			* CountDownLatch doneSignal = new CountDownLatch(N);
94			* Executor e = ...
95			*
96			* for (int i = 0; i < N; ++i) // create and start threads
97			* e.execute(new WorkerRunnable(doneSignal, i));
98			*
99			* doneSignal.await(); // wait for all to finish
100			* }
101			* }
102			*
103			* class WorkerRunnable implements Runnable {
104			* private final CountDownLatch doneSignal;
105			* private final int i;
106	dl	1.13	* WorkerRunnable(CountDownLatch doneSignal, int i) {
107	brian	1.4	* this.doneSignal = doneSignal;
108			* this.i = i;
109			* }
110			* public void run() {
111			* try {
112			* doWork(i);
113			* doneSignal.countDown();
114	tim	1.7	* } catch (InterruptedException ex) {} // return;
115	brian	1.4	* }
116			*
117			* void doWork() { ... }
118			* }
119			*
120			* </pre>
121			*
122	dl	1.35	* <p>Memory consistency effects: Until the count reaches
123			* zero, actions in a thread prior to calling
124	jsr166	1.31	* {@code countDown()}
125			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
126	brian	1.29	* actions following a successful return from a corresponding
127	jsr166	1.31	* {@code await()} in another thread.
128	brian	1.29	*
129	tim	1.1	* @since 1.5
130	dl	1.5	* @author Doug Lea
131	tim	1.1	*/
132			public class CountDownLatch {
133	dl	1.16	/**
134			* Synchronization control For CountDownLatch.
135			* Uses AQS state to represent count.
136			*/
137			private static final class Sync extends AbstractQueuedSynchronizer {
138	dl	1.27	private static final long serialVersionUID = 4982264981922014374L;
139
140	dl	1.16	Sync(int count) {
141	jsr166	1.25	setState(count);
142	dl	1.16	}
143	jsr166	1.25
144	dl	1.17	int getCount() {
145			return getState();
146			}
147
148	jsr166	1.34	protected int tryAcquireShared(int acquires) {
149	jsr166	1.36	return (getState() == 0) ? 1 : -1;
150	dl	1.16	}
151	jsr166	1.25
152	jsr166	1.34	protected boolean tryReleaseShared(int releases) {
153	dl	1.16	// Decrement count; signal when transition to zero
154	dl	1.17	for (;;) {
155			int c = getState();
156			if (c == 0)
157			return false;
158	dl	1.19	int nextc = c-1;
159	jsr166	1.25	if (compareAndSetState(c, nextc))
160	dl	1.19	return nextc == 0;
161	dl	1.17	}
162	dl	1.16	}
163			}
164	tim	1.1
165	dl	1.16	private final Sync sync;
166	jsr166	1.33
167	tim	1.1	/**
168	jsr166	1.33	* Constructs a {@code CountDownLatch} initialized with the given count.
169	jsr166	1.25	*
170	tim	1.1	* @param count the number of times {@link #countDown} must be invoked
171	jsr166	1.33	* before threads can pass through {@link #await}
172			* @throws IllegalArgumentException if {@code count} is negative
173	tim	1.1	*/
174	jsr166	1.25	public CountDownLatch(int count) {
175	dl	1.2	if (count < 0) throw new IllegalArgumentException("count < 0");
176	dl	1.16	this.sync = new Sync(count);
177	dl	1.2	}
178	tim	1.1
179			/**
180	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
181	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
182			*
183			* <p>If the current count is zero then this method returns immediately.
184	tim	1.1	*
185	jsr166	1.33	* <p>If the current count is greater than zero then the current
186			* thread becomes disabled for thread scheduling purposes and lies
187			* dormant until one of two things happen:
188	tim	1.1	* <ul>
189	dholmes	1.9	* <li>The count reaches zero due to invocations of the
190	tim	1.1	* {@link #countDown} method; or
191	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
192			* the current thread.
193	tim	1.1	* </ul>
194	jsr166	1.33	*
195	tim	1.1	* <p>If the current thread:
196			* <ul>
197	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
198	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
199	tim	1.1	* </ul>
200	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
201			* interrupted status is cleared.
202	tim	1.1	*
203			* @throws InterruptedException if the current thread is interrupted
204	jsr166	1.33	* while waiting
205	tim	1.1	*/
206	dl	1.2	public void await() throws InterruptedException {
207	dl	1.16	sync.acquireSharedInterruptibly(1);
208	dl	1.2	}
209
210	tim	1.1	/**
211	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
212	jsr166	1.33	* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
213	tim	1.1	* or the specified waiting time elapses.
214			*
215	jsr166	1.33	* <p>If the current count is zero then this method returns immediately
216			* with the value {@code true}.
217	tim	1.1	*
218	jsr166	1.33	* <p>If the current count is greater than zero then the current
219			* thread becomes disabled for thread scheduling purposes and lies
220			* dormant until one of three things happen:
221	tim	1.1	* <ul>
222			* <li>The count reaches zero due to invocations of the
223			* {@link #countDown} method; or
224	jsr166	1.33	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
225			* the current thread; or
226	tim	1.1	* <li>The specified waiting time elapses.
227			* </ul>
228	jsr166	1.33	*
229	tim	1.1	* <p>If the count reaches zero then the method returns with the
230	jsr166	1.33	* value {@code true}.
231			*
232	tim	1.1	* <p>If the current thread:
233			* <ul>
234	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
235	jsr166	1.33	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
236	tim	1.1	* </ul>
237	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
238			* interrupted status is cleared.
239	tim	1.1	*
240	jsr166	1.33	* <p>If the specified waiting time elapses then the value {@code false}
241			* is returned. If the time is less than or equal to zero, the method
242			* will not wait at all.
243	tim	1.1	*
244			* @param timeout the maximum time to wait
245	jsr166	1.33	* @param unit the time unit of the {@code timeout} argument
246			* @return {@code true} if the count reached zero and {@code false}
247			* if the waiting time elapsed before the count reached zero
248	tim	1.1	* @throws InterruptedException if the current thread is interrupted
249	jsr166	1.33	* while waiting
250	tim	1.1	*/
251	jsr166	1.25	public boolean await(long timeout, TimeUnit unit)
252	tim	1.1	throws InterruptedException {
253	dl	1.23	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
254	tim	1.1	}
255
256			/**
257			* Decrements the count of the latch, releasing all waiting threads if
258			* the count reaches zero.
259	jsr166	1.33	*
260			* <p>If the current count is greater than zero then it is decremented.
261			* If the new count is zero then all waiting threads are re-enabled for
262			* thread scheduling purposes.
263			*
264			* <p>If the current count equals zero then nothing happens.
265	tim	1.1	*/
266	dl	1.2	public void countDown() {
267	dl	1.16	sync.releaseShared(1);
268	dl	1.2	}
269	tim	1.1
270			/**
271			* Returns the current count.
272	jsr166	1.33	*
273	tim	1.1	* <p>This method is typically used for debugging and testing purposes.
274	jsr166	1.33	*
275			* @return the current count
276	tim	1.1	*/
277			public long getCount() {
278	dl	1.17	return sync.getCount();
279	tim	1.1	}
280	dl	1.21
281			/**
282	dl	1.24	* Returns a string identifying this latch, as well as its state.
283	jsr166	1.33	* The state, in brackets, includes the String {@code "Count ="}
284			* followed by the current count.
285			*
286			* @return a string identifying this latch, as well as its state
287	dl	1.21	*/
288			public String toString() {
289			return super.toString() + "[Count = " + sync.getCount() + "]";
290			}
291	tim	1.1	}