util/concurrent/CountDownLatch.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util.concurrent;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 *
 * <p>A <tt>CountDownLatch</tt> is initialized with a given
 * <em>count</em>.  The {@link #await} methods block until the current
 * {@link #getCount 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} 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 <tt>CountDownLatch</tt> is a versatile synchronization tool
 * and can be used for a number of purposes. 
 * A <tt>CountDownLatch</tt> initialized with a count of one serves as a simple on/off
 * latch, or gate: all threads invoking {@link #await} wait at the gate until
 * it is opened by a thread invoking {@link #countDown}.
 * A <tt>CountDownLatch</tt> 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 <tt>CountDownLatch</tt> is that it doesn't
 * require that threads calling <tt>countDown</tt> wait for the count to reach zero
 * before proceeding, it simply
 * prevents any thread from proceeding past the {@link #await wait} 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.
 *
 * <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;
 *   Worker(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>
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/06/07 18:20:20 $
 * @editor $Author: dl $
 */
public class CountDownLatch {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition zero = lock.newCondition();
    private int count;

    /**
     * Constructs a <tt>CountDownLatch</tt> 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 <tt>count</tt> is less than zero.
     */
    public CountDownLatch(int count) { 
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.count = count; 
    }

    /**
     * Causes the current thread to wait until the latch has counted down to 
     * zero, unless the thread is {@link Thread#interrupt interrupted}.
     *
     * <p>If the current {@link #getCount count} is zero then this method
     * returns immediately.
     * <p>If the current {@link #getCount 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 {@link 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 {@link 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 {
        lock.lock();
        try {
            while (count != 0)
                zero.await();
        }
        finally {
            lock.unlock();
        }
    }


    /**
     * Causes the current thread to wait until the latch has counted down to 
     * zero, unless the thread is {@link Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current {@link #getCount count} is zero then this method
     * returns immediately with the value <tt>true</tt>.
     *
     * <p>If the current {@link #getCount 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 {@link 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 <tt>true</tt>.
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link 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 <tt>false</tt>
     * is returned.
     * The given waiting time is a best-effort lower bound. 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 <tt>timeout</tt> argument.
     * @return <tt>true</tt> if the count reached zero  and <tt>false</tt>
     * 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 {
        long nanos = unit.toNanos(timeout);
        lock.lock();
        try {
            for (;;) {
                if (count == 0)
                    return true;
                nanos = zero.awaitNanos(nanos);
                if (nanos <= 0)
                    return false;
            }
        }
        finally {
            lock.unlock();
        }
    }


    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     * <p>If the current {@link #getCount 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 {@link #getCount count} equals zero then nothing
     * happens.
     */
    public void countDown() {
        lock.lock();
        try {
            if (count > 0 && --count == 0)
                zero.signalAll();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Returns the current count.
     * <p>This method is typically used for debugging and testing purposes.
     * @return the current count.
     */
    public long getCount() {
        lock.lock();
        try {
            return count;
        }
        finally {
            lock.unlock();
        }
    }
}
Revision:	1.4
Committed:	Mon Jun 23 02:26:16 2003 UTC (20 years, 11 months ago) by brian
Branch:	MAIN
Changes since 1.3:	+47 -8 lines
Log Message:	Partial javadoc pass
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain. Use, modify, and
4			* redistribute this code in any way without acknowledgement.
5			*/
6
7	tim	1.1	package java.util.concurrent;
8
9			/**
10	brian	1.4	* A synchronization aid that allows one or more threads to wait until
11			* a set of operations being performed in other threads completes.
12	dl	1.3	*
13			* <p>A <tt>CountDownLatch</tt> is initialized with a given
14			* <em>count</em>. The {@link #await} methods block until the current
15			* {@link #getCount count} reaches zero due to invocations of the
16			* {@link #countDown} method, after which all waiting threads are
17	tim	1.1	* released and any subsequent invocations of {@link #await} return
18	dl	1.3	* immediately. This is a one-shot phenomenon -- the count cannot be
19			* reset. If you need a version that resets the count, consider using
20			* a {@link CyclicBarrier}.
21	tim	1.1	*
22			* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
23			* and can be used for a number of purposes.
24	brian	1.4	* A <tt>CountDownLatch</tt> initialized with a count of one serves as a simple on/off
25	tim	1.1	* latch, or gate: all threads invoking {@link #await} wait at the gate until
26			* it is opened by a thread invoking {@link #countDown}.
27			* A <tt>CountDownLatch</tt> initialized to <em>N</em> can be used to make
28	brian	1.4	* one thread wait until <em>N</em> threads have completed some action, or some
29			* action has been completed N times.
30			* <p>A useful property of a <tt>CountDownLatch</tt> is that it doesn't
31			* require that threads calling <tt>countDown</tt> wait for the count to reach zero
32			* before proceeding, it simply
33	tim	1.1	* prevents any thread from proceeding past the {@link #await wait} until
34			* all threads could pass.
35			*
36			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
37			* of worker threads use two countdown latches:
38			* <ul>
39			* <li> The first is a start signal that prevents any worker from proceeding
40			* until the driver is ready for them to proceed;
41			* <li> The second is a completion signal that allows the driver to wait
42			* until all workers have completed.
43			* </ul>
44			*
45			* <pre>
46			* class Driver { // ...
47			* void main() throws InterruptedException {
48			* CountDownLatch startSignal = new CountDownLatch(1);
49			* CountDownLatch doneSignal = new CountDownLatch(N);
50			*
51			* for (int i = 0; i < N; ++i) // create and start threads
52			* new Thread(new Worker(startSignal, doneSignal)).start();
53			*
54			* doSomethingElse(); // don't let run yet
55			* startSignal.countDown(); // let all threads proceed
56			* doSomethingElse();
57			* doneSignal.await(); // wait for all to finish
58			* }
59			* }
60			*
61			* class Worker implements Runnable {
62			* private final CountDownLatch startSignal;
63			* private final CountDownLatch doneSignal;
64			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
65			* this.startSignal = startSignal;
66			* this.doneSignal = doneSignal;
67			* }
68			* public void run() {
69			* try {
70			* startSignal.await();
71			* doWork();
72			* doneSignal.countDown();
73			* }
74			* catch (InterruptedException ex) {} // return;
75			* }
76			*
77			* void doWork() { ... }
78			* }
79			*
80			* </pre>
81			*
82	brian	1.4	* <p>Another typical usage would be to divide a problem into N parts, describe each part
83			* with a Runnable that executes that portion and counts down on the latch, and queue all
84			* the Runnables to an Executor. When all sub-parts are complete, the coordinating thread
85			* will be able to pass through await.
86			*
87			* <pre>
88			* class Driver2 { // ...
89			* void main() throws InterruptedException {
90			* CountDownLatch doneSignal = new CountDownLatch(N);
91			* Executor e = ...
92			*
93			* for (int i = 0; i < N; ++i) // create and start threads
94			* e.execute(new WorkerRunnable(doneSignal, i));
95			*
96			* doneSignal.await(); // wait for all to finish
97			* }
98			* }
99			*
100			* class WorkerRunnable implements Runnable {
101			* private final CountDownLatch doneSignal;
102			* private final int i;
103			* Worker(CountDownLatch doneSignal, int i) {
104			* this.doneSignal = doneSignal;
105			* this.i = i;
106			* }
107			* public void run() {
108			* try {
109			* doWork(i);
110			* doneSignal.countDown();
111			* }
112			* catch (InterruptedException ex) {} // return;
113			* }
114			*
115			* void doWork() { ... }
116			* }
117			*
118			* </pre>
119			*
120	tim	1.1	* @since 1.5
121			* @spec JSR-166
122	brian	1.4	* @revised $Date: 2003/06/07 18:20:20 $
123	dl	1.3	* @editor $Author: dl $
124	tim	1.1	*/
125			public class CountDownLatch {
126	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
127			private final Condition zero = lock.newCondition();
128			private int count;
129	tim	1.1
130			/**
131			* Constructs a <tt>CountDownLatch</tt> initialized with the given
132			* count.
133			*
134			* @param count the number of times {@link #countDown} must be invoked
135			* before threads can pass through {@link #await}.
136			*
137	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
138	tim	1.1	*/
139	dl	1.2	public CountDownLatch(int count) {
140			if (count < 0) throw new IllegalArgumentException("count < 0");
141			this.count = count;
142			}
143	tim	1.1
144			/**
145			* Causes the current thread to wait until the latch has counted down to
146			* zero, unless the thread is {@link Thread#interrupt interrupted}.
147			*
148			* <p>If the current {@link #getCount count} is zero then this method
149			* returns immediately.
150			* <p>If the current {@link #getCount count} is greater than zero then
151			* the current thread becomes disabled for thread scheduling
152			* purposes and lies dormant until one of two things happen:
153			* <ul>
154			* <li> The count reaches zero due to invocations of the
155			* {@link #countDown} method; or
156			* <li> Some other thread {@link Thread#interrupt interrupts} the current
157			* thread.
158			* </ul>
159			* <p>If the current thread:
160			* <ul>
161			* <li>has its interrupted status set on entry to this method; or
162			* <li>is {@link Thread#interrupt interrupted} while waiting,
163			* </ul>
164			* then {@link InterruptedException} is thrown and the current thread's
165			* interrupted status is cleared.
166			*
167			* @throws InterruptedException if the current thread is interrupted
168			* while waiting.
169			*/
170	dl	1.2	public void await() throws InterruptedException {
171			lock.lock();
172			try {
173			while (count != 0)
174			zero.await();
175			}
176			finally {
177			lock.unlock();
178			}
179			}
180
181	tim	1.1
182			/**
183			* Causes the current thread to wait until the latch has counted down to
184			* zero, unless the thread is {@link Thread#interrupt interrupted},
185			* or the specified waiting time elapses.
186			*
187			* <p>If the current {@link #getCount count} is zero then this method
188			* returns immediately with the value <tt>true</tt>.
189			*
190			* <p>If the current {@link #getCount count} is greater than zero then
191			* the current thread becomes disabled for thread scheduling
192			* purposes and lies dormant until one of three things happen:
193			* <ul>
194			* <li>The count reaches zero due to invocations of the
195			* {@link #countDown} method; or
196			* <li>Some other thread {@link Thread#interrupt interrupts} the current
197			* thread; or
198			* <li>The specified waiting time elapses.
199			* </ul>
200			* <p>If the count reaches zero then the method returns with the
201			* value <tt>true</tt>.
202			* <p>If the current thread:
203			* <ul>
204			* <li>has its interrupted status set on entry to this method; or
205			* <li>is {@link Thread#interrupt interrupted} while waiting,
206			* </ul>
207			* then {@link InterruptedException} is thrown and the current thread's
208			* interrupted status is cleared.
209			*
210			* <p>If the specified waiting time elapses then the value <tt>false</tt>
211			* is returned.
212			* The given waiting time is a best-effort lower bound. If the time is
213			* less than or equal to zero, the method will not wait at all.
214			*
215			* @param timeout the maximum time to wait
216	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
217	tim	1.1	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
218			* if the waiting time elapsed before the count reached zero.
219			*
220			* @throws InterruptedException if the current thread is interrupted
221			* while waiting.
222			*/
223	dl	1.2	public boolean await(long timeout, TimeUnit unit)
224	tim	1.1	throws InterruptedException {
225	dl	1.2	long nanos = unit.toNanos(timeout);
226			lock.lock();
227			try {
228			for (;;) {
229			if (count == 0)
230			return true;
231			nanos = zero.awaitNanos(nanos);
232			if (nanos <= 0)
233			return false;
234			}
235			}
236			finally {
237			lock.unlock();
238			}
239	tim	1.1	}
240
241
242	dl	1.2
243	tim	1.1	/**
244			* Decrements the count of the latch, releasing all waiting threads if
245			* the count reaches zero.
246			* <p>If the current {@link #getCount count} is greater than zero then
247			* it is decremented. If the new count is zero then all waiting threads
248			* are re-enabled for thread scheduling purposes.
249			* <p>If the current {@link #getCount count} equals zero then nothing
250			* happens.
251			*/
252	dl	1.2	public void countDown() {
253			lock.lock();
254			try {
255			if (count > 0 && --count == 0)
256			zero.signalAll();
257			}
258			finally {
259			lock.unlock();
260			}
261			}
262	tim	1.1
263			/**
264			* Returns the current count.
265			* <p>This method is typically used for debugging and testing purposes.
266			* @return the current count.
267			*/
268			public long getCount() {
269	dl	1.2	lock.lock();
270			try {
271			return count;
272			}
273			finally {
274			lock.unlock();
275			}
276	tim	1.1	}
277			}