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: 2003/06/23 02:26:16 $
 * @editor $Author: brian $
 * @author Doug Lea
 */
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:	1.5
Committed:	Tue Jun 24 14:34:47 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.4:	+19 -17 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	dl	1.5	* and can be used for a number of purposes. A
24			* <tt>CountDownLatch</tt> initialized with a count of one serves as a
25			* simple on/off latch, or gate: all threads invoking {@link #await}
26			* wait at the gate until it is opened by a thread invoking {@link
27			* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
28			* 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			* <p>A useful property of a <tt>CountDownLatch</tt> is that it
31			* doesn't require that threads calling <tt>countDown</tt> wait for
32			* the count to reach zero before proceeding, it simply prevents any
33			* thread from proceeding past the {@link #await wait} until all
34			* threads could pass.
35	tim	1.1	*
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	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	brian	1.4	* will be able to pass through await.
87			*
88			* <pre>
89			* class Driver2 { // ...
90			* void main() throws InterruptedException {
91			* CountDownLatch doneSignal = new CountDownLatch(N);
92			* Executor e = ...
93			*
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			* Worker(CountDownLatch doneSignal, int i) {
105			* this.doneSignal = doneSignal;
106			* this.i = i;
107			* }
108			* public void run() {
109			* try {
110			* doWork(i);
111			* doneSignal.countDown();
112			* }
113			* catch (InterruptedException ex) {} // return;
114			* }
115			*
116			* void doWork() { ... }
117			* }
118			*
119			* </pre>
120			*
121	tim	1.1	* @since 1.5
122			* @spec JSR-166
123	dl	1.5	* @revised $Date: 2003/06/23 02:26:16 $
124			* @editor $Author: brian $
125			* @author Doug Lea
126	tim	1.1	*/
127			public class CountDownLatch {
128	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
129			private final Condition zero = lock.newCondition();
130			private int count;
131	tim	1.1
132			/**
133			* Constructs a <tt>CountDownLatch</tt> initialized with the given
134			* count.
135			*
136			* @param count the number of times {@link #countDown} must be invoked
137			* before threads can pass through {@link #await}.
138			*
139	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
140	tim	1.1	*/
141	dl	1.2	public CountDownLatch(int count) {
142			if (count < 0) throw new IllegalArgumentException("count < 0");
143			this.count = count;
144			}
145	tim	1.1
146			/**
147			* Causes the current thread to wait until the latch has counted down to
148			* zero, unless the thread is {@link Thread#interrupt interrupted}.
149			*
150			* <p>If the current {@link #getCount count} is zero then this method
151			* returns immediately.
152			* <p>If the current {@link #getCount count} is greater than zero then
153			* the current thread becomes disabled for thread scheduling
154			* purposes and lies dormant until one of two things happen:
155			* <ul>
156			* <li> The count reaches zero due to invocations of the
157			* {@link #countDown} method; or
158			* <li> Some other thread {@link Thread#interrupt interrupts} the current
159			* thread.
160			* </ul>
161			* <p>If the current thread:
162			* <ul>
163			* <li>has its interrupted status set on entry to this method; or
164			* <li>is {@link Thread#interrupt interrupted} while waiting,
165			* </ul>
166			* then {@link InterruptedException} is thrown and the current thread's
167			* interrupted status is cleared.
168			*
169			* @throws InterruptedException if the current thread is interrupted
170			* while waiting.
171			*/
172	dl	1.2	public void await() throws InterruptedException {
173			lock.lock();
174			try {
175			while (count != 0)
176			zero.await();
177			}
178			finally {
179			lock.unlock();
180			}
181			}
182
183	tim	1.1
184			/**
185			* Causes the current thread to wait until the latch has counted down to
186			* zero, unless the thread is {@link Thread#interrupt interrupted},
187			* or the specified waiting time elapses.
188			*
189			* <p>If the current {@link #getCount count} is zero then this method
190			* returns immediately with the value <tt>true</tt>.
191			*
192			* <p>If the current {@link #getCount count} is greater than zero then
193			* the current thread becomes disabled for thread scheduling
194			* purposes and lies dormant until one of three things happen:
195			* <ul>
196			* <li>The count reaches zero due to invocations of the
197			* {@link #countDown} method; or
198			* <li>Some other thread {@link Thread#interrupt interrupts} the current
199			* thread; or
200			* <li>The specified waiting time elapses.
201			* </ul>
202			* <p>If the count reaches zero then the method returns with the
203			* value <tt>true</tt>.
204			* <p>If the current thread:
205			* <ul>
206			* <li>has its interrupted status set on entry to this method; or
207			* <li>is {@link Thread#interrupt interrupted} while waiting,
208			* </ul>
209			* then {@link InterruptedException} is thrown and the current thread's
210			* interrupted status is cleared.
211			*
212			* <p>If the specified waiting time elapses then the value <tt>false</tt>
213			* is returned.
214			* The given waiting time is a best-effort lower bound. If the time is
215			* less than or equal to zero, the method will not wait at all.
216			*
217			* @param timeout the maximum time to wait
218	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
219	tim	1.1	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
220			* if the waiting time elapsed before the count reached zero.
221			*
222			* @throws InterruptedException if the current thread is interrupted
223			* while waiting.
224			*/
225	dl	1.2	public boolean await(long timeout, TimeUnit unit)
226	tim	1.1	throws InterruptedException {
227	dl	1.2	long nanos = unit.toNanos(timeout);
228			lock.lock();
229			try {
230			for (;;) {
231			if (count == 0)
232			return true;
233			nanos = zero.awaitNanos(nanos);
234			if (nanos <= 0)
235			return false;
236			}
237			}
238			finally {
239			lock.unlock();
240			}
241	tim	1.1	}
242
243
244	dl	1.2
245	tim	1.1	/**
246			* Decrements the count of the latch, releasing all waiting threads if
247			* the count reaches zero.
248			* <p>If the current {@link #getCount count} is greater than zero then
249			* it is decremented. If the new count is zero then all waiting threads
250			* are re-enabled for thread scheduling purposes.
251			* <p>If the current {@link #getCount count} equals zero then nothing
252			* happens.
253			*/
254	dl	1.2	public void countDown() {
255			lock.lock();
256			try {
257			if (count > 0 && --count == 0)
258			zero.signalAll();
259			}
260			finally {
261			lock.unlock();
262			}
263			}
264	tim	1.1
265			/**
266			* Returns the current count.
267			* <p>This method is typically used for debugging and testing purposes.
268			* @return the current count.
269			*/
270			public long getCount() {
271	dl	1.2	lock.lock();
272			try {
273			return count;
274			}
275			finally {
276			lock.unlock();
277			}
278	tim	1.1	}
279			}