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;
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 <tt>CountDownLatch</tt> is initialized with a given
 * <em>count</em>.  The {@link #await 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 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 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 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>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    private final ReentrantLock lock = new ReentrantLock();
    private final ReentrantLock.ConditionObject 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.
     * 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:	1.13
Committed:	Mon Nov 3 00:11:46 2003 UTC (20 years, 7 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_NOV3_FREEZE, JSR166_DEC9_PRE_ES_SUBMIT, JSR166_DEC9_POST_ES_SUBMIT
Changes since 1.12:	+3 -2 lines
Log Message:	Fixed typo in example
#	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	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			* <p>A <tt>CountDownLatch</tt> is initialized with a given
15	dholmes	1.9	* <em>count</em>. The {@link #await await} methods block until the current
16	dl	1.3	* {@link #getCount count} reaches zero due to invocations of the
17			* {@link #countDown} method, after which all waiting threads are
18	dholmes	1.9	* released and any subsequent invocations of {@link #await await} return
19	dl	1.3	* immediately. This is a one-shot phenomenon -- the count cannot be
20			* reset. If you need a version that resets the count, consider using
21			* a {@link CyclicBarrier}.
22	tim	1.1	*
23			* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
24	dl	1.5	* and can be used for a number of purposes. A
25			* <tt>CountDownLatch</tt> 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			* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
29			* 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			* <p>A useful property of a <tt>CountDownLatch</tt> is that it
32			* doesn't require that threads calling <tt>countDown</tt> wait for
33			* 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	tim	1.1	* @since 1.5
122	dl	1.5	* @author Doug Lea
123	tim	1.1	*/
124			public class CountDownLatch {
125	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
126	dl	1.12	private final ReentrantLock.ConditionObject zero = lock.newCondition();
127	dl	1.2	private int count;
128	tim	1.1
129			/**
130			* Constructs a <tt>CountDownLatch</tt> initialized with the given
131			* count.
132			*
133			* @param count the number of times {@link #countDown} must be invoked
134			* before threads can pass through {@link #await}.
135			*
136	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
137	tim	1.1	*/
138	dl	1.2	public CountDownLatch(int count) {
139			if (count < 0) throw new IllegalArgumentException("count < 0");
140			this.count = count;
141			}
142	tim	1.1
143			/**
144			* Causes the current thread to wait until the latch has counted down to
145			* zero, unless the thread is {@link Thread#interrupt interrupted}.
146			*
147			* <p>If the current {@link #getCount count} is zero then this method
148			* returns immediately.
149			* <p>If the current {@link #getCount count} is greater than zero then
150			* the current thread becomes disabled for thread scheduling
151			* purposes and lies dormant until one of two things happen:
152			* <ul>
153	dholmes	1.9	* <li>The count reaches zero due to invocations of the
154	tim	1.1	* {@link #countDown} method; or
155	dholmes	1.9	* <li>Some other thread {@link Thread#interrupt interrupts} the current
156	tim	1.1	* thread.
157			* </ul>
158			* <p>If the current thread:
159			* <ul>
160			* <li>has its interrupted status set on entry to this method; or
161			* <li>is {@link Thread#interrupt interrupted} while waiting,
162			* </ul>
163			* then {@link InterruptedException} is thrown and the current thread's
164			* interrupted status is cleared.
165			*
166			* @throws InterruptedException if the current thread is interrupted
167			* while waiting.
168			*/
169	dl	1.2	public void await() throws InterruptedException {
170			lock.lock();
171			try {
172			while (count != 0)
173			zero.await();
174	tim	1.7	} finally {
175	dl	1.2	lock.unlock();
176			}
177			}
178
179	tim	1.1
180			/**
181			* Causes the current thread to wait until the latch has counted down to
182			* zero, unless the thread is {@link Thread#interrupt interrupted},
183			* or the specified waiting time elapses.
184			*
185			* <p>If the current {@link #getCount count} is zero then this method
186			* returns immediately with the value <tt>true</tt>.
187			*
188			* <p>If the current {@link #getCount count} is greater than zero then
189			* the current thread becomes disabled for thread scheduling
190			* purposes and lies dormant until one of three things happen:
191			* <ul>
192			* <li>The count reaches zero due to invocations of the
193			* {@link #countDown} method; or
194			* <li>Some other thread {@link Thread#interrupt interrupts} the current
195			* thread; or
196			* <li>The specified waiting time elapses.
197			* </ul>
198			* <p>If the count reaches zero then the method returns with the
199			* value <tt>true</tt>.
200			* <p>If the current thread:
201			* <ul>
202			* <li>has its interrupted status set on entry to this method; or
203			* <li>is {@link Thread#interrupt interrupted} while waiting,
204			* </ul>
205			* then {@link InterruptedException} is thrown and the current thread's
206			* interrupted status is cleared.
207			*
208			* <p>If the specified waiting time elapses then the value <tt>false</tt>
209			* is returned.
210	dholmes	1.10	* If the time is
211	tim	1.1	* less than or equal to zero, the method will not wait at all.
212			*
213			* @param timeout the maximum time to wait
214	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
215	tim	1.1	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
216			* if the waiting time elapsed before the count reached zero.
217			*
218			* @throws InterruptedException if the current thread is interrupted
219			* while waiting.
220			*/
221	dl	1.2	public boolean await(long timeout, TimeUnit unit)
222	tim	1.1	throws InterruptedException {
223	dl	1.2	long nanos = unit.toNanos(timeout);
224			lock.lock();
225			try {
226			for (;;) {
227			if (count == 0)
228			return true;
229			nanos = zero.awaitNanos(nanos);
230			if (nanos <= 0)
231			return false;
232			}
233	tim	1.7	} finally {
234	dl	1.2	lock.unlock();
235			}
236	tim	1.1	}
237
238
239	dl	1.2
240	tim	1.1	/**
241			* Decrements the count of the latch, releasing all waiting threads if
242			* the count reaches zero.
243			* <p>If the current {@link #getCount count} is greater than zero then
244			* it is decremented. If the new count is zero then all waiting threads
245			* are re-enabled for thread scheduling purposes.
246			* <p>If the current {@link #getCount count} equals zero then nothing
247			* happens.
248			*/
249	dl	1.2	public void countDown() {
250			lock.lock();
251			try {
252			if (count > 0 && --count == 0)
253			zero.signalAll();
254	tim	1.7	} finally {
255	dl	1.2	lock.unlock();
256			}
257			}
258	tim	1.1
259			/**
260			* Returns the current count.
261			* <p>This method is typically used for debugging and testing purposes.
262			* @return the current count.
263			*/
264			public long getCount() {
265	dl	1.2	lock.lock();
266			try {
267			return count;
268	tim	1.7	} finally {
269	dl	1.2	lock.unlock();
270			}
271	tim	1.1	}
272			}