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 <tt>CountDownLatch</tt> allows one set of threads to wait until
 * the actions of another set of threads allow the first set to proceed.
 * <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 to 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.
 * A useful property of a <tt>CountDownLatch</tt> is that it doesn't
 * require that all threads wait before any can proceed, 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>
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/30 22:12:13 $
 * @editor $Author: dholmes $
 */
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:	1.2
Committed:	Tue May 27 18:14:39 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Changes since 1.1:	+59 -8 lines
Log Message:	re-check-in initial implementations
#	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			* A <tt>CountDownLatch</tt> allows one set of threads to wait until
11			* the actions of another set of threads allow the first set to proceed.
12			* <p>A <tt>CountDownLatch</tt> is initialized with a given <em>count</em>.
13			* The {@link #await} methods block until the current {@link #getCount count}
14			* reaches zero due to invocations of the {@link #countDown} method,
15			* after which all waiting threads are
16			* released and any subsequent invocations of {@link #await} return
17			* immediately. This is a one-shot phenomenon -- the count
18			* cannot be reset. If you need a version that resets the count,
19			* consider using a {@link CyclicBarrier}.
20			*
21			* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
22			* and can be used for a number of purposes.
23			* A <tt>CountDownLatch</tt> initialized to one serves as a simple on/off
24			* latch, or gate: all threads invoking {@link #await} wait at the gate until
25			* it is opened by a thread invoking {@link #countDown}.
26			* A <tt>CountDownLatch</tt> initialized to <em>N</em> can be used to make
27			* one thread wait until <em>N</em> threads have completed some action.
28			* A useful property of a <tt>CountDownLatch</tt> is that it doesn't
29			* require that all threads wait before any can proceed, it simply
30			* prevents any thread from proceeding past the {@link #await wait} until
31			* all threads could pass.
32			*
33			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
34			* of worker threads use two countdown latches:
35			* <ul>
36			* <li> The first is a start signal that prevents any worker from proceeding
37			* until the driver is ready for them to proceed;
38			* <li> The second is a completion signal that allows the driver to wait
39			* until all workers have completed.
40			* </ul>
41			*
42			* <pre>
43			* class Driver { // ...
44			* void main() throws InterruptedException {
45			* CountDownLatch startSignal = new CountDownLatch(1);
46			* CountDownLatch doneSignal = new CountDownLatch(N);
47			*
48			* for (int i = 0; i < N; ++i) // create and start threads
49			* new Thread(new Worker(startSignal, doneSignal)).start();
50			*
51			* doSomethingElse(); // don't let run yet
52			* startSignal.countDown(); // let all threads proceed
53			* doSomethingElse();
54			* doneSignal.await(); // wait for all to finish
55			* }
56			* }
57			*
58			* class Worker implements Runnable {
59			* private final CountDownLatch startSignal;
60			* private final CountDownLatch doneSignal;
61			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
62			* this.startSignal = startSignal;
63			* this.doneSignal = doneSignal;
64			* }
65			* public void run() {
66			* try {
67			* startSignal.await();
68			* doWork();
69			* doneSignal.countDown();
70			* }
71			* catch (InterruptedException ex) {} // return;
72			* }
73			*
74			* void doWork() { ... }
75			* }
76			*
77			* </pre>
78			*
79			* @since 1.5
80			* @spec JSR-166
81			* @revised $Date: 2003/01/30 22:12:13 $
82			* @editor $Author: dholmes $
83			*/
84			public class CountDownLatch {
85	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
86			private final Condition zero = lock.newCondition();
87			private int count;
88	tim	1.1
89			/**
90			* Constructs a <tt>CountDownLatch</tt> initialized with the given
91			* count.
92			*
93			* @param count the number of times {@link #countDown} must be invoked
94			* before threads can pass through {@link #await}.
95			*
96	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
97	tim	1.1	*/
98	dl	1.2	public CountDownLatch(int count) {
99			if (count < 0) throw new IllegalArgumentException("count < 0");
100			this.count = count;
101			}
102	tim	1.1
103			/**
104			* Causes the current thread to wait until the latch has counted down to
105			* zero, unless the thread is {@link Thread#interrupt interrupted}.
106			*
107			* <p>If the current {@link #getCount count} is zero then this method
108			* returns immediately.
109			* <p>If the current {@link #getCount count} is greater than zero then
110			* the current thread becomes disabled for thread scheduling
111			* purposes and lies dormant until one of two things happen:
112			* <ul>
113			* <li> The count reaches zero due to invocations of the
114			* {@link #countDown} method; or
115			* <li> Some other thread {@link Thread#interrupt interrupts} the current
116			* thread.
117			* </ul>
118			* <p>If the current thread:
119			* <ul>
120			* <li>has its interrupted status set on entry to this method; or
121			* <li>is {@link Thread#interrupt interrupted} while waiting,
122			* </ul>
123			* then {@link InterruptedException} is thrown and the current thread's
124			* interrupted status is cleared.
125			*
126			* @throws InterruptedException if the current thread is interrupted
127			* while waiting.
128			*/
129	dl	1.2	public void await() throws InterruptedException {
130			lock.lock();
131			try {
132			while (count != 0)
133			zero.await();
134			}
135			finally {
136			lock.unlock();
137			}
138			}
139
140	tim	1.1
141			/**
142			* Causes the current thread to wait until the latch has counted down to
143			* zero, unless the thread is {@link Thread#interrupt interrupted},
144			* or the specified waiting time elapses.
145			*
146			* <p>If the current {@link #getCount count} is zero then this method
147			* returns immediately with the value <tt>true</tt>.
148			*
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 three things happen:
152			* <ul>
153			* <li>The count reaches zero due to invocations of the
154			* {@link #countDown} method; or
155			* <li>Some other thread {@link Thread#interrupt interrupts} the current
156			* thread; or
157			* <li>The specified waiting time elapses.
158			* </ul>
159			* <p>If the count reaches zero then the method returns with the
160			* value <tt>true</tt>.
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			* <p>If the specified waiting time elapses then the value <tt>false</tt>
170			* is returned.
171			* The given waiting time is a best-effort lower bound. If the time is
172			* less than or equal to zero, the method will not wait at all.
173			*
174			* @param timeout the maximum time to wait
175	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
176	tim	1.1	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
177			* if the waiting time elapsed before the count reached zero.
178			*
179			* @throws InterruptedException if the current thread is interrupted
180			* while waiting.
181			*/
182	dl	1.2	public boolean await(long timeout, TimeUnit unit)
183	tim	1.1	throws InterruptedException {
184	dl	1.2	long nanos = unit.toNanos(timeout);
185			lock.lock();
186			try {
187			for (;;) {
188			if (count == 0)
189			return true;
190			nanos = zero.awaitNanos(nanos);
191			if (nanos <= 0)
192			return false;
193			}
194			}
195			finally {
196			lock.unlock();
197			}
198	tim	1.1	}
199
200
201	dl	1.2
202	tim	1.1	/**
203			* Decrements the count of the latch, releasing all waiting threads if
204			* the count reaches zero.
205			* <p>If the current {@link #getCount count} is greater than zero then
206			* it is decremented. If the new count is zero then all waiting threads
207			* are re-enabled for thread scheduling purposes.
208			* <p>If the current {@link #getCount count} equals zero then nothing
209			* happens.
210			*/
211	dl	1.2	public void countDown() {
212			lock.lock();
213			try {
214			if (count > 0 && --count == 0)
215			zero.signalAll();
216			}
217			finally {
218			lock.unlock();
219			}
220			}
221	tim	1.1
222			/**
223			* Returns the current count.
224			* <p>This method is typically used for debugging and testing purposes.
225			* @return the current count.
226			*/
227			public long getCount() {
228	dl	1.2	lock.lock();
229			try {
230			return count;
231			}
232			finally {
233			lock.unlock();
234			}
235	tim	1.1	}
236			}