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 <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 {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        Sync(int count) {
            setState(count); 
        }
        
        int getCount() {
            return getState();
        }

        public int tryAcquireShared(boolean isQueued, int acquires) {
            return getState() == 0? 1 : -1;
        }
        
        public boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                if (compareAndSetState(c, c-1)) 
                    return c == 1;
            }
        }
    }

    private final Sync sync;
    /**
     * 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.sync = new Sync(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 {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * 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 {
        return sync.acquireSharedTimed(1, unit.toNanos(timeout));
    }

    /**
     * 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() {
        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();
    }
}
Revision:	1.18
Committed:	Fri Jan 2 21:02:31 2004 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.17:	+2 -2 lines
Log Message:	Avoid timeout problems in fair modes; improve AQS method names
#	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			* <p>A <tt>CountDownLatch</tt> is initialized with a given
16	dholmes	1.9	* <em>count</em>. The {@link #await await} methods block until the current
17	dl	1.3	* {@link #getCount count} reaches zero due to invocations of the
18			* {@link #countDown} method, after which all waiting threads are
19	dholmes	1.9	* released and any subsequent invocations of {@link #await await} return
20	dl	1.3	* immediately. This is a one-shot phenomenon -- the count cannot be
21			* reset. If you need a version that resets the count, consider using
22			* a {@link CyclicBarrier}.
23	tim	1.1	*
24			* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
25	dl	1.5	* and can be used for a number of purposes. A
26			* <tt>CountDownLatch</tt> initialized with a count of one serves as a
27	dholmes	1.9	* simple on/off latch, or gate: all threads invoking {@link #await await}
28	dl	1.5	* wait at the gate until it is opened by a thread invoking {@link
29			* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
30			* can be used to make one thread wait until <em>N</em> threads have
31			* completed some action, or some action has been completed N times.
32			* <p>A useful property of a <tt>CountDownLatch</tt> is that it
33			* doesn't require that threads calling <tt>countDown</tt> wait for
34			* 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	tim	1.1	* @since 1.5
123	dl	1.5	* @author Doug Lea
124	tim	1.1	*/
125			public class CountDownLatch {
126	dl	1.16	/**
127			* Synchronization control For CountDownLatch.
128			* Uses AQS state to represent count.
129			*/
130			private static final class Sync extends AbstractQueuedSynchronizer {
131			Sync(int count) {
132	dl	1.17	setState(count);
133	dl	1.16	}
134
135	dl	1.17	int getCount() {
136			return getState();
137			}
138
139	dl	1.18	public int tryAcquireShared(boolean isQueued, int acquires) {
140	dl	1.17	return getState() == 0? 1 : -1;
141	dl	1.16	}
142
143	dl	1.18	public boolean tryReleaseShared(int releases) {
144	dl	1.16	// Decrement count; signal when transition to zero
145	dl	1.17	for (;;) {
146			int c = getState();
147			if (c == 0)
148			return false;
149			if (compareAndSetState(c, c-1))
150			return c == 1;
151			}
152	dl	1.16	}
153			}
154	tim	1.1
155	dl	1.16	private final Sync sync;
156	tim	1.1	/**
157			* Constructs a <tt>CountDownLatch</tt> initialized with the given
158			* count.
159			*
160			* @param count the number of times {@link #countDown} must be invoked
161			* before threads can pass through {@link #await}.
162			*
163	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
164	tim	1.1	*/
165	dl	1.2	public CountDownLatch(int count) {
166			if (count < 0) throw new IllegalArgumentException("count < 0");
167	dl	1.16	this.sync = new Sync(count);
168	dl	1.2	}
169	tim	1.1
170			/**
171			* Causes the current thread to wait until the latch has counted down to
172			* zero, unless the thread is {@link Thread#interrupt interrupted}.
173			*
174			* <p>If the current {@link #getCount count} is zero then this method
175			* returns immediately.
176			* <p>If the current {@link #getCount count} is greater than zero then
177			* the current thread becomes disabled for thread scheduling
178			* purposes and lies dormant until one of two things happen:
179			* <ul>
180	dholmes	1.9	* <li>The count reaches zero due to invocations of the
181	tim	1.1	* {@link #countDown} method; or
182	dholmes	1.9	* <li>Some other thread {@link Thread#interrupt interrupts} the current
183	tim	1.1	* thread.
184			* </ul>
185			* <p>If the current thread:
186			* <ul>
187			* <li>has its interrupted status set on entry to this method; or
188			* <li>is {@link Thread#interrupt interrupted} while waiting,
189			* </ul>
190			* then {@link InterruptedException} is thrown and the current thread's
191			* interrupted status is cleared.
192			*
193			* @throws InterruptedException if the current thread is interrupted
194			* while waiting.
195			*/
196	dl	1.2	public void await() throws InterruptedException {
197	dl	1.16	sync.acquireSharedInterruptibly(1);
198	dl	1.2	}
199
200	tim	1.1	/**
201			* Causes the current thread to wait until the latch has counted down to
202			* zero, unless the thread is {@link Thread#interrupt interrupted},
203			* or the specified waiting time elapses.
204			*
205			* <p>If the current {@link #getCount count} is zero then this method
206			* returns immediately with the value <tt>true</tt>.
207			*
208			* <p>If the current {@link #getCount count} is greater than zero then
209			* the current thread becomes disabled for thread scheduling
210			* purposes and lies dormant until one of three things happen:
211			* <ul>
212			* <li>The count reaches zero due to invocations of the
213			* {@link #countDown} method; or
214			* <li>Some other thread {@link Thread#interrupt interrupts} the current
215			* thread; or
216			* <li>The specified waiting time elapses.
217			* </ul>
218			* <p>If the count reaches zero then the method returns with the
219			* value <tt>true</tt>.
220			* <p>If the current thread:
221			* <ul>
222			* <li>has its interrupted status set on entry to this method; or
223			* <li>is {@link Thread#interrupt interrupted} while waiting,
224			* </ul>
225			* then {@link InterruptedException} is thrown and the current thread's
226			* interrupted status is cleared.
227			*
228			* <p>If the specified waiting time elapses then the value <tt>false</tt>
229			* is returned.
230	dholmes	1.10	* If the time is
231	tim	1.1	* less than or equal to zero, the method will not wait at all.
232			*
233			* @param timeout the maximum time to wait
234	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
235	tim	1.1	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
236			* if the waiting time elapsed before the count reached zero.
237			*
238			* @throws InterruptedException if the current thread is interrupted
239			* while waiting.
240			*/
241	dl	1.2	public boolean await(long timeout, TimeUnit unit)
242	tim	1.1	throws InterruptedException {
243	dl	1.16	return sync.acquireSharedTimed(1, unit.toNanos(timeout));
244	tim	1.1	}
245
246			/**
247			* Decrements the count of the latch, releasing all waiting threads if
248			* the count reaches zero.
249			* <p>If the current {@link #getCount count} is greater than zero then
250			* it is decremented. If the new count is zero then all waiting threads
251			* are re-enabled for thread scheduling purposes.
252			* <p>If the current {@link #getCount count} equals zero then nothing
253			* happens.
254			*/
255	dl	1.2	public void countDown() {
256	dl	1.16	sync.releaseShared(1);
257	dl	1.2	}
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	1.17	return sync.getCount();
266	tim	1.1	}
267			}