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.*; // for javadoc (till 6280605 is fixed)
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>
 *
 * <p> Memory consistency effects: Actions in a thread prior to calling
 * <tt>countDown()</tt> <a
 * href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * <tt>await()</tt> in another thread.
 *
 * @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 {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        public int tryAcquireShared(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;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    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.tryAcquireSharedNanos(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();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String
     * &quot;Count =&quot; followed by the current count.
     * @return a string identifying this latch, as well as its
     * state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }

}
Revision:	1.30
Committed:	Thu Sep 8 00:04:00 2005 UTC (18 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.29:	+1 -1 lines
Log Message:	Edit pass for happens-before descriptions
#	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	jsr166	1.28	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
9	dl	1.6	import java.util.concurrent.locks.*;
10	dl	1.16	import java.util.concurrent.atomic.*;
11	tim	1.1
12			/**
13	brian	1.4	* A synchronization aid that allows one or more threads to wait until
14			* a set of operations being performed in other threads completes.
15	dl	1.3	*
16			* <p>A <tt>CountDownLatch</tt> is initialized with a given
17	dholmes	1.9	* <em>count</em>. The {@link #await await} methods block until the current
18	dl	1.3	* {@link #getCount count} reaches zero due to invocations of the
19			* {@link #countDown} method, after which all waiting threads are
20	dholmes	1.9	* released and any subsequent invocations of {@link #await await} return
21	dl	1.3	* immediately. This is a one-shot phenomenon -- the count cannot be
22			* reset. If you need a version that resets the count, consider using
23			* a {@link CyclicBarrier}.
24	tim	1.1	*
25			* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
26	dl	1.5	* and can be used for a number of purposes. A
27			* <tt>CountDownLatch</tt> initialized with a count of one serves as a
28	dholmes	1.9	* simple on/off latch, or gate: all threads invoking {@link #await await}
29	dl	1.5	* wait at the gate until it is opened by a thread invoking {@link
30			* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
31			* can be used to make one thread wait until <em>N</em> threads have
32			* completed some action, or some action has been completed N times.
33			* <p>A useful property of a <tt>CountDownLatch</tt> is that it
34			* doesn't require that threads calling <tt>countDown</tt> wait for
35			* the count to reach zero before proceeding, it simply prevents any
36	dholmes	1.9	* thread from proceeding past an {@link #await await} until all
37	dl	1.5	* threads could pass.
38	tim	1.1	*
39			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
40			* of worker threads use two countdown latches:
41			* <ul>
42	dholmes	1.9	* <li>The first is a start signal that prevents any worker from proceeding
43	tim	1.1	* until the driver is ready for them to proceed;
44	dholmes	1.9	* <li>The second is a completion signal that allows the driver to wait
45	tim	1.1	* until all workers have completed.
46			* </ul>
47			*
48			* <pre>
49			* class Driver { // ...
50			* void main() throws InterruptedException {
51			* CountDownLatch startSignal = new CountDownLatch(1);
52			* CountDownLatch doneSignal = new CountDownLatch(N);
53			*
54			* for (int i = 0; i < N; ++i) // create and start threads
55			* new Thread(new Worker(startSignal, doneSignal)).start();
56			*
57			* doSomethingElse(); // don't let run yet
58			* startSignal.countDown(); // let all threads proceed
59			* doSomethingElse();
60			* doneSignal.await(); // wait for all to finish
61			* }
62			* }
63			*
64			* class Worker implements Runnable {
65			* private final CountDownLatch startSignal;
66			* private final CountDownLatch doneSignal;
67			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
68			* this.startSignal = startSignal;
69			* this.doneSignal = doneSignal;
70			* }
71			* public void run() {
72			* try {
73			* startSignal.await();
74			* doWork();
75			* doneSignal.countDown();
76	tim	1.7	* } catch (InterruptedException ex) {} // return;
77	tim	1.1	* }
78			*
79			* void doWork() { ... }
80			* }
81			*
82			* </pre>
83			*
84	dl	1.5	* <p>Another typical usage would be to divide a problem into N parts,
85			* describe each part with a Runnable that executes that portion and
86			* counts down on the latch, and queue all the Runnables to an
87			* Executor. When all sub-parts are complete, the coordinating thread
88	dl	1.13	* will be able to pass through await. (When threads must repeatedly
89			* count down in this way, instead use a {@link CyclicBarrier}.)
90	brian	1.4	*
91			* <pre>
92			* class Driver2 { // ...
93			* void main() throws InterruptedException {
94			* CountDownLatch doneSignal = new CountDownLatch(N);
95			* Executor e = ...
96			*
97			* for (int i = 0; i < N; ++i) // create and start threads
98			* e.execute(new WorkerRunnable(doneSignal, i));
99			*
100			* doneSignal.await(); // wait for all to finish
101			* }
102			* }
103			*
104			* class WorkerRunnable implements Runnable {
105			* private final CountDownLatch doneSignal;
106			* private final int i;
107	dl	1.13	* WorkerRunnable(CountDownLatch doneSignal, int i) {
108	brian	1.4	* this.doneSignal = doneSignal;
109			* this.i = i;
110			* }
111			* public void run() {
112			* try {
113			* doWork(i);
114			* doneSignal.countDown();
115	tim	1.7	* } catch (InterruptedException ex) {} // return;
116	brian	1.4	* }
117			*
118			* void doWork() { ... }
119			* }
120			*
121			* </pre>
122			*
123	dl	1.30	* <p> Memory consistency effects: Actions in a thread prior to calling
124	brian	1.29	* <tt>countDown()</tt> <a
125			* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
126			* actions following a successful return from a corresponding
127			* <tt>await()</tt> in another thread.
128			*
129	tim	1.1	* @since 1.5
130	dl	1.5	* @author Doug Lea
131	tim	1.1	*/
132			public class CountDownLatch {
133	dl	1.16	/**
134			* Synchronization control For CountDownLatch.
135			* Uses AQS state to represent count.
136			*/
137			private static final class Sync extends AbstractQueuedSynchronizer {
138	dl	1.27	private static final long serialVersionUID = 4982264981922014374L;
139
140	dl	1.16	Sync(int count) {
141	jsr166	1.25	setState(count);
142	dl	1.16	}
143	jsr166	1.25
144	dl	1.17	int getCount() {
145			return getState();
146			}
147
148	dl	1.19	public int tryAcquireShared(int acquires) {
149	dl	1.17	return getState() == 0? 1 : -1;
150	dl	1.16	}
151	jsr166	1.25
152	dl	1.18	public boolean tryReleaseShared(int releases) {
153	dl	1.16	// Decrement count; signal when transition to zero
154	dl	1.17	for (;;) {
155			int c = getState();
156			if (c == 0)
157			return false;
158	dl	1.19	int nextc = c-1;
159	jsr166	1.25	if (compareAndSetState(c, nextc))
160	dl	1.19	return nextc == 0;
161	dl	1.17	}
162	dl	1.16	}
163			}
164	tim	1.1
165	dl	1.16	private final Sync sync;
166	tim	1.1	/**
167			* Constructs a <tt>CountDownLatch</tt> initialized with the given
168			* count.
169	jsr166	1.25	*
170	tim	1.1	* @param count the number of times {@link #countDown} must be invoked
171			* before threads can pass through {@link #await}.
172			*
173	dl	1.2	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
174	tim	1.1	*/
175	jsr166	1.25	public CountDownLatch(int count) {
176	dl	1.2	if (count < 0) throw new IllegalArgumentException("count < 0");
177	dl	1.16	this.sync = new Sync(count);
178	dl	1.2	}
179	tim	1.1
180			/**
181	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
182	tim	1.1	* zero, unless the thread is {@link Thread#interrupt interrupted}.
183			*
184			* <p>If the current {@link #getCount count} is zero then this method
185			* returns immediately.
186			* <p>If the current {@link #getCount count} is greater than zero then
187	jsr166	1.25	* the current thread becomes disabled for thread scheduling
188	tim	1.1	* purposes and lies dormant until one of two things happen:
189			* <ul>
190	dholmes	1.9	* <li>The count reaches zero due to invocations of the
191	tim	1.1	* {@link #countDown} method; or
192	dholmes	1.9	* <li>Some other thread {@link Thread#interrupt interrupts} the current
193	tim	1.1	* thread.
194			* </ul>
195			* <p>If the current thread:
196			* <ul>
197	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
198			* <li>is {@link Thread#interrupt interrupted} while waiting,
199	tim	1.1	* </ul>
200	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
201			* interrupted status is cleared.
202	tim	1.1	*
203			* @throws InterruptedException if the current thread is interrupted
204			* while waiting.
205			*/
206	dl	1.2	public void await() throws InterruptedException {
207	dl	1.16	sync.acquireSharedInterruptibly(1);
208	dl	1.2	}
209
210	tim	1.1	/**
211	jsr166	1.25	* Causes the current thread to wait until the latch has counted down to
212	tim	1.1	* zero, unless the thread is {@link Thread#interrupt interrupted},
213			* or the specified waiting time elapses.
214			*
215			* <p>If the current {@link #getCount count} is zero then this method
216			* returns immediately with the value <tt>true</tt>.
217			*
218			* <p>If the current {@link #getCount count} is greater than zero then
219	jsr166	1.25	* the current thread becomes disabled for thread scheduling
220	tim	1.1	* purposes and lies dormant until one of three things happen:
221			* <ul>
222			* <li>The count reaches zero due to invocations of the
223			* {@link #countDown} method; or
224			* <li>Some other thread {@link Thread#interrupt interrupts} the current
225			* thread; or
226			* <li>The specified waiting time elapses.
227			* </ul>
228			* <p>If the count reaches zero then the method returns with the
229			* value <tt>true</tt>.
230			* <p>If the current thread:
231			* <ul>
232	jsr166	1.25	* <li>has its interrupted status set on entry to this method; or
233			* <li>is {@link Thread#interrupt interrupted} while waiting,
234	tim	1.1	* </ul>
235	jsr166	1.25	* then {@link InterruptedException} is thrown and the current thread's
236			* interrupted status is cleared.
237	tim	1.1	*
238			* <p>If the specified waiting time elapses then the value <tt>false</tt>
239			* is returned.
240	jsr166	1.25	* If the time is
241	tim	1.1	* less than or equal to zero, the method will not wait at all.
242			*
243			* @param timeout the maximum time to wait
244	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
245	jsr166	1.26	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
246	tim	1.1	* if the waiting time elapsed before the count reached zero.
247			*
248			* @throws InterruptedException if the current thread is interrupted
249			* while waiting.
250			*/
251	jsr166	1.25	public boolean await(long timeout, TimeUnit unit)
252	tim	1.1	throws InterruptedException {
253	dl	1.23	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
254	tim	1.1	}
255
256			/**
257			* Decrements the count of the latch, releasing all waiting threads if
258			* the count reaches zero.
259			* <p>If the current {@link #getCount count} is greater than zero then
260			* it is decremented. If the new count is zero then all waiting threads
261			* are re-enabled for thread scheduling purposes.
262			* <p>If the current {@link #getCount count} equals zero then nothing
263			* happens.
264			*/
265	dl	1.2	public void countDown() {
266	dl	1.16	sync.releaseShared(1);
267	dl	1.2	}
268	tim	1.1
269			/**
270			* Returns the current count.
271			* <p>This method is typically used for debugging and testing purposes.
272			* @return the current count.
273			*/
274			public long getCount() {
275	dl	1.17	return sync.getCount();
276	tim	1.1	}
277	dl	1.21
278			/**
279	dl	1.24	* Returns a string identifying this latch, as well as its state.
280	jsr166	1.25	* The state, in brackets, includes the String
281	dl	1.22	* "Count =" followed by the current count.
282			* @return a string identifying this latch, as well as its
283			* state
284	dl	1.21	*/
285			public String toString() {
286			return super.toString() + "[Count = " + sync.getCount() + "]";
287			}
288
289	tim	1.1	}