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
#	Content
1	/*
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	package java.util.concurrent;
8
9	/**
10	* A synchronization aid that allows one or more threads to wait until
11	* a set of operations being performed in other threads completes.
12	*
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	* released and any subsequent invocations of {@link #await} return
18	* 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	*
22	* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
23	* 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	*
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	* <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	* 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	* @since 1.5
122	* @spec JSR-166
123	* @revised $Date: 2003/06/23 02:26:16 $
124	* @editor $Author: brian $
125	* @author Doug Lea
126	*/
127	public class CountDownLatch {
128	private final ReentrantLock lock = new ReentrantLock();
129	private final Condition zero = lock.newCondition();
130	private int count;
131
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	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
140	*/
141	public CountDownLatch(int count) {
142	if (count < 0) throw new IllegalArgumentException("count < 0");
143	this.count = count;
144	}
145
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	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
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	* @param unit the time unit of the <tt>timeout</tt> argument.
219	* @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	public boolean await(long timeout, TimeUnit unit)
226	throws InterruptedException {
227	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	}
242
243
244
245	/**
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	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
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	lock.lock();
272	try {
273	return count;
274	}
275	finally {
276	lock.unlock();
277	}
278	}
279	}