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: 2003/06/07 18:20:20 $
 * @editor $Author: dl $
 */
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:	1.4
Committed:	Mon Jun 23 02:26:16 2003 UTC (20 years, 11 months ago) by brian
Branch:	MAIN
Changes since 1.3:	+47 -8 lines
Log Message:	Partial javadoc pass
#	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.
24	* A <tt>CountDownLatch</tt> initialized with a count of one serves as a simple on/off
25	* latch, or gate: all threads invoking {@link #await} wait at the gate until
26	* it is opened by a thread invoking {@link #countDown}.
27	* A <tt>CountDownLatch</tt> initialized to <em>N</em> can be used to make
28	* one thread wait until <em>N</em> threads have completed some action, or some
29	* action has been completed N times.
30	* <p>A useful property of a <tt>CountDownLatch</tt> is that it doesn't
31	* require that threads calling <tt>countDown</tt> wait for the count to reach zero
32	* before proceeding, it simply
33	* prevents any thread from proceeding past the {@link #await wait} until
34	* all 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, describe each part
83	* with a Runnable that executes that portion and counts down on the latch, and queue all
84	* the Runnables to an Executor. When all sub-parts are complete, the coordinating thread
85	* will be able to pass through await.
86	*
87	* <pre>
88	* class Driver2 { // ...
89	* void main() throws InterruptedException {
90	* CountDownLatch doneSignal = new CountDownLatch(N);
91	* Executor e = ...
92	*
93	* for (int i = 0; i < N; ++i) // create and start threads
94	* e.execute(new WorkerRunnable(doneSignal, i));
95	*
96	* doneSignal.await(); // wait for all to finish
97	* }
98	* }
99	*
100	* class WorkerRunnable implements Runnable {
101	* private final CountDownLatch doneSignal;
102	* private final int i;
103	* Worker(CountDownLatch doneSignal, int i) {
104	* this.doneSignal = doneSignal;
105	* this.i = i;
106	* }
107	* public void run() {
108	* try {
109	* doWork(i);
110	* doneSignal.countDown();
111	* }
112	* catch (InterruptedException ex) {} // return;
113	* }
114	*
115	* void doWork() { ... }
116	* }
117	*
118	* </pre>
119	*
120	* @since 1.5
121	* @spec JSR-166
122	* @revised $Date: 2003/06/07 18:20:20 $
123	* @editor $Author: dl $
124	*/
125	public class CountDownLatch {
126	private final ReentrantLock lock = new ReentrantLock();
127	private final Condition zero = lock.newCondition();
128	private int count;
129
130	/**
131	* Constructs a <tt>CountDownLatch</tt> initialized with the given
132	* count.
133	*
134	* @param count the number of times {@link #countDown} must be invoked
135	* before threads can pass through {@link #await}.
136	*
137	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
138	*/
139	public CountDownLatch(int count) {
140	if (count < 0) throw new IllegalArgumentException("count < 0");
141	this.count = count;
142	}
143
144	/**
145	* Causes the current thread to wait until the latch has counted down to
146	* zero, unless the thread is {@link Thread#interrupt interrupted}.
147	*
148	* <p>If the current {@link #getCount count} is zero then this method
149	* returns immediately.
150	* <p>If the current {@link #getCount count} is greater than zero then
151	* the current thread becomes disabled for thread scheduling
152	* purposes and lies dormant until one of two things happen:
153	* <ul>
154	* <li> The count reaches zero due to invocations of the
155	* {@link #countDown} method; or
156	* <li> Some other thread {@link Thread#interrupt interrupts} the current
157	* thread.
158	* </ul>
159	* <p>If the current thread:
160	* <ul>
161	* <li>has its interrupted status set on entry to this method; or
162	* <li>is {@link Thread#interrupt interrupted} while waiting,
163	* </ul>
164	* then {@link InterruptedException} is thrown and the current thread's
165	* interrupted status is cleared.
166	*
167	* @throws InterruptedException if the current thread is interrupted
168	* while waiting.
169	*/
170	public void await() throws InterruptedException {
171	lock.lock();
172	try {
173	while (count != 0)
174	zero.await();
175	}
176	finally {
177	lock.unlock();
178	}
179	}
180
181
182	/**
183	* Causes the current thread to wait until the latch has counted down to
184	* zero, unless the thread is {@link Thread#interrupt interrupted},
185	* or the specified waiting time elapses.
186	*
187	* <p>If the current {@link #getCount count} is zero then this method
188	* returns immediately with the value <tt>true</tt>.
189	*
190	* <p>If the current {@link #getCount count} is greater than zero then
191	* the current thread becomes disabled for thread scheduling
192	* purposes and lies dormant until one of three things happen:
193	* <ul>
194	* <li>The count reaches zero due to invocations of the
195	* {@link #countDown} method; or
196	* <li>Some other thread {@link Thread#interrupt interrupts} the current
197	* thread; or
198	* <li>The specified waiting time elapses.
199	* </ul>
200	* <p>If the count reaches zero then the method returns with the
201	* value <tt>true</tt>.
202	* <p>If the current thread:
203	* <ul>
204	* <li>has its interrupted status set on entry to this method; or
205	* <li>is {@link Thread#interrupt interrupted} while waiting,
206	* </ul>
207	* then {@link InterruptedException} is thrown and the current thread's
208	* interrupted status is cleared.
209	*
210	* <p>If the specified waiting time elapses then the value <tt>false</tt>
211	* is returned.
212	* The given waiting time is a best-effort lower bound. If the time is
213	* less than or equal to zero, the method will not wait at all.
214	*
215	* @param timeout the maximum time to wait
216	* @param unit the time unit of the <tt>timeout</tt> argument.
217	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
218	* if the waiting time elapsed before the count reached zero.
219	*
220	* @throws InterruptedException if the current thread is interrupted
221	* while waiting.
222	*/
223	public boolean await(long timeout, TimeUnit unit)
224	throws InterruptedException {
225	long nanos = unit.toNanos(timeout);
226	lock.lock();
227	try {
228	for (;;) {
229	if (count == 0)
230	return true;
231	nanos = zero.awaitNanos(nanos);
232	if (nanos <= 0)
233	return false;
234	}
235	}
236	finally {
237	lock.unlock();
238	}
239	}
240
241
242
243	/**
244	* Decrements the count of the latch, releasing all waiting threads if
245	* the count reaches zero.
246	* <p>If the current {@link #getCount count} is greater than zero then
247	* it is decremented. If the new count is zero then all waiting threads
248	* are re-enabled for thread scheduling purposes.
249	* <p>If the current {@link #getCount count} equals zero then nothing
250	* happens.
251	*/
252	public void countDown() {
253	lock.lock();
254	try {
255	if (count > 0 && --count == 0)
256	zero.signalAll();
257	}
258	finally {
259	lock.unlock();
260	}
261	}
262
263	/**
264	* Returns the current count.
265	* <p>This method is typically used for debugging and testing purposes.
266	* @return the current count.
267	*/
268	public long getCount() {
269	lock.lock();
270	try {
271	return count;
272	}
273	finally {
274	lock.unlock();
275	}
276	}
277	}