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;
import java.util.concurrent.locks.*;

/**
 * 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.
 *
 * <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
 * @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.
     * 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:	1.11
Committed:	Sun Aug 31 13:33:13 2003 UTC (20 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.10:	+0 -3 lines
Log Message:	Removed non-standard tags and misc javadoc cleanup
#	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	import java.util.concurrent.locks.*;
9
10	/**
11	* A synchronization aid that allows one or more threads to wait until
12	* a set of operations being performed in other threads completes.
13	*
14	* <p>A <tt>CountDownLatch</tt> is initialized with a given
15	* <em>count</em>. The {@link #await await} methods block until the current
16	* {@link #getCount count} reaches zero due to invocations of the
17	* {@link #countDown} method, after which all waiting threads are
18	* released and any subsequent invocations of {@link #await await} return
19	* immediately. This is a one-shot phenomenon -- the count cannot be
20	* reset. If you need a version that resets the count, consider using
21	* a {@link CyclicBarrier}.
22	*
23	* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
24	* and can be used for a number of purposes. A
25	* <tt>CountDownLatch</tt> initialized with a count of one serves as a
26	* simple on/off latch, or gate: all threads invoking {@link #await await}
27	* wait at the gate until it is opened by a thread invoking {@link
28	* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
29	* can be used to make one thread wait until <em>N</em> threads have
30	* completed some action, or some action has been completed N times.
31	* <p>A useful property of a <tt>CountDownLatch</tt> is that it
32	* doesn't require that threads calling <tt>countDown</tt> wait for
33	* the count to reach zero before proceeding, it simply prevents any
34	* thread from proceeding past an {@link #await await} until all
35	* threads could pass.
36	*
37	* <p><b>Sample usage:</b> Here is a pair of classes in which a group
38	* of worker threads use two countdown latches:
39	* <ul>
40	* <li>The first is a start signal that prevents any worker from proceeding
41	* until the driver is ready for them to proceed;
42	* <li>The second is a completion signal that allows the driver to wait
43	* until all workers have completed.
44	* </ul>
45	*
46	* <pre>
47	* class Driver { // ...
48	* void main() throws InterruptedException {
49	* CountDownLatch startSignal = new CountDownLatch(1);
50	* CountDownLatch doneSignal = new CountDownLatch(N);
51	*
52	* for (int i = 0; i < N; ++i) // create and start threads
53	* new Thread(new Worker(startSignal, doneSignal)).start();
54	*
55	* doSomethingElse(); // don't let run yet
56	* startSignal.countDown(); // let all threads proceed
57	* doSomethingElse();
58	* doneSignal.await(); // wait for all to finish
59	* }
60	* }
61	*
62	* class Worker implements Runnable {
63	* private final CountDownLatch startSignal;
64	* private final CountDownLatch doneSignal;
65	* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
66	* this.startSignal = startSignal;
67	* this.doneSignal = doneSignal;
68	* }
69	* public void run() {
70	* try {
71	* startSignal.await();
72	* doWork();
73	* doneSignal.countDown();
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	* } catch (InterruptedException ex) {} // return;
113	* }
114	*
115	* void doWork() { ... }
116	* }
117	*
118	* </pre>
119	*
120	* @since 1.5
121	* @author Doug Lea
122	*/
123	public class CountDownLatch {
124	private final ReentrantLock lock = new ReentrantLock();
125	private final Condition zero = lock.newCondition();
126	private int count;
127
128	/**
129	* Constructs a <tt>CountDownLatch</tt> initialized with the given
130	* count.
131	*
132	* @param count the number of times {@link #countDown} must be invoked
133	* before threads can pass through {@link #await}.
134	*
135	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
136	*/
137	public CountDownLatch(int count) {
138	if (count < 0) throw new IllegalArgumentException("count < 0");
139	this.count = count;
140	}
141
142	/**
143	* Causes the current thread to wait until the latch has counted down to
144	* zero, unless the thread is {@link Thread#interrupt interrupted}.
145	*
146	* <p>If the current {@link #getCount count} is zero then this method
147	* returns immediately.
148	* <p>If the current {@link #getCount count} is greater than zero then
149	* the current thread becomes disabled for thread scheduling
150	* purposes and lies dormant until one of two things happen:
151	* <ul>
152	* <li>The count reaches zero due to invocations of the
153	* {@link #countDown} method; or
154	* <li>Some other thread {@link Thread#interrupt interrupts} the current
155	* thread.
156	* </ul>
157	* <p>If the current thread:
158	* <ul>
159	* <li>has its interrupted status set on entry to this method; or
160	* <li>is {@link Thread#interrupt interrupted} while waiting,
161	* </ul>
162	* then {@link InterruptedException} is thrown and the current thread's
163	* interrupted status is cleared.
164	*
165	* @throws InterruptedException if the current thread is interrupted
166	* while waiting.
167	*/
168	public void await() throws InterruptedException {
169	lock.lock();
170	try {
171	while (count != 0)
172	zero.await();
173	} finally {
174	lock.unlock();
175	}
176	}
177
178
179	/**
180	* Causes the current thread to wait until the latch has counted down to
181	* zero, unless the thread is {@link Thread#interrupt interrupted},
182	* or the specified waiting time elapses.
183	*
184	* <p>If the current {@link #getCount count} is zero then this method
185	* returns immediately with the value <tt>true</tt>.
186	*
187	* <p>If the current {@link #getCount count} is greater than zero then
188	* the current thread becomes disabled for thread scheduling
189	* purposes and lies dormant until one of three things happen:
190	* <ul>
191	* <li>The count reaches zero due to invocations of the
192	* {@link #countDown} method; or
193	* <li>Some other thread {@link Thread#interrupt interrupts} the current
194	* thread; or
195	* <li>The specified waiting time elapses.
196	* </ul>
197	* <p>If the count reaches zero then the method returns with the
198	* value <tt>true</tt>.
199	* <p>If the current thread:
200	* <ul>
201	* <li>has its interrupted status set on entry to this method; or
202	* <li>is {@link Thread#interrupt interrupted} while waiting,
203	* </ul>
204	* then {@link InterruptedException} is thrown and the current thread's
205	* interrupted status is cleared.
206	*
207	* <p>If the specified waiting time elapses then the value <tt>false</tt>
208	* is returned.
209	* If the time is
210	* less than or equal to zero, the method will not wait at all.
211	*
212	* @param timeout the maximum time to wait
213	* @param unit the time unit of the <tt>timeout</tt> argument.
214	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
215	* if the waiting time elapsed before the count reached zero.
216	*
217	* @throws InterruptedException if the current thread is interrupted
218	* while waiting.
219	*/
220	public boolean await(long timeout, TimeUnit unit)
221	throws InterruptedException {
222	long nanos = unit.toNanos(timeout);
223	lock.lock();
224	try {
225	for (;;) {
226	if (count == 0)
227	return true;
228	nanos = zero.awaitNanos(nanos);
229	if (nanos <= 0)
230	return false;
231	}
232	} finally {
233	lock.unlock();
234	}
235	}
236
237
238
239	/**
240	* Decrements the count of the latch, releasing all waiting threads if
241	* the count reaches zero.
242	* <p>If the current {@link #getCount count} is greater than zero then
243	* it is decremented. If the new count is zero then all waiting threads
244	* are re-enabled for thread scheduling purposes.
245	* <p>If the current {@link #getCount count} equals zero then nothing
246	* happens.
247	*/
248	public void countDown() {
249	lock.lock();
250	try {
251	if (count > 0 && --count == 0)
252	zero.signalAll();
253	} finally {
254	lock.unlock();
255	}
256	}
257
258	/**
259	* Returns the current count.
260	* <p>This method is typically used for debugging and testing purposes.
261	* @return the current count.
262	*/
263	public long getCount() {
264	lock.lock();
265	try {
266	return count;
267	} finally {
268	lock.unlock();
269	}
270	}
271	}