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} 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: 2003/06/24 14:34:47 $
 * @editor $Author: dl $
 * @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:	1.6
Committed:	Tue Jul 8 00:46:33 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_CR1, JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.5:	+3 -2 lines
Log Message:	Locks in subpackage; fairness params added
#	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} 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} 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}
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 the {@link #await wait} 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	* }
75	* catch (InterruptedException ex) {} // return;
76	* }
77	*
78	* void doWork() { ... }
79	* }
80	*
81	* </pre>
82	*
83	* <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	* will be able to pass through await.
88	*
89	* <pre>
90	* class Driver2 { // ...
91	* void main() throws InterruptedException {
92	* CountDownLatch doneSignal = new CountDownLatch(N);
93	* Executor e = ...
94	*
95	* for (int i = 0; i < N; ++i) // create and start threads
96	* e.execute(new WorkerRunnable(doneSignal, i));
97	*
98	* doneSignal.await(); // wait for all to finish
99	* }
100	* }
101	*
102	* class WorkerRunnable implements Runnable {
103	* private final CountDownLatch doneSignal;
104	* private final int i;
105	* Worker(CountDownLatch doneSignal, int i) {
106	* this.doneSignal = doneSignal;
107	* this.i = i;
108	* }
109	* public void run() {
110	* try {
111	* doWork(i);
112	* doneSignal.countDown();
113	* }
114	* catch (InterruptedException ex) {} // return;
115	* }
116	*
117	* void doWork() { ... }
118	* }
119	*
120	* </pre>
121	*
122	* @since 1.5
123	* @spec JSR-166
124	* @revised $Date: 2003/06/24 14:34:47 $
125	* @editor $Author: dl $
126	* @author Doug Lea
127	*/
128	public class CountDownLatch {
129	private final ReentrantLock lock = new ReentrantLock();
130	private final Condition zero = lock.newCondition();
131	private int count;
132
133	/**
134	* Constructs a <tt>CountDownLatch</tt> initialized with the given
135	* count.
136	*
137	* @param count the number of times {@link #countDown} must be invoked
138	* before threads can pass through {@link #await}.
139	*
140	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
141	*/
142	public CountDownLatch(int count) {
143	if (count < 0) throw new IllegalArgumentException("count < 0");
144	this.count = count;
145	}
146
147	/**
148	* Causes the current thread to wait until the latch has counted down to
149	* zero, unless the thread is {@link Thread#interrupt interrupted}.
150	*
151	* <p>If the current {@link #getCount count} is zero then this method
152	* returns immediately.
153	* <p>If the current {@link #getCount count} is greater than zero then
154	* the current thread becomes disabled for thread scheduling
155	* purposes and lies dormant until one of two things happen:
156	* <ul>
157	* <li> The count reaches zero due to invocations of the
158	* {@link #countDown} method; or
159	* <li> Some other thread {@link Thread#interrupt interrupts} the current
160	* thread.
161	* </ul>
162	* <p>If the current thread:
163	* <ul>
164	* <li>has its interrupted status set on entry to this method; or
165	* <li>is {@link Thread#interrupt interrupted} while waiting,
166	* </ul>
167	* then {@link InterruptedException} is thrown and the current thread's
168	* interrupted status is cleared.
169	*
170	* @throws InterruptedException if the current thread is interrupted
171	* while waiting.
172	*/
173	public void await() throws InterruptedException {
174	lock.lock();
175	try {
176	while (count != 0)
177	zero.await();
178	}
179	finally {
180	lock.unlock();
181	}
182	}
183
184
185	/**
186	* Causes the current thread to wait until the latch has counted down to
187	* zero, unless the thread is {@link Thread#interrupt interrupted},
188	* or the specified waiting time elapses.
189	*
190	* <p>If the current {@link #getCount count} is zero then this method
191	* returns immediately with the value <tt>true</tt>.
192	*
193	* <p>If the current {@link #getCount count} is greater than zero then
194	* the current thread becomes disabled for thread scheduling
195	* purposes and lies dormant until one of three things happen:
196	* <ul>
197	* <li>The count reaches zero due to invocations of the
198	* {@link #countDown} method; or
199	* <li>Some other thread {@link Thread#interrupt interrupts} the current
200	* thread; or
201	* <li>The specified waiting time elapses.
202	* </ul>
203	* <p>If the count reaches zero then the method returns with the
204	* value <tt>true</tt>.
205	* <p>If the current thread:
206	* <ul>
207	* <li>has its interrupted status set on entry to this method; or
208	* <li>is {@link Thread#interrupt interrupted} while waiting,
209	* </ul>
210	* then {@link InterruptedException} is thrown and the current thread's
211	* interrupted status is cleared.
212	*
213	* <p>If the specified waiting time elapses then the value <tt>false</tt>
214	* is returned.
215	* The given waiting time is a best-effort lower bound. If the time is
216	* less than or equal to zero, the method will not wait at all.
217	*
218	* @param timeout the maximum time to wait
219	* @param unit the time unit of the <tt>timeout</tt> argument.
220	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
221	* if the waiting time elapsed before the count reached zero.
222	*
223	* @throws InterruptedException if the current thread is interrupted
224	* while waiting.
225	*/
226	public boolean await(long timeout, TimeUnit unit)
227	throws InterruptedException {
228	long nanos = unit.toNanos(timeout);
229	lock.lock();
230	try {
231	for (;;) {
232	if (count == 0)
233	return true;
234	nanos = zero.awaitNanos(nanos);
235	if (nanos <= 0)
236	return false;
237	}
238	}
239	finally {
240	lock.unlock();
241	}
242	}
243
244
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	public void countDown() {
256	lock.lock();
257	try {
258	if (count > 0 && --count == 0)
259	zero.signalAll();
260	}
261	finally {
262	lock.unlock();
263	}
264	}
265
266	/**
267	* Returns the current count.
268	* <p>This method is typically used for debugging and testing purposes.
269	* @return the current count.
270	*/
271	public long getCount() {
272	lock.lock();
273	try {
274	return count;
275	}
276	finally {
277	lock.unlock();
278	}
279	}
280	}