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. (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>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    private final ReentrantLock lock = new ReentrantLock();
    private final ReentrantLock.ConditionObject 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 {
        final ReentrantLock lock = this.lock;
        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);
        final ReentrantLock lock = this.lock;
        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() {
        final ReentrantLock lock = this.lock;
        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() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }
}
Revision:	1.14
Committed:	Tue Dec 23 19:38:09 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.13:	+4 -0 lines
Log Message:	cache finals across volatiles; avoid readResolve; doc improvments; timed invokeAll interleaves
#	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. (When threads must repeatedly
87	* count down in this way, instead use a {@link CyclicBarrier}.)
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	* WorkerRunnable(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	* } catch (InterruptedException ex) {} // return;
114	* }
115	*
116	* void doWork() { ... }
117	* }
118	*
119	* </pre>
120	*
121	* @since 1.5
122	* @author Doug Lea
123	*/
124	public class CountDownLatch {
125	private final ReentrantLock lock = new ReentrantLock();
126	private final ReentrantLock.ConditionObject zero = lock.newCondition();
127	private int count;
128
129	/**
130	* Constructs a <tt>CountDownLatch</tt> initialized with the given
131	* count.
132	*
133	* @param count the number of times {@link #countDown} must be invoked
134	* before threads can pass through {@link #await}.
135	*
136	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
137	*/
138	public CountDownLatch(int count) {
139	if (count < 0) throw new IllegalArgumentException("count < 0");
140	this.count = count;
141	}
142
143	/**
144	* Causes the current thread to wait until the latch has counted down to
145	* zero, unless the thread is {@link Thread#interrupt interrupted}.
146	*
147	* <p>If the current {@link #getCount count} is zero then this method
148	* returns immediately.
149	* <p>If the current {@link #getCount count} is greater than zero then
150	* the current thread becomes disabled for thread scheduling
151	* purposes and lies dormant until one of two things happen:
152	* <ul>
153	* <li>The count reaches zero due to invocations of the
154	* {@link #countDown} method; or
155	* <li>Some other thread {@link Thread#interrupt interrupts} the current
156	* thread.
157	* </ul>
158	* <p>If the current thread:
159	* <ul>
160	* <li>has its interrupted status set on entry to this method; or
161	* <li>is {@link Thread#interrupt interrupted} while waiting,
162	* </ul>
163	* then {@link InterruptedException} is thrown and the current thread's
164	* interrupted status is cleared.
165	*
166	* @throws InterruptedException if the current thread is interrupted
167	* while waiting.
168	*/
169	public void await() throws InterruptedException {
170	final ReentrantLock lock = this.lock;
171	lock.lock();
172	try {
173	while (count != 0)
174	zero.await();
175	} finally {
176	lock.unlock();
177	}
178	}
179
180
181	/**
182	* Causes the current thread to wait until the latch has counted down to
183	* zero, unless the thread is {@link Thread#interrupt interrupted},
184	* or the specified waiting time elapses.
185	*
186	* <p>If the current {@link #getCount count} is zero then this method
187	* returns immediately with the value <tt>true</tt>.
188	*
189	* <p>If the current {@link #getCount count} is greater than zero then
190	* the current thread becomes disabled for thread scheduling
191	* purposes and lies dormant until one of three things happen:
192	* <ul>
193	* <li>The count reaches zero due to invocations of the
194	* {@link #countDown} method; or
195	* <li>Some other thread {@link Thread#interrupt interrupts} the current
196	* thread; or
197	* <li>The specified waiting time elapses.
198	* </ul>
199	* <p>If the count reaches zero then the method returns with the
200	* value <tt>true</tt>.
201	* <p>If the current thread:
202	* <ul>
203	* <li>has its interrupted status set on entry to this method; or
204	* <li>is {@link Thread#interrupt interrupted} while waiting,
205	* </ul>
206	* then {@link InterruptedException} is thrown and the current thread's
207	* interrupted status is cleared.
208	*
209	* <p>If the specified waiting time elapses then the value <tt>false</tt>
210	* is returned.
211	* If the time is
212	* less than or equal to zero, the method will not wait at all.
213	*
214	* @param timeout the maximum time to wait
215	* @param unit the time unit of the <tt>timeout</tt> argument.
216	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
217	* if the waiting time elapsed before the count reached zero.
218	*
219	* @throws InterruptedException if the current thread is interrupted
220	* while waiting.
221	*/
222	public boolean await(long timeout, TimeUnit unit)
223	throws InterruptedException {
224	long nanos = unit.toNanos(timeout);
225	final ReentrantLock lock = this.lock;
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	} finally {
236	lock.unlock();
237	}
238	}
239
240
241
242	/**
243	* Decrements the count of the latch, releasing all waiting threads if
244	* the count reaches zero.
245	* <p>If the current {@link #getCount count} is greater than zero then
246	* it is decremented. If the new count is zero then all waiting threads
247	* are re-enabled for thread scheduling purposes.
248	* <p>If the current {@link #getCount count} equals zero then nothing
249	* happens.
250	*/
251	public void countDown() {
252	final ReentrantLock lock = this.lock;
253	lock.lock();
254	try {
255	if (count > 0 && --count == 0)
256	zero.signalAll();
257	} finally {
258	lock.unlock();
259	}
260	}
261
262	/**
263	* Returns the current count.
264	* <p>This method is typically used for debugging and testing purposes.
265	* @return the current count.
266	*/
267	public long getCount() {
268	final ReentrantLock lock = this.lock;
269	lock.lock();
270	try {
271	return count;
272	} finally {
273	lock.unlock();
274	}
275	}
276	}