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 a set of threads to wait until
 * the actions of another set of threads allow the first set to
 * proceed.
 *
 * <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 to 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.
 * A useful property of a <tt>CountDownLatch</tt> is that it doesn't
 * require that all threads wait before any can proceed, 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>
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/05/27 18:14:39 $
 * @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:	1.3
Committed:	Sat Jun 7 18:20:20 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_1
Changes since 1.2:	+13 -11 lines
Log Message:	Misc documentation updates
#	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 a set of threads to wait until
11	* the actions of another set of threads allow the first set to
12	* proceed.
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.
25	* A <tt>CountDownLatch</tt> initialized to one serves as a simple on/off
26	* latch, or gate: all threads invoking {@link #await} wait at the gate until
27	* it is opened by a thread invoking {@link #countDown}.
28	* A <tt>CountDownLatch</tt> initialized to <em>N</em> can be used to make
29	* one thread wait until <em>N</em> threads have completed some action.
30	* A useful property of a <tt>CountDownLatch</tt> is that it doesn't
31	* require that all threads wait before any can proceed, it simply
32	* prevents any thread from proceeding past the {@link #await wait} until
33	* all threads could pass.
34	*
35	* <p><b>Sample usage:</b> Here is a pair of classes in which a group
36	* of worker threads use two countdown latches:
37	* <ul>
38	* <li> The first is a start signal that prevents any worker from proceeding
39	* until the driver is ready for them to proceed;
40	* <li> The second is a completion signal that allows the driver to wait
41	* until all workers have completed.
42	* </ul>
43	*
44	* <pre>
45	* class Driver { // ...
46	* void main() throws InterruptedException {
47	* CountDownLatch startSignal = new CountDownLatch(1);
48	* CountDownLatch doneSignal = new CountDownLatch(N);
49	*
50	* for (int i = 0; i < N; ++i) // create and start threads
51	* new Thread(new Worker(startSignal, doneSignal)).start();
52	*
53	* doSomethingElse(); // don't let run yet
54	* startSignal.countDown(); // let all threads proceed
55	* doSomethingElse();
56	* doneSignal.await(); // wait for all to finish
57	* }
58	* }
59	*
60	* class Worker implements Runnable {
61	* private final CountDownLatch startSignal;
62	* private final CountDownLatch doneSignal;
63	* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
64	* this.startSignal = startSignal;
65	* this.doneSignal = doneSignal;
66	* }
67	* public void run() {
68	* try {
69	* startSignal.await();
70	* doWork();
71	* doneSignal.countDown();
72	* }
73	* catch (InterruptedException ex) {} // return;
74	* }
75	*
76	* void doWork() { ... }
77	* }
78	*
79	* </pre>
80	*
81	* @since 1.5
82	* @spec JSR-166
83	* @revised $Date: 2003/05/27 18:14:39 $
84	* @editor $Author: dl $
85	*/
86	public class CountDownLatch {
87	private final ReentrantLock lock = new ReentrantLock();
88	private final Condition zero = lock.newCondition();
89	private int count;
90
91	/**
92	* Constructs a <tt>CountDownLatch</tt> initialized with the given
93	* count.
94	*
95	* @param count the number of times {@link #countDown} must be invoked
96	* before threads can pass through {@link #await}.
97	*
98	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
99	*/
100	public CountDownLatch(int count) {
101	if (count < 0) throw new IllegalArgumentException("count < 0");
102	this.count = count;
103	}
104
105	/**
106	* Causes the current thread to wait until the latch has counted down to
107	* zero, unless the thread is {@link Thread#interrupt interrupted}.
108	*
109	* <p>If the current {@link #getCount count} is zero then this method
110	* returns immediately.
111	* <p>If the current {@link #getCount count} is greater than zero then
112	* the current thread becomes disabled for thread scheduling
113	* purposes and lies dormant until one of two things happen:
114	* <ul>
115	* <li> The count reaches zero due to invocations of the
116	* {@link #countDown} method; or
117	* <li> Some other thread {@link Thread#interrupt interrupts} the current
118	* thread.
119	* </ul>
120	* <p>If the current thread:
121	* <ul>
122	* <li>has its interrupted status set on entry to this method; or
123	* <li>is {@link Thread#interrupt interrupted} while waiting,
124	* </ul>
125	* then {@link InterruptedException} is thrown and the current thread's
126	* interrupted status is cleared.
127	*
128	* @throws InterruptedException if the current thread is interrupted
129	* while waiting.
130	*/
131	public void await() throws InterruptedException {
132	lock.lock();
133	try {
134	while (count != 0)
135	zero.await();
136	}
137	finally {
138	lock.unlock();
139	}
140	}
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	* or the specified waiting time elapses.
147	*
148	* <p>If the current {@link #getCount count} is zero then this method
149	* returns immediately with the value <tt>true</tt>.
150	*
151	* <p>If the current {@link #getCount count} is greater than zero then
152	* the current thread becomes disabled for thread scheduling
153	* purposes and lies dormant until one of three things happen:
154	* <ul>
155	* <li>The count reaches zero due to invocations of the
156	* {@link #countDown} method; or
157	* <li>Some other thread {@link Thread#interrupt interrupts} the current
158	* thread; or
159	* <li>The specified waiting time elapses.
160	* </ul>
161	* <p>If the count reaches zero then the method returns with the
162	* value <tt>true</tt>.
163	* <p>If the current thread:
164	* <ul>
165	* <li>has its interrupted status set on entry to this method; or
166	* <li>is {@link Thread#interrupt interrupted} while waiting,
167	* </ul>
168	* then {@link InterruptedException} is thrown and the current thread's
169	* interrupted status is cleared.
170	*
171	* <p>If the specified waiting time elapses then the value <tt>false</tt>
172	* is returned.
173	* The given waiting time is a best-effort lower bound. If the time is
174	* less than or equal to zero, the method will not wait at all.
175	*
176	* @param timeout the maximum time to wait
177	* @param unit the time unit of the <tt>timeout</tt> argument.
178	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
179	* if the waiting time elapsed before the count reached zero.
180	*
181	* @throws InterruptedException if the current thread is interrupted
182	* while waiting.
183	*/
184	public boolean await(long timeout, TimeUnit unit)
185	throws InterruptedException {
186	long nanos = unit.toNanos(timeout);
187	lock.lock();
188	try {
189	for (;;) {
190	if (count == 0)
191	return true;
192	nanos = zero.awaitNanos(nanos);
193	if (nanos <= 0)
194	return false;
195	}
196	}
197	finally {
198	lock.unlock();
199	}
200	}
201
202
203
204	/**
205	* Decrements the count of the latch, releasing all waiting threads if
206	* the count reaches zero.
207	* <p>If the current {@link #getCount count} is greater than zero then
208	* it is decremented. If the new count is zero then all waiting threads
209	* are re-enabled for thread scheduling purposes.
210	* <p>If the current {@link #getCount count} equals zero then nothing
211	* happens.
212	*/
213	public void countDown() {
214	lock.lock();
215	try {
216	if (count > 0 && --count == 0)
217	zero.signalAll();
218	}
219	finally {
220	lock.unlock();
221	}
222	}
223
224	/**
225	* Returns the current count.
226	* <p>This method is typically used for debugging and testing purposes.
227	* @return the current count.
228	*/
229	public long getCount() {
230	lock.lock();
231	try {
232	return count;
233	}
234	finally {
235	lock.unlock();
236	}
237	}
238	}