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 <tt>CountDownLatch</tt> allows one 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: 2003/01/30 22:12:13 $
 * @editor $Author: dholmes $
 */
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:	1.2
Committed:	Tue May 27 18:14:39 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Changes since 1.1:	+59 -8 lines
Log Message:	re-check-in initial implementations
#	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 <tt>CountDownLatch</tt> allows one set of threads to wait until
11	* the actions of another set of threads allow the first set to proceed.
12	* <p>A <tt>CountDownLatch</tt> is initialized with a given <em>count</em>.
13	* The {@link #await} methods block until the current {@link #getCount count}
14	* reaches zero due to invocations of the {@link #countDown} method,
15	* after which all waiting threads are
16	* released and any subsequent invocations of {@link #await} return
17	* immediately. This is a one-shot phenomenon -- the count
18	* cannot be reset. If you need a version that resets the count,
19	* consider using a {@link CyclicBarrier}.
20	*
21	* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
22	* and can be used for a number of purposes.
23	* A <tt>CountDownLatch</tt> initialized to one serves as a simple on/off
24	* latch, or gate: all threads invoking {@link #await} wait at the gate until
25	* it is opened by a thread invoking {@link #countDown}.
26	* A <tt>CountDownLatch</tt> initialized to <em>N</em> can be used to make
27	* one thread wait until <em>N</em> threads have completed some action.
28	* A useful property of a <tt>CountDownLatch</tt> is that it doesn't
29	* require that all threads wait before any can proceed, it simply
30	* prevents any thread from proceeding past the {@link #await wait} until
31	* all threads could pass.
32	*
33	* <p><b>Sample usage:</b> Here is a pair of classes in which a group
34	* of worker threads use two countdown latches:
35	* <ul>
36	* <li> The first is a start signal that prevents any worker from proceeding
37	* until the driver is ready for them to proceed;
38	* <li> The second is a completion signal that allows the driver to wait
39	* until all workers have completed.
40	* </ul>
41	*
42	* <pre>
43	* class Driver { // ...
44	* void main() throws InterruptedException {
45	* CountDownLatch startSignal = new CountDownLatch(1);
46	* CountDownLatch doneSignal = new CountDownLatch(N);
47	*
48	* for (int i = 0; i < N; ++i) // create and start threads
49	* new Thread(new Worker(startSignal, doneSignal)).start();
50	*
51	* doSomethingElse(); // don't let run yet
52	* startSignal.countDown(); // let all threads proceed
53	* doSomethingElse();
54	* doneSignal.await(); // wait for all to finish
55	* }
56	* }
57	*
58	* class Worker implements Runnable {
59	* private final CountDownLatch startSignal;
60	* private final CountDownLatch doneSignal;
61	* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
62	* this.startSignal = startSignal;
63	* this.doneSignal = doneSignal;
64	* }
65	* public void run() {
66	* try {
67	* startSignal.await();
68	* doWork();
69	* doneSignal.countDown();
70	* }
71	* catch (InterruptedException ex) {} // return;
72	* }
73	*
74	* void doWork() { ... }
75	* }
76	*
77	* </pre>
78	*
79	* @since 1.5
80	* @spec JSR-166
81	* @revised $Date: 2003/01/30 22:12:13 $
82	* @editor $Author: dholmes $
83	*/
84	public class CountDownLatch {
85	private final ReentrantLock lock = new ReentrantLock();
86	private final Condition zero = lock.newCondition();
87	private int count;
88
89	/**
90	* Constructs a <tt>CountDownLatch</tt> initialized with the given
91	* count.
92	*
93	* @param count the number of times {@link #countDown} must be invoked
94	* before threads can pass through {@link #await}.
95	*
96	* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
97	*/
98	public CountDownLatch(int count) {
99	if (count < 0) throw new IllegalArgumentException("count < 0");
100	this.count = count;
101	}
102
103	/**
104	* Causes the current thread to wait until the latch has counted down to
105	* zero, unless the thread is {@link Thread#interrupt interrupted}.
106	*
107	* <p>If the current {@link #getCount count} is zero then this method
108	* returns immediately.
109	* <p>If the current {@link #getCount count} is greater than zero then
110	* the current thread becomes disabled for thread scheduling
111	* purposes and lies dormant until one of two things happen:
112	* <ul>
113	* <li> The count reaches zero due to invocations of the
114	* {@link #countDown} method; or
115	* <li> Some other thread {@link Thread#interrupt interrupts} the current
116	* thread.
117	* </ul>
118	* <p>If the current thread:
119	* <ul>
120	* <li>has its interrupted status set on entry to this method; or
121	* <li>is {@link Thread#interrupt interrupted} while waiting,
122	* </ul>
123	* then {@link InterruptedException} is thrown and the current thread's
124	* interrupted status is cleared.
125	*
126	* @throws InterruptedException if the current thread is interrupted
127	* while waiting.
128	*/
129	public void await() throws InterruptedException {
130	lock.lock();
131	try {
132	while (count != 0)
133	zero.await();
134	}
135	finally {
136	lock.unlock();
137	}
138	}
139
140
141	/**
142	* Causes the current thread to wait until the latch has counted down to
143	* zero, unless the thread is {@link Thread#interrupt interrupted},
144	* or the specified waiting time elapses.
145	*
146	* <p>If the current {@link #getCount count} is zero then this method
147	* returns immediately with the value <tt>true</tt>.
148	*
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 three 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; or
157	* <li>The specified waiting time elapses.
158	* </ul>
159	* <p>If the count reaches zero then the method returns with the
160	* value <tt>true</tt>.
161	* <p>If the current thread:
162	* <ul>
163	* <li>has its interrupted status set on entry to this method; or
164	* <li>is {@link Thread#interrupt interrupted} while waiting,
165	* </ul>
166	* then {@link InterruptedException} is thrown and the current thread's
167	* interrupted status is cleared.
168	*
169	* <p>If the specified waiting time elapses then the value <tt>false</tt>
170	* is returned.
171	* The given waiting time is a best-effort lower bound. If the time is
172	* less than or equal to zero, the method will not wait at all.
173	*
174	* @param timeout the maximum time to wait
175	* @param unit the time unit of the <tt>timeout</tt> argument.
176	* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
177	* if the waiting time elapsed before the count reached zero.
178	*
179	* @throws InterruptedException if the current thread is interrupted
180	* while waiting.
181	*/
182	public boolean await(long timeout, TimeUnit unit)
183	throws InterruptedException {
184	long nanos = unit.toNanos(timeout);
185	lock.lock();
186	try {
187	for (;;) {
188	if (count == 0)
189	return true;
190	nanos = zero.awaitNanos(nanos);
191	if (nanos <= 0)
192	return false;
193	}
194	}
195	finally {
196	lock.unlock();
197	}
198	}
199
200
201
202	/**
203	* Decrements the count of the latch, releasing all waiting threads if
204	* the count reaches zero.
205	* <p>If the current {@link #getCount count} is greater than zero then
206	* it is decremented. If the new count is zero then all waiting threads
207	* are re-enabled for thread scheduling purposes.
208	* <p>If the current {@link #getCount count} equals zero then nothing
209	* happens.
210	*/
211	public void countDown() {
212	lock.lock();
213	try {
214	if (count > 0 && --count == 0)
215	zero.signalAll();
216	}
217	finally {
218	lock.unlock();
219	}
220	}
221
222	/**
223	* Returns the current count.
224	* <p>This method is typically used for debugging and testing purposes.
225	* @return the current count.
226	*/
227	public long getCount() {
228	lock.lock();
229	try {
230	return count;
231	}
232	finally {
233	lock.unlock();
234	}
235	}
236	}