util/concurrent/CountDownLatch.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 *
 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
 * The {@link #await await} methods block until the current 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 {@code CountDownLatch} is a versatile synchronization tool
 * and can be used for a number of purposes.  A
 * {@code CountDownLatch} 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 {@code CountDownLatch} 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 {@code CountDownLatch} is that it
 * doesn't require that threads calling {@code countDown} 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> {@code
 * 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> {@code
 * 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>
 *
 * <p>Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful return from a corresponding
 * {@code await()} in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * Constructs a {@code CountDownLatch} 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 {@code count} is negative
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>If the current count is zero then this method returns immediately.
     *
     * <p>If the current 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 {@linkplain 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 {@linkplain 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 {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * <p>If the current 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 {@linkplain 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 {@code true}.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain 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 {@code false}
     * 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 {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         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 {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * <p>If the current 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 count equals zero then nothing happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String {@code "Count ="}
     * followed by the current count.
     *
     * @return a string identifying this latch, as well as its state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}
Revision:	1.1
Committed:	Sun Dec 16 20:55:15 2012 UTC (11 years, 5 months ago) by dl
Branch:	MAIN
Log Message:	Create src/jdk7 package
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent;
8			import java.util.concurrent.locks.AbstractQueuedSynchronizer;
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 {@code CountDownLatch} is initialized with a given <em>count</em>.
15			* The {@link #await await} methods block until the current count reaches
16			* zero due to invocations of the {@link #countDown} method, after which
17			* all waiting threads are released and any subsequent invocations of
18			* {@link #await await} return immediately. This is a one-shot phenomenon
19			* -- the count cannot be reset. If you need a version that resets the
20			* count, consider using a {@link CyclicBarrier}.
21			*
22			* <p>A {@code CountDownLatch} is a versatile synchronization tool
23			* and can be used for a number of purposes. A
24			* {@code CountDownLatch} initialized with a count of one serves as a
25			* simple on/off latch, or gate: all threads invoking {@link #await await}
26			* wait at the gate until it is opened by a thread invoking {@link
27			* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
28			* can be used to make one thread wait until <em>N</em> threads have
29			* completed some action, or some action has been completed N times.
30			*
31			* <p>A useful property of a {@code CountDownLatch} is that it
32			* doesn't require that threads calling {@code countDown} 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> {@code
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			* }}</pre>
79			*
80			* <p>Another typical usage would be to divide a problem into N parts,
81			* describe each part with a Runnable that executes that portion and
82			* counts down on the latch, and queue all the Runnables to an
83			* Executor. When all sub-parts are complete, the coordinating thread
84			* will be able to pass through await. (When threads must repeatedly
85			* count down in this way, instead use a {@link CyclicBarrier}.)
86			*
87			* <pre> {@code
88			* class Driver2 { // ...
89			* void main() throws InterruptedException {
90			* CountDownLatch doneSignal = new CountDownLatch(N);
91			* Executor e = ...
92			*
93			* for (int i = 0; i < N; ++i) // create and start threads
94			* e.execute(new WorkerRunnable(doneSignal, i));
95			*
96			* doneSignal.await(); // wait for all to finish
97			* }
98			* }
99			*
100			* class WorkerRunnable implements Runnable {
101			* private final CountDownLatch doneSignal;
102			* private final int i;
103			* WorkerRunnable(CountDownLatch doneSignal, int i) {
104			* this.doneSignal = doneSignal;
105			* this.i = i;
106			* }
107			* public void run() {
108			* try {
109			* doWork(i);
110			* doneSignal.countDown();
111			* } catch (InterruptedException ex) {} // return;
112			* }
113			*
114			* void doWork() { ... }
115			* }}</pre>
116			*
117			* <p>Memory consistency effects: Until the count reaches
118			* zero, actions in a thread prior to calling
119			* {@code countDown()}
120			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
121			* actions following a successful return from a corresponding
122			* {@code await()} in another thread.
123			*
124			* @since 1.5
125			* @author Doug Lea
126			*/
127			public class CountDownLatch {
128			/**
129			* Synchronization control For CountDownLatch.
130			* Uses AQS state to represent count.
131			*/
132			private static final class Sync extends AbstractQueuedSynchronizer {
133			private static final long serialVersionUID = 4982264981922014374L;
134
135			Sync(int count) {
136			setState(count);
137			}
138
139			int getCount() {
140			return getState();
141			}
142
143			protected int tryAcquireShared(int acquires) {
144			return (getState() == 0) ? 1 : -1;
145			}
146
147			protected boolean tryReleaseShared(int releases) {
148			// Decrement count; signal when transition to zero
149			for (;;) {
150			int c = getState();
151			if (c == 0)
152			return false;
153			int nextc = c-1;
154			if (compareAndSetState(c, nextc))
155			return nextc == 0;
156			}
157			}
158			}
159
160			private final Sync sync;
161
162			/**
163			* Constructs a {@code CountDownLatch} initialized with the given count.
164			*
165			* @param count the number of times {@link #countDown} must be invoked
166			* before threads can pass through {@link #await}
167			* @throws IllegalArgumentException if {@code count} is negative
168			*/
169			public CountDownLatch(int count) {
170			if (count < 0) throw new IllegalArgumentException("count < 0");
171			this.sync = new Sync(count);
172			}
173
174			/**
175			* Causes the current thread to wait until the latch has counted down to
176			* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
177			*
178			* <p>If the current count is zero then this method returns immediately.
179			*
180			* <p>If the current count is greater than zero then the current
181			* thread becomes disabled for thread scheduling purposes and lies
182			* dormant until one of two things happen:
183			* <ul>
184			* <li>The count reaches zero due to invocations of the
185			* {@link #countDown} method; or
186			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
187			* the current thread.
188			* </ul>
189			*
190			* <p>If the current thread:
191			* <ul>
192			* <li>has its interrupted status set on entry to this method; or
193			* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
194			* </ul>
195			* then {@link InterruptedException} is thrown and the current thread's
196			* interrupted status is cleared.
197			*
198			* @throws InterruptedException if the current thread is interrupted
199			* while waiting
200			*/
201			public void await() throws InterruptedException {
202			sync.acquireSharedInterruptibly(1);
203			}
204
205			/**
206			* Causes the current thread to wait until the latch has counted down to
207			* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
208			* or the specified waiting time elapses.
209			*
210			* <p>If the current count is zero then this method returns immediately
211			* with the value {@code true}.
212			*
213			* <p>If the current count is greater than zero then the current
214			* thread becomes disabled for thread scheduling purposes and lies
215			* dormant until one of three things happen:
216			* <ul>
217			* <li>The count reaches zero due to invocations of the
218			* {@link #countDown} method; or
219			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
220			* the current thread; or
221			* <li>The specified waiting time elapses.
222			* </ul>
223			*
224			* <p>If the count reaches zero then the method returns with the
225			* value {@code true}.
226			*
227			* <p>If the current thread:
228			* <ul>
229			* <li>has its interrupted status set on entry to this method; or
230			* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
231			* </ul>
232			* then {@link InterruptedException} is thrown and the current thread's
233			* interrupted status is cleared.
234			*
235			* <p>If the specified waiting time elapses then the value {@code false}
236			* is returned. If the time is less than or equal to zero, the method
237			* will not wait at all.
238			*
239			* @param timeout the maximum time to wait
240			* @param unit the time unit of the {@code timeout} argument
241			* @return {@code true} if the count reached zero and {@code false}
242			* if the waiting time elapsed before the count reached zero
243			* @throws InterruptedException if the current thread is interrupted
244			* while waiting
245			*/
246			public boolean await(long timeout, TimeUnit unit)
247			throws InterruptedException {
248			return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
249			}
250
251			/**
252			* Decrements the count of the latch, releasing all waiting threads if
253			* the count reaches zero.
254			*
255			* <p>If the current count is greater than zero then it is decremented.
256			* If the new count is zero then all waiting threads are re-enabled for
257			* thread scheduling purposes.
258			*
259			* <p>If the current count equals zero then nothing happens.
260			*/
261			public void countDown() {
262			sync.releaseShared(1);
263			}
264
265			/**
266			* Returns the current count.
267			*
268			* <p>This method is typically used for debugging and testing purposes.
269			*
270			* @return the current count
271			*/
272			public long getCount() {
273			return sync.getCount();
274			}
275
276			/**
277			* Returns a string identifying this latch, as well as its state.
278			* The state, in brackets, includes the String {@code "Count ="}
279			* followed by the current count.
280			*
281			* @return a string identifying this latch, as well as its state
282			*/
283			public String toString() {
284			return super.toString() + "[Count = " + sync.getCount() + "]";
285			}
286			}