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:	1.2
Committed:	Sun Jan 18 20:17:32 2015 UTC (9 years, 4 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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	jsr166	1.2
9	dl	1.1	import java.util.concurrent.locks.AbstractQueuedSynchronizer;
10
11			/**
12			* A synchronization aid that allows one or more threads to wait until
13			* a set of operations being performed in other threads completes.
14			*
15			* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
16			* The {@link #await await} methods block until the current count reaches
17			* zero due to invocations of the {@link #countDown} method, after which
18			* all waiting threads are released and any subsequent invocations of
19			* {@link #await await} return immediately. This is a one-shot phenomenon
20			* -- the count cannot be reset. If you need a version that resets the
21			* count, consider using a {@link CyclicBarrier}.
22			*
23			* <p>A {@code CountDownLatch} is a versatile synchronization tool
24			* and can be used for a number of purposes. A
25			* {@code CountDownLatch} 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 {@code CountDownLatch} 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			*
32			* <p>A useful property of a {@code CountDownLatch} is that it
33			* doesn't require that threads calling {@code countDown} wait for
34			* the count to reach zero before proceeding, it simply prevents any
35			* thread from proceeding past an {@link #await await} until all
36			* threads could pass.
37			*
38			* <p><b>Sample usage:</b> Here is a pair of classes in which a group
39			* of worker threads use two countdown latches:
40			* <ul>
41			* <li>The first is a start signal that prevents any worker from proceeding
42			* until the driver is ready for them to proceed;
43			* <li>The second is a completion signal that allows the driver to wait
44			* until all workers have completed.
45			* </ul>
46			*
47			* <pre> {@code
48			* class Driver { // ...
49			* void main() throws InterruptedException {
50			* CountDownLatch startSignal = new CountDownLatch(1);
51			* CountDownLatch doneSignal = new CountDownLatch(N);
52			*
53			* for (int i = 0; i < N; ++i) // create and start threads
54			* new Thread(new Worker(startSignal, doneSignal)).start();
55			*
56			* doSomethingElse(); // don't let run yet
57			* startSignal.countDown(); // let all threads proceed
58			* doSomethingElse();
59			* doneSignal.await(); // wait for all to finish
60			* }
61			* }
62			*
63			* class Worker implements Runnable {
64			* private final CountDownLatch startSignal;
65			* private final CountDownLatch doneSignal;
66			* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
67			* this.startSignal = startSignal;
68			* this.doneSignal = doneSignal;
69			* }
70			* public void run() {
71			* try {
72			* startSignal.await();
73			* doWork();
74			* doneSignal.countDown();
75			* } catch (InterruptedException ex) {} // return;
76			* }
77			*
78			* void doWork() { ... }
79			* }}</pre>
80			*
81			* <p>Another typical usage would be to divide a problem into N parts,
82			* describe each part with a Runnable that executes that portion and
83			* counts down on the latch, and queue all the Runnables to an
84			* Executor. When all sub-parts are complete, the coordinating thread
85			* will be able to pass through await. (When threads must repeatedly
86			* count down in this way, instead use a {@link CyclicBarrier}.)
87			*
88			* <pre> {@code
89			* class Driver2 { // ...
90			* void main() throws InterruptedException {
91			* CountDownLatch doneSignal = new CountDownLatch(N);
92			* Executor e = ...
93			*
94			* for (int i = 0; i < N; ++i) // create and start threads
95			* e.execute(new WorkerRunnable(doneSignal, i));
96			*
97			* doneSignal.await(); // wait for all to finish
98			* }
99			* }
100			*
101			* class WorkerRunnable implements Runnable {
102			* private final CountDownLatch doneSignal;
103			* private final int i;
104			* WorkerRunnable(CountDownLatch doneSignal, int i) {
105			* this.doneSignal = doneSignal;
106			* this.i = i;
107			* }
108			* public void run() {
109			* try {
110			* doWork(i);
111			* doneSignal.countDown();
112			* } catch (InterruptedException ex) {} // return;
113			* }
114			*
115			* void doWork() { ... }
116			* }}</pre>
117			*
118			* <p>Memory consistency effects: Until the count reaches
119			* zero, actions in a thread prior to calling
120			* {@code countDown()}
121			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
122			* actions following a successful return from a corresponding
123			* {@code await()} in another thread.
124			*
125			* @since 1.5
126			* @author Doug Lea
127			*/
128			public class CountDownLatch {
129			/**
130			* Synchronization control For CountDownLatch.
131			* Uses AQS state to represent count.
132			*/
133			private static final class Sync extends AbstractQueuedSynchronizer {
134			private static final long serialVersionUID = 4982264981922014374L;
135
136			Sync(int count) {
137			setState(count);
138			}
139
140			int getCount() {
141			return getState();
142			}
143
144			protected int tryAcquireShared(int acquires) {
145			return (getState() == 0) ? 1 : -1;
146			}
147
148			protected boolean tryReleaseShared(int releases) {
149			// Decrement count; signal when transition to zero
150			for (;;) {
151			int c = getState();
152			if (c == 0)
153			return false;
154			int nextc = c-1;
155			if (compareAndSetState(c, nextc))
156			return nextc == 0;
157			}
158			}
159			}
160
161			private final Sync sync;
162
163			/**
164			* Constructs a {@code CountDownLatch} initialized with the given count.
165			*
166			* @param count the number of times {@link #countDown} must be invoked
167			* before threads can pass through {@link #await}
168			* @throws IllegalArgumentException if {@code count} is negative
169			*/
170			public CountDownLatch(int count) {
171			if (count < 0) throw new IllegalArgumentException("count < 0");
172			this.sync = new Sync(count);
173			}
174
175			/**
176			* Causes the current thread to wait until the latch has counted down to
177			* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
178			*
179			* <p>If the current count is zero then this method returns immediately.
180			*
181			* <p>If the current count is greater than zero then the current
182			* thread becomes disabled for thread scheduling purposes and lies
183			* dormant until one of two things happen:
184			* <ul>
185			* <li>The count reaches zero due to invocations of the
186			* {@link #countDown} method; or
187			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
188			* the current thread.
189			* </ul>
190			*
191			* <p>If the current thread:
192			* <ul>
193			* <li>has its interrupted status set on entry to this method; or
194			* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
195			* </ul>
196			* then {@link InterruptedException} is thrown and the current thread's
197			* interrupted status is cleared.
198			*
199			* @throws InterruptedException if the current thread is interrupted
200			* while waiting
201			*/
202			public void await() throws InterruptedException {
203			sync.acquireSharedInterruptibly(1);
204			}
205
206			/**
207			* Causes the current thread to wait until the latch has counted down to
208			* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
209			* or the specified waiting time elapses.
210			*
211			* <p>If the current count is zero then this method returns immediately
212			* with the value {@code true}.
213			*
214			* <p>If the current count is greater than zero then the current
215			* thread becomes disabled for thread scheduling purposes and lies
216			* dormant until one of three things happen:
217			* <ul>
218			* <li>The count reaches zero due to invocations of the
219			* {@link #countDown} method; or
220			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
221			* the current thread; or
222			* <li>The specified waiting time elapses.
223			* </ul>
224			*
225			* <p>If the count reaches zero then the method returns with the
226			* value {@code true}.
227			*
228			* <p>If the current thread:
229			* <ul>
230			* <li>has its interrupted status set on entry to this method; or
231			* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
232			* </ul>
233			* then {@link InterruptedException} is thrown and the current thread's
234			* interrupted status is cleared.
235			*
236			* <p>If the specified waiting time elapses then the value {@code false}
237			* is returned. If the time is less than or equal to zero, the method
238			* will not wait at all.
239			*
240			* @param timeout the maximum time to wait
241			* @param unit the time unit of the {@code timeout} argument
242			* @return {@code true} if the count reached zero and {@code false}
243			* if the waiting time elapsed before the count reached zero
244			* @throws InterruptedException if the current thread is interrupted
245			* while waiting
246			*/
247			public boolean await(long timeout, TimeUnit unit)
248			throws InterruptedException {
249			return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
250			}
251
252			/**
253			* Decrements the count of the latch, releasing all waiting threads if
254			* the count reaches zero.
255			*
256			* <p>If the current count is greater than zero then it is decremented.
257			* If the new count is zero then all waiting threads are re-enabled for
258			* thread scheduling purposes.
259			*
260			* <p>If the current count equals zero then nothing happens.
261			*/
262			public void countDown() {
263			sync.releaseShared(1);
264			}
265
266			/**
267			* Returns the current count.
268			*
269			* <p>This method is typically used for debugging and testing purposes.
270			*
271			* @return the current count
272			*/
273			public long getCount() {
274			return sync.getCount();
275			}
276
277			/**
278			* Returns a string identifying this latch, as well as its state.
279			* The state, in brackets, includes the String {@code "Count ="}
280			* followed by the current count.
281			*
282			* @return a string identifying this latch, as well as its state
283			*/
284			public String toString() {
285			return super.toString() + "[Count = " + sync.getCount() + "]";
286			}
287			}