CountDownLatch.java revision 72e93344b4d1ffc71e9c832ec23de0657e5b04a5 
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/licenses/publicdomain
5 */
6
7package java.util.concurrent;
8import java.util.concurrent.locks.*;
9import java.util.concurrent.atomic.*;
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>
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 * }
80 *
81 * </pre>
82 *
83 * <p>Another typical usage would be to divide a problem into N parts,
84 * describe each part with a Runnable that executes that portion and
85 * counts down on the latch, and queue all the Runnables to an
86 * Executor.  When all sub-parts are complete, the coordinating thread
87 * will be able to pass through await. (When threads must repeatedly
88 * count down in this way, instead use a {@link CyclicBarrier}.)
89 *
90 * <pre>
91 * class Driver2 { // ...
92 *   void main() throws InterruptedException {
93 *     CountDownLatch doneSignal = new CountDownLatch(N);
94 *     Executor e = ...
95 *
96 *     for (int i = 0; i < N; ++i) // create and start threads
97 *       e.execute(new WorkerRunnable(doneSignal, i));
98 *
99 *     doneSignal.await();           // wait for all to finish
100 *   }
101 * }
102 *
103 * class WorkerRunnable implements Runnable {
104 *   private final CountDownLatch doneSignal;
105 *   private final int i;
106 *   WorkerRunnable(CountDownLatch doneSignal, int i) {
107 *      this.doneSignal = doneSignal;
108 *      this.i = i;
109 *   }
110 *   public void run() {
111 *      try {
112 *        doWork(i);
113 *        doneSignal.countDown();
114 *      } catch (InterruptedException ex) {} // return;
115 *   }
116 *
117 *   void doWork() { ... }
118 * }
119 *
120 * </pre>
121 *
122 * <p>Memory consistency effects: Until the count reaches
123 * zero, actions in a thread prior to calling
124 * {@code countDown()}
125 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
126 * actions following a successful return from a corresponding
127 * {@code await()} in another thread.
128 *
129 * @since 1.5
130 * @author Doug Lea
131 */
132public class CountDownLatch {
133    /**
134     * Synchronization control For CountDownLatch.
135     * Uses AQS state to represent count.
136     */
137    private static final class Sync extends AbstractQueuedSynchronizer {
138        private static final long serialVersionUID = 4982264981922014374L;
139
140        Sync(int count) {
141            setState(count);
142        }
143
144        int getCount() {
145            return getState();
146        }
147
148        protected int tryAcquireShared(int acquires) {
149            return getState() == 0? 1 : -1;
150        }
151
152        protected boolean tryReleaseShared(int releases) {
153            // Decrement count; signal when transition to zero
154            for (;;) {
155                int c = getState();
156                if (c == 0)
157                    return false;
158                int nextc = c-1;
159                if (compareAndSetState(c, nextc))
160                    return nextc == 0;
161            }
162        }
163    }
164
165    private final Sync sync;
166
167    /**
168     * Constructs a {@code CountDownLatch} initialized with the given count.
169     *
170     * @param count the number of times {@link #countDown} must be invoked
171     *        before threads can pass through {@link #await}
172     * @throws IllegalArgumentException if {@code count} is negative
173     */
174    public CountDownLatch(int count) {
175        if (count < 0) throw new IllegalArgumentException("count < 0");
176        this.sync = new Sync(count);
177    }
178
179    /**
180     * Causes the current thread to wait until the latch has counted down to
181     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
182     *
183     * <p>If the current count is zero then this method returns immediately.
184     *
185     * <p>If the current count is greater than zero then the current
186     * thread becomes disabled for thread scheduling purposes and lies
187     * dormant until one of two things happen:
188     * <ul>
189     * <li>The count reaches zero due to invocations of the
190     * {@link #countDown} method; or
191     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
192     * the current thread.
193     * </ul>
194     *
195     * <p>If the current thread:
196     * <ul>
197     * <li>has its interrupted status set on entry to this method; or
198     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
199     * </ul>
200     * then {@link InterruptedException} is thrown and the current thread's
201     * interrupted status is cleared.
202     *
203     * @throws InterruptedException if the current thread is interrupted
204     *         while waiting
205     */
206    public void await() throws InterruptedException {
207        sync.acquireSharedInterruptibly(1);
208    }
209
210    /**
211     * Causes the current thread to wait until the latch has counted down to
212     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
213     * or the specified waiting time elapses.
214     *
215     * <p>If the current count is zero then this method returns immediately
216     * with the value {@code true}.
217     *
218     * <p>If the current count is greater than zero then the current
219     * thread becomes disabled for thread scheduling purposes and lies
220     * dormant until one of three things happen:
221     * <ul>
222     * <li>The count reaches zero due to invocations of the
223     * {@link #countDown} method; or
224     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
225     * the current thread; or
226     * <li>The specified waiting time elapses.
227     * </ul>
228     *
229     * <p>If the count reaches zero then the method returns with the
230     * value {@code true}.
231     *
232     * <p>If the current thread:
233     * <ul>
234     * <li>has its interrupted status set on entry to this method; or
235     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
236     * </ul>
237     * then {@link InterruptedException} is thrown and the current thread's
238     * interrupted status is cleared.
239     *
240     * <p>If the specified waiting time elapses then the value {@code false}
241     * is returned.  If the time is less than or equal to zero, the method
242     * will not wait at all.
243     *
244     * @param timeout the maximum time to wait
245     * @param unit the time unit of the {@code timeout} argument
246     * @return {@code true} if the count reached zero and {@code false}
247     *         if the waiting time elapsed before the count reached zero
248     * @throws InterruptedException if the current thread is interrupted
249     *         while waiting
250     */
251    public boolean await(long timeout, TimeUnit unit)
252        throws InterruptedException {
253        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
254    }
255
256    /**
257     * Decrements the count of the latch, releasing all waiting threads if
258     * the count reaches zero.
259     *
260     * <p>If the current count is greater than zero then it is decremented.
261     * If the new count is zero then all waiting threads are re-enabled for
262     * thread scheduling purposes.
263     *
264     * <p>If the current count equals zero then nothing happens.
265     */
266    public void countDown() {
267        sync.releaseShared(1);
268    }
269
270    /**
271     * Returns the current count.
272     *
273     * <p>This method is typically used for debugging and testing purposes.
274     *
275     * @return the current count
276     */
277    public long getCount() {
278        return sync.getCount();
279    }
280
281    /**
282     * Returns a string identifying this latch, as well as its state.
283     * The state, in brackets, includes the String {@code "Count ="}
284     * followed by the current count.
285     *
286     * @return a string identifying this latch, as well as its state
287     */
288    public String toString() {
289        return super.toString() + "[Count = " + sync.getCount() + "]";
290    }
291}
292