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
7package java.util.concurrent;
8import 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 */
127public 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}
287