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 
Sync(int count)135         Sync(int count) {
136             setState(count);
137         }
138 
getCount()139         int getCount() {
140             return getState();
141         }
142 
tryAcquireShared(int acquires)143         protected int tryAcquireShared(int acquires) {
144             return (getState() == 0) ? 1 : -1;
145         }
146 
tryReleaseShared(int releases)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      */
CountDownLatch(int count)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      */
await()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      */
await(long timeout, TimeUnit unit)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      */
countDown()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      */
getCount()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      */
toString()283     public String toString() {
284         return super.toString() + "[Count = " + sync.getCount() + "]";
285     }
286 }
287