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
7 package java.util.concurrent;
8 import java.util.concurrent.locks.*;
9 import java.util.concurrent.atomic.*;
12 * A synchronization aid that allows one or more threads to wait until
13 * a set of operations being performed in other threads completes.
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}.
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.
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
38 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
39 * of worker threads use two countdown latches:
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.
48 * class Driver { // ...
49 * void main() throws InterruptedException {
50 * CountDownLatch startSignal = new CountDownLatch(1);
51 * CountDownLatch doneSignal = new CountDownLatch(N);
53 * for (int i = 0; i < N; ++i) // create and start threads
54 * new Thread(new Worker(startSignal, doneSignal)).start();
56 * doSomethingElse(); // don't let run yet
57 * startSignal.countDown(); // let all threads proceed
59 * doneSignal.await(); // wait for all to finish
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;
72 * startSignal.await();
74 * doneSignal.countDown();
75 * } catch (InterruptedException ex) {} // return;
78 * void doWork() { ... }
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}.)
91 * class Driver2 { // ...
92 * void main() throws InterruptedException {
93 * CountDownLatch doneSignal = new CountDownLatch(N);
96 * for (int i = 0; i < N; ++i) // create and start threads
97 * e.execute(new WorkerRunnable(doneSignal, i));
99 * doneSignal.await(); // wait for all to finish
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;
110 * public void run() {
113 * doneSignal.countDown();
114 * } catch (InterruptedException ex) {} // return;
117 * void doWork() { ... }
122 * <p>Memory consistency effects: Actions in a thread prior to calling
123 * {@code countDown()}
124 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
125 * actions following a successful return from a corresponding
126 * {@code await()} in another thread.
131 public class CountDownLatch {
133 * Synchronization control For CountDownLatch.
134 * Uses AQS state to represent count.
136 private static final class Sync extends AbstractQueuedSynchronizer {
137 private static final long serialVersionUID = 4982264981922014374L;
147 public int tryAcquireShared(int acquires) {
148 return getState() == 0? 1 : -1;
151 public boolean tryReleaseShared(int releases) {
152 // Decrement count; signal when transition to zero
158 if (compareAndSetState(c, nextc))
164 private final Sync sync;
167 * Constructs a {@code CountDownLatch} initialized with the given count.
169 * @param count the number of times {@link #countDown} must be invoked
170 * before threads can pass through {@link #await}
171 * @throws IllegalArgumentException if {@code count} is negative
173 public CountDownLatch(int count) {
174 if (count < 0) throw new IllegalArgumentException("count < 0");
175 this.sync = new Sync(count);
179 * Causes the current thread to wait until the latch has counted down to
180 * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
182 * <p>If the current count is zero then this method returns immediately.
184 * <p>If the current count is greater than zero then the current
185 * thread becomes disabled for thread scheduling purposes and lies
186 * dormant until one of two things happen:
188 * <li>The count reaches zero due to invocations of the
189 * {@link #countDown} method; or
190 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
191 * the current thread.
194 * <p>If the current thread:
196 * <li>has its interrupted status set on entry to this method; or
197 * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
199 * then {@link InterruptedException} is thrown and the current thread's
200 * interrupted status is cleared.
202 * @throws InterruptedException if the current thread is interrupted
205 public void await() throws InterruptedException {
206 sync.acquireSharedInterruptibly(1);
210 * Causes the current thread to wait until the latch has counted down to
211 * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
212 * or the specified waiting time elapses.
214 * <p>If the current count is zero then this method returns immediately
215 * with the value {@code true}.
217 * <p>If the current count is greater than zero then the current
218 * thread becomes disabled for thread scheduling purposes and lies
219 * dormant until one of three things happen:
221 * <li>The count reaches zero due to invocations of the
222 * {@link #countDown} method; or
223 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
224 * the current thread; or
225 * <li>The specified waiting time elapses.
228 * <p>If the count reaches zero then the method returns with the
229 * value {@code true}.
231 * <p>If the current thread:
233 * <li>has its interrupted status set on entry to this method; or
234 * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
236 * then {@link InterruptedException} is thrown and the current thread's
237 * interrupted status is cleared.
239 * <p>If the specified waiting time elapses then the value {@code false}
240 * is returned. If the time is less than or equal to zero, the method
241 * will not wait at all.
243 * @param timeout the maximum time to wait
244 * @param unit the time unit of the {@code timeout} argument
245 * @return {@code true} if the count reached zero and {@code false}
246 * if the waiting time elapsed before the count reached zero
247 * @throws InterruptedException if the current thread is interrupted
250 public boolean await(long timeout, TimeUnit unit)
251 throws InterruptedException {
252 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
256 * Decrements the count of the latch, releasing all waiting threads if
257 * the count reaches zero.
259 * <p>If the current count is greater than zero then it is decremented.
260 * If the new count is zero then all waiting threads are re-enabled for
261 * thread scheduling purposes.
263 * <p>If the current count equals zero then nothing happens.
265 public void countDown() {
266 sync.releaseShared(1);
270 * Returns the current count.
272 * <p>This method is typically used for debugging and testing purposes.
274 * @return the current count
276 public long getCount() {
277 return sync.getCount();
281 * Returns a string identifying this latch, as well as its state.
282 * The state, in brackets, includes the String {@code "Count ="}
283 * followed by the current count.
285 * @return a string identifying this latch, as well as its state
287 public String toString() {
288 return super.toString() + "[Count = " + sync.getCount() + "]";