/* * 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. * *

A {@code CountDownLatch} is initialized with a given count. * 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}. * *

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 N * can be used to make one thread wait until N threads have * completed some action, or some action has been completed N times. * *

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. * *

Sample usage: Here is a pair of classes in which a group * of worker threads use two countdown latches: *

* *
 {@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() { ... }
 * }}
* *

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}.) * *

 {@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() { ... }
 * }}
* *

Memory consistency effects: Until the count reaches * zero, actions in a thread prior to calling * {@code countDown()} * happen-before * 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}. * *

If the current count is zero then this method returns immediately. * *

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: *

* *

If the current thread: *

* 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. * *

If the current count is zero then this method returns immediately * with the value {@code true}. * *

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: *

* *

If the count reaches zero then the method returns with the * value {@code true}. * *

If the current thread: *

* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

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. * *

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. * *

If the current count equals zero then nothing happens. */ public void countDown() { sync.releaseShared(1); } /** * Returns the current count. * *

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() + "]"; } }