CountDownLatch允许一个或者多个线程一直等待，直到一组其它操作执行完成。在使用CountDownLatch时，需要指定一个整数值，此值是线程将要等待的操作数。当某个线程为了要执行这些操作而等待时，需要调用await方法。await方法让线程进入休眠状态直到所有等待的操作完成为止。当等待的某个操作执行完成，它使用countDown方法来减少CountDownLatch类的内部计数器。当内部计数器递减为0时，CountDownLatch会唤醒所有调用await方法而休眠的线程们。
下面代码演示了CountDownLatch简单使用。演示的场景是5位运动员参加跑步比赛，发令枪打响后，5个计时器开始分别计时，直到所有运动员都到达终点。
public class CountDownLatchDemo { public static void main(String[] args) { Timer timer = new Timer(5); new Thread(timer).start(); for (int athleteNo = 0; athleteNo < 5; athleteNo++) { new Thread(new Athlete(timer, "athlete" + athleteNo)).start(); } } } class Timer implements Runnable { CountDownLatch timerController; public Timer(int numOfAthlete) { this.timerController = new CountDownLatch(numOfAthlete); } public void recordResult(String athleteName) { System.out.println(athleteName + " has arrived"); timerController.countDown(); System.out.println("There are " + timerController.getCount() + " athletes did not reach the end"); } @Override public void run() { try { System.out.println("Start..."); timerController.await(); System.out.println("All the athletes have arrived"); } catch (InterruptedException e) { e.printStackTrace(); } } } class Athlete implements Runnable { Timer timer; String athleteName; public Athlete(Timer timer, String athleteName) { this.timer = timer; this.athleteName = athleteName; } @Override public void run() { try { System.out.println(athleteName + " start running"); long duration = (long) (Math.random() * 10); Thread.sleep(duration * 1000); timer.recordResult(athleteName); } catch (InterruptedException e) { e.printStackTrace(); } } } 输出结果如下所示：
Start... athlete0 start running athlete1 start running athlete2 start running athlete3 start running athlete4 start running athlete0 has arrived There are 4 athletes did not reach the end athlete3 has arrived There are 3 athletes did not reach the end athlete2 has arrived athlete1 has arrived There are 1 athletes did not reach the end There are 2 athletes did not reach the end athlete4 has arrived There are 0 athletes did not reach the end All the athletes have arrived
1.构造方法
CountDownLatch(int count)构造一个指定计数的CountDownLatch，count为线程将要等待的操作数。
2.await()
调用await方法后，使当前线程在锁存器(内部计数器)倒计数至零之前一直等待，进入休眠状态，除非线程被中断。如果当前计数递减为零，则此方法立即返回，继续执行。
3.await(long timeout, TimeUnit unit)
调用await方法后，使当前线程在锁存器(内部计数器)倒计数至零之前一直等待，进入休眠状态，除非线程被 中断或超出了指定的等待时间。如果当前计数为零，则此方法立刻返回true值。
3.acountDown()
acountDown方法递减锁存器的计数，如果计数到达零，则释放所有等待的线程。如果当前计数大于零，则将计数减少。如果新的计数为零，出于线程调度目的，将重新启用所有的等待线程。
4.getCount()
调用此方法后，返回当前计数，即还未完成的操作数，此方法通常用于调试和测试。
进入源码分析之前先看一下CountDownLatch的类图，