CountDownLatch 它被用来同步一个或者多个任务，强制他们等待由其他任务执行的一组操作完成。
你可以向CountDownLatch对象设置一个初始计数值，任何在这个对象上调用wait（）的方法都将阻塞，直到这个计数值达到0。其他任务在结束其工作时，可以在该对象上调用countDown()来减小这个计数值。CountDownLatch被设计为只出发一次，计数值不能被重置。如果你需要能够重置计数值的版本，可以使用CycliicBarrier。
调用countDown()的任务在产生这个调用时并没有被阻塞，只有对await（）的调用会被阻塞，直至计数器到达0。
CountDownLatch的典型用法是将程序分为n个相互独立的可解决任务，并创建值为0的CountDownLatch。当每个任务完成时，都会在这个锁存器上调用countDown()。等待问题被解决的任务在这个锁存器上调用await()，将他们自己拦住，直至锁存器计数结束。下面是演示这种技术的一个框架事例：
package concurrency;//: concurrency/CountDownLatchDemo.java
import java.util.concurrent.*;
import java.util.*;
import static net.mindview.util.Print.*;// Performs some portion of a task:
class TaskPortion implements Runnable {private static int counter = 0;private final int id = counter++;private static Random rand = new Random(47);private final CountDownLatch latch;TaskPortion(CountDownLatch latch) {this.latch = latch;}public void run() {try {doWork();latch.countDown();} catch(InterruptedException ex) {// Acceptable way to exit}}public void doWork() throws InterruptedException {TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));print(this + "completed");}public String toString() {return String.format("%1$-3d ", id);}
}// Waits on the CountDownLatch:
class WaitingTask implements Runnable {private static int counter = 0;private final int id = counter++;private final CountDownLatch latch;WaitingTask(CountDownLatch latch) {this.latch = latch;}public void run() {try {latch.await();print("Latch barrier passed for " + this);} catch(InterruptedException ex) {print(this + " interrupted");}}public String toString() {return String.format("WaitingTask %1$-3d ", id);}
}public class CountDownLatchDemo {static final int SIZE = 100;public static void main(String[] args) throws Exception {ExecutorService exec = Executors.newCachedThreadPool();// All must share a single CountDownLatch object:CountDownLatch latch = new CountDownLatch(SIZE);for(int i = 0; i < 10; i++)exec.execute(new WaitingTask(latch));for(int i = 0; i < SIZE; i++)exec.execute(new TaskPortion(latch));print("Launched all tasks");exec.shutdown(); // Quit when all tasks complete}
} /* (Execute to see output) *///:~
TaskPortion将随机的休眠一段时间，以模拟这部分工作的完成，erWaitingTask表示系统中必须等待的部分，它要等待到问题的初始部分完成为止。所有的任务都使用了main()中定义的同一个单一的CountDownLatch。