CountDownLatch 是一个极其简单但又极其常用的实用工具，用于在保持给定数目的信号、事件或条件前阻塞执行，通过调用await()，countDown()方法，实现同步功能。
例子：有三个线程，等待主线程下发命令，当主线程下发命令时，三个线程会接收命令，并执行，主线程接收执行完的结果
public class CountdownLatchTest {
public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final CountDownLatch cdOrder = new CountDownLatch(1);
final CountDownLatch cdAnswer = new CountDownLatch(3);
for(int i=0;i<3;i++){
Runnable runnable = new Runnable(){
public void run(){
try {
System.out.println("线程" + Thread.currentThread().getName() +
"正准备接受命令");
cdOrder.await();
System.out.println("线程" + Thread.currentThread().getName() +
"已接受命令");
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"回应命令处理结果");
cdAnswer.countDown();
} catch (Exception e) {
e.printStackTrace();
}
}
};
service.execute(runnable);
}
try {
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程" + Thread.currentThread().getName() +
"即将发布命令");
cdOrder.countDown();
System.out.println("线程" + Thread.currentThread().getName() +
"已发送命令，正在等待结果");
cdAnswer.await();
System.out.println("线程" + Thread.currentThread().getName() +
"已收到所有响应结果");
} catch (Exception e) {
e.printStackTrace();
}
service.shutdown();
}
}