import java.util.concurrent.CountDownLatch;CountDownLatch  c = new CountDownLatch(5); //初始化一个计数值为5的进程c.await();     //进入等待状态，直到计数值为0c.countDown(); //计数值减1，如果计数值为0则释放所有等待的线程

例子：

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() {
     ... }}

第一个类是一个启动信号，在 driver 为继续执行 worker 做好准备之前，它会阻止所有的 worker 继续执行。

第二个类是一个完成信号，它允许 driver 在完成所有 worker 之前一直等待。