java：同步线程，等待

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0
c.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。