/** *  * Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches: The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed; The second is a completion signal that allows the driver to wait until all workers have completed. */public class CountDownLatchDemo {    public static void main(String[] args) throws InterruptedException {        new Driver().run();    }}class Driver {    static final int N = 5;    public void run() throws InterruptedException {        CountDownLatch startSignal = new CountDownLatch(1);        CountDownLatch doneSignal = new CountDownLatch(N);        for (int i=0; i<N; i++) {            new Thread(new Worker(startSignal, doneSignal))                    .start();        }        doSomething(); // don‘t let run yet        startSignal.countDown(); // let all threads proceed        doneSignal.await(); // wait for all to finish        doSomething();    }    private void doSomething() {        System.out.println("doSomething");    }}class Worker implements Runnable {    private CountDownLatch startSignal;    private CountDownLatch doneSignal;    public Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {        this.startSignal = startSignal;        this.doneSignal = doneSignal;    }    public void run() {        try {            startSignal.await();            doWork();            doneSignal.countDown();        } catch (InterruptedException e) {            e.printStackTrace();        }    }    private void doWork() {        System.out.println("worker work.");    }}