CountDownLatch 多線程，等待所有線程結束

CountDownLatch，一個同步輔助類，在完成一組正在其他線程中執行的操作之前，它允許一個或多個線程一直等待。

主要方法
public CountDownLatch(int count);
構造方法參數指定了計數的次數
public void countDown();
當前線程調用此方法，則計數減一
public void await() throws InterruptedException
調用此方法會一直阻塞當前線程，直到計時器的值為0

public class CountDownLatchDemo {
    final static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(2);// 兩個工人的協作
        Worker worker1 = new Worker("zhang san", 5000, latch);
        Worker worker2 = new Worker("li si", 8000, latch);
        worker1.start();//
        worker2.start();//
        latch.await();// 等待所有工人完成工作
        System.out.println("all work done at " + sdf.format(new Date()));
    }

    static class Worker extends Thread {
        String workerName;
        int workTime;
        CountDownLatch latch;

        public Worker(String workerName, int workTime, CountDownLatch latch) {
            this.workerName = workerName;
            this.workTime = workTime;
            this.latch = latch;
        }

        public void run() {
            System.out.println("Worker " + workerName + " do work begin at " + sdf.format(new Date()));
            try {
                Thread.sleep(workTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                latch.countDown();
            }
            System.out.println("Worker " + workerName + " do work complete at " + sdf.format(new Date()));

        }

    }

}