Java countDownLatch 線程輔助類

countDownLatch 線程輔助類，是一個線程計數類，用來線程同步，每當一個線程執行完成后，就是加1，當達到計數，就算執行完成。初始值必須小于等于要執行的線程數，為0 是，不其作用，大于0，小于等于線程數時，為同步線程數，當大于線程數時，為死循環（可能不對）反正就是一直等待。

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.CountDownLatch;

public class T1 {
    final static SimpleDateFormat sdf = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");

public static void main(String[] args) throws Exception {
    CountDownLatch countDownLatch = new CountDownLatch(3);
    Worker w1 = new Worker("w1", 2000, countDownLatch);
    Worker w2 = new Worker("w2", 5000, countDownLatch);
    w1.start();//
    w2.start();//
    countDownLatch.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;
    }

    @Override
    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();
        }
        System.out.println("Worker " + workerName + " do work complete at "
                + sdf.format(new Date()));
        latch.countDown();// 工人完成工作，計數器減一
    }
}

}</pre>