场景题：有三个线程t1、t2、t3。确保三个线程t1执行完后t2执行，t2执行完成后t3执行_编写程序task51.c,主线程创建3个对等线程t1、t2、t3,每个线程利用循环执行5次prin

场景题：有三个线程t1、t2、t3。确保三个线程t1执行完后t2执行，t2执行完成后t3执行

主要有两种方法：

1. 通过在当前线程传入上一线程对象，调用join()方法让当前线程阻塞等待上一线程执行完毕的方式，实现代码如下：

public class Main{

	public static void main(String[] args) {
		Thread t1 = new Thread(new Task(null));
        Thread t2 = new Thread(new Task(t1));
        Thread t3 = new Thread(new Task(t2));
        t1.start();
        t2.start();
        t3.start();
	}
	
	static class Task implements Runnable {
        private Thread preThread;
        public Task(Thread preThread) {
            this.preThread = preThread;
        }
        public void run() {
            if (preThread != null) {
                try {
                    preThread.join();
                    System.out.println("thread start:" + Thread.currentThread().getName());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                System.out.println("thread start:" + Thread.currentThread().getName());
            }
        }
    }
}
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2. 通过CountDownLatch计数器去实现，初始化上一线程与当前线程的计数器，通过await()方法阻塞等待，countDown()方法减计数器的值，当计数器的值减到0时当前线程开始运行。实现代码如下：

public class Main02{
	public static void main(String[] args) {
		CountDownLatch countDownLatch1 = new CountDownLatch(0);// 在这里count为0，表示该线程立马开始执行
        CountDownLatch countDownLatch2 = new CountDownLatch(1);// 在这里count为1，表示等待上一线程执行完，也就是要等countDownLatch2减去1为0时
        CountDownLatch countDownLatch3 = new CountDownLatch(1);// 在这里count为1，表示等待上一线程执行完
        Thread t1 = new Thread(new Count(countDownLatch1,countDownLatch2)); 
        Thread t2 = new Thread(new Count(countDownLatch2, countDownLatch3));
        Thread t3 = new Thread(new Count(countDownLatch3, countDownLatch3));
	}
	static class Count implements Runnable {

        CountDownLatch countDownLatch1;
        CountDownLatch countDownLatch2;

        /**
         * @param countDownLatch1 上一个线程的计数器
         * @param countDownLatch2 当前线程的计数器
         */
        public Count(CountDownLatch countDownLatch1, CountDownLatch countDownLatch2) {
            super();
            this.countDownLatch1 = countDownLatch1;
            this.countDownLatch2 = countDownLatch2;
        }

        @Override
        public void run() {
            try {
                countDownLatch1.await();//线程t1进来的时候,因为count=0,所以不用等直接运行
                System.out.println("thread start:" + Thread.currentThread().getName());
                Thread.sleep(1000);
                countDownLatch2.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }
	
}
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