面试官：如何控制多线程执行顺序？

面试官：如何控制多线程执行顺序？

介绍

先看如下代码：

public class Test { static Thread thread1 = new Thread(()-> { System.out.println("thread1"); }); static Thread thread2 = new Thread(()-> { System.out.println("thread2"); }); static Thread thread3 = new Thread(()-> { System.out.println("thread3"); }); public static void main(String[] args) throws InterruptedException { thread1.start(); thread2.start(); thread3.start(); }}

重复执行多次，发现输出并不是按照线程的启动顺序来执行。因为这个里面涉及到CPU对线程的调度问题。

thread1thread3thread2

如何让thread1，thread2，thread3顺序执行呢？

方法1

通过join方法去保证多线程顺序执行

public class Test { static Thread thread1 = new Thread(()-> { System.out.println("thread1"); }); static Thread thread2 = new Thread(()-> { System.out.println("thread2"); }); static Thread thread3 = new Thread(()-> { System.out.println("thread3"); }); public static void main(String[] args) throws InterruptedException { thread1.start(); thread1.join(); thread2.start(); thread2.join(); thread3.start(); }}

可以看到输出一直是如下

thread1thread2thread3

join是怎么实现这个功能的呢？ join方法让主线程等待子线程结束以后才能继续运行，因此保证了线程的顺序执行

方法2

使用单例线程池，用唯一的工作线程执行任务，保证所有任务按照指定顺序执行

ExecutorService executorService = Executors.newSingleThreadExecutor();

这个会把线程放在一个FIFO队列，依次执行线程

public class Test { static Thread thread1 = new Thread(()-> { System.out.println("thread1"); }); static Thread thread2 = new Thread(()-> { System.out.println("thread2"); }); static Thread thread3 = new Thread(()-> { System.out.println("thread3"); }); static ExecutorService executorService = Executors.newSingleThreadExecutor(); public static void main(String[] args) throws InterruptedException { executorService.submit(thread1); executorService.submit(thread2); executorService.submit(thread3); executorService.shutdown(); }}

输出一直为

thread1thread2thread3

目的达到

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参考博客

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