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。

thread1
thread3
thread2

如何让thread1，thread2，thread3顺序执行呢？

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

thread1
thread2
thread3

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

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

thread1
thread2
thread3