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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?_多线程

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。

thread1
thread3
thread2

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

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

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

thread1
thread2
thread3

目的达到

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面试官:如何控制多线程执行顺序?_d3_02

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