关于Thread.sleep()的疑问：调用自定义线程引用为何让主线程休眠？

Thread.sleep()方法会让执行它的线程进入休眠。但在下面的代码中，通过自定义线程的引用调用sleep()方法后，休眠的却是主线程，这让人困惑。

代码示例 #

public class ThreadDemo {

    public static Thread t1 = null;
    public static Thread t2 = null;
    public static void main(String[] args) throws InterruptedException{

        System.out.println("start of main function, Thread : "+Thread.currentThread().getName());
        t1 = Thread.currentThread(); //main thread
        MyThread thread = new MyThread();
        t2 = thread;
        thread.start();

        System.out.println("Before sleep in main method : "+System.currentTimeMillis());
        thread.sleep(1000);
        System.out.println("After sleep in main method : "+System.currentTimeMillis());

        System.out.println("In main method, Thread : "+t1.getName()+" ,In state: "+t1.getState());
        System.out.println("In main method, Thread : "+t2.getName()+" ,In state: "+t2.getState());

        System.out.println("Is thread, "+thread.getName() + " alive : "+ thread.isAlive());

        System.out.println("end of main function, Thread : "+Thread.currentThread().getName());
    }

    public static class MyThread extends Thread{
        int sum = 0;
        @Override
        public void run() {
            System.out.println("In run method, Thread : "+t1.getName()+" ,In state: "+t1.getState());
            System.out.println("In run method, Thread : "+t2.getName()+" ,In state: "+t2.getState());
            System.out.println("start of run method, Thread : "+Thread.currentThread().getName());
            for(int i=0; i<10000; i++){
                sum += i;
            }
            try{
                Thread.sleep(10000);
                System.out.println("After sleep in run method : "+System.currentTimeMillis());
            }catch (InterruptedException e){
                e.printStackTrace();
            }
            System.out.println("end of run method, "+ Thread.currentThread().getName() +", returns : "+sum);
        }
    }
}

输出结果 #

start of main function, Thread : main
Before sleep in main method : 1701105524066
In run method, Thread : main ,In state: TIMED_WAITING
In run method, Thread : Thread-0 ,In state: RUNNABLE
start of run method, Thread : Thread-0
After sleep in main method : 1701105525070
In main method, Thread : main ,In state: RUNNABLE
In main method, Thread : Thread-0 ,In state: TIMED_WAITING
Is thread, Thread-0 alive : true
end of main function, Thread : main
After sleep in run method : 1701105534071
end of run method, Thread-0, returns : 49995000

问题核心 #

为什么通过自定义线程的引用调用sleep()方法，休眠的却是主线程？

解答 #

原因很直接：Thread.sleep()是一个静态方法。

不管你用哪个Thread实例去调用它，本质上都是调用了Thread类的静态方法，它只会让当前执行这段代码的线程进入休眠，和你用来调用的那个线程实例没有任何关系。

在你的代码里，thread.sleep(1000)这段代码是在主线程的main方法里执行的，所以休眠的自然是主线程。哪怕你用一个已经终止的线程实例去调用sleep()，休眠的依然是当前执行代码的线程。

Java设计这个静态方法的逻辑就是：sleep是让当前线程暂停，而不是让某个指定的线程暂停——毕竟直接控制另一个线程的休眠状态会涉及线程调度的安全性问题。

如果想要让自定义线程MyThread休眠，你需要在它的run()方法里调用Thread.sleep()，就像你代码里已经做的那样，这样休眠的才是MyThread线程本身。

内容的提问来源于stack exchange，提问作者Bh_04