用java Thread interrupt的时候为啥会抛出异常

在用java Thread interrupt的时候为啥会抛出异常？
```
public class Demo1 extends Thread {

    @Override
    public void run(){
        while (!interrupted()) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(this.getName() + " is executing");
        }
    }

    public static void main(String[] args) {
        Demo1 t1 = new Demo1();
        Demo1 t2 = new Demo1();
        t1.start();
        t2.start();
        t1.interrupt();
    }
}

```
输出结果：

```
java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at t2.Demo1.run(Demo1.java:9)
Thread-0 is executing
Thread-1 is executing
Thread-0 is executing
Thread-1 is executing
Thread-0 is executing
Thread-1 is executing
Thread-0 is executing
Thread-1 is executing
Thread-0 is executing
Thread-1 is executing
Thread-0 is executing
Thread-1 is executing
.
.
.
```

为什么没有直接中断呢？ 都是按照教程敲的，搞不明白。。。希望大神指点~