通过扩展 Thread 类的线程行为

Question

I have following class‍ :

public class Thread1 extends Thread {
  @Override
  public void run() {
    System.out.println("I am the first thread");
  }

 public static void main(String args[]) {
    Thread1 t = new Thread1();
    t.start();  
    }
}

If I run the above program it will print in new thread

"I am the first thread"

Now in the main if I try to start the same thread twice it will throw java.lang.IllegalThreadStateException

Now I will rewrite the Thread1 class as below by overriding the start() method

public class Thread1 extends Thread {
  @Override
  public void run() {
    System.out.println("I am the first thread");
  }

  @Override
  public void start() {
    System.out.println("I am the Start");
  }

  public static void main(String args[]) {
    Thread1 t = new Thread1();
    t.start();  
    t.start();  
    }
  }

Now if I call start() method twice the output will be :

  I am in start
  I am in start

The above program is throwing any exception. Can anyone explain me this behavior. Why it is not starting the new thread. Seems be to simple but not able understand it.

Answer 1

When @Override ing a method, your new method that you are creating is what is going to be called instead of the previous method.

In class Thread , start() is a method which does some stuff and starts the Thread.

In your new class Thread1 , start() is a method which just prints out a line and does nothing else

---

Your understanding is correct - that calling super.start() is calling the superclass's version of start() which is allowing the Thread to actually start running.

Note though, that you are still going to be hitting that exception when trying to run it twice. This is the way Thread is designed.

From link in comment on question:

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.