如果在线程的InterruptedException的catch块中使用“返回”，该返回哪里？

Question

It might be very simple question for you but I would much appreciate if you please help to clear my doubt on this.

Suppose I created a thread that uses a sleep() method and from my main method I am interrupting it.

Here is my code fragment

class myThread implements Runnable{
    volatile boolean checking=true;
    int counter =1;
    public void run()
    {
         while(checking)
         {
              System.out.println(Thread.currentThread().getName() + " - count - "+counter++);
              try{
                  Thread.sleep(1000);
              }catch(InterruptedException e){
                  System.out.println("Interrupted");
                  Thread.currentThread().interrupt();
                  return;  //line of confusion
              }

         }
    }
    public void stopCounting()
    {
        checking = false;
    }
    public boolean getCheker()
    {
        return checking ;
    }

}
//main class 
public class Demo{
    public static void main(string args[])
    {
        myThread th = new myThread();
        Thread t1 = new Thread(th);
        t1.start();
        for(int i=0;i<50000;i++)
        {
            //do nothing
            System.out.println("Do nothing...");
        }
        t1.interrupt();
        System.out.println("Press enter to stop the counter....");

        Scanner sc = new Scanner(System.in);
        sc.nextLine();
        th.stopCounting();

        //this line is executed when I am not using return statement
        // in thread
        System.out.println("value of while loop cheker is: "+th.getCounter());
    }
}

Where does the "return" return in the above case?

Case 1 : I checked if I keep the "return" keyword in the catch block, the last executed line again is from the main method which is "Press enter to stop the counter...."

Case 2: If I omit the return statement then the last executed line from main is "System.out.println("value of counter is "+th.getCheker());"

My question is, why in the first case also line "System.out.println("value of counter is "+th.getCheker());" is not executed?

I thought while return is called the control should return from the "run()" method of the thread to the place in main from where the thread is started. So, statements that are not executed in main by then, should be executed now. But it seems if I use the return statement the application is ending. The main is also returning. No statement is getting executed after that. Could you please explain that? What I am missing?

Answer 1

Your "line of confusion" will cause the run() method of the thread to return. That will cause the thread to die because threads always die when run() returns.

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"Case 1:" vs. "Case 2:"

Your "line of confusion" is contained within a loop. If you take out the return statement, then the run() method won't return at that point: It will go back to the top of the while(checking) loop.

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I thought while return is called the control should return from the "run()" method of the thread to the place in main from where the thread is started.

That's not how threads work. THere's nothing for the new thread to go back to. It's the main() thread's job to execute the statements that comes after the one that started the new thread. When the new thread's run() method is finished, there's nothing left for it to do except die.

Answer 2

它将结束函数，并且当函数返回void时，它将不返回任何内容

Answer 3

return; will break the while loop.

You can use break; or checking = false and you'd have the same affect.

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the last executed line again is from the main method which is "Press enter to stop the counter...."

Maybe that is because it is waiting for you to actually press Enter ?

If I omit the return statement then the last executed line from main is "System.out.println("value of counter is "+th.getCheker());"

That line of code doesn't match your question...

should return from the "run()" method of the thread to the place in main from where the thread is started

Nope, it's a separate Thread, it executes parallelly in the background. You just happen to be printing to the same output stream, so that's hard to see.

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The Thread executes in its own thread with its own call stack. When you return from run() , it ends, therefore stopping it, and it doesn't "return to somewhere else" in your code.

Both print statements you have should run if you would remove the Scanner.