Question

I am in doubt, what happens when a thread joins itself. i.e thread calls the join method on its own. I am not getting any error.

Sample :

public class JoinItself extends Thread {

    public void run() {
        System.out.println("Inside the run method ");
        System.out.println(Thread.currentThread().isAlive());
        for(int i=0;i<5;i++) {
            try {
                System.out.println("Joining itself ...");
                Thread.currentThread().join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {

        JoinItself j = new JoinItself();

        System.out.println(j.isAlive());
        j.start();
        System.out.println(j.isAlive());
        System.out.println("Thread started ...");

    }

}

But Why? Should I get any error?

Answer 1

The concept of a thread joining itself does not make sense.

It happens out that the join() method uses the isAlive() method to
determine when to return from the join() method. In the current implementation, it also
does not check to see if the thread is joining itself.
In other words, the join() method returns when and only when the thread is no longer alive. This will have the effect of
waiting forever.