isAlive() method of java thread is not working properly?

Question

I was trying a example of isAlive() method of java threading. But i found that isAlive() method is returning false even if thread has been already started. Can someone please tell me what am i doing wrong? Here is the code snippet.

package app;

public class ThreadAliveDemo {

    public static void main(String[] args) {

        Thread myThread;

        myThread = new Thread()
        {
            public void run()
            {
                            Thread.sleep(3000);
                System.out.println("My Thread.");
            }
        };

        myThread.setName("My Thread");
        myThread.start();

        if(!myThread.isAlive())
        {
            myThread.setName("My Thread");
            myThread.start();
        }

    }

}

Answer 1

There's a good chance the thread will have started, executed, and finished, between your call to start() and your call to isAlive().

Java offers no guarantees on the sequence in which these things happen. It could execute the spawned thread immediately, or it may choose to defer it until a bit later.

Incidentally, your code is trying to re-start the thread after it has died. This is not permitted:

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

So calling start() after checking isAlive() is never going to work.

Answer 2

I haven't done any multithreading in java yet, but it looks to me like your thread probably will have run and exited before the isAlive() check. After all, looks like your thread just prints something out and then dies.

Answer 3

If my memory serves me well java has quite long periods between thread switching so it is possible that the isAlive fails because the thread is not yet alive. Try to add some waiting time between thread.start() and thread.isAlive()

Answer 4

I don't see the point of the code you have posted. Thread.start() starts the thread: you don't need to start it twice. I don't see how your code can realistically into a situation where it has a Thread and doesn't know whether it has been started or not; anyway there are plenty of ways to code around that so it can't happen.