Why does this simple threaded program get stuck?

Question

Take a look at this simple Java program:

import java.lang.*;

class A {
    static boolean done;
    public static void main(String args[]) {
        done = false;
        new Thread() {
            public void run() {
            try {
                Thread.sleep(1000); // dummy work load
            } catch (Exception e) {
                done = true;
            }
            done = true;
            }
        }.start();
        while (!done);
        System.out.println("bye");
    }
}

On one machine, it prints "bye" and exits right away, while on another machine, it doesn't print anything and sits there forever. Why?

Answer 1

This is because your boolean is not volatile, therefore Threads are allowed to cache copies of it and never update them. I would recommend an AtomicBoolean - that will prevent any issues you may have.

public static void main(String args[]) {
    final AtomicBoolean done = new AtomicBoolean(false);
    new Thread() {
        public void run() {
            done.set(true);
        }
    }.start();
    while (!done.get());
    System.out.println("bye");
}

Answer 2

By the time the main program's while loop is reached (which is also a Thread), the new Thread might be finishing its run() where done flag is set to true. Just to confirm this, you can add a sleep in the run() before done is set to true and then see if your bye is displayed on other machine also. Hope this would help.