java thread visibility

Question

When I read the "java concurrency in practice" c03, I was confused by the following program:

public class NoVisibility { 
    private static boolean ready; 
    private static int number; 

    private static class ReaderThread extends Thread { 
        public void run() { 
            while (!ready) 
                Thread.yield(); 
            System.out.println(number); 
        } 
    } 

    public static void main(String[] args) { 
        new ReaderThread().start(); 
        number = 42; 
        ready = true; 
    } 
}

Because of the reordering and thread visibility, the loop may never stop, or the output may be zero, but I have tried many times, and the output is always 42. All the reason is I'm too lucky?

Answer 1

All the reason is I'm too lucky?

Not necessarily. It will depend on your processor architecture and JVM implementation too. That's one of the problems with subtle memory model issues: they can be very hard to reproduce in the wild.