Sharing variable without synchronization

Diskun Zhu :

I read it from Java Concurrency in Practice, that it is bad to share variables in threads without synchronisation. However, for some examples as following which only have one read thread and one write thread, I can't find errors in it. From my perspective, the result for the following program will definitely terminate and print 42 because ReaderThread can go through only when ready becomes true, and that means number is 42. Could somebody give me some explanation why I am wrong?

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;
    } 
}

Kayaman :

Since ready isn't volatile, there's no guarantee that ReaderThread will see that your main thread has changed it. When you mark ready as volatile, all writes from one thread will be seen in other threads reading it.

You always need some sort of synchronization / visibility control when communicating between threads. Whether it's volatile, explicitly using synchronized or using the java.util.concurrent.* classes.