question about "Java Concurrency in Practice" example

Question

I'm looking at a code sample from "Java Concurrency in Practice" by Brian Goetz. He says that it is possible that this code will stay in an infinite loop because "the value of 'ready' might never become visible to the reader thread". I don't understand how this can happen...

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;
    } 
}

Answer 1

Because ready isn't marked as volatile and the value may be cached at the start of the while loop because it isn't changed within the while loop. It's one of the ways the jitter optimizes the code.

So it's possible that the thread starts before ready = true and reads ready = false caches that thread-locally and never reads it again.

Check out the volatile keyword.