Multithreading: volatile makes a write visible to other threads

Without volatile, a worker thread might spin forever because it never observes a plain field change made by another thread. Marking the flag volatile guarantees the write is published, so the worker sees it and exits its loop.

Code

class Flag {
    volatile boolean stop = false;
}
Flag flag = new Flag();
int[] loops = new int[1];
Thread worker = new Thread(() -> {
    while (!flag.stop) {
        loops[0]++;
        try {
            Thread.sleep(5);
        } catch (InterruptedException e) {
            break;
        }
    }
});
worker.start();
Thread.sleep(30);
flag.stop = true;
worker.join();
System.out.println("Worker saw the volatile flag change: " + (loops[0] > 0));

Output

Worker saw the volatile flag change: true