issue-visibility

Problem Example (Without volatile)with volatile

    @Slf4j
    public class Main {
        static boolean running = true;

        public static void main(String[] args) throws InterruptedException {
            Thread t = new Thread(() -> {
                while (running) {
                    // do nothing
                }
                log.info("Thread stopped.");
            });

            t.start();
            Thread.sleep(1000);
            running = false;
            log.info("Main thread changed flag.");
        }
    }

Expected Problem: t may never stop, because it doesn't see running = false.

@Slf4j
public class Main {
    static volatile boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(() -> {
            while (running) {
                // do nothing
            }
            log.info("Thread stopped.");
        });

        t.start();
        Thread.sleep(1000);
        running = false;
        log.info("Main thread changed flag.");
    }
}

Now it stops correctly — the change to running is visible to the thread t.