volatile

Definition

A volatile field is always read from and written to main memory, so a change made by one thread becomes visible to the others straight away. It guarantees visibility, but not that compound steps such as count++ are atomic.

Example code

A stop flag that another thread is guaranteed to see

class Demo {
    static volatile boolean running = true;                 // without volatile the loop might never see the change

    public static void main(String[] args) throws InterruptedException {
        Thread worker = new Thread(() -> {
            long loops = 0;
            while (running) loops++;                         // keeps looping until the flag changes
            System.out.println("worker stopped");
        });
        worker.start();

        Thread.sleep(100);
        running = false;                                     // visible to the worker immediately
        worker.join();
    }
}

// Output:
// worker stopped