Complete example for use of volatile key word in Java?

Question

I need a simple example of use of the volatile keyword in Java, behaving inconsistently as a result of not using volatile.

The theory part of volatile usage is already clear to me.

Answer 1

First of all, there's no guaranteed way of exposing caching due to non-volatile variables. Your JVM might just be very kind to you all the time and effectively treat every variable as volatile.

That being said, there are a few ways to increase probability of having threads caching their own versions of a non-volatile variable. Here is a program that exposes the importance of volatile in most machines I've tested it on (adapted version from here):

class Test extends Thread {

    boolean keepRunning = true;

    public void run() {
        while (keepRunning) {
        }

        System.out.println("Thread terminated.");
    }

    public static void main(String[] args) throws InterruptedException {
        Test t = new Test();
        t.start();
        Thread.sleep(1000);
        t.keepRunning = false;
        System.out.println("keepRunning set to false.");
    }
}

This program will typically just output

keepRunning set to false.

and continue running. Making keepRunning volatile causes it to print

keepRunning set to false.
Thread terminated.

and terminate.