Program not terminating after loop completion

Question

In the following scenario, the boolean 'done' gets set to true which should end the program. Instead the program just keeps going on even though the while(!done) is no longer a valid scenario thus it should have halted. Now if I were to add in a Thread sleep even with zero sleep time, the program terminates as expected. Why is that?

public class Sample {

    private static boolean done;

    public static void main(String[] args) throws InterruptedException {
        done = false;
        new Thread(() -> {
            System.out.println("Running...");
            int count = 0;
            while (!done) {
                count++;
                try {
                    Thread.sleep(0); // program only ends if I add this line. 
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();

        Thread.sleep(2000);

        done = true; // this is set to true after 2 seconds so program should end.
        System.out.println("Done!"); // this gets printed after 2 seconds
    }

}

EDIT: I am looking to understand why the above needs Thread.sleep(0) to terminate. I do not want to use volatile keyword unless it is an absolute must and I do understand that would work by exposing my value to all threads which is not my intention to expose.

Answer 1

Each thread have a different cached version of done created for performance, your counter thread is too busy making the calculations for count that it doesnt give a chance to reload done.

volatile ensures that any read/write is done on the main memory, always update the cpu cache copy.

Thread.sleep always pause the current thread, so even if 0 your counter thread is interrupted by some time <1ms, that is enough time for the thread to be adviced of done variable change.