The code example which can prove “volatile” declare should be used

Question

Currently I can\'t understand when we should use volatile to declare variable.

I have do some study and searched some materials about it for a long time

Answer 1

To expand on the answer from @jed-wesley-smith, if you drop this into a new project, take out the volatile keyword from the iterationCount, and run it, it will never stop. Adding the volatile keyword to either str or iterationCount would cause the code to end successfully. I've also noticed that the sleep can't be smaller than 5, using Java 8, but perhaps your mileage may vary with other JVMs / Java versions.

public static class DelayWrite implements Runnable
{
    private String str;
    public volatile int iterationCount = 0;

    void setStr(String str)
    {
        this.str = str;
    }

    public void run()
    {
        while (str == null)
        {
            iterationCount++;
        }
        System.out.println(str + " after " + iterationCount + " iterations.");
    }
}

public static void main(String[] args) throws InterruptedException
{
    System.out.println("This should print 'Hello world!' and exit if str or iterationCount is volatile.");
    DelayWrite delay = new DelayWrite();
    new Thread(delay).start();
    Thread.sleep(5);
    System.out.println("Thread sleep gave the thread " + delay.iterationCount + " iterations.");
    delay.setStr("Hello world!!");
}