java多线程程序中的可变变量

为什么多线程小程序中的原始变量表现为易变变量?请帮我输入代码。

/**
 * Practice with threads problem visibility.
 * @author Matevosyan Vardan
 * @version 1.0
 * created on 21.09.2017
 */

public class VisibilityProblem {
    private static int countingVal = 0;

    public static int getCountingVal() {
        return countingVal;
    }

在main中启动

    public static void main(String[] args) throws InterruptedException {
       Thread looperR = new VisibilityProblem.Looper();
        Thread listener = new VisibilityProblem.Listener();

        listener.start();
        looperR.start();

        listener.join();
        looperR.join();
    }

在睡眠500毫秒后,向wright学习并增加计数变量

    public static class Looper extends Thread {
        @Override
        public void run() {
            while (VisibilityProblem.countingVal < 5) {
                VisibilityProblem.countingVal++;
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("increase " + VisibilityProblem.countingVal);
            }
        }
    }

    public static class Listener extends Thread {
        @Override
        public void run() {
            int localCount = VisibilityProblem.countingVal;
            while (localCount < 5) {
                if (localCount != VisibilityProblem.countingVal) {
                    System.out.println("It is " + localCount + " now");
                    localCount = VisibilityProblem.countingVal;
                }
            }
        }
    }
}