Java volatile 的测试（Java代码实战-004）

 

package Threads;

/**
 * Created by xfyou 2018/5/21 16:07.
 */
public class VolatileTest {

    private static volatile int MY_INT = 0;

    /**
     * 主线程启动2个测试子线程
     *
     * @param args null
     */
    public static void main(String[] args) {
        new ChangeListener().start();
        new ChangeMaker().start();
    }

    /**
     * 此线程负责对MY_INT值改变的侦听，如果有改变就会打印出来
     */
    static class ChangeListener extends Thread {
        @Override
        public void run() {
            int local_value = MY_INT;
            while (local_value < 5) {
                if (local_value != MY_INT) {
                    System.out.println("Got Change for MY_INT : " + MY_INT);
                    local_value = MY_INT;
                }
            }
        }
    }

    /**
     * 此线程负责改变MY_INT的值
     */
    static class ChangeMaker extends Thread {
        @Override
        public void run() {
            int local_value = MY_INT;
            while (MY_INT < 5) {
                System.out.println("Incrementing MY_INT to " + (local_value + 1));
                MY_INT = ++local_value;
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

当使用volatile时，修改线程对static MY_INT值的修改，另一个侦听线程（读MY_INT的值）是可以及时读取到的，输出结果如下：

Incrementing MY_INT to 1
Got Change for MY_INT : 1
Incrementing MY_INT to 2
Got Change for MY_INT : 2
Incrementing MY_INT to 3
Got Change for MY_INT : 3
Incrementing MY_INT to 4
Got Change for MY_INT : 4
Incrementing MY_INT to 5
Got Change for MY_INT : 5

当不使用volatile时，修改线程对static MY_INT值的修改，另一个侦听线程（读MY_INT的值）可能完全读取不到（无感知），可能的输出结果如下：

Incrementing MY_INT to 1
Incrementing MY_INT to 2
Incrementing MY_INT to 3
Incrementing MY_INT to 4
Incrementing MY_INT to 5