Java volatile 的測試（Java代碼實戰-004）

 

package Threads; /** * Created by xfyou 2018/5/21 16:07. */
public class VolatileTest { private static volatile int MY_INT = 0; /** * 主線程啟動2個測試子線程 * * @param args null */
    public static void main(String[] args) { new ChangeListener().start(); new ChangeMaker().start(); } /** * 此線程負責對MY_INT值改變的偵聽，如果有改變就會打印出來 */
    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; } } } } /** * 此線程負責改變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(); } } } } }

當使用volatile時，修改線程對static MY_INT值的修改，另一個偵聽線程（讀MY_INT的值）是可以及時讀取到的，輸出結果如下：

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

當不使用volatile時，修改線程對static MY_INT值的修改，另一個偵聽線程（讀MY_INT的值）可能完全讀取不到（無感知），可能的輸出結果如下：

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