volatile修饰全局变量，可以保证线程并发安全吗？

 

今天被人问到volatile能不能保证并发安全？

呵，这能难倒我？

上代码：（代码太长建议跳到页末）

//电脑太好，100线程起步~
public class ThreadTest {
    private static volatile int num = 0; public static void main(String[] args) throws InterruptedException { new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); Thread.sleep(500); System.out.println(num); } }

期望最终值：
　　100 * 100 = 10000

输出结果：
　　...
　　...
　　9998
　　9998
　

分析：

　　100个线程对volatilei修饰的num++，会被编译成以下三步：
　　 1.获取i的值；2.执行i+1；3.将结果赋值给i。
　　volatile只能保证可见性，并不能保证原子性。

结论：
　　volatile只能保证这3步在编译后指令不会被重新排序，并不能保证线程并发的数据安全。建议搭配上synchronized或其他Lock锁使用。