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:

//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);
}
}
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,并不能保证并发数据安全。建议搭配上synchronized或其他Lock锁使用。

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