volatile修饰全局变量，可以保证并发安全吗？
2024-09-08 02:32:15
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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