java - volatile 关键字原子性

Question

我正在努力学习volatile field modifier在多线程中。我看到这样的说法:

Volatile is preferred in cases when one thread reads and writes a shared variable and other threads just read the same. Whereas if there are more than 2 threads performing read and write both on the shared variable then only volatile is not enough, you need to have synchronization as well.

我知道 volatile 提供了可见性和发生前保证，但是是否可以给出一个简单的小代码示例来演示上述语句，其中需要同步块(synchronized block)？

Answer 1

public class TwoInts {
    private volatile int i1; 
    private volatile int i2;  

    public void set(int i1, int i2) {
        this.i1 = i1;
        this.i2 = i2;
    }

    public boolean same() {
        return i1 == i2;
    }
}

现在，如果您有一个线程执行此操作:

while (true) {
    twoInts.set(i, i); 
    i++;
}

第二个线程执行此操作:

while (true) {
    if (!twoInts.same()) {
        System.out.println("Ooops!!");
    }
}

然后你就会观察到引用文本所讨论的问题。如果您重写 TwoInts 类以使方法同步，那么“Oooops!!”消息将停止。