初始化期间使用的静态非最终变量

Question

See this incomplete code fragment:

public class Singleton implements Serializable {

    private static class SingletonHolder {

        private static final Singleton SINGLETON_INSTANCE;

        static {
            Singleton tmp = singletonTMP;
            if (tmp != null) {
                SINGLETON_INSTANCE = tmp;
            } else {
                // etc.
            }
        }
    }

    private static volatile Singleton singletonTMP;

    // etc.

}

I get a strange warning in NetBeans at line Singleton tmp = singletonTMP; : "Usage of static non-final variable used during initialization".

So, yes. This is true, of course, but why would this be a problem?

Answer 1

The problem is that the variable :

private static final Singleton SINGLETON_INSTANCE;

might not have been initialized in that static block of yours, you can inititalize it to null(say) in the else clause inside that static block, ie:

    static 
    {
        Singleton tmp = singletonTMP;
        if (tmp != null) 
        {
            SINGLETON_INSTANCE = tmp;
        } else
        {
            **SINGLETON_INSTANCE = null;**
        }
    }

Answer 2

After reading a bit about security I think my previous comment was correct.

I think it is a security issue, an attacker could replace your static field with a new object. This happens when the attribute is also defined as public. I assume netbeans considers it a risk and displays the warning even if you declared it private.

More details here: https://www.securecoding.cert.org/confluence/display/java/OBJ10-J.+Do+not+use+public+static+nonfinal+variables