Be Careful with Statics

Thursday, Jul 16, 2015 |

I recently came across an interesting piece of code at work:

private static final DateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd");

What struck me as odd was the private qualifier and that the fact that SimpleDateFormat is not thread-safe. Is the private some odd attempt to work around concurrency issues, or was thread safety just overlooked? That led me to this question: Is a private staticstill one instance per JVM, or does the private actually change anything? My understanding was that this was a bug, but I thought I'd write a test just to make sure.

Let's start with a class:

public class SomeClass {
    private static int number;

    public SomeClass(int number) {
        this.number = number;
    }

    public int getNumber() {
        return number;
    }
}

and here's our simple test:

public class StaticTest {
    public static void main(String[] args) {
        SomeClass a = new SomeClass(1);
        SomeClass b = new SomeClass(2);

        System.out.println("a.number = " + a.getNumber());
        System.out.println("b.number = " + b.getNumber());
    }

}

As expected, here's the output:

a.number = 2
b.number = 2

If you've been around the block a few times, this probably doesn't come as a surprise. If you're newer, though, or haven't ever had to give it thought, you might be mildly surprised. Either way, the bottom line is this: scope qualifiers don't modify the behavior of a static, so if the type is not thread-safe, you'll need a better (read as: correct) way to handle the concurrency concerns.