Question

I am looking at some code in our app that I think may be encountering a case of “Double-checked locking“. I have written some sample code that is similar to what we do.

Can anyone see how this can be experiencing double-checked locking? Or is this safe?

class Foo {
    private Helper helper = null;
    public Helper getHelper() {
        Helper result;
        synchronized(this) {
            result = helper;
        }

        if (helper == null) {
            synchronized(this) {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
        return helper;
    }
}

Base code borrowed from wiki.

Answer 1

The whole point of double-checked locking is that the fast path (when you don’t need to instantiate the object) isn’t synchronized. So what you have isn’t double-checked locking.

You need to get rid of the first synchronized block in order to get a broken double-checked lock. Then you need to make helper volatile to fix it.