I guess your real problem is how to test any classes with static methods and all these ugly things. The best idea is to completely avoid them
Another very helpful testing framework I found some time ago is JMockit. It offers tools to overcome all the typical problems like static methods, final classes or attributes, private members etc. With a good application design and test driven development this shouldn't be necessary very often but it can be very handy if you really need such functionality during tests. This should also make it possible to make singletons more accessible for test code.
These singleton classes are legacy classes in our java code. I do not want to re-write those singleton classes. We are using junit for unit testing. Is there a way to test singleton classes using junit test framework ?
You've already been given two frameworks that help. What other information are you looking for?
I mean, testing the singleton class itself seems really easy--just call its methods in the test case and make sure you get what you want. Testing things that *use* singletons is where JMockit and PowerMock come in to play.
The frameworks David and I mentioned should be usable as add-ons for JUnit. So you could use your existing test infrastructure.
With only the JUnit framework it depends... What exactly do you want to test? The "singleton behavior"? Or just ordinary methods? Methods with or without side-effects? Just because a class is designed as a singleton doesn't necessarily mean that you can't test it as usual. Where do you get stuck when you try to test this class?
PS: OK, David was obviously faster But as you see from both of our posts there's really no magic involved in testing singletons in general. The difficulty is how to obtain an instance of your singleton class as David already pointed out.