spring-boot-exception-handling | Final source code accompanying the article on the Toptal | REST library

Your syntax is correct, but your specific problem here is that getRootBeanClass returns a Class, and Mockito can't mock final classes. Under the hood, when you ask to mock a class, Mockito generates a subclass that (effectively) overrides all methods and redirects them to Mockito control. This override mechanism is why you'll have trouble mocking final methods, as well as final classes like java.lang.Class.

Though you might be able to get this done with PowerMock, it's inadvisable, as in Johan Haleby's post from powermock@googlegroups.com in 2011:

I don't think there's a possibility to mock Class because that will mess up a lot of PowerMock internal stuff. You can how ever mock the "getClass()" method of Objects if you set MockGateway.MOCK_GET_CLASS_METHOD = true.

I would strongly advice [sic] against mocking Class or getClass method though. Try to verify it in some other way.

Because you're only concerned about making getSimpleName return a predictable value, consider creating a new class ("MyBean") and setting getRootBeanClass() to return it. It'll be very easy to mock a return value on an interface, and you'll get the same kind of simple name return value you're looking to use.