Mockito Spy'ing on the object being unit tested

Question

Is it a code smell to spy on an object that is being unit tested? For example say I have a LineCounter class whose job is to simply count the number of lines in

Answer 1

In this situation, it is perfectly legitimate to stub the method that is called by the method under test. It is even the only way I can think of to test it in isolation. You just don't want to extract a single method into it's own class for the sole purpose of testing.

Beware of the side effects in the stubbed method though. It might not be sufficient to stub the returned value, if the stubbed method has side effects then you have to stub the side effects as well. It might even be a reason against it in some situations where the side effects are very complex, but that would most likely be an indication of a code smell in the implementation of the class under test itself.

To answer your question, I find it easy to find reasons for it, but hard to find reasons against it. It's the technique I use every day, it helps me split my implementation in small methods that are tested individually in complete isolation, and I haven't seen any limitation to it yet.