Today, Bob Martin tweeted :
I use mocking frameworks as little as possible. I think heavy reliance on a mocking framework is a smell. I usually write my own mocks.
I agree with this. Of course the 140 characters allowed in a tweet don’t really give much space to explain what that smell is. So let’s refine our nose.
Mocks let you test implementation. Anyone who ever gets uppity about the use of stubs, fakes & mocks is paying a lot of attention to this. With a mock you can verify that something has been called. While this can be useful, it is rarely part of the Behavior of a system, and often part of the implementation. Since there are literally 1,000’s of way to program the exact same behavior, Unit tests that lock a particular implementation will actually in up “protecting” your code from being refactored. This smell tends to come up in newly written code, and is usually accompanied by the phrase
“anytime I want to change anything a bunch of tests break and I have to go and fix them”.
Mocks let you fake very hard to fake calls. Ever need to mock out a call to HttpServletRequest? There are over 40 methods on that interface. Even with the help of your IDE, that’s a pain. A good mocking framework (I prefer EasyMock) will let you do it in 2-3 lines. This is great, especially in legacy code, or API’s you don’t control. But it’s a cover for a much more insidious smell that exists in HttpServletRequest. Simply put, an interface should not have 40 methods. Now you might argue that was how many were needed to handle something as complex as a web call. And you would be wrong. Take a look at Rack (or our Port of it - JRack) it handles everything with 1 simple abstraction.
When mocks are the easiest way to gain an handle into your code, you have coupled you code too tightly, and not left enough inserts points.
Mocks let you fake calls. Lastly, mocks let you fake easy calls too. One example I use a lot is Loaders. My Loader interface looks like this:
public interface Loader<T>
public T load() throws Exception;
I will constantly be making calls like
But here a mocking framework is overkill. It’s so easy to write the above line, than
Loader<Member> loader = EasyMock.createMock(Loader.class);
If I’ve kept my code clean, mocking frameworks just aren’t that useful.
So hopefully you will start to sense the same things in your code. Of course if your nose is prickling over some hard to test piece of code, grab your mocking framework. It’s your first line of attack. Like grabbing some cologne when you are a bit smelly and guest are coming over, just realize that eventually you have to take a bath.