February 9, 2015

...Learn TDD with Codemanship

Mock Abuse: How Powerful Mocking Tools Can Make Code Even Harder To Change

Conversation turned today to that perennial question about mock abuse; namely that there are some things mocking frameworks enable us to do that we probably shouldn't ought to.

In particular, as frameworks have become more powerful, they've made it possible for us to substitute the un-substitutable in our tests.

Check out this example:

Because Orders invokes the static database access method getAllOrders(), it's not possible for us to use dependency injection to make it so we can unit test Orders without hitting the database. Boo! Hiss!

Along comes our mocking knight in shining armour, enabling me to stub out that static method to give a test-specific response:

Problem solved. Right?

Well, for now, maybe yes. But the mocking tool has not solved the problem that I still couldn't substitute CustomerData.getAllOrders() in the actual design if I wanted to (say, to use a different kind of back-end data store or a web service). So it's solved the "how do I unit test this?" problem, but not in a way that buys me any flexibility or solves the underlying design problem.

If anything, it's made things a bit worse. Now, if I want to refactor Orders to make the database back end swappable, I've got a bunch of test code that also depends on that static method (and in arguably a bigger way - more code depends on that internal dependency. If you catch my drift.)

I warn very strongly against using tools and techniques like these to get around inherent internal dependency problems, because - when it comes to refactoring (and what's the point in having fast-running unit tests if we can't refactor?) all that extra test code can actually bake in the design problems.

Multiply this one toy example by 1,000 to get the real scale I sometimes see this one in real code bases. This approach can make rigid and brittle designs even more rigid and more brittle. In the long term, it's better to make the code unit-testable by fixing the dependency problem, even if this means living with slow-running (or even - gasp! - manual) tests for a while.

Posted 2 years, 10 months ago on February 9, 2015