At my last client I got the unusual chance to try three Java mocking frameworks within the same project. The project had started to use EasyMock as the project team felt that Mockito didn’t really have any decent documentation (it does but it’s in the Mockito codebase, not on the Google Code wiki). However as a ThoughtWorks team was working next door there was a strong push to use that.
My personal preference is still for JMock so I also selfishly pushed that into the project to round out the selection. With all three there; we were ready for a Mock Off!
The first distinctive thing is that EasyMock and Mockito can mock concrete classes not just interfaces. The second thing that all three have very different methods of constructing and verifying mocks. EasyMock is record-replay, JMock constructs expectations that are automatically verified by a JUnit runner when the test stops executing, Mockito uses a TestSpy where you can verify what happened to the mock whenever you want.
The record-replay paradigm lead to EasyMock being discarded early on. It has two kinds of problems as far as I was concerned. Firstly you have the feeling of “inside out” where the test is copying the internals of the class under test. Secondly I was confused several times as to what state my mock was in and having to switch the mocks to replay mode felt noisy, particularly where you were setting multiple collaborators in the test (do you switch them to replay once their recording is done or do you record all mocks then switch all of the them to replay?).
Mockito’s easy mocking of concrete classes made it a valuable addition to the toolkit and it does deliver the promised noise free setup and verificiation. However its use of global state was frustrating, particularly in that you can create a compiling use of the API that fails at runtime. If you call verify without a following method then you get an error about illegal use of the framework. Although this is meant to happen in the test that follows the test with the illegal construction, in practice this is hideous pain when the test suite is a non-trivial size. It also meant that a test was appearing to pass when actually nothing was being verified and the error appeared in another pair’s test (the one that implemented the next test) making them think that something was wrong with their code.
Mockito also uses a lot of static imports (which I do have a weaknesses for) but it also means that you have to remember the entry points into the framework.
JMock by comparision to Mockito feels quite verbose, the price for having all that IDE support and a discoverable API is that you have to have a Mockery or two in all your classes and you are defining all your Expectations for the Mockery. There’s no denying that even with the IDE autocomplete you’re doing a lot more typing with JMock. From a lazy programmer point of view you are going to go with Mockito every time.
And that is pretty much what happened in the project. Which I think is a shame. Now in explaining this I am going to go into a bit of ThoughtWorks testing geekery so if you are lazy feel free to go off and use Mockito because the pain I’m talking about will happen in the future not now.
I feel that Mockito is a framework for Test Driven Development and JMock is a framework for Test Driven Design. A lot of times you want the former: tight-deadline work and brownfield work. You want to verify the work you are doing but often design is getting pushed to the sidelines or you actually lack the ability to change the design as you might want. Mockito is great for that.
However Mockito doesn’t let the code speak to you. It takes care of so much of the detail that you often lose feedback on your code. One thing that making you type out your Expectations in JMock does is make you think, really think, about the way your code is collaborating. I think you are much more likely to design your code with JMock because it offers lots of opportunities to refactor. For example if two services are constantly being mocked together then maybe they contain services that are so closely related they should be unified, logically or physically. You don’t see that opportunity for change with Mockito.
By using both JMock and Mockito I was able to see that Mockito did tend to make us a bit lazy and our classes tended to grow fatter in terms of functionality and collaborators because there was no penalty for doing so. I was also concerned that sometimes Mockito’s ability to mock concrete classes meant that sometimes we mocked objects that should have been real or stub implementations.
Mockito is a powerful framework and I suspect that Java developers will take to it as soon as they encounter it. However for genuine Test Driven Design I think Mockito suffocates the code and requires developers with a lot more discipline and design-nous, almost certainly not the people that wanted an easy to use framework in the first place.