Friday, 23 January 2009

Are mocks just stubs by another name, or something more?

[An older article that I published internally to a client community, reprinted with changes to remove client references]

An opinion that I've run into among some of my clients is that mock objects are just what we've always called stubs, and that it's an old approach. It's actually a quite common perspective - one I have held myself for part of my career. In fact early mock object approaches were very much like "fake implementations", but modern mocks are different. While they can both be considered "test doubles" or "test stand-ins", stubbed out interfaces or provide expected data for the system under test, mock objects provide behavioural expectation. These terminologies can be confusing, but we can sort that out.

jMock and EasyMock are two examples of mock object frameworks which allow for the specification of behaviour. jMock uses a domain specific language (DSL) such that you code the expectations in a fairly verbal syntax (often called a fluent interface). Something along the lines of


... which should be vaguely like english to the initiated. EasyMock, on the other hand, uses a "proxy/record/replay" approach instead, which some find easier. The point is that they both define a set of expected interactions, rather than a first this state, then the next state only.

Martin Fowler, around the middle of his article "Mocks Aren't Stubs", after describing fakes vs. mocks approaches in more detail. He starts to use a clarifying terminology which I like:

"In the two styles of testing I've shown above, the first case uses a real warehouse object and the second case uses a mock warehouse, which of course isn't a real warehouse object. Using mocks is one way to not use a real warehouse in the test, but there are other forms of unreal objects used in testing like this.

"The vocabulary for talking about this soon gets messy - all sorts of words are used: stub, mock, fake, dummy. For this article I'm going to follow the vocabulary of Gerard Meszaros's book. It's not what everyone uses, but I think it's a good vocabulary and since it's my essay I get to pick which words to use.

"Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes. The name comes from the notion of a Stunt Double in movies. (One of his aims was to avoid using any name that was already widely used.) Meszaros then defined four particular kinds of double:

  • Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
  • Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
  • Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
  • Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

"Of these kinds of doubles, only mocks insist upon behavior verification. The other doubles can, and usually do, use state verification. Mocks actually do behave like other doubles during the exercise phase, as they need to make the SUT believe it's talking with its real collaborators - but mocks differ in the setup and the verification phases."

This use of behavioural expectations for "Test Double" objects is quite handy, especially in systems where you have components which make use of heavy service objects or (gasp) singletons which may do more than a simple "call-and-answer" method-call on the object. Deep call chains may require stronger behavioural testing. Being able to provide an object to the system under test that expects to be called a certain way and in a certain order can create a much more precise test, and reduce the amount of code you have to write in a "fake" object. Otherwise, to fully and precisely test, one ends up with severely large number of Test Dummies and ballooning "Fake Objects", which themselves could have error and often have to be tested. Having a system like jMock or EasyMock can reduce the size of your testing code, thus removing some code that could easily become out-of-sync with the system under test, and either introduce false-positive errors or become insufficient and therefore meaningless. So less code, less maintenance, less syncing problems, and the tests are able to be more precise at the same time.

Another approach is to radically re-factor your code into smaller components, each of which is substantially simpler to test. If a component does one thing and does it well, and components interact and collaborate, then you often can use simpler mock behaviours, or simple fakes without as much effort on the testing. Great resources for testable code are available here .

Mocks aren't always easily intuitive for someone used to building fakes (it wasn't for me, and it still occasionally trips me up), but once you're comfortable with the approach, it can be much crisper. This is especially true as people try to get better coverage in isolated unit tests, or who are trying to test-drive their software. The linked Fowler article is a good one, and certainly worth reading for those trying to figure out how to more meaningfully test components without having to start up external servers or simulators.

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