Forming Mockito “grammars”

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There are several disadvantages to the when/thenReturn, when/thenThrow and when/then syntaxes. For example,

  • In the case of when/thenReturn, if the return type is a generic with a
    wildcard, and you wish to return a mock of the same type, you will be unable
    to avoid a compile warning.
  • You can’t use when/thenThrow and when/then for a void method.
  • You can’t use these syntaxes on Mockito spies.
  • You can only call when once for each combination of mock object,
    method and arguments, unless you call reset on the mock.
  • Calling when multiple times for one combination of mock
    object and method, when you are using argument matchers, can lead to problems.

I find these cases difficult to remember. So instead of trying to keep track of when the
when/thenReturn, when/thenThrow and when/then syntaxes will and won’t work, I prefer to avoid them completely, in favour of the doReturn/when, doThrow/when and doAnswer/when alternatives. That is to say, since you’ll occasionally need doReturn/when, doThrow/when and doAnswer/when, and you can ALWAYS use these methods, there is no point in learning how to use when/thenReturn, when/thenThrow and when/then.

Note that doReturn, doThrow and doAnswer can be chained together in the same way as thenReturn, thenThrow and then. What they don’t have is an option for returning several values (or throwing several exceptions, or running several answers) within a single call to doReturn, doThrow and doAnswer. But I find that I need to do this so seldom, that it doesn’t really matter.

There’s one more disadvantage to doReturn, which I consider insignificant. You don’t get compile time checking of the type of its argument, like you do with when/thenReturn. So if you get the argument type wrong, you won’t find out until you run your test. Frankly, I don’t care.

In summary then, I have been using Mockito for more than two years, and I consider the consistent use of doReturn, doThrow and doAnswer to be a Mockito best practice. Other Mockito users disagree.

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