Coming Up for Air

Dear MockK, Repeat After Me

The project I’m working on is using Mockk in the unit tests. It’s a great library that has made true unit testing so much easier. I ran into a problem, though, where I needed a method I was mocking to return the value it was receiving. To be more specific, we were passing an object to a Spring repository method that had been built inside the method to test, and, to test thoroughly, I needed to get to that object. Turns out, that’s pretty easy to do with Mockk. Let’s take a look…​

First, a clear set up. The code basically looks like something like this:

fun post(@RequestBody foo : Foo) : Bar {
    val bar = Bar()
    bar.prop1 = foo.prop1
    bar.prop2 = calculatedValue(foo)

    var newBar =

    // Do some more work with newBar

    return newBar

For the purposes of the test, I don’t much care about the "Do some more work" part. I need the return value of I set up the test like this:

fun testCreateFoo() {
    val barSlot = slot<Bar>()

    every { } answers { barSlot.captured }

    val result = mockMvc.perform(MockMvcRequestBuilders
    // ...

    val savedBar = gson.fromJson(result.response.contentAsString,

The interesting part is the call to every. Using a CapturingSlot<T>, I instruct the system to capture the Bar that the method under test passes in, and I simply return it in the lambda I pass to answers. It is true that we don’t have access to the what the repository might do to the Bar instance (filling in related objects, etc.), but that’s OK. I just want to look at the value of prop2, and this lets me do that nice and neat. :)

tags: Kotlin Mock


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