A New Mocking Tool for Kotlin

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A New Mocking Tool for Kotlin

Take a look at MockK, a new mocking tool designed specifically for use with Kotlin code, helping you avoid cumbersome Java wrappers.

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DinoOne pain point in testing Kotlin code is mocking. Have you ever tried to use a Mockito wrapper? It tries to hide this Java dinosaur in a DSL, but it still feels so unnatural and Java-ish

In this aMockKrticle, I'd like to present new shiny pure Kotlin mocking library — MockK. Its main philosophy is first-class support for Kotlin features. Thus, your code using coroutines or lambda blocks naturally fits into a simple DSL describing the behavior of objects.


First, you need to create a mock or spy:

val mock = mockk<Type>()
val spy = spyk<Type>(Type(...))

Then, you can stub some calls with argument matchers or regular arguments:

every {
  mock.call(any(), eq(3), more(4))
} returns 5

Stubbed mocks can now be used in some tested code and called as regular objects.

mock.call(22, 3, 6) // returns 5

After testing is done, you can verify calls again with matchers or regular arguments:

verify {
    mock.call(22, 3, more(4))

That's it for the basics, but there is a lot more, like two dozen types of matchers, coroutine coEvery/coVerify, verification modes, etc. Check out the documentation here and examples here.


Some things are very similar to Mockito, and that is inevitable. But it's not about features themselves — Mockito and PowerMock have tons of features. It's about making it look and feel like Kotlin code.

Let me describe few things that help write clean code.

1. Chained Call Mocking

For example, you are going to test some web-handling code. Like this one:


This value is then used for building your request. To test it, you need a mocked ctx and to provide behavior for each of three method calls: getRequestgetHeaders, and get.

In MockK, you can simply write:

every { request.headers[HttpHeaders.Host] } returns "host"

And the chain of mocking calls is captured and stubbed. Similar to deep stubs, but better.

2. Partial Matcher Specification

Many things can be simply better just because we use Kotlin. One of them is the specification of parts of arguments as matchers. Yes, truly you can do it. Only in Kotlin! And it fits Kotlin very well. Why? Because there is such things as named and default parameters.

Let's get back to another example:

fun response(html: String = "",
    contentType: String = "text/html",
    charset: Charset = Charset.forName("UTF-8"),
    status: HttpResponseStatus = HttpResponseStatus.OK): HttpResponse

So this is a very simple function to build an HTTP response. In dinosaur wrappers, you need to specify all arguments as matchers or none of them. That's not the case with MockK

every {
    response(html = match {
} returns HttpResponse(...)


every { 
    response(html = any(),
           status = HttpResponseStatus.BadRequest)
} returns HttpResponse(...)

Or actually any combination of matchers and regular arguments.

The tricky thing is that it is just impossible to do that in Java. In Kotlin this block is a lambda, and it can be executed several times. And thus a library can randomize values and guess what matchers are placed at what argument positions. Regular arguments are always constants. Almost always. Actually, they are until the moment when a default value is some sophisticated function returning time, for example, or random value. In that case, you do need to manually override it via matcher. But come on, the idea is very cool, isn't it? You just need to deal with a few corner cases. Okay, one corner case. And as I said before, it is very natural to Kotlin.  

3. Matcher Expressions

The idea is simple. Just allow combining matchers into composite matchers. Partial argument specifications make it even more powerful. Additionally, you can create a matcher that extends CompositeMatcher and create your own expression langauge. 

fun sum(a: Int, b:Int): Int

every { 
    obj.sum(any(), any()) 
} returns 0

every {
        or(eq(3), more(5)), 
} answers { firstArg<Int>() + secondArg<Int>() }

Image title

4. Coroutines

First-class support of coroutines as matchers and as stubbing/verification blocks:

// as stubbing block

suspend fun respond(message: Any)

coEvery {
} just Runs

// both as matcher and as block

class HtmlContent {
   suspend fun writeTo(channel: WriteChannel)

val channel = ... some channel ...
coVerify {
    appCall.respond(coAny<HtmlContent> { content ->

So you don't need to manually wrap all the calls in runBlocking or similar. Just works.

5. Lambdas

Very often, when mocking DSLs, you need to call lambdas. There are plenty of ways to do that, capturing or answering alike, but the simplest way is to just use the special invoke matcher and provide arguments.

class A {
    fun b(block: () -> Unit)

every {
} just Runs

6. Verification Modes

If you need some ordering or checking of a full sequence of calls, that is easily achievable:

verify { // unordered by default
    obj.sum(1, 2)
    obj.sum(1, 3)
    obj.sum(2, 2)

verifyOrder { // calls where ordered like that
    obj.sum(1, 2)
    obj.sum(1, 3)
    obj.sum(2, 2)

verifySequence { // this was the exact sequence of calls
    obj.sum(1, 2)
    obj.sum(1, 3)
    obj.sum(2, 2)


All these tiny details make writing a MockK DSL in Kotlin a natural fit. I bet the wrapper syntax is something that can never be so nice as a native implementation in regards to clean and concise code. Besides that, mocking wrappers do not have the possibilities for future maneuvers as an implementation from scratch has.

Good luck and please star my repo MockK. Thanks!

java, kotlin, kotlin dsl, mocking frameworks, mockk, tutorial

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