MockK
- Kotlin course
- Intermediate
- Lesson 110/116
- 5 min
Learn mocking with MockK.
MockK: Mocking library
MockK is a mocking library for Kotlin. It mimics the behavior of dependencies during tests; so you can write tests without real database or network calls. mockk<T>() creates a mock object, every { } defines behavior, verify { } checks calls.
val service = mockk<WeatherService>()
every { service.temperature(any()) } returns 20
verify { service.temperature("Istanbul") }Here WeatherService is mocked; calling temperature always returns 20. verify checks that the method was called with the specified argument. MockK also supports Kotlin's suspend and extension functions; this makes it a common choice in Kotlin.
The fake pattern
Before using MockK, the fake pattern is useful to understand mocking. A fake is a hand-written class that implements an interface and offers predictable behavior in tests. It shows explicitly what MockK does behind the scenes.
interface Greeter {
fun greet(name: String): String
}
class FakeGreeter : Greeter {
override fun greet(name: String) = "Hello, $name!"
}Here FakeGreeter implements the Greeter interface and always returns a fixed result. The test uses this fake; it does not depend on a real resource. MockK produces this fake automatically; mockk creates a mock for any interface.
Practice Apply the fake service pattern
Define interface WeatherService; inside it add fun temperature(place: String): Int. Define class FakeWeatherService : WeatherService whose temperature always returns 20. Define class WeatherApp(private val service: WeatherService) whose fun report(place: String): String body returns "$place: ${service.temperature(place)}C". In main create val app = WeatherApp(FakeWeatherService()); print println(app.report("Istanbul")).
Starter code
interface WeatherService {
fun temperature(place: String): Int
}
// FakeWeatherService: implements WeatherService as a fake, returns 20
// WeatherApp: takes service, report(place) method returns "$place: {temp}C"
// main: create WeatherApp with FakeWeatherService, print report for "Istanbul"
fun main() {
// write here
}Sample solution
interface WeatherService {
fun temperature(place: String): Int
}
class FakeWeatherService : WeatherService {
override fun temperature(place: String): Int = 20
}
class WeatherApp(private val service: WeatherService) {
fun report(place: String): String = "$place: ${service.temperature(place)}C"
}
fun main() {
val app = WeatherApp(FakeWeatherService())
println(app.report("Istanbul"))
}Expected output
Istanbul: 20C
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is MockK used for?
- Mocking dependencies in tests Correct answer
- Drawing interfaces
- Querying databases
- Compressing files
Explanation: MockK mimics the behavior of dependencies during tests.
2 Create a mock object
val service = ____<WeatherService>()val service = mockk<WeatherService>()Explanation: mockk<WeatherService>() creates a mock object.
3 Match the MockK functions
| Term | Meaning |
|---|---|
mockk<T>() |
Create a mock |
every { } |
Define behavior |
verify { } |
Check a call |
returns |
Return value |