Mockito-Kotlin is a small library that provides helper functions to work with Mockito in Kotlin. It is a great choice for Java devs already familiar with Mockito. Mockito is a widely used Java testing framework that helps create mock objects for unit testing. It allows developers to isolate code from dependencies, define behaviors, and verify method calls.

Creating Mocks

🤮 Legacy Java Mockito syntax. Don't do it.

val mock = mock(MyDependency::class.java)

😍 Now, don't these look better?

val mock: MyDependency = mock()

val mock = mock<MyDependency>

😍 Creating mocks inline can be quite handy!

val subject = MyClass(mock())

Stubbing

🤮 Mockito stubbing method when requires backticks since when is a reserved keyword in Kotlin.

`when`(mock.foo()).doReturn(null)

🙂 Mockito-Kotlin adds a whenever alias so you don't have to type or look at those ugly backticks.

whenever(mock.foo()).doReturn(null)

😍 Even better, Mockito-Kotlin adds a new stubbing DSL! It can be used when creating a new mock and on existing mocks.

val mock: MyDependency = mock {
    on { foo() } doReturn { "default" }
    on { bar() } doReturn { 1 }
}

@Test
fun test() {
    mock.stub {
        on { foo() } doReturn { "override" }
    }
}

😍 It even supports stubbing of suspend functions (Kotlin Coroutines).

mock {
    onBlocking { apiCall() } doReturn "Hello World"
}