Testing in Kotlin: Unit, Integration, E2E — Complete Guide

Introduction to Testing in Kotlin: from Unit to End-to-End

A high-quality codebase is impossible without automated testing. Kotlin, as a modern language for the JVM, provides developers with a rich toolkit for creating reliable tests. In this article, we will break down three levels of the testing pyramid as applied to Kotlin: unit, integration, and e2e. You will learn how to write effective tests in Kotlin using popular frameworks such as JUnit 5, MockK, Ktor Client, and Selenium.

1. Unit Testing in Kotlin: The Foundation of Stability

Unit tests verify the operation of individual functions, classes, or methods in isolation from external dependencies. In Kotlin, JUnit 5 is most often used for this, in combination with the mocking library MockK.

1.1. Project Setup

Add the dependencies to the build.gradle.kts file:

dependencies {    testImplementation("org.junit.jupiter:junit-jupiter:5.10.0")    testImplementation("io.mockk:mockk:1.13.8")}

1.2. Unit Test Example with MockK

Suppose we have a UserService that depends on a repository:

class UserService(private val repository: UserRepository) {    fun getUserName(id: Long): String {        val user = repository.findById(id)        return user?.name ?: "Unknown"    }}

Test using MockK:

import io.mockk.everyimport io.mockk.mockkimport org.junit.jupiter.api.Testimport kotlin.test.assertEquals

class UserServiceTest {    @Test    fun `return userName when user exists`() {        val mockRepo = mockk<UserRepository>()        every { mockRepo.findById(1L) } returns User(1L, "Alice")        val service = UserService(mockRepo)

        val result = service.getUserName(1L)

        assertEquals("Alice", result)    }}

1.3. Best Practices for Unit Testing

• Isolate dependencies — use mocks for all external calls (database, API).
• Test edge cases: null, empty strings, negative numbers.
• Use clear test names — they should describe the scenario and the expected result.
• Follow the AAA principle: Arrange, Act, Assert.

2. Integration Testing in Kotlin: Checking Interaction

Integration tests verify how several system components work together. In Kotlin, this could be testing a REST API using Ktor Client or checking database operations via Exposed.

2.1. Testing REST API with Ktor

Create a simple Ktor server and test it:

// Serverfun Application.module() {    routing {        get("/hello") {            call.respondText("Hello, Kotlin!")        }    }}

// Testclass ApiTest {    @Test    fun `test hello endpoint`() = testApplication {        application { module() }        val response = client.get("/hello")        assertEquals(HttpStatusCode.OK, response.status)        assertEquals("Hello, Kotlin!", response.bodyAsText())    }}

2.2. Working with a Test Database

For integration tests with a database, use H2 (in-memory) or Testcontainers for PostgreSQL:

@Testfun `test user creation`() {    withTestDatabase {        val user = User.new {            name = "Bob"            email = "bob@test.com"        }        assertEquals("Bob", user.name)        assertNotNull(user.id)    }}

private fun withTestDatabase(block: () -> Unit) {    Database.connect("jdbc:h2:mem: