kotlin-testing skill

Kotlin Testing Patterns

Comprehensive Kotlin testing patterns for writing reliable, maintainable tests following TDD methodology with Kotest and MockK.

When to Use

• Writing new Kotlin functions or classes
• Adding test coverage to existing Kotlin code
• Implementing property-based tests
• Following TDD workflow in Kotlin projects
• Configuring Kover for code coverage

How It Works

1. Identify target code — Find the function, class, or module to test
2. Write a Kotest spec — Choose a spec style (StringSpec, FunSpec, BehaviorSpec) matching the test scope
3. Mock dependencies — Use MockK to isolate the unit under test
4. Run tests (RED) — Verify the test fails with the expected error
5. Implement code (GREEN) — Write minimal code to pass the test
6. Refactor — Improve the implementation while keeping tests green
7. Check coverage — Run ./gradlew koverHtmlReport and verify 80%+ coverage

Examples

The following sections contain detailed, runnable examples for each testing pattern:

Quick Reference

• Kotest specs — StringSpec, FunSpec, BehaviorSpec, DescribeSpec examples in Kotest Spec Styles
• Mocking — MockK setup, coroutine mocking, argument capture in MockK
• TDD walkthrough — Full RED/GREEN/REFACTOR cycle with EmailValidator in TDD Workflow for Kotlin
• Coverage — Kover configuration and commands in Kover Coverage
• Ktor testing — testApplication setup in Ktor testApplication Testing

TDD Workflow for Kotlin

The RED-GREEN-REFACTOR Cycle

RED     -> Write a failing test first
GREEN   -> Write minimal code to pass the test
REFACTOR -> Improve code while keeping tests green
REPEAT  -> Continue with next requirement

Step-by-Step TDD in Kotlin

// Step 1: Define the interface/signature
// EmailValidator.kt
package com.example.validator

fun validateEmail(email: String): Result<String> {
    TODO("not implemented")
}

// Step 2: Write failing test (RED)
// EmailValidatorTest.kt
package com.example.validator

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.result.shouldBeFailure
import io.kotest.matchers.result.shouldBeSuccess

class EmailValidatorTest : StringSpec({
    "valid email returns success" {
        validateEmail("user@example.com").shouldBeSuccess("user@example.com")
    }

    "empty email returns failure" {
        validateEmail("").shouldBeFailure()
    }

    "email without @ returns failure" {
        validateEmail("userexample.com").shouldBeFailure()
    }
})

// Step 3: Run tests - verify FAIL
// $ ./gradlew test
// EmailValidatorTest > valid email returns success FAILED
//   kotlin.NotImplementedError: An operation is not implemented

// Step 4: Implement minimal code (GREEN)
fun validateEmail(email: String): Result<String> {
    if (email.isBlank()) return Result.failure(IllegalArgumentException("Email cannot be blank"))
    if ('@' !in email) return Result.failure(IllegalArgumentE

Kotest Spec Styles

StringSpec (Simplest)

class CalculatorTest : StringSpec({
    "add two positive numbers" {
        Calculator.add(2, 3) shouldBe 5
    }

    "add negative numbers" {
        Calculator.add(-1, -2) shouldBe -3
    }

    "add zero" {
        Calculator.add(0, 5) shouldBe 5
    }
})

FunSpec (JUnit-like)

class UserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val service = UserService(repository)

    test("getUser returns user when found") {
        val expected = User(id = "1", name = "Alice")
        coEvery { repository.findById("1") } returns expected

        val result = service.getUser("1")

        result shouldBe expected
    }

    test("getUser throws when not found") {
        coEvery { repository.findById("999") } returns null

        shouldThrow<UserNotFoundException> {
            service.getUser("999")
        }
    }
})

BehaviorSpec (BDD Style)

class OrderServiceTest : BehaviorSpec({
    val repository = mockk<OrderRepository>()
    val paymentService = mockk<PaymentService>()
    val service = OrderService(repository, paymentService)

    Given("a valid order request") {
        val request = CreateOrderRequest(
            userId = "user-1",
            items = listOf(OrderItem("product-1", quantity = 2)),
        )

        When("the order is placed") {
            coEvery { paymentService.charge(any()) } returns PaymentResult.Success
            coEvery { repository.save(any()) } answers { firstArg() }

            val result = service.placeOrder(request)

            Then("it should return a confirmed order") {
                result.status shouldBe OrderStatus.CONFIRMED
            }

            Then("it should charge payment") {
                coVerify(exactly = 1) { paymentService.charge(any()) }
            }
        }

        When("payment fails") {
            coEvery { paymentService.charge(any()) } returns PaymentResult.Declined

            Then("it should throw PaymentException") {
                shouldThrow<PaymentException> {
                    service.placeOrder(request)
                }
          

DescribeSpec (RSpec Style)

class UserValidatorTest : DescribeSpec({
    describe("validateUser") {
        val validator = UserValidator()

        context("with valid input") {
            it("accepts a normal user") {
                val user = CreateUserRequest("Alice", "alice@example.com")
                validator.validate(user).shouldBeValid()
            }
        }

        context("with invalid name") {
            it("rejects blank name") {
                val user = CreateUserRequest("", "alice@example.com")
                validator.validate(user).shouldBeInvalid()
            }

            it("rejects name exceeding max length") {
                val user = CreateUserRequest("A".repeat(256), "alice@example.com")
                validator.validate(user).shouldBeInvalid()
            }
        }
    }
})

Kotest Matchers

Core Matchers

import io.kotest.matchers.shouldBe
import io.kotest.matchers.shouldNotBe
import io.kotest.matchers.string.*
import io.kotest.matchers.collections.*
import io.kotest.matchers.nulls.*

// Equality
result shouldBe expected
result shouldNotBe unexpected

// Strings
name shouldStartWith "Al"
name shouldEndWith "ice"
name shouldContain "lic"
name shouldMatch Regex("[A-Z][a-z]+")
name.shouldBeBlank()

// Collections
list shouldContain "item"
list shouldHaveSize 3
list.shouldBeSorted()
list.shouldContainAll("a", "b", "c")
list.shouldBeEmpty()

// Nulls
result.shouldNotBeNull()
result.shouldBeNull()

// Types
result.shouldBeInstanceOf<User>()

// Numbers
count shouldBeGreaterThan 0
price shouldBeInRange 1.0..100.0

// Exceptions
shouldThrow<IllegalArgumentException> {
    validateAge(-1)
}.message shouldBe "Age must be positive"

shouldNotThrow<Exception> {
    validateAge(25)
}

Custom Matchers

fun beActiveUser() = object : Matcher<User> {
    override fun test(value: User) = MatcherResult(
        value.isActive && value.lastLogin != null,
        { "User ${value.id} should be active with a last login" },
        { "User ${value.id} should not be active" },
    )
}

// Usage
user should beActiveUser()

MockK

Basic Mocking

class UserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val logger = mockk<Logger>(relaxed = true) // Relaxed: returns defaults
    val service = UserService(repository, logger)

    beforeTest {
        clearMocks(repository, logger)
    }

    test("findUser delegates to repository") {
        val expected = User(id = "1", name = "Alice")
        every { repository.findById("1") } returns expected

        val result = service.findUser("1")

        result shouldBe expected
        verify(exactly = 1) { repository.findById("1") }
    }

    test("findUser returns null for unknown id") {
        every { repository.findById(any()) } returns null

        val result = service.findUser("unknown")

        result.shouldBeNull()
    }
})

Coroutine Mocking

class AsyncUserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val service = UserService(repository)

    test("getUser suspending function") {
        coEvery { repository.findById("1") } returns User(id = "1", name = "Alice")

        val result = service.getUser("1")

        result.name shouldBe "Alice"
        coVerify { repository.findById("1") }
    }

    test("getUser with delay") {
        coEvery { repository.findById("1") } coAnswers {
            delay(100) // Simulate async work
            User(id = "1", name = "Alice")
        }

        val result = service.getUser("1")
        result.name shouldBe "Alice"
    }
})

Argument Capture

test("save captures the user argument") {
    val slot = slot<User>()
    coEvery { repository.save(capture(slot)) } returns Unit

    service.createUser(CreateUserRequest("Alice", "alice@example.com"))

    slot.captured.name shouldBe "Alice"
    slot.captured.email shouldBe "alice@example.com"
    slot.captured.id.shouldNotBeNull()
}

Spy and Partial Mocking

test("spy on real object") {
    val realService = UserService(repository)
    val spy = spyk(realService)

    every { spy.generateId() } returns "fixed-id"

    spy.createUser(request)

    verify { spy.generateId() } // Overridden
    // Other methods use real implementation
}

Coroutine Testing

runTest for Suspend Functions

import kotlinx.coroutines.test.runTest

class CoroutineServiceTest : FunSpec({
    test("concurrent fetches complete together") {
        runTest {
            val service = DataService(testScope = this)

            val result = service.fetchAllData()

            result.users.shouldNotBeEmpty()
            result.products.shouldNotBeEmpty()
        }
    }

    test("timeout after delay") {
        runTest {
            val service = SlowService()

            shouldThrow<TimeoutCancellationException> {
                withTimeout(100) {
                    service.slowOperation() // Takes > 100ms
                }
            }
        }
    }
})

Testing Flows

import io.kotest.matchers.collections.shouldContainInOrder
import kotlinx.coroutines.flow.MutableSharedFlow
import kotlinx.coroutines.flow.toList
import kotlinx.coroutines.launch
import kotlinx.coroutines.test.advanceTimeBy
import kotlinx.coroutines.test.runTest

class FlowServiceTest : FunSpec({
    test("observeUsers emits updates") {
        runTest {
            val service = UserFlowService()

            val emissions = service.observeUsers()
                .take(3)
                .toList()

            emissions shouldHaveSize 3
            emissions.last().shouldNotBeEmpty()
        }
    }

    test("searchUsers debounces input") {
        runTest {
            val service = SearchService()
            val queries = MutableSharedFlow<String>()

            val results = mutableListOf<List<User>>()
            val job = launch {
                service.searchUsers(queries).collect { results.add(it) }
            }

            queries.emit("a")
            queries.emit("ab")
            queries.emit("abc") // Only this should trigger search
            advanceTimeBy(500)

            results shouldHaveSize 1
            job.cancel()
        }
    }
})

TestDispatcher

import kotlinx.coroutines.test.StandardTestDispatcher
import kotlinx.coroutines.test.advanceUntilIdle

class DispatcherTest : FunSpec({
    test("uses test dispatcher for controlled execution") {
        val dispatcher = StandardTestDispatcher()

        runTest(dispatcher) {
            var completed = false

            launch {
                delay(1000)
                completed = true
            }

            completed shouldBe false
            advanceTimeBy(1000)
            completed shouldBe true
        }
    }
})

Property-Based Testing

Kotest Property Testing

import io.kotest.core.spec.style.FunSpec
import io.kotest.property.Arb
import io.kotest.property.arbitrary.*
import io.kotest.property.forAll
import io.kotest.property.checkAll
import kotlinx.serialization.json.Json
import kotlinx.serialization.encodeToString
import kotlinx.serialization.decodeFromString

// Note: The serialization roundtrip test below requires the User data class
// to be annotated with @Serializable (from kotlinx.serialization).

class PropertyTest : FunSpec({
    test("string reverse is involutory") {
        forAll<String> { s ->
            s.reversed().reversed() == s
        }
    }

    test("list sort is idempotent") {
        forAll(Arb.list(Arb.int())) { list ->
            list.sorted() == list.sorted().sorted()
        }
    }

    test("serialization roundtrip preserves data") {
        checkAll(Arb.bind(Arb.string(1..50), Arb.string(5..100)) { name, email ->
            User(name = name, email = "$email@test.com")
        }) { user ->
            val json = Json.encodeToString(user)
            val decoded = Json.decodeFromString<User>(json)
            decoded shouldBe user
        }
    }
})

Custom Generators

val userArb: Arb<User> = Arb.bind(
    Arb.string(minSize = 1, maxSize = 50),
    Arb.email(),
    Arb.enum<Role>(),
) { name, email, role ->
    User(
        id = UserId(UUID.randomUUID().toString()),
        name = name,
        email = Email(email),
        role = role,
    )
}

val moneyArb: Arb<Money> = Arb.bind(
    Arb.long(1L..1_000_000L),
    Arb.enum<Currency>(),
) { amount, currency ->
    Money(amount, currency)
}

Data-Driven Testing

withData in Kotest

class ParserTest : FunSpec({
    context("parsing valid dates") {
        withData(
            "2026-01-15" to LocalDate(2026, 1, 15),
            "2026-12-31" to LocalDate(2026, 12, 31),
            "2000-01-01" to LocalDate(2000, 1, 1),
        ) { (input, expected) ->
            parseDate(input) shouldBe expected
        }
    }

    context("rejecting invalid dates") {
        withData(
            nameFn = { "rejects '$it'" },
            "not-a-date",
            "2026-13-01",
            "2026-00-15",
            "",
        ) { input ->
            shouldThrow<DateParseException> {
                parseDate(input)
            }
        }
    }
})

Test Lifecycle and Fixtures

BeforeTest / AfterTest

class DatabaseTest : FunSpec({
    lateinit var db: Database

    beforeSpec {
        db = Database.connect("jdbc:h2:mem:test;DB_CLOSE_DELAY=-1")
        transaction(db) {
            SchemaUtils.create(UsersTable)
        }
    }

    afterSpec {
        transaction(db) {
            SchemaUtils.drop(UsersTable)
        }
    }

    beforeTest {
        transaction(db) {
            UsersTable.deleteAll()
        }
    }

    test("insert and retrieve user") {
        transaction(db) {
            UsersTable.insert {
                it[name] = "Alice"
                it[email] = "alice@example.com"
            }
        }

        val users = transaction(db) {
            UsersTable.selectAll().map { it[UsersTable.name] }
        }

        users shouldContain "Alice"
    }
})

Kotest Extensions

// Reusable test extension
class DatabaseExtension : BeforeSpecListener, AfterSpecListener {
    lateinit var db: Database

    override suspend fun beforeSpec(spec: Spec) {
        db = Database.connect("jdbc:h2:mem:test;DB_CLOSE_DELAY=-1")
    }

    override suspend fun afterSpec(spec: Spec) {
        // cleanup
    }
}

class UserRepositoryTest : FunSpec({
    val dbExt = DatabaseExtension()
    register(dbExt)

    test("save and find user") {
        val repo = UserRepository(dbExt.db)
        // ...
    }
})

Kover Coverage

Gradle Configuration