Kotlin Testing

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 or data-driven 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 (FunSpec, StringSpec, BehaviorSpec, DescribeSpec) 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

TDD 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

For full step-by-step EmailValidator walkthrough, see references/code-examples.md.

Kotest Spec Styles

FunSpec (Recommended for Most Tests)

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")
        }
    }
})

For StringSpec, BehaviorSpec, and DescribeSpec examples, see references/code-examples.md.

Kotest Matchers

Core Matchers

// 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)
}

For custom matchers, see references/code-examples.md.

MockK Basics

class UserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val logger = mockk<Logger>(relaxed = true) // Returns defaults for all calls
    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("suspend function mocking") {
        coEvery { repository.findById("1") } returns User(id = "1", name = "Alice")

        val result = service.getUser("1")

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

For advanced MockK patterns (argument capture, spy, coroutine mocking), see references/code-examples.md.

Coroutine Testing

Use runTest from kotlinx.coroutines.test for suspend functions and coroutines:

test("concurrent fetches complete together") {
    runTest {
        val service = DataService(testScope = this)
        val result = service.fetchAllData()
        result.users.shouldNotBeEmpty()
    }
}

For Flow testing, TestDispatcher, and advanced coroutine patterns, see references/code-examples.md.

Property-Based & Data-Driven Testing

Property-based testing: Automatically generates test cases using Kotest's Arb (arbitrary) generators. Great for pure functions.

Data-driven testing: withData allows parameterized test cases for multiple inputs.

Examples and custom generators available in references/code-examples.md.

Test Lifecycle and Fixtures

Use beforeTest/afterTest for test-level setup/cleanup, and beforeSpec/afterSpec for suite-level initialization.

beforeSpec { db = setupDatabase() }
afterSpec { db.close() }
beforeTest { db.clear() }

Kotest extensions enable reusable test infrastructure. See references/code-examples.md for DatabaseExtension example.

Kover Coverage Configuration

Gradle Setup

// build.gradle.kts
plugins {
    id("org.jetbrains.kotlinx.kover") version "0.9.7"
}

kover {
    reports {
        total {
            html { }
            xml { }
        }
        filters {
            excludes {
                classes("*.generated.*", "*.config.*")
            }
        }
        verify {
            rule {
                minBound(80) // Fail build below 80% coverage
            }
        }
    }
}

Coverage Commands

./gradlew koverHtmlReport     # Generate HTML report
./gradlew koverVerify         # Verify coverage meets thresholds
./gradlew koverXmlReport      # Generate XML for CI

# View report (macOS)
open build/reports/kover/html/index.html

Coverage Targets

Testing Commands

./gradlew test                                  # Run all tests
./gradlew test --tests "com.example.UserServiceTest"  # Run specific class
./gradlew test --tests "com.example.UserServiceTest.getUser*"  # Run by pattern
./gradlew test --info                          # Verbose output
./gradlew koverHtmlReport                      # Tests + coverage report
./gradlew detekt                               # Static analysis
./gradlew ktlintCheck                          # Formatting check
./gradlew test --continuous                    # Watch mode

Effective Patterns

• Write tests FIRST — Follow TDD strictly. RED phase catches bugs early.
• Use Kotest consistently — Pick a spec style (FunSpec recommended) and stick with it across the project.
• Mock suspend functions with coEvery/coVerify — Never use every/verify for coroutines.
• Use runTest for coroutines — Never use Thread.sleep() in coroutine tests; use advanceTimeBy or advanceUntilIdle.
• Test behavior, not implementation — Focus on what the code does, not how it does it.
• Use real instances for immutable objects — Don't mock data classes; create real test fixtures.
• Leverage property-based testing — Excellent for pure functions and edge case discovery.
• Keep fixtures in data classes — Makes test intent clear and setup reusable.
• Use relaxed = true for loggers/observers — They're usually not critical to test assertions.

Patterns to Avoid

• Mixing test frameworks — Kotest and JUnit don't play well together; commit fully to Kotest.
• Mocking immutable objects — Use real instances instead for simplicity.
• Complex test setup — If setup takes >10 lines, extract to a helper or factory.
• Flaky tests from timing — Use advanceTimeBy or runTest scheduler control instead of delays.
• Testing private functions directly — Private functions are tested indirectly via public APIs.
• Ignoring test failures — Flaky tests hide real bugs; fix or skip immediately.
• Tight coupling to implementation — Refactorings should not require rewriting tests.

Quick References

• Kotest spec styles: See references/code-examples.md for StringSpec, BehaviorSpec, DescribeSpec
• EmailValidator TDD walkthrough: Full RED-GREEN-REFACTOR example in references/code-examples.md
• Advanced MockK: Argument capture, spy, coroutine mocking in references/code-examples.md
• Flow testing: Testing Kotlin Flows and debounce patterns in references/code-examples.md
• Custom matchers: Build domain-specific assertions in references/code-examples.md
• CI/CD integration: GitHub Actions YAML workflow in references/code-examples.md

Remember: Tests are documentation. They show how your Kotlin code is meant to be used. Use Kotest's expressive matchers to make tests readable and MockK for clean mocking of dependencies.