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skills/kotlin-testing/SKILL.md

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+---
+name: kotlin-testing
+description: Kotlin testing patterns with Kotest, MockK, coroutine testing, property-based testing, and Kover coverage. Follows TDD methodology with idiomatic Kotlin practices.
+origin: ECC
+---
+
+# 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](#kotest-spec-styles)
+- **Mocking** — MockK setup, coroutine mocking, argument capture in [MockK](#mockk)
+- **TDD walkthrough** — Full RED/GREEN/REFACTOR cycle with EmailValidator in [TDD Workflow for Kotlin](#tdd-workflow-for-kotlin)
+- **Coverage** — Kover configuration and commands in [Kover Coverage](#kover-coverage)
+- **Ktor testing** — testApplication setup in [Ktor testApplication Testing](#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
+
+```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(IllegalArgumentException("Email must contain @"))
+    val regex = Regex("^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$")
+    if (!regex.matches(email)) return Result.failure(IllegalArgumentException("Invalid email format"))
+    return Result.success(email)
+}
+
+// Step 5: Run tests - verify PASS
+// $ ./gradlew test
+// EmailValidatorTest > valid email returns success PASSED
+// EmailValidatorTest > empty email returns failure PASSED
+// EmailValidatorTest > email without @ returns failure PASSED
+
+// Step 6: Refactor if needed, verify tests still pass
+```
+
+## Kotest Spec Styles
+
+### StringSpec (Simplest)
+
+```kotlin
+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)
+
+```kotlin
+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)
+
+```kotlin
+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)
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+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
+
+```kotlin
+// 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
+
+```kotlin
+// build.gradle.kts
+plugins {
+    id("org.jetbrains.kotlinx.kover") version "0.9.7"
+}
+
+kover {
+    reports {
+        total {
+            html { onCheck = true }
+            xml { onCheck = true }
+        }
+        filters {
+            excludes {
+                classes("*.generated.*", "*.config.*")
+            }
+        }
+        verify {
+            rule {
+                minBound(80) // Fail build below 80% coverage
+            }
+        }
+    }
+}
+```
+
+### Coverage Commands
+
+```bash
+# Run tests with coverage
+./gradlew koverHtmlReport
+
+# Verify coverage thresholds
+./gradlew koverVerify
+
+# XML report for CI
+./gradlew koverXmlReport
+
+# View HTML report (use the command for your OS)
+# macOS:   open build/reports/kover/html/index.html
+# Linux:   xdg-open build/reports/kover/html/index.html
+# Windows: start build/reports/kover/html/index.html
+```
+
+### Coverage Targets
+
+| Code Type | Target |
+|-----------|--------|
+| Critical business logic | 100% |
+| Public APIs | 90%+ |
+| General code | 80%+ |
+| Generated / config code | Exclude |
+
+## Ktor testApplication Testing
+
+```kotlin
+class ApiRoutesTest : FunSpec({
+    test("GET /users returns list") {
+        testApplication {
+            application {
+                configureRouting()
+                configureSerialization()
+            }
+
+            val response = client.get("/users")
+
+            response.status shouldBe HttpStatusCode.OK
+            val users = response.body<List<UserResponse>>()
+            users.shouldNotBeEmpty()
+        }
+    }
+
+    test("POST /users creates user") {
+        testApplication {
+            application {
+                configureRouting()
+                configureSerialization()
+            }
+
+            val response = client.post("/users") {
+                contentType(ContentType.Application.Json)
+                setBody(CreateUserRequest("Alice", "alice@example.com"))
+            }
+
+            response.status shouldBe HttpStatusCode.Created
+        }
+    }
+})
+```
+
+## Testing Commands
+
+```bash
+# Run all tests
+./gradlew test
+
+# Run specific test class
+./gradlew test --tests "com.example.UserServiceTest"
+
+# Run specific test
+./gradlew test --tests "com.example.UserServiceTest.getUser returns user when found"
+
+# Run with verbose output
+./gradlew test --info
+
+# Run with coverage
+./gradlew koverHtmlReport
+
+# Run detekt (static analysis)
+./gradlew detekt
+
+# Run ktlint (formatting check)
+./gradlew ktlintCheck
+
+# Continuous testing
+./gradlew test --continuous
+```
+
+## Best Practices
+
+**DO:**
+- Write tests FIRST (TDD)
+- Use Kotest's spec styles consistently across the project
+- Use MockK's `coEvery`/`coVerify` for suspend functions
+- Use `runTest` for coroutine testing
+- Test behavior, not implementation
+- Use property-based testing for pure functions
+- Use `data class` test fixtures for clarity
+
+**DON'T:**
+- Mix testing frameworks (pick Kotest and stick with it)
+- Mock data classes (use real instances)
+- Use `Thread.sleep()` in coroutine tests (use `advanceTimeBy`)
+- Skip the RED phase in TDD
+- Test private functions directly
+- Ignore flaky tests
+
+## Integration with CI/CD
+
+```yaml
+# GitHub Actions example
+test:
+  runs-on: ubuntu-latest
+  steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
+      with:
+        distribution: 'temurin'
+        java-version: '21'
+
+    - name: Run tests with coverage
+      run: ./gradlew test koverXmlReport
+
+    - name: Verify coverage
+      run: ./gradlew koverVerify
+
+    - name: Upload coverage
+      uses: codecov/codecov-action@v5
+      with:
+        files: build/reports/kover/report.xml
+        token: ${{ secrets.CODECOV_TOKEN }}
+```
+
+**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.