Testing

Testing on the JVM is mature, powerful, and surprisingly different from what you’re used to in TypeScript or Go. This module covers the entire testing stack: JUnit 5, MockK, Kotest, Testcontainers, and framework-specific testing for Spring Boot and Ktor.

Testing ecosystem overview

The JVM testing world is layered: a platform (JUnit Platform) runs engines (JUnit Jupiter, Kotest), and libraries (MockK, Testcontainers) plug into them. Here’s how the pieces map to what you already know:

Concept	TypeScript	Go	Kotlin/JVM
Test runner	Jest / Vitest	go test	JUnit 5 (JUnit Platform)
Assertions	expect(x).toBe(y)	if x != y { t.Errorf(...) }	assertEquals(y, x) / Kotest matchers
Mocking	jest.mock() / vi.mock()	interfaces + manual / testify	MockK
BDD style	describe/it blocks	subtests t.Run()	Kotest BehaviorSpec / DescribeSpec
Property testing	fast-check	rapid / gopter	Kotest property testing
Integration (Docker)	testcontainers-node	testcontainers-go	Testcontainers Java/Kotlin
Coverage	--coverage flag	go test -cover	JaCoCo
Framework testing	supertest	httptest	@SpringBootTest / Ktor testApplication

Dependency setup (Gradle)

Every Kotlin project using tests needs these in build.gradle.kts:

build.gradle.kts

plugins {
    kotlin("jvm") version "2.1.0"
}

dependencies {
    // JUnit 5
    testImplementation(platform("org.junit:junit-bom:5.11.4"))
    testImplementation("org.junit.jupiter:junit-jupiter")

    // MockK
    testImplementation("io.mockk:mockk:1.13.13")

    // Kotest
    testImplementation("io.kotest:kotest-runner-junit5:5.9.1")
    testImplementation("io.kotest:kotest-assertions-core:5.9.1")
    testImplementation("io.kotest:kotest-property:5.9.1")

    // Testcontainers
    testImplementation(platform("org.testcontainers:testcontainers-bom:1.20.4"))
    testImplementation("org.testcontainers:junit-jupiter")
    testImplementation("org.testcontainers:postgresql")

    // Coroutine testing
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.9.0")
}

tasks.test {
    useJUnitPlatform()
}

JUnit 5 fundamentals

JUnit 5 is the default test engine on the JVM — the go test / Jest of this world. Here’s the same two tests (a passing assertion and an expected exception) written in each ecosystem:

TypeScript

describe('Calculator', () => {
  test('adds two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });

  test('throws on division by zero', () => {
    expect(() => divide(1, 0)).toThrow('Division by zero');
  });
});

Go

func TestAdd(t *testing.T) {
    result := Add(2, 3)
    if result != 5 {
        t.Errorf("Add(2, 3) = %d; want 5", result)
    }
}

func TestDivideByZero(t *testing.T) {
    _, err := Divide(1, 0)
    if err == nil {
        t.Fatal("expected error for division by zero")
    }
}

Kotlin

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.assertThrows
import org.junit.jupiter.api.Assertions.*

class CalculatorTest {

    private val calculator = Calculator()

    @Test
    fun `adds two numbers`() {
        assertEquals(5, calculator.add(2, 3))
    }

    @Test
    fun `throws on division by zero`() {
        val exception = assertThrows<ArithmeticException> {
            calculator.divide(1, 0)
        }
        assertEquals("Division by zero", exception.message)
    }
}

Key differences:

• Kotlin test method names can use backticks with spaces: `adds two numbers`().
• No describe blocks in JUnit 5 — use @Nested classes instead (shown below).
• Assertions are static imports: assertEquals(expected, actual) — note the order.
• assertThrows<T> is a Kotlin-specific JUnit 5 extension that uses reified generics.

The class under test:

class Calculator {
    fun add(a: Int, b: Int): Int = a + b

    fun divide(a: Int, b: Int): Int {
        if (b == 0) throw ArithmeticException("Division by zero")
        return a / b
    }
}

Display names

With backtick method names you rarely need @DisplayName, but it exists for when you want richer descriptions or non-ASCII characters:

import org.junit.jupiter.api.DisplayName

@DisplayName("Calculator")
class CalculatorTest {

    @Test
    @DisplayName("should add two positive numbers")
    fun addPositive() {
        assertEquals(5, Calculator().add(2, 3))
    }

    @Test
    @DisplayName("should handle negative numbers")
    fun addNegative() {
        assertEquals(-1, Calculator().add(2, -3))
    }
}

Assertions in depth

JUnit ships its own assertions, but you can also use Kotest’s matchers inside plain JUnit tests for a more readable, Jest-like style.

JUnit 5 built-in assertions

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*

class AssertionExamplesTest {

    @Test
    fun `basic assertions`() {
        // Equality
        assertEquals(4, 2 + 2)
        assertEquals(3.14, Math.PI, 0.01) // delta for doubles

        // Boolean
        assertTrue(listOf(1, 2, 3).contains(2))
        assertFalse("hello".isEmpty())

        // Null
        val result: String? = findUser("alice")
        assertNotNull(result)
        assertNull(findUser("nobody"))

        // Same reference
        val list1 = listOf(1, 2)
        val list2 = list1
        assertSame(list1, list2)
    }

    @Test
    fun `grouped assertions -- all run even if some fail`() {
        val user = User("Alice", 30, "alice@example.com")

        assertAll(
            "user properties",
            { assertEquals("Alice", user.name) },
            { assertEquals(30, user.age) },
            { assertTrue(user.email.contains("@")) }
        )
    }

    @Test
    fun `exception assertions`() {
        val exception = assertThrows<IllegalArgumentException> {
            validateAge(-1)
        }
        assertTrue(exception.message!!.contains("negative"))
    }

    @Test
    fun `timeout assertion`() {
        assertTimeout(java.time.Duration.ofSeconds(2)) {
            Thread.sleep(100) // must complete within 2 seconds
        }
    }

    private fun findUser(name: String): String? =
        if (name == "alice") "Alice" else null

    private fun validateAge(age: Int) {
        require(age >= 0) { "Age cannot be negative: $age" }
    }

    data class User(val name: String, val age: Int, val email: String)
}

assertAll is worth highlighting: it runs every assertion even if some fail, then reports all failures together — handy for checking several properties of one object.

Kotest assertions (more readable)

Kotest provides matchers that read like natural language — similar to Jest’s expect API. You can use them inside JUnit 5 tests:

import io.kotest.matchers.shouldBe
import io.kotest.matchers.string.shouldContain
import io.kotest.matchers.string.shouldStartWith
import io.kotest.matchers.collections.shouldContainAll
import io.kotest.matchers.collections.shouldHaveSize
import io.kotest.matchers.nulls.shouldBeNull
import io.kotest.matchers.nulls.shouldNotBeNull
import io.kotest.matchers.types.shouldBeInstanceOf
import org.junit.jupiter.api.Test

class KotestMatchersInJUnitTest {

    @Test
    fun `string matchers`() {
        val greeting = "Hello, World!"
        greeting shouldContain "World"
        greeting shouldStartWith "Hello"
        greeting.length shouldBe 13
    }

    @Test
    fun `collection matchers`() {
        val numbers = listOf(1, 2, 3, 4, 5)
        numbers shouldHaveSize 5
        numbers shouldContainAll listOf(2, 4)
    }

    @Test
    fun `null matchers`() {
        val present: String? = "hello"
        val absent: String? = null

        present.shouldNotBeNull()
        absent.shouldBeNull()
    }

    @Test
    fun `type matchers`() {
        val result: Any = "hello"
        result.shouldBeInstanceOf<String>()
    }
}

Jest expect vs Kotest shouldBe:

Jest / Vitest	Kotest
expect(x).toBe(5)	x shouldBe 5
expect(x).toContain("a")	x shouldContain "a"
expect(x).toHaveLength(3)	x shouldHaveSize 3
expect(x).toBeNull()	x.shouldBeNull()
expect(x).toBeInstanceOf(Foo)	x.shouldBeInstanceOf<Foo>()
expect(x).toThrow()	shouldThrow<Exception> { ... }

Test lifecycle and organization

Every framework has setup/teardown hooks. Here’s the lifecycle in each ecosystem:

TypeScript

describe('UserService', () => {
  let db: Database;

  beforeAll(async () => { db = await connectDb(); });
  afterAll(async () => { await db.close(); });
  beforeEach(() => { db.clear(); });
  afterEach(() => { /* cleanup */ });

  test('creates user', () => { /* ... */ });
});

Go

func TestMain(m *testing.M) {
    // setup
    db := connectDb()
    code := m.Run()
    // teardown
    db.Close()
    os.Exit(code)
}

Kotlin

import org.junit.jupiter.api.*

class UserServiceTest {

    companion object {
        private lateinit var db: Database

        @JvmStatic
        @BeforeAll
        fun setupAll() {
            db = Database.connect()
        }

        @JvmStatic
        @AfterAll
        fun teardownAll() {
            db.close()
        }
    }

    @BeforeEach
    fun setup() {
        db.clear()
    }

    @AfterEach
    fun cleanup() {
        // per-test cleanup
    }

    @Test
    fun `creates a user`() {
        // ...
    }
}

Key points:

• @BeforeAll / @AfterAll must be on companion object methods with @JvmStatic.
• Or use @TestInstance(TestInstance.Lifecycle.PER_CLASS) to avoid companion objects.
• By default, JUnit 5 creates a new test class instance per test method (like Go).

The per-class lifecycle is simpler — no companion object needed:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class UserServiceTest {

    private lateinit var db: Database

    @BeforeAll
    fun setupAll() {
        db = Database.connect()  // no companion object needed
    }

    @AfterAll
    fun teardownAll() {
        db.close()
    }

    @Test
    fun `creates a user`() { /* ... */ }
}

Nested tests (like describe blocks)

This is the JUnit 5 equivalent of Jest’s nested describe — an inner class annotated with @Nested:

import org.junit.jupiter.api.Nested
import org.junit.jupiter.api.DisplayName

@DisplayName("UserService")
class UserServiceTest {

    private val service = UserService()

    @Nested
    @DisplayName("when creating users")
    inner class CreateUser {

        @Test
        fun `saves valid user`() {
            val user = service.create("alice", "alice@test.com")
            assertNotNull(user.id)
        }

        @Test
        fun `rejects duplicate email`() {
            service.create("alice", "alice@test.com")
            assertThrows<DuplicateEmailException> {
                service.create("bob", "alice@test.com")
            }
        }
    }

    @Nested
    @DisplayName("when finding users")
    inner class FindUser {

        @Test
        fun `finds existing user by email`() {
            service.create("alice", "alice@test.com")
            val found = service.findByEmail("alice@test.com")
            assertNotNull(found)
        }

        @Test
        fun `returns null for unknown email`() {
            assertNull(service.findByEmail("nobody@test.com"))
        }
    }
}

Disabling and conditional execution

import org.junit.jupiter.api.Disabled
import org.junit.jupiter.api.condition.*

@Test
@Disabled("Waiting for API v2 -- JIRA-1234")
fun `handles rate limiting`() { /* ... */ }

@Test
@EnabledOnOs(OS.LINUX)
fun `uses epoll on Linux`() { /* ... */ }

@Test
@EnabledIfEnvironmentVariable(named = "CI", matches = "true")
fun `only runs in CI`() { /* ... */ }

@Test
@EnabledIfSystemProperty(named = "java.version", matches = "21.*")
fun `needs JDK 21`() { /* ... */ }

Test execution order

import org.junit.jupiter.api.MethodOrderer
import org.junit.jupiter.api.Order
import org.junit.jupiter.api.TestMethodOrder

@TestMethodOrder(MethodOrderer.OrderAnnotation::class)
class OrderedTest {

    @Test @Order(1)
    fun `step 1 -- create resource`() { /* ... */ }

    @Test @Order(2)
    fun `step 2 -- modify resource`() { /* ... */ }

    @Test @Order(3)
    fun `step 3 -- delete resource`() { /* ... */ }
}

Anti-pattern alert

Ordered tests are an anti-pattern for unit tests (tests should be independent). Use them only for integration/E2E test sequences.

Parameterized tests

Parameterized tests let you run the same test logic with multiple inputs — like Jest’s test.each or Go’s table-driven tests.

TypeScript

test.each([
  [1, 1, 2],
  [2, 3, 5],
  [0, 0, 0],
])('add(%i, %i) = %i', (a, b, expected) => {
  expect(add(a, b)).toBe(expected);
});

Go

func TestAdd(t *testing.T) {
    tests := []struct {
        a, b, want int
    }{
        {1, 1, 2},
        {2, 3, 5},
        {0, 0, 0},
    }
    for _, tt := range tests {
        t.Run(fmt.Sprintf("%d+%d", tt.a, tt.b), func(t *testing.T) {
            if got := Add(tt.a, tt.b); got != tt.want {
                t.Errorf("Add(%d, %d) = %d, want %d", tt.a, tt.b, got, tt.want)
            }
        })
    }
}

Kotlin

import org.junit.jupiter.params.ParameterizedTest
import org.junit.jupiter.params.provider.*

class ParameterizedExamplesTest {

    // Method 1: @ValueSource -- single parameter
    @ParameterizedTest
    @ValueSource(strings = ["hello", "world", "kotlin"])
    fun `string is not empty`(input: String) {
        assertTrue(input.isNotEmpty())
    }

    // Method 2: @CsvSource -- multiple parameters (like table-driven)
    @ParameterizedTest(name = "add({0}, {1}) = {2}")
    @CsvSource(
        "1, 1, 2",
        "2, 3, 5",
        "0, 0, 0",
        "-1, 1, 0"
    )
    fun `adds two numbers`(a: Int, b: Int, expected: Int) {
        assertEquals(expected, Calculator().add(a, b))
    }

    // Method 3: @EnumSource
    @ParameterizedTest
    @EnumSource(Status::class)
    fun `all statuses have a display name`(status: Status) {
        assertTrue(status.displayName.isNotEmpty())
    }

    // Method 4: @MethodSource -- complex objects
    @ParameterizedTest(name = "{0}")
    @MethodSource("userTestCases")
    fun `validates user email`(testName: String, email: String, isValid: Boolean) {
        assertEquals(isValid, EmailValidator.isValid(email))
    }

    companion object {
        @JvmStatic
        fun userTestCases(): List<Arguments> = listOf(
            Arguments.of("valid email", "user@example.com", true),
            Arguments.of("missing @", "userexample.com", false),
            Arguments.of("missing domain", "user@", false),
            Arguments.of("empty string", "", false),
        )
    }
}

enum class Status(val displayName: String) {
    ACTIVE("Active"),
    INACTIVE("Inactive"),
    PENDING("Pending")
}

@MethodSource with data classes

This is the closest to Go’s table-driven test pattern — a data class per case:

data class TestCase(
    val name: String,
    val input: String,
    val expected: String
)

class SlugifyTest {

    @ParameterizedTest(name = "{0}")
    @MethodSource("testCases")
    fun `converts string to slug`(case: TestCase) {
        assertEquals(case.expected, slugify(case.input))
    }

    companion object {
        @JvmStatic
        fun testCases() = listOf(
            TestCase("simple words", "Hello World", "hello-world"),
            TestCase("special chars", "Hello, World!", "hello-world"),
            TestCase("extra spaces", "  too   many   spaces  ", "too-many-spaces"),
            TestCase("already a slug", "hello-world", "hello-world"),
            TestCase("mixed case", "CamelCaseString", "camelcasestring"),
        ).map { Arguments.of(it) }
    }
}

MockK: Kotlin-native mocking

MockK is the standard mocking library for Kotlin. It’s designed for Kotlin’s language features (extension functions, coroutines, companion objects) in a way that Mockito cannot match.

Feature	Mockito	MockK
Kotlin-first DSL	No	Yes
Extension functions	Partial	Full support
Coroutine support	Via mockito-kotlin	Native
Companion object mocking	No	Yes
Top-level function mocking	No	Yes
Relaxed mocks	No	Yes

Basic mocking

The shape is the same everywhere: build a fake repository, inject it into the service, assert on behavior. MockK creates the fake from an interface with mockk<UserRepository>():

TypeScript

const userRepo = {
  findById: jest.fn().mockResolvedValue({ id: '1', name: 'Alice' }),
  save: jest.fn().mockResolvedValue(undefined),
};

const service = new UserService(userRepo);

Go

type mockUserRepo struct {
    findByIDFunc func(id string) (*User, error)
}

func (m *mockUserRepo) FindByID(id string) (*User, error) {
    return m.findByIDFunc(id)
}

Kotlin

import io.mockk.*
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*

interface UserRepository {
    fun findById(id: String): User?
    fun save(user: User): User
    fun delete(id: String)
    fun findAll(): List<User>
}

data class User(val id: String, val name: String, val email: String)

class UserService(private val repo: UserRepository) {
    fun getUser(id: String): User =
        repo.findById(id) ?: throw NoSuchElementException("User not found: $id")

    fun createUser(name: String, email: String): User {
        val user = User(id = java.util.UUID.randomUUID().toString(), name = name, email = email)
        return repo.save(user)
    }
}

class UserServiceTest {

    private val repo = mockk<UserRepository>()
    private val service = UserService(repo)

    @Test
    fun `returns user when found`() {
        // Arrange: define behavior
        val alice = User("1", "Alice", "alice@test.com")
        every { repo.findById("1") } returns alice

        // Act
        val result = service.getUser("1")

        // Assert
        assertEquals("Alice", result.name)

        // Verify the mock was called
        verify(exactly = 1) { repo.findById("1") }
    }

    @Test
    fun `throws when user not found`() {
        every { repo.findById("999") } returns null

        assertThrows<NoSuchElementException> {
            service.getUser("999")
        }
    }
}

Stubbing: every / returns / throws / answers

The every { … } returns … block is MockK’s core stubbing DSL. Variants cover exceptions, sequences, custom logic, and void functions:

class MockBehaviorTest {

    private val repo = mockk<UserRepository>()

    @Test
    fun `throws an exception`() {
        every { repo.findById("bad") } throws IllegalArgumentException("Invalid ID")
    }

    @Test
    fun `returns different values on consecutive calls`() {
        every { repo.findById("1") } returnsMany listOf(
            User("1", "Alice", "a@b.com"),
            User("1", "Alice Updated", "a@b.com")
        )

        assertEquals("Alice", repo.findById("1")!!.name)
        assertEquals("Alice Updated", repo.findById("1")!!.name)
    }

    @Test
    fun `custom answer logic`() {
        every { repo.save(any()) } answers {
            val user = firstArg<User>()
            user.copy(id = "generated-id")
        }

        val saved = repo.save(User("", "Bob", "bob@test.com"))
        assertEquals("generated-id", saved.id)
    }

    @Test
    fun `void functions -- use just Runs`() {
        every { repo.delete(any()) } just Runs

        repo.delete("1")  // no exception

        verify { repo.delete("1") }
    }
}

Argument matchers and capturing

Use any() or match { … } to match arguments loosely, and a slot<User>() to capture exactly what was passed:

@Test
fun `argument matchers`() {
    every { repo.findById(any()) } returns User("x", "Any", "any@test.com")
    every { repo.findById(match { it.startsWith("user-") }) } returns User("u", "Matched", "m@test.com")

    assertEquals("Any", repo.findById("random")!!.name)
    assertEquals("Matched", repo.findById("user-123")!!.name)
}

@Test
fun `captures arguments with slot`() {
    val slot = slot<User>()
    every { repo.save(capture(slot)) } answers { slot.captured }

    service.createUser("Charlie", "charlie@test.com")

    // Inspect what was passed to repo.save()
    assertEquals("Charlie", slot.captured.name)
    assertEquals("charlie@test.com", slot.captured.email)
    assertTrue(slot.captured.id.isNotEmpty())
}

To capture multiple calls, pass a mutableListOf<User>() to capture(...) instead of a single slot.

Verification

@Test
fun `verification options`() {
    val repo = mockk<UserRepository>()
    every { repo.findById(any()) } returns null
    every { repo.findAll() } returns emptyList()
    every { repo.save(any()) } answers { firstArg() }

    repo.findById("1")
    repo.findById("2")
    repo.findAll()

    // Verify call count
    verify(exactly = 2) { repo.findById(any()) }
    verify(exactly = 1) { repo.findAll() }
    verify(exactly = 0) { repo.save(any()) }

    // Verify call order
    verifyOrder {
        repo.findById("1")
        repo.findById("2")
        repo.findAll()
    }

    // Verify no other calls were made
    confirmVerified(repo)
}

Relaxed mocks

A relaxed mock returns default values (0, “”, empty list, null) for any unstubbed call, instead of throwing. This is useful for tests where you only care about some interactions:

@Test
fun `relaxed mock returns defaults`() {
    val repo = mockk<UserRepository>(relaxed = true)

    // These all work without `every` -- they return defaults
    val user = repo.findById("1")    // returns null (nullable return)
    val users = repo.findAll()       // returns emptyList()
    repo.delete("1")                 // just runs

    assertNull(user)
    assertTrue(users.isEmpty())
}

relaxUnitFun = true only relaxes Unit-returning functions: with mockk<UserRepository>(relaxUnitFun = true), repo.delete("1") works without stubbing, but repo.findById("1") still throws — it returns a value type.

Spying (spyk)

A spy wraps a real object and lets you override specific methods:

TypeScript

const service = new UserService();
jest.spyOn(service, 'getUser').mockReturnValue({ id: '1', name: 'Alice' });

Kotlin

class EmailService {
    fun sendWelcome(email: String): Boolean {
        // Actually sends email -- we don't want this in tests
        return sendEmail(email, "Welcome!", "Hello!")
    }

    fun sendEmail(to: String, subject: String, body: String): Boolean {
        println("Sending email to $to")
        return true
    }
}

@Test
fun `spy overrides specific methods`() {
    val emailService = spyk(EmailService())

    // Override sendEmail but let sendWelcome call real logic
    every { emailService.sendEmail(any(), any(), any()) } returns true

    val result = emailService.sendWelcome("alice@test.com")

    assertTrue(result)
    verify { emailService.sendEmail("alice@test.com", "Welcome!", "Hello!") }
}

Mocking companion objects and coroutines

MockK can mock things Mockito can’t — companion objects with mockkObject, and suspending functions with coEvery / coVerify:

class IdGenerator {
    companion object {
        fun generate(): String = java.util.UUID.randomUUID().toString()
    }
}

@Test
fun `mock companion object`() {
    mockkObject(IdGenerator.Companion)
    every { IdGenerator.generate() } returns "fixed-id"

    assertEquals("fixed-id", IdGenerator.generate())

    unmockkObject(IdGenerator.Companion)
}

interface AsyncUserRepository {
    suspend fun findById(id: String): User?
    suspend fun save(user: User): User
}

@Test
fun `mock suspending functions`() = runTest {
    val repo = mockk<AsyncUserRepository>()

    coEvery { repo.findById("1") } returns User("1", "Alice", "a@b.com")
    coEvery { repo.save(any()) } coAnswers { firstArg() }

    val user = repo.findById("1")
    assertEquals("Alice", user!!.name)

    coVerify { repo.findById("1") }
}

coEvery and coVerify are the suspend equivalents of every and verify.

Clean up between tests

Call clearAllMocks() in an @AfterEach to clear recorded calls and stubbing. For static/companion mocks, unmockkAll() restores original behavior.

Kotest: BDD and property testing

Kotest is a Kotlin-native test framework that provides BDD-style specs (like Jest/Mocha) and powerful property-based testing (like fast-check or Go’s rapid).

Spec styles

Kotest offers multiple spec styles — pick the one that matches your team’s preference. StringSpec is the simplest, DescribeSpec reads exactly like Jest, and BehaviorSpec gives you Given/When/Then BDD:

StringSpec

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.shouldBe
import io.kotest.matchers.string.shouldHaveLength

class StringSpecExample : StringSpec({

    "string length should be correct" {
        "hello".length shouldBe 5
    }

    "string should have expected length" {
        "kotlin" shouldHaveLength 6
    }
})

DescribeSpec

import io.kotest.core.spec.style.DescribeSpec
import io.kotest.matchers.shouldBe
import io.kotest.matchers.shouldNotBe

class UserServiceDescribeSpec : DescribeSpec({

    // This reads exactly like Jest!
    describe("UserService") {
        val service = UserService(InMemoryUserRepo())

        describe("createUser") {
            it("should create a user with valid data") {
                val user = service.createUser("Alice", "alice@test.com")
                user.name shouldBe "Alice"
                user.id shouldNotBe null
            }

            it("should reject empty name") {
                shouldThrow<IllegalArgumentException> {
                    service.createUser("", "alice@test.com")
                }
            }
        }

        describe("getUser") {
            it("should return null for unknown user") {
                service.getUser("unknown") shouldBe null
            }
        }
    }
})

BehaviorSpec

import io.kotest.core.spec.style.BehaviorSpec
import io.kotest.matchers.shouldBe

class ShoppingCartBehaviorSpec : BehaviorSpec({

    Given("an empty shopping cart") {
        val cart = ShoppingCart()

        When("adding an item") {
            cart.add(Item("Widget", 9.99))

            Then("cart should have 1 item") {
                cart.itemCount shouldBe 1
            }

            Then("total should be item price") {
                cart.total shouldBe 9.99
            }
        }

        When("checking out with no items") {
            Then("should throw EmptyCartException") {
                shouldThrow<EmptyCartException> {
                    ShoppingCart().checkout()
                }
            }
        }
    }
})

Property-based testing

Property testing generates random inputs to verify that properties hold for all inputs, not just the ones you think of. This is like fast-check in TypeScript or rapid in Go:

TypeScript

import fc from 'fast-check';

test('reverse of reverse is identity', () => {
  fc.assert(
    fc.property(fc.array(fc.integer()), (arr) => {
      expect(arr.reverse().reverse()).toEqual(arr);
    })
  );
});

Kotlin

import io.kotest.core.spec.style.StringSpec
import io.kotest.property.forAll
import io.kotest.property.Arb
import io.kotest.property.arbitrary.*
import io.kotest.property.checkAll
import io.kotest.matchers.shouldBe

class PropertyTestExamples : StringSpec({

    "reverse of reverse is identity" {
        forAll<List<Int>> { list ->
            list.reversed().reversed() == list
        }
    }

    "string concatenation length" {
        forAll<String, String> { a, b ->
            (a + b).length == a.length + b.length
        }
    }

    "absolute value is always non-negative" {
        forAll(Arb.int()) { n ->
            kotlin.math.abs(n.toLong()) >= 0
        }
    }

    "parsed email has valid parts" {
        checkAll(Arb.email()) { email ->
            val parts = email.split("@")
            parts.size shouldBe 2
            parts[0].isNotEmpty() shouldBe true
            parts[1].contains(".") shouldBe true
        }
    }
})

// Custom Arb (Arbitrary) for domain objects
fun Arb.Companion.email(): Arb<String> =
    Arb.bind(
        Arb.string(minSize = 1, maxSize = 10, codepoints = Arb.az()),
        Arb.string(minSize = 1, maxSize = 5, codepoints = Arb.az()),
        Arb.of("com", "org", "net", "io")
    ) { local, domain, tld ->
        "$local@$domain.$tld"
    }

fun Arb.Companion.az() = Arb.of(('a'..'z').map { Codepoint(it.code) })

The key generators are forAll (returns a Boolean property) and checkAll (runs assertions inside the block). Arb (Arbitrary) describes how to generate values, and you can compose your own for domain objects:

import io.kotest.property.Arb
import io.kotest.property.arbitrary.*

data class Money(val amount: Long, val currency: String) {
    init {
        require(amount >= 0) { "Amount must be non-negative" }
    }

    operator fun plus(other: Money): Money {
        require(currency == other.currency) { "Currency mismatch" }
        return Money(amount + other.amount, currency)
    }
}

fun Arb.Companion.money(currency: String = "USD"): Arb<Money> =
    Arb.long(0L..1_000_000L).map { Money(it, currency) }

class MoneyPropertyTest : StringSpec({

    "addition is commutative" {
        forAll(Arb.money(), Arb.money()) { a, b ->
            a + b == b + a
        }
    }

    "addition with zero is identity" {
        forAll(Arb.money()) { money ->
            money + Money(0, "USD") == money
        }
    }

    "addition is associative" {
        forAll(Arb.money(), Arb.money(), Arb.money()) { a, b, c ->
            (a + b) + c == a + (b + c)
        }
    }
})

Testcontainers: Docker-based integration tests

Testcontainers spins up real Docker containers (PostgreSQL, Redis, Kafka, etc.) for your tests. No more mocking databases — test against the real thing. Each container’s lifecycle is bound to the test run:

Testcontainers lifecycle

Rendering diagram…

For comparison, this is the equivalent in testcontainers-go:

container, err := testcontainers.GenericContainer(ctx, testcontainers.GenericContainerRequest{
    ContainerRequest: testcontainers.ContainerRequest{
        Image:        "postgres:16",
        ExposedPorts: []string{"5432/tcp"},
        WaitingFor:   wait.ForListeningPort("5432/tcp"),
    },
    Started: true,
})

PostgreSQL testcontainer

The @Testcontainers annotation manages the container lifecycle; @Container on a @JvmStatic field starts one container shared by all tests in the class:

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*
import org.testcontainers.containers.PostgreSQLContainer
import org.testcontainers.junit.jupiter.Container
import org.testcontainers.junit.jupiter.Testcontainers
import java.sql.DriverManager

@Testcontainers
class PostgresIntegrationTest {

    companion object {
        @Container
        @JvmStatic
        val postgres = PostgreSQLContainer("postgres:16-alpine").apply {
            withDatabaseName("testdb")
            withUsername("test")
            withPassword("test")
        }
    }

    @Test
    fun `can connect and query`() {
        val conn = DriverManager.getConnection(
            postgres.jdbcUrl,
            postgres.username,
            postgres.password
        )

        conn.createStatement().execute("""
            CREATE TABLE IF NOT EXISTS users (
                id SERIAL PRIMARY KEY,
                name TEXT NOT NULL,
                email TEXT UNIQUE NOT NULL
            )
        """)

        conn.prepareStatement("INSERT INTO users (name, email) VALUES (?, ?)").apply {
            setString(1, "Alice")
            setString(2, "alice@test.com")
            executeUpdate()
        }

        val rs = conn.createStatement().executeQuery("SELECT name FROM users WHERE email = 'alice@test.com'")
        assertTrue(rs.next())
        assertEquals("Alice", rs.getString("name"))

        conn.close()
    }
}

Redis and Kafka testcontainers

Redis uses the GenericContainer with an exposed port; Kafka has a dedicated KafkaContainer. Both expose connection details via mapped ports:

@Testcontainers
class RedisIntegrationTest {

    companion object {
        @Container
        @JvmStatic
        val redis = GenericContainer("redis:7-alpine").apply {
            withExposedPorts(6379)
        }
    }

    @Test
    fun `stores and retrieves values`() {
        val jedis = Jedis(redis.host, redis.getMappedPort(6379))
        jedis.set("greeting", "hello")
        assertEquals("hello", jedis.get("greeting"))
        jedis.close()
    }
}

@Testcontainers
class KafkaIntegrationTest {

    companion object {
        @Container
        @JvmStatic
        val kafka = KafkaContainer("apache/kafka:3.8.1")
    }

    @Test
    fun `produces and consumes messages`() {
        val topic = "test-topic"

        val producerProps = Properties().apply {
            put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafka.bootstrapServers)
            put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer::class.java.name)
            put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer::class.java.name)
        }
        val producer = KafkaProducer<String, String>(producerProps)
        producer.send(ProducerRecord(topic, "key", "hello-kafka")).get()
        producer.close()

        val consumerProps = Properties().apply {
            put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, kafka.bootstrapServers)
            put(ConsumerConfig.GROUP_ID_CONFIG, "test-group")
            put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest")
            put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer::class.java.name)
            put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer::class.java.name)
        }
        val consumer = KafkaConsumer<String, String>(consumerProps)
        consumer.subscribe(listOf(topic))

        val records = consumer.poll(Duration.ofSeconds(10))
        assertEquals(1, records.count())
        assertEquals("hello-kafka", records.first().value())

        consumer.close()
    }
}

Reusable containers (faster tests)

Starting containers for every test class is slow. Share them across test classes by holding the container in an object with withReuse(true):

src/test/kotlin/testutil/TestContainers.kt

object TestPostgres {
    val container = PostgreSQLContainer("postgres:16-alpine").apply {
        withDatabaseName("testdb")
        withUsername("test")
        withPassword("test")
        withReuse(true)  // Keep container running between test runs
        start()
    }
}

Then add this to ~/.testcontainers.properties to enable reuse:

~/.testcontainers.properties

testcontainers.reuse.enable=true

Spring Boot testing

Spring Boot provides comprehensive test infrastructure with test slices that load only the parts of the application you need:

Annotation	What it loads	Use for
@SpringBootTest	Full application context	Integration / E2E tests
@WebMvcTest	Controllers + MVC infra	Controller unit tests
@DataJpaTest	JPA + embedded DB	Repository tests
@WebFluxTest	WebFlux controllers	Reactive controller tests
None (plain JUnit)	Nothing	Unit tests (preferred)

Full integration test with @SpringBootTest

@SpringBootTest plus @AutoConfigureMockMvc boots the full context and gives you a MockMvc to drive HTTP requests:

import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.http.MediaType
import org.springframework.test.web.servlet.MockMvc
import org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*
import org.springframework.test.web.servlet.result.MockMvcResultMatchers.*

@SpringBootTest
@AutoConfigureMockMvc
class TaskControllerIntegrationTest {

    @Autowired
    lateinit var mockMvc: MockMvc

    @Test
    fun `creates and retrieves a task`() {
        val createResult = mockMvc.perform(
            post("/api/tasks")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""{"title": "Write tests", "description": "Add test coverage"}""")
        )
            .andExpect(status().isCreated)
            .andExpect(jsonPath("$.title").value("Write tests"))
            .andReturn()

        val id = com.jayway.jsonpath.JsonPath.read<Int>(
            createResult.response.contentAsString, "$.id"
        )

        mockMvc.perform(get("/api/tasks/$id"))
            .andExpect(status().isOk)
            .andExpect(jsonPath("$.title").value("Write tests"))
    }

    @Test
    fun `returns 404 for unknown task`() {
        mockMvc.perform(get("/api/tasks/99999"))
            .andExpect(status().isNotFound)
    }
}

Controller unit test with @WebMvcTest

@WebMvcTest loads only the controller layer. You mock the service layer with @MockkBean (from springmockk):

import io.mockk.every
import io.mockk.verify
import com.ninjasquad.springmockk.MockkBean
import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest
import org.springframework.test.web.servlet.MockMvc
import org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*
import org.springframework.test.web.servlet.result.MockMvcResultMatchers.*

@WebMvcTest(TaskController::class)
class TaskControllerUnitTest {

    @Autowired
    lateinit var mockMvc: MockMvc

    @MockkBean
    lateinit var taskService: TaskService

    @Test
    fun `returns task by id`() {
        val task = Task(id = 1, title = "Test", description = "A task", completed = false)
        every { taskService.findById(1) } returns task

        mockMvc.perform(get("/api/tasks/1"))
            .andExpect(status().isOk)
            .andExpect(jsonPath("$.title").value("Test"))
            .andExpect(jsonPath("$.completed").value(false))

        verify(exactly = 1) { taskService.findById(1) }
    }
}

Note

Use springmockk (com.ninjasquad:springmockk) instead of Spring’s @MockBean, which uses Mockito. The @MockkBean annotation integrates MockK with Spring’s test context.

Repository test with @DataJpaTest

@DataJpaTest configures an embedded DB and gives you a TestEntityManager to set up fixtures:

import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest
import org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager

@DataJpaTest
class TaskRepositoryTest {

    @Autowired
    lateinit var entityManager: TestEntityManager

    @Autowired
    lateinit var repository: TaskRepository

    @Test
    fun `finds tasks by completed status`() {
        entityManager.persist(TaskEntity(title = "Done", description = "d", completed = true))
        entityManager.persist(TaskEntity(title = "Todo", description = "d", completed = false))
        entityManager.persist(TaskEntity(title = "Also done", description = "d", completed = true))
        entityManager.flush()

        val completed = repository.findByCompleted(true)
        assertEquals(2, completed.size)
        assertTrue(completed.all { it.completed })
    }
}

Spring Boot + Testcontainers

The most common integration pattern — use a real PostgreSQL database, wiring the container’s connection details into Spring with @DynamicPropertySource:

@SpringBootTest
@AutoConfigureMockMvc
@Testcontainers
class TaskApiIntegrationTest {

    companion object {
        @Container
        @JvmStatic
        val postgres = PostgreSQLContainer("postgres:16-alpine").apply {
            withDatabaseName("testdb")
            withUsername("test")
            withPassword("test")
        }

        @JvmStatic
        @DynamicPropertySource
        fun configureProperties(registry: DynamicPropertyRegistry) {
            registry.add("spring.datasource.url") { postgres.jdbcUrl }
            registry.add("spring.datasource.username") { postgres.username }
            registry.add("spring.datasource.password") { postgres.password }
            registry.add("spring.jpa.hibernate.ddl-auto") { "create-drop" }
        }
    }

    @Autowired
    lateinit var mockMvc: MockMvc

    @Test
    fun `full CRUD cycle against real PostgreSQL`() {
        // Create
        val json = """{"title": "Integration test", "description": "Testing with real DB"}"""
        val result = mockMvc.perform(
            post("/api/tasks")
                .contentType(MediaType.APPLICATION_JSON)
                .content(json)
        )
            .andExpect(status().isCreated)
            .andExpect(jsonPath("$.id").exists())
            .andReturn()

        val id = com.jayway.jsonpath.JsonPath.read<Int>(
            result.response.contentAsString, "$.id"
        )

        // Read / Update / Delete
        mockMvc.perform(get("/api/tasks/$id"))
            .andExpect(status().isOk)
            .andExpect(jsonPath("$.title").value("Integration test"))

        mockMvc.perform(
            put("/api/tasks/$id")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""{"title": "Updated", "description": "Updated desc", "completed": true}""")
        )
            .andExpect(status().isOk)
            .andExpect(jsonPath("$.completed").value(true))

        mockMvc.perform(delete("/api/tasks/$id"))
            .andExpect(status().isNoContent)

        mockMvc.perform(get("/api/tasks/$id"))
            .andExpect(status().isNotFound)
    }
}

To avoid duplicating container setup, extract it into an abstract base class that your test classes extend:

src/test/kotlin/testutil/IntegrationTestBase.kt

abstract class IntegrationTestBase {

    companion object {
        @JvmStatic
        val postgres = PostgreSQLContainer("postgres:16-alpine").apply {
            withDatabaseName("testdb")
            withUsername("test")
            withPassword("test")
            start()
        }

        @JvmStatic
        @DynamicPropertySource
        fun configureProperties(registry: DynamicPropertyRegistry) {
            registry.add("spring.datasource.url") { postgres.jdbcUrl }
            registry.add("spring.datasource.username") { postgres.username }
            registry.add("spring.datasource.password") { postgres.password }
            registry.add("spring.jpa.hibernate.ddl-auto") { "create-drop" }
        }
    }
}

WebTestClient (alternative to MockMvc)

WebTestClient provides a fluent API and works with both MVC and WebFlux:

import org.springframework.boot.test.context.SpringBootTest.WebEnvironment
import org.springframework.test.web.reactive.server.WebTestClient

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class TaskApiWebTestClientTest {

    @Autowired
    lateinit var webClient: WebTestClient

    @Test
    fun `creates a task`() {
        webClient.post()
            .uri("/api/tasks")
            .contentType(org.springframework.http.MediaType.APPLICATION_JSON)
            .bodyValue("""{"title": "Test", "description": "desc"}""")
            .exchange()
            .expectStatus().isCreated
            .expectBody()
            .jsonPath("$.title").isEqualTo("Test")
    }
}

Ktor testing

Ktor provides a built-in test engine that runs your application in-process without starting a real HTTP server. This is fast and simple — comparable to Go’s httptest:

req := httptest.NewRequest("GET", "/api/tasks", nil)
w := httptest.NewRecorder()
handler.ServeHTTP(w, req)
assert.Equal(t, 200, w.Code)

testApplication

The testApplication { … } builder configures your app and exposes a client for making requests:

import io.ktor.client.request.*
import io.ktor.client.statement.*
import io.ktor.http.*
import io.ktor.server.testing.*
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*

class TaskRoutesTest {

    @Test
    fun `GET tasks returns empty list`() = testApplication {
        application {
            configureSerialization()
            configureRouting()
        }

        val response = client.get("/api/tasks")
        assertEquals(HttpStatusCode.OK, response.status)
        assertEquals("[]", response.bodyAsText())
    }

    @Test
    fun `POST creates a task`() = testApplication {
        application {
            configureSerialization()
            configureRouting()
        }

        val response = client.post("/api/tasks") {
            contentType(ContentType.Application.Json)
            setBody("""{"title": "Test task", "description": "A test"}""")
        }

        assertEquals(HttpStatusCode.Created, response.status)
        assertTrue(response.bodyAsText().contains("Test task"))
    }

    @Test
    fun `GET unknown task returns 404`() = testApplication {
        application {
            configureSerialization()
            configureRouting()
        }

        val response = client.get("/api/tasks/99999")
        assertEquals(HttpStatusCode.NotFound, response.status)
    }
}

Typed clients, auth, and Testcontainers

Install ContentNegotiation on a custom client to send and receive typed bodies with body<TaskResponse>():

import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.json.*

@Test
fun `create and retrieve task with typed client`() = testApplication {
    application {
        configureSerialization()
        configureRouting()
    }

    val jsonClient = createClient {
        install(ContentNegotiation) { json() }
    }

    val created = jsonClient.post("/api/tasks") {
        contentType(ContentType.Application.Json)
        setBody(CreateTaskRequest(title = "Learn Ktor", description = "Testing module"))
    }
    assertEquals(HttpStatusCode.Created, created.status)

    val task = created.body<TaskResponse>()
    assertEquals("Learn Ktor", task.title)
}

Authenticated endpoints are tested by sending the Authorization header, and a database-backed app can wire in a Testcontainers Postgres:

@Test
fun `protected endpoint requires auth`() = testApplication {
    application {
        configureSerialization()
        configureAuth()
        configureRouting()
    }

    val unauthed = client.get("/api/admin/users")
    assertEquals(HttpStatusCode.Unauthorized, unauthed.status)

    val authed = client.get("/api/admin/users") {
        header(HttpHeaders.Authorization, "Bearer ${generateTestToken()}")
    }
    assertEquals(HttpStatusCode.OK, authed.status)
}

Coroutine testing

Testing coroutines requires special support to control virtual time and concurrency. The problem: regular runBlocking waits real time for delay() calls. runTest skips delays automatically.

runTest basics

import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.delay
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*

class CoroutineTestExamples {

    suspend fun fetchWithRetry(fetch: suspend () -> String, retries: Int = 3): String {
        repeat(retries) { attempt ->
            try {
                return fetch()
            } catch (e: Exception) {
                if (attempt == retries - 1) throw e
                delay(1000L * (attempt + 1))  // Exponential backoff
            }
        }
        throw IllegalStateException("Unreachable")
    }

    @Test
    fun `retries on failure then succeeds`() = runTest {
        var attempts = 0
        val result = fetchWithRetry(
            fetch = {
                attempts++
                if (attempts < 3) throw RuntimeException("fail")
                "success after retries"
            }
        )
        assertEquals("success after retries", result)
        assertEquals(3, attempts)
        // Note: delays were skipped -- this test runs instantly
    }

    @Test
    fun `throws after all retries exhausted`() = runTest {
        assertThrows<RuntimeException> {
            fetchWithRetry(
                fetch = { throw RuntimeException("always fails") },
                retries = 3
            )
        }
    }
}

Here the function under test takes a suspend () -> String lambda — the retry delays are skipped by runTest, so the test completes instantly.

TestScope and virtual time

runTest gives you a TestScope with virtual-time controls: advanceTimeBy, runCurrent, and advanceUntilIdle:

import kotlinx.coroutines.test.*
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch

class VirtualTimeTest {

    @Test
    fun `controls virtual time`() = runTest {
        var value = 0

        launch {
            delay(1000)
            value = 1
            delay(1000)
            value = 2
        }

        assertEquals(0, value)   // time hasn't advanced yet

        advanceTimeBy(1000)
        runCurrent()
        assertEquals(1, value)

        advanceTimeBy(1000)
        runCurrent()
        assertEquals(2, value)
    }

    @Test
    fun `advanceUntilIdle completes all work`() = runTest {
        val results = mutableListOf<Int>()

        launch { delay(100); results.add(1) }
        launch { delay(200); results.add(2) }
        launch { delay(300); results.add(3) }

        advanceUntilIdle()

        assertEquals(listOf(1, 2, 3), results)
    }
}

Testing flows

Flows collect inside runTest like any other suspending code. For infinite flows, combine take(n) with toList():

import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.runTest

class FlowTestExamples {

    fun numberFlow(): Flow<Int> = flow {
        emit(1); emit(2); emit(3)
    }

    @Test
    fun `collects all values from flow`() = runTest {
        val values = numberFlow().toList()
        assertEquals(listOf(1, 2, 3), values)
    }

    @Test
    fun `tests flow transformations`() = runTest {
        val doubled = numberFlow()
            .map { it * 2 }
            .filter { it > 2 }
            .toList()
        assertEquals(listOf(4, 6), doubled)
    }
}

Turbine for flows

The Turbine library (app.cash.turbine:turbine) gives a nicer API for asserting on emissions one at a time — awaitItem(), awaitComplete(), etc. — inside a flow.test { … } block.

Injecting test dispatchers

When your code uses specific dispatchers, inject a TestDispatcher so virtual time controls it:

import kotlinx.coroutines.*
import kotlinx.coroutines.test.*

class UserService(private val dispatcher: CoroutineDispatcher = Dispatchers.IO) {
    suspend fun loadUser(id: String): User = withContext(dispatcher) {
        delay(1000)  // simulate slow I/O
        User(id, "User $id")
    }
}

class UserServiceTest {

    @Test
    fun `loads user with test dispatcher`() = runTest {
        val testDispatcher = StandardTestDispatcher(testScheduler)
        val service = UserService(dispatcher = testDispatcher)

        val deferred = async { service.loadUser("1") }
        advanceUntilIdle()

        val user = deferred.await()
        assertEquals("User 1", user.name)
    }
}

Test organization and code coverage

A common convention is to split fast unit tests from slow integration tests by directory:

• Directorysrc/

  • Directorymain/kotlin/com/example/

    • Directorymodel/

      • …
    • Directoryrepository/

      • …
    • Directoryservice/

      • …
    • Directorycontroller/

      • …
  • Directorytest/kotlin/com/example/

    • Directoryunit/ Fast, no external deps

      • Directoryservice/

        • UserServiceTest.kt
        • TaskServiceTest.kt
    • Directoryintegration/ Testcontainers, real DB

      • Directoryrepository/

        • UserRepositoryTest.kt
      • Directoryapi/

        • TaskApiIntegrationTest.kt
    • Directorytestutil/ Shared test helpers

      • IntegrationTestBase.kt
      • TestFixtures.kt

Separating unit and integration tests with Gradle

Create a separate source set and Gradle task so integration tests run on demand:

build.gradle.kts

sourceSets {
    create("integrationTest") {
        kotlin.srcDir("src/integrationTest/kotlin")
        resources.srcDir("src/integrationTest/resources")
        compileClasspath += sourceSets["main"].output + sourceSets["test"].output
        runtimeClasspath += sourceSets["main"].output + sourceSets["test"].output
    }
}

configurations["integrationTestImplementation"].extendsFrom(configurations["testImplementation"])
configurations["integrationTestRuntimeOnly"].extendsFrom(configurations["testRuntimeOnly"])

tasks.register<Test>("integrationTest") {
    description = "Runs integration tests."
    group = "verification"
    testClassesDirs = sourceSets["integrationTest"].output.classesDirs
    classpath = sourceSets["integrationTest"].runtimeClasspath
    useJUnitPlatform()
    shouldRunAfter(tasks.test)
}

tasks.check {
    dependsOn(tasks.named("integrationTest"))
}

Tagging tests (alternative to separate source sets)

Tag classes with @Tag(...), then include or exclude tags at run time:

import org.junit.jupiter.api.Tag

@Tag("unit")
class UserServiceTest { /* ... */ }

@Tag("integration")
class UserApiIntegrationTest { /* ... */ }

@Tag("slow")
class FullE2ETest { /* ... */ }

./gradlew test -PincludeTags=unit   # only unit tests
./gradlew test -PexcludeTags=slow   # skip slow tests

build.gradle.kts

tasks.test {
    useJUnitPlatform {
        val includeTags = project.findProperty("includeTags") as String?
        val excludeTags = project.findProperty("excludeTags") as String?
        includeTags?.let { includeTags(it) }
        excludeTags?.let { excludeTags(it) }
    }
}

JaCoCo code coverage

JaCoCo is the JVM’s coverage tool (the equivalent of go test -cover or Jest’s --coverage). Apply the plugin and wire reports onto the test task:

build.gradle.kts

plugins {
    jacoco
}

tasks.test {
    useJUnitPlatform()
    finalizedBy(tasks.jacocoTestReport)
}

tasks.jacocoTestReport {
    dependsOn(tasks.test)
    reports {
        xml.required.set(true)   // For CI tools
        html.required.set(true)  // For humans
        csv.required.set(false)
    }
}

tasks.jacocoTestCoverageVerification {
    violationRules {
        rule {
            limit { minimum = "0.80".toBigDecimal() }  // 80% line coverage
        }
        rule {
            element = "CLASS"
            excludes = listOf("*.Application*", "*.config.*")
            limit {
                counter = "BRANCH"
                minimum = "0.70".toBigDecimal()
            }
        }
    }
}

build/reports/jacoco/test/html/index.html

./gradlew test jacocoTestReport
./gradlew jacocoTestCoverageVerification
# Fails build if coverage is below threshold

Test fixtures and factories

Centralize test-data construction in factory functions with sensible defaults — the name: String = "Test User" defaults let each test override only what it cares about:

src/test/kotlin/testutil/TestFixtures.kt

object TestFixtures {
    fun user(
        id: String = "user-${java.util.UUID.randomUUID()}",
        name: String = "Test User",
        email: String = "$id@test.com"
    ) = User(id = id, name = name, email = email)

    fun task(
        id: Long = 0,
        title: String = "Test Task",
        description: String = "A test task",
        completed: Boolean = false
    ) = Task(id = id, title = title, description = description, completed = completed)
}

Summary

Concept	TypeScript	Go	Kotlin/JVM
Test framework	Jest / Vitest	go test	JUnit 5
Assertions	expect().toBe()	if/t.Error	assertEquals / Kotest shouldBe
Mocking	jest.mock()	interfaces + manual	MockK mockk<T>()
BDD style	describe/it	t.Run() subtests	Kotest DescribeSpec / BehaviorSpec
Table tests	test.each	[]struct{} loop	@ParameterizedTest / @MethodSource
Property testing	fast-check	rapid	Kotest forAll / checkAll
Docker tests	testcontainers-node	testcontainers-go	Testcontainers @Container
Controller tests	supertest	httptest	MockMvc / testApplication
Coroutine tests	—	—	runTest / TestScope
Coverage	--coverage	go test -cover	JaCoCo

Practice

Put the testing stack to work — write a full suite against a real database, then prove invariants with generated inputs.

Spring Boot REST API Test SuiteTest a Spring Boot CRUD API against a real PostgreSQL container with Testcontainers, mocking the service layer with MockK and driving HTTP with MockMvc.

Kotest Property-Based TestingVerify invariants of a Money and email-validation domain model with Kotest generators, checking properties across thousands of generated inputs.