Kotlin Testing

Kotlin Testing

Testing patterns for Spring Boot 4 + Kotlin 2 projects.

When to Use

• Writing unit tests for services/​repositories
• Setting up integration tests with Testcontainers
• Mocking dependencies with MockK
• Selecting appropriate Spring test slices
• Debugging flaky or failing tests

MCP Workflow

# 1. Find existing test patterns
serena.search_for_pattern("@Test|@MockK|@SpringBootTest", relative_path="src/​test/")

# 2. Check test utilities
serena.get_symbols_overview(relative_path="src/​test/​kotlin/​.../​support/")

# 3. Find test slice usage
jetbrains.search_in_files_by_text("@DataJpaTest|@WebMvcTest", fileMask="*Test.kt")

# 4. Spring testing docs
context7.get-library-docs("/​spring/​spring-boot", "testing")

Test Slice Selection

Rule: Start with unit tests (no slice), add slices only when testing integration points.

MockK Patterns

Basic Mocking

@ExtendWith(MockKExtension::class)
class PipelineServiceTest {
    @MockK
    private lateinit var repository: PipelineRepositoryJpa

    @InjectMockKs
    private lateinit var service: PipelineService

    @Test
    fun `should create pipeline`() {
        // Arrange
        val command = CreatePipelineCommand(name = "test")
        val entity = PipelineEntity(id = 1, name = "test")
        every { repository.save(any()) } returns entity

        // Act
        val result = service.create(command)

        // Assert
        assertThat(result.name).isEqualTo("test")
        verify(exactly = 1) { repository.save(any()) }
    }
}

Relaxed Mocks (Stubs)

@MockK(relaxed = true)  // Returns sensible defaults
private lateinit var logger: Logger

// Or inline
val mockRepo = mockk<Repository>(relaxed = true)

Capturing Arguments

val slot = slot<PipelineEntity>()
every { repository.save(capture(slot)) } returns mockEntity

service.create(command)

assertThat(slot.captured.name).isEqualTo("test")

Coroutines (coEvery/​coVerify)

coEvery { asyncService.process(any()) } returns Result.success()
coVerify { asyncService.process(match { it.id == 1L }) }

Spring Test Patterns

@DataJpaTest (Repository Testing)

@DataJpaTest
@AutoConfigureTestDatabase(replace = Replace.NONE)
@Testcontainers
class UserRepositoryTest {
    companion object {
        @Container
        val mysql = MySQLContainer("mysql:8.0")
            .withDatabaseName("test")

        @DynamicPropertySource
        @JvmStatic
        fun properties(registry: DynamicPropertyRegistry) {
            registry.add("spring.datasource.url", mysql::getJdbcUrl)
            registry.add("spring.datasource.username", mysql::getUsername)
            registry.add("spring.datasource.password", mysql::getPassword)
        }
    }

    @Autowired
    private lateinit var repository: UserRepositoryJpaSpringData

    @Test
    fun `should find by email`() {
        val user = UserEntity(email = "[email protected]")
        repository.save(user)

        val found = repository.findByEmail("[email protected]")

        assertThat(found?.email).isEqualTo("[email protected]")
    }
}

@WebMvcTest (Controller Testing)

@WebMvcTest(PipelineController::class)
class PipelineControllerTest {
    @Autowired
    private lateinit var mockMvc: MockMvc

    @MockkBean
    private lateinit var pipelineService: PipelineService

    @Test
    fun `should return pipeline by id`() {
        every { pipelineService.findById(1L) } returns PipelineDto(id = 1, name = "test")

        mockMvc.get("/​api/​pipelines/​1")
            .andExpect {
                status { isOk() }
                jsonPath("$.name") { value("test") }
            }
    }
}

TDD Workflow for Kotlin

1. RED: Write Failing Test

@Test
fun `should reject duplicate pipeline names`() {
    every { repository.existsByName("existing") } returns true

    assertThrows<DuplicateNameException> {
        service.create(CreatePipelineCommand(name = "existing"))
    }
}

2. GREEN: Implement

fun create(command: CreatePipelineCommand): PipelineDto {
    if (repository.existsByName(command.name)) {
        throw DuplicateNameException("Pipeline '${command.name}' exists")
    }
    return repository.save(command.toEntity()).toDto()
}

3. REFACTOR: Improve

• Extract validation to domain
• Add more edge case tests
• Verify exception messages

Anti-Patterns

Quality Checklist

• Unit tests use no Spring context (fast)
• Integration tests use appropriate slice
• Mocks verified with verify blocks
• Testcontainers for database tests
• No Thread.sleep() (use awaitility)
• Tests follow AAA pattern (Arrange-Act-Assert)
• Test names describe behavior: should X when Y