spring-boot-test-patterns

SKILL.md

Spring Boot Testing Patterns

Overview

Comprehensive guidance for writing robust test suites for Spring Boot applications using JUnit 5, Mockito, Testcontainers, and performance-optimized slice testing patterns.

When to Use

Writing unit tests for services or repositories with mocked dependencies

Implementing integration tests with real databases via Testcontainers

Testing REST APIs with @WebMvcTest or MockMvc

Configuring @ServiceConnection for container management in Spring Boot 3.5+

Quick Reference

Test Type
Annotation
Target Time
Use Case

Unit Tests
@ExtendWith(MockitoExtension.class)
< 50ms
Business logic without Spring context

Repository Tests
@DataJpaTest
< 100ms
Database operations with minimal context

Controller Tests
@WebMvcTest / @WebFluxTest
< 100ms
REST API layer testing

Integration Tests
@SpringBootTest
< 500ms
Full application context with containers

Testcontainers
@ServiceConnection / @Testcontainers
Varies
Real database/message broker containers

Core Concepts

Test Architecture Philosophy

Unit Tests — Fast, isolated tests without Spring context (< 50ms)

Slice Tests — Minimal Spring context for specific layers (< 100ms)

Integration Tests — Full Spring context with real dependencies (< 500ms)

Key Annotations

Spring Boot Test:

@SpringBootTest — Full application context (use sparingly)

@DataJpaTest — JPA components only (repositories, entities)

@WebMvcTest — MVC layer only (controllers, @ControllerAdvice)

@WebFluxTest — WebFlux layer only (reactive controllers)

@JsonTest — JSON serialization components only

Testcontainers:

@ServiceConnection — Wire Testcontainer to Spring Boot (3.5+)

@DynamicPropertySource — Register dynamic properties at runtime

@Testcontainers — Enable Testcontainers lifecycle management

Instructions

1. Unit Testing Pattern

Test business logic with mocked dependencies:

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Test
    void shouldFindUserByIdWhenExists() {
        when(userRepository.findById(1L)).thenReturn(Optional.of(user));
        Optional<User> result = userService.findById(1L);
        assertThat(result).isPresent();
        verify(userRepository).findById(1L);
    }
}

See unit-testing.md for advanced patterns.

2. Slice Testing Pattern

Use focused test slices for specific layers:

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@TestContainerConfig
class UserRepositoryIntegrationTest {
    @Autowired
    private UserRepository userRepository;

    @Test
    void shouldSaveAndRetrieveUser() {
        User saved = userRepository.save(user);
        assertThat(userRepository.findByEmail("test@example.com")).isPresent();
    }
}

See slice-testing.md for all slice patterns.

3. REST API Testing Pattern

Test controllers with MockMvc:

@WebMvcTest(UserController.class)
class UserControllerTest {
    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private UserService userService;

    @Test
    void shouldGetUserById() throws Exception {
        mockMvc.perform(get("/api/users/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.email").value("test@example.com"));
    }
}

4. Testcontainers with @ServiceConnection

Configure containers with Spring Boot 3.5+:

@TestConfiguration
public class TestContainerConfig {
    @Bean
    @ServiceConnection
    public PostgreSQLContainer<?> postgresContainer() {
        return new PostgreSQLContainer<>("postgres:16-alpine");
    }
}

Apply with @Import(TestContainerConfig.class) on test classes.
See testcontainers-setup.md for detailed configuration.

5. Add Dependencies

Include required testing dependencies:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.testcontainers</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>1.19.0</version>
    <scope>test</scope>
</dependency>

See test-dependencies.md for complete dependency list.

6. Configure CI/CD

Set up GitHub Actions for automated testing:

name: Tests
on: [push, pull_request]
jobs:
  test:
    runs-on: ubuntu-latest
    services:
      docker:
        image: docker:20-dind
    steps:
    - uses: actions/checkout@v4
    - name: Set up JDK 17
      uses: actions/setup-java@v4
      with:
        distribution: 'temurin'
    - name: Run tests
      run: ./mvnw test

See ci-cd-configuration.md for full CI/CD patterns.

Validation Checkpoints

After implementing tests, verify:

Container running: docker ps (look for testcontainer images)

Context loaded: check startup logs for "Started Application in X.XX seconds"

Test isolation: run tests individually and confirm no cross-contamination

Examples

Full Integration Test with @ServiceConnection

@SpringBootTest
@Import(TestContainerConfig.class)
class OrderServiceIntegrationTest {

    @Autowired
    private OrderService orderService;

    @Autowired
    private UserRepository userRepository;

    @Test
    void shouldCreateOrderForExistingUser() {
        User user = userRepository.save(User.builder()
            .email("order-test@example.com")
            .build());

        Order order = orderService.createOrder(user.getId(), List.of(
            new OrderItem("SKU-001", 2)
        ));

        assertThat(order.getId()).isNotNull();
        assertThat(order.getStatus()).isEqualTo(OrderStatus.PENDING);
    }
}

@DataJpaTest with Real Database

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@TestContainerConfig
class UserRepositoryTest {

    @Autowired
    private UserRepository userRepository;

    @Test
    void shouldFindByEmail() {
        userRepository.save(User.builder()
            .email("jpa-test@example.com")
            .build());
        assertThat(userRepository.findByEmail("jpa-test@example.com"))
            .isPresent();
    }
}

See workflow-patterns.md for complete end-to-end examples.

Best Practices

Use the right test type: @DataJpaTest for repositories, @WebMvcTest for controllers, @SpringBootTest only for full integration

Prefer @ServiceConnection on Spring Boot 3.5+ for cleaner container management over @DynamicPropertySource

Keep tests deterministic: Initialize all test data explicitly in @BeforeEach

Organize by layer: Group tests by layer to maximize context caching

Reuse Testcontainers at JVM level (withReuse(true) + TESTCONTAINERS_REUSE_ENABLE=true)

Avoid @DirtiesContext: Forces context rebuild, significantly hurts performance

Mock external services, use real databases only when necessary

Performance targets: Unit < 50ms, Slice < 100ms, Integration < 500ms

Constraints and Warnings

Never use @DirtiesContext unless absolutely necessary (forces context rebuild)

Avoid mixing @MockBean with different configurations (creates separate contexts)

Testcontainers require Docker; ensure CI/CD pipelines have Docker support

Do not rely on test execution order; each test must be independent

Be cautious with @TestPropertySource (creates separate contexts)

Do not use @SpringBootTest for unit tests; use plain Mockito instead

Context caching can be invalidated by different @MockBean configurations

Avoid static mutable state in tests (causes flaky tests)

References

test-dependencies.md — Maven/Gradle test dependencies

unit-testing.md — Unit testing with Mockito patterns

slice-testing.md — Repository, controller, and JSON slice tests

testcontainers-setup.md — Testcontainers configuration patterns

ci-cd-configuration.md — GitHub Actions, GitLab CI, Docker Compose

api-reference.md — Complete test annotations and utilities

best-practices.md — Testing patterns and optimization

workflow-patterns.md — Complete integration test examples