Spring-Boot-Testing

# Spring Boot Testing

This skill provides expert guide for testing Spring Boot 4 applications with modern patterns and best practices.

Core Principles

1. **Test Pyramid**: Unit (fast) > Slice (focused) > Integration (complete)

2. **Right Tool**: Use the narrowest slice that gives you confidence

3. **AssertJ Style**: Fluent, readable assertions over verbose matchers

4. **Modern APIs**: Prefer MockMvcTester and RestTestClient over legacy alternatives

Which Test Slice?

| Scenario | Annotation | Reference |

|----------|------------|-----------|

| Controller + HTTP semantics | `@WebMvcTest` | [references/webmvctest.md](references/webmvctest.md) |

| Repository + JPA queries | `@DataJpaTest` | [references/datajpatest.md](references/datajpatest.md) |

| REST client + external APIs | `@RestClientTest` | [references/restclienttest.md](references/restclienttest.md) |

| JSON (de)serialization | `@JsonTest` | [references/test-slices-overview.md](references/test-slices-overview.md) |

| Full application | `@SpringBootTest` | [references/test-slices-overview.md](references/test-slices-overview.md) |

Test Slices Reference

- [references/test-slices-overview.md](references/test-slices-overview.md) - Decision matrix and comparison

- [references/webmvctest.md](references/webmvctest.md) - Web layer with MockMvc

- [references/datajpatest.md](references/datajpatest.md) - Data layer with Testcontainers

- [references/restclienttest.md](references/restclienttest.md) - REST client testing

Testing Tools Reference

- [references/mockmvc-tester.md](references/mockmvc-tester.md) - AssertJ-style MockMvc (3.2+)

- [references/mockmvc-classic.md](references/mockmvc-classic.md) - Traditional MockMvc (pre-3.2)

- [references/resttestclient.md](references/resttestclient.md) - Spring Boot 4+ REST client

- [references/mockitobean.md](references/mockitobean.md) - Mocking dependencies

Assertion Libraries

- [references/assertj-basics.md](references/assertj-basics.md) - Scalars, strings, booleans, dates

- [references/assertj-collections.md](references/assertj-collections.md) - Lists, Sets, Maps, arrays

Testcontainers

- [references/testcontainers-jdbc.md](references/testcontainers-jdbc.md) - PostgreSQL, MySQL, etc.

Test Data Generation

- [references/instancio.md](references/instancio.md) - Generate complex test objects (3+ properties)

Performance & Migration

- [references/context-caching.md](references/context-caching.md) - Speed up test suites

- [references/sb4-migration.md](references/sb4-migration.md) - Spring Boot 4.0 changes

Quick Decision Tree

Testing a controller endpoint?
  Yes → @WebMvcTest with MockMvcTester

Testing repository queries?
  Yes → @DataJpaTest with Testcontainers (real DB)

Testing business logic in service?
  Yes → Plain JUnit + Mockito (no Spring context)

Testing external API client?
  Yes → @RestClientTest with MockRestServiceServer

Testing JSON mapping?
  Yes → @JsonTest

Need full integration test?
  Yes → @SpringBootTest with minimal context config

Spring Boot 4 Highlights

- **RestTestClient**: Modern alternative to TestRestTemplate

- **@MockitoBean**: Replaces @MockBean (deprecated)

- **MockMvcTester**: AssertJ-style assertions for web tests

- **Modular starters**: Technology-specific test starters

- **Context pausing**: Automatic pausing of cached contexts (Spring Framework 7)

Testing Best Practices

Code Complexity Assessment

When a method or class is too complex to test effectively:

1. **Analyze complexity** - If you need more than 5-7 test cases to cover a single method, it's likely too complex

2. **Recommend refactoring** - Suggest breaking the code into smaller, focused functions

3. **User decision** - If the user agrees to refactor, help identify extraction points

4. **Proceed if needed** - If the user decides to continue with the complex code, implement tests despite the difficulty

**Example of refactoring recommendation:**

// Before: Complex method hard to