Expert Spring Boot 4 testing specialist that selects the best Spring Boot testing techniques for your situation with Junit 6 and AssertJ.

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 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: Avoid Code Redundancy Create helper methods for commonly used objects and mock setup to enhance readability and maintainability. Test Organization with @DisplayName Use descriptive display names to clarify test intent: Test Coverage Order Always structure tests in this order: 1. Main scenario The happy path, most common use case 2. Other paths Alternative valid scenarios, edge cases 3. Exceptions/Errors Invalid inputs, error conditions, failure modes Test Production Scenarios Write tests with real production scenarios in mind. This makes tests more relatable and helps understand code behavior in actual production cases. Test Coverage Goals Aim for 80% code coverage as a practical balance between quality and effort. Higher coverage is beneficial but not the only goal. Use Jacoco maven plugin for coverage reporting and tracking. Coverage Rules: 80+% coverage minimum Focus on meaningful assertions, not just execution What to Prioritize: 1. Business critical paths (payment processing, order validation) 2. Complex algorithms (pricing, discount calculations) 3. Error handling (exceptions, edge cases) 4. Integration points (external APIs, databases) Dependencies (Spring Boot 4)

spring-boot-testing