spring-boot-testing

name spring-boot-testing description 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 Test Pyramid : Unit (fast) > Slice (focused) > Integration (complete) Right Tool : Use the narrowest slice that gives you confidence AssertJ Style : Fluent, readable assertions over verbose matchers Modern APIs : Prefer MockMvcTester and RestTestClient over legacy alternatives Which Test Slice? Scenario Annotation Reference Controller + HTTP semantics @WebMvcTest references/webmvctest.md Repository + JPA queries @DataJpaTest references/datajpatest.md REST client + external APIs @RestClientTest references/restclienttest.md JSON (de)serialization @JsonTest references/test-slices-overview.md Full application @SpringBootTest references/test-slices-overview.md Test Slices Reference references/test-slices-overview.md - Decision matrix and comparison references/webmvctest.md - Web layer with MockMvc references/datajpatest.md - Data layer with Testcontainers references/restclienttest.md - REST client testing Testing Tools Reference references/mockmvc-tester.md - AssertJ-style MockMvc (3.2+) references/mockmvc-classic.md - Traditional MockMvc (pre-3.2) references/resttestclient.md - Spring Boot 4+ REST client references/mockitobean.md - Mocking dependencies Assertion Libraries references/assertj-basics.md - Scalars, strings, booleans, dates references/assertj-collections.md - Lists, Sets, Maps, arrays Testcontainers references/testcontainers-jdbc.md - PostgreSQL, MySQL, etc. Test Data Generation references/instancio.md - Generate complex test objects (3+ properties) Performance & Migration references/context-caching.md - Speed up test suites 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: Analyze complexity - If you need more than 5-7 test cases to cover a single method, it's likely too complex Recommend refactoring - Suggest breaking the code into smaller, focused functions User decision - If the user agrees to refactor, help identify extraction points 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 test public Order processOrder (OrderRequest request) { // Validation, discount calculation, payment, inventory, notification... // 50+ lines of mixed concerns } // After: Refactored into testable units public Order processOrder (OrderRequest request) { validateOrder(request); var order = createOrder(request); applyDiscount(order); processPayment(order); updateInventory(order); sendNotification(order); return order; } 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 @DisplayName("Should calculate discount for VIP customer") void shouldCalculateDiscountForVip () { } @Test @DisplayName("Should reject order when customer has insufficient credit") void shouldRejectOrderForInsufficientCredit () { } Test Coverage Order Always structure tests in this order: Main scenario - The happy path, most common use case Other paths - Alternative valid scenarios, edge cases 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: Business-critical paths (payment processing, order validation) Complex algorithms (pricing, discount calculations) Error handling (exceptions, edge cases) Integration points (external APIs, databases) Dependencies (Spring Boot 4) < dependency > < groupId > org.springframework.boot </ groupId > < artifactId > spring-boot-starter-test </ artifactId > < scope > test </ scope > </ dependency > <!-- For WebMvc tests --> < dependency > < groupId > org.springframework.boot </ groupId > < artifactId > spring-boot-starter-webmvc-test </ artifactId > < scope > test </ scope > </ dependency > <!-- For Testcontainers --> < dependency > < groupId > org.springframework.boot </ groupId > < artifactId > spring-boot-testcontainers </ artifactId > < scope > test </ scope > </ dependency >