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feat: add spring-boot-testing skill for Spring Boot 4.0 (#1085)
- Introduced MockMvcTester for AssertJ-style assertions in Spring MVC testing. - Added @RestClientTest for testing REST clients with MockRestServiceServer. - Implemented RestTestClient as a modern alternative to TestRestTemplate. - Documented migration steps from Spring Boot 3.x to 4.0, including dependency and annotation changes. - Created an overview of test slices to guide testing strategies. - Included Testcontainers setup for JDBC testing with PostgreSQL and MySQL. - Enhanced @WebMvcTest documentation with examples for various HTTP methods and validation.
This commit is contained in:
Kartik Dhiman
GitHub
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skills/spring-boot-testing/SKILL.md
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skills/spring-boot-testing/SKILL.md
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---
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name: spring-boot-testing
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description: Expert Spring Boot 4 testing specialist that selects the best Spring Boot testing techniques for your situation with Junit 6 and AssertJ.
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---
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# Spring Boot Testing
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This skill provides expert guide for testing Spring Boot 4 applications with modern patterns and best practices.
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## Core Principles
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1. **Test Pyramid**: Unit (fast) > Slice (focused) > Integration (complete)
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2. **Right Tool**: Use the narrowest slice that gives you confidence
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3. **AssertJ Style**: Fluent, readable assertions over verbose matchers
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4. **Modern APIs**: Prefer MockMvcTester and RestTestClient over legacy alternatives
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## Which Test Slice?
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| Scenario | Annotation | Reference |
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|----------|------------|-----------|
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| Controller + HTTP semantics | `@WebMvcTest` | [references/webmvctest.md](references/webmvctest.md) |
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| Repository + JPA queries | `@DataJpaTest` | [references/datajpatest.md](references/datajpatest.md) |
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| REST client + external APIs | `@RestClientTest` | [references/restclienttest.md](references/restclienttest.md) |
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| JSON (de)serialization | `@JsonTest` | [references/test-slices-overview.md](references/test-slices-overview.md) |
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| Full application | `@SpringBootTest` | [references/test-slices-overview.md](references/test-slices-overview.md) |
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## Test Slices Reference
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- [references/test-slices-overview.md](references/test-slices-overview.md) - Decision matrix and comparison
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- [references/webmvctest.md](references/webmvctest.md) - Web layer with MockMvc
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- [references/datajpatest.md](references/datajpatest.md) - Data layer with Testcontainers
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- [references/restclienttest.md](references/restclienttest.md) - REST client testing
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## Testing Tools Reference
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- [references/mockmvc-tester.md](references/mockmvc-tester.md) - AssertJ-style MockMvc (3.2+)
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- [references/mockmvc-classic.md](references/mockmvc-classic.md) - Traditional MockMvc (pre-3.2)
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- [references/resttestclient.md](references/resttestclient.md) - Spring Boot 4+ REST client
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- [references/mockitobean.md](references/mockitobean.md) - Mocking dependencies
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## Assertion Libraries
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- [references/assertj-basics.md](references/assertj-basics.md) - Scalars, strings, booleans, dates
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- [references/assertj-collections.md](references/assertj-collections.md) - Lists, Sets, Maps, arrays
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## Testcontainers
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- [references/testcontainers-jdbc.md](references/testcontainers-jdbc.md) - PostgreSQL, MySQL, etc.
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## Test Data Generation
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- [references/instancio.md](references/instancio.md) - Generate complex test objects (3+ properties)
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## Performance & Migration
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- [references/context-caching.md](references/context-caching.md) - Speed up test suites
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- [references/sb4-migration.md](references/sb4-migration.md) - Spring Boot 4.0 changes
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## Quick Decision Tree
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```
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Testing a controller endpoint?
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Yes → @WebMvcTest with MockMvcTester
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Testing repository queries?
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Yes → @DataJpaTest with Testcontainers (real DB)
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Testing business logic in service?
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Yes → Plain JUnit + Mockito (no Spring context)
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Testing external API client?
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Yes → @RestClientTest with MockRestServiceServer
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Testing JSON mapping?
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Yes → @JsonTest
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Need full integration test?
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Yes → @SpringBootTest with minimal context config
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```
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## Spring Boot 4 Highlights
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- **RestTestClient**: Modern alternative to TestRestTemplate
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- **@MockitoBean**: Replaces @MockBean (deprecated)
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- **MockMvcTester**: AssertJ-style assertions for web tests
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- **Modular starters**: Technology-specific test starters
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- **Context pausing**: Automatic pausing of cached contexts (Spring Framework 7)
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## Testing Best Practices
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### Code Complexity Assessment
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When a method or class is too complex to test effectively:
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1. **Analyze complexity** - If you need more than 5-7 test cases to cover a single method, it's likely too complex
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2. **Recommend refactoring** - Suggest breaking the code into smaller, focused functions
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3. **User decision** - If the user agrees to refactor, help identify extraction points
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4. **Proceed if needed** - If the user decides to continue with the complex code, implement tests despite the difficulty
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**Example of refactoring recommendation:**
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```java
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// Before: Complex method hard to test
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public Order processOrder(OrderRequest request) {
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// Validation, discount calculation, payment, inventory, notification...
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// 50+ lines of mixed concerns
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}
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// After: Refactored into testable units
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public Order processOrder(OrderRequest request) {
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validateOrder(request);
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var order = createOrder(request);
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applyDiscount(order);
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processPayment(order);
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updateInventory(order);
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sendNotification(order);
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return order;
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}
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```
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### Avoid Code Redundancy
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Create helper methods for commonly used objects and mock setup to enhance readability and maintainability.
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### Test Organization with @DisplayName
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Use descriptive display names to clarify test intent:
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```java
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@Test
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@DisplayName("Should calculate discount for VIP customer")
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void shouldCalculateDiscountForVip() { }
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@Test
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@DisplayName("Should reject order when customer has insufficient credit")
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void shouldRejectOrderForInsufficientCredit() { }
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```
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### Test Coverage Order
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Always structure tests in this order:
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1. **Main scenario** - The happy path, most common use case
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2. **Other paths** - Alternative valid scenarios, edge cases
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3. **Exceptions/Errors** - Invalid inputs, error conditions, failure modes
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### Test Production Scenarios
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Write tests with real production scenarios in mind. This makes tests more relatable and helps understand code behavior in actual production cases.
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### Test Coverage Goals
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Aim for 80% code coverage as a practical balance between quality and effort. Higher coverage is beneficial but not the only goal.
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Use Jacoco maven plugin for coverage reporting and tracking.
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**Coverage Rules:**
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- 80+% coverage minimum
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- Focus on meaningful assertions, not just execution
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**What to Prioritize:**
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1. Business-critical paths (payment processing, order validation)
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2. Complex algorithms (pricing, discount calculations)
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3. Error handling (exceptions, edge cases)
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4. Integration points (external APIs, databases)
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## Dependencies (Spring Boot 4)
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```xml
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-test</artifactId>
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<scope>test</scope>
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</dependency>
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<!-- For WebMvc tests -->
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-webmvc-test</artifactId>
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<scope>test</scope>
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</dependency>
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<!-- For Testcontainers -->
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-testcontainers</artifactId>
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<scope>test</scope>
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</dependency>
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```
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