feat: add spring-boot-testing skill for Spring Boot 4.0

docs/README.skills.md

@@ -224,6 +224,7 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md#adding-skills) for guidelines on how to
 | [shuffle-json-data](../skills/shuffle-json-data/SKILL.md) | Shuffle repetitive JSON objects safely by validating schema consistency before randomising entries. | None |
 | [snowflake-semanticview](../skills/snowflake-semanticview/SKILL.md) | Create, alter, and validate Snowflake semantic views using Snowflake CLI (snow). Use when asked to build or troubleshoot semantic views/semantic layer definitions with CREATE/ALTER SEMANTIC VIEW, to validate semantic-view DDL against Snowflake via CLI, or to guide Snowflake CLI installation and connection setup. | None |
 | [sponsor-finder](../skills/sponsor-finder/SKILL.md) | Find which of a GitHub repository's dependencies are sponsorable via GitHub Sponsors. Uses deps.dev API for dependency resolution across npm, PyPI, Cargo, Go, RubyGems, Maven, and NuGet. Checks npm funding metadata, FUNDING.yml files, and web search. Verifies every link. Shows direct and transitive dependencies with OSSF Scorecard health data. Invoke with /sponsor followed by a GitHub owner/repo (e.g. "/sponsor expressjs/express"). | None |
+| [spring-boot-testing](../skills/spring-boot-testing/SKILL.md) | Expert Spring Boot 4 testing specialist that selects the best Spring Boot testing techniques for your situation with Junit 6 and AssertJ. | `references/assertj-basics.md`<br />`references/assertj-collections.md`<br />`references/context-caching.md`<br />`references/datajpatest.md`<br />`references/instancio.md`<br />`references/mockitobean.md`<br />`references/mockmvc-classic.md`<br />`references/mockmvc-tester.md`<br />`references/restclienttest.md`<br />`references/resttestclient.md`<br />`references/sb4-migration.md`<br />`references/test-slices-overview.md`<br />`references/testcontainers-jdbc.md`<br />`references/webmvctest.md` |
 | [sql-code-review](../skills/sql-code-review/SKILL.md) | Universal SQL code review assistant that performs comprehensive security, maintainability, and code quality analysis across all SQL databases (MySQL, PostgreSQL, SQL Server, Oracle). Focuses on SQL injection prevention, access control, code standards, and anti-pattern detection. Complements SQL optimization prompt for complete development coverage. | None |
 | [sql-optimization](../skills/sql-optimization/SKILL.md) | Universal SQL performance optimization assistant for comprehensive query tuning, indexing strategies, and database performance analysis across all SQL databases (MySQL, PostgreSQL, SQL Server, Oracle). Provides execution plan analysis, pagination optimization, batch operations, and performance monitoring guidance. | None |
 | [structured-autonomy-generate](../skills/structured-autonomy-generate/SKILL.md) | Structured Autonomy Implementation Generator Prompt | None |

skills/spring-boot-testing/SKILL.md

@@ -0,0 +1,189 @@
+---
+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
+
+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:**
+```java
+// 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:
+
+```java
+@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:
+
+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)
+
+```xml
+<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>
+```