SKILL.md

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
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:

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 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:

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)

<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>