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?

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

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