spring-boot-testing

SKILL.md

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>