Spring Boot Unit Testing with Mockito

Apply Spring Boot unit testing guidelines with Mockito.

What is covered in this Skill?

• Pure unit tests: @ExtendWith(MockitoExtension.class), @Mock, @InjectMocks for @Service/@Component (no Spring context)
• Slice tests: @WebMvcTest, @MockitoBean for controllers (Spring Boot 4.0.x — @MockBean removed)
• @JsonTest for JSON serialization with correct JSON value assertions (extractingJsonPathStringValue, etc.)
• Parameterized tests: @ParameterizedTest with @CsvSource or @MethodSource over copy-pasted methods
• Java records for domain objects in tests
• @TestConfiguration, @ActiveProfiles, @Primary fixed-clock beans for deterministic test setup
• Modern mocking API: @MockitoBean / @MockitoSpyBean; @MockBean/@SpyBean are deprecated/removed in Boot 4.0.x

Scope: Apply recommendations based on the reference rules and good/bad code examples. For integration tests use @322-frameworks-spring-boot-testing-integration-tests.

Constraints

Before applying any test changes, ensure the project compiles. If compilation fails, stop immediately. After applying improvements, run full verification.

• MANDATORY: Run ./mvnw compile or mvn compile before applying any change
• SAFETY: If compilation fails, stop immediately
• VERIFY: Run ./mvnw clean verify or mvn clean verify after applying improvements
• BEFORE APPLYING: Read the reference for detailed rules and good/bad patterns
• EDGE CASE: If request scope is ambiguous, stop and ask a clarifying question before applying changes
• EDGE CASE: If required inputs, files, or tooling are missing, report what is missing and ask whether to proceed with setup guidance

When to use this skill

• Review Java code for Spring Boot unit tests
• Apply best practices for Spring Boot unit tests in Java code
• Write Mockito-first unit tests for Spring Boot services
• Avoid unnecessary @SpringBootTest in Spring unit tests
• Review Spring Boot slice-free unit test design

Workflow

1. Read reference and assess project context

Read references/321-frameworks-spring-boot-testing-unit-tests.md and inspect the current project setup before proposing changes.

2. Gather scope and decide target improvements

Identify requested outcomes, constraints, and the minimum safe set of changes to apply.

3. Apply framework-aligned changes

Implement or refactor configuration/code following the reference patterns and project conventions.

4. Run verification and report results

Execute appropriate build/tests and summarize what changed, what was verified, and any follow-up actions.

Reference

For detailed guidance, examples, and constraints, see references/321-frameworks-spring-boot-testing-unit-tests.md.