Spring Boot Testing

Spring Boot testing

Choose the narrowest Spring Boot testing technique that provides confidence, then write maintainable tests with JUnit 6, AssertJ-style assertions, modern Spring Boot 4 test APIs, and meaningful coverage.

When to invoke

• "Write tests for this Spring Boot controller."
• "Should this use @WebMvcTest or @SpringBootTest?"
• "Migrate these tests from @MockBean to @MockitoBean."
• "Add repository tests with Testcontainers."
• "Improve Spring Boot test coverage without brittle tests."

Prerequisites and context

• Use the project's existing test runner and build tool.
• Add dependencies only when the project lacks the required Spring Boot test starter.
• Read bundled references for detailed API examples rather than expanding the main skill.

Test slice selection

Decision tree:

Testing a controller endpoint? -> @WebMvcTest with MockMvcTester
Testing repository queries? -> @DataJpaTest with Testcontainers (real DB)
Testing business logic in service? -> Plain JUnit + Mockito (no Spring context)
Testing external API client? -> @RestClientTest with MockRestServiceServer
Testing JSON mapping? -> @JsonTest
Need full integration test? -> @SpringBootTest with minimal context config

Core principles

Spring Boot 4 highlights

• RestTestClient: modern alternative to TestRestTemplate.
• @MockitoBean: replaces deprecated @MockBean.
• MockMvcTester: AssertJ-style web test assertions.
• Modular starters: technology-specific test starters such as spring-boot-starter-webmvc-test.
• Context pausing: Spring Framework 7 can pause cached contexts.

Test quality rules

• Create helper methods for common objects and mock setup to reduce redundancy.
• Use production-realistic scenarios: payment processing, order validation, discount calculations, error handling, external APIs, and databases.
• Use @DisplayName to state behavior: @DisplayName("Should calculate discount for VIP customer").
• Avoid tests that only import classes or execute code without assertions.
• Aim for 80+% coverage as a practical minimum, but prioritize meaningful assertions over numeric coverage.
• Use the Jacoco Maven plugin for coverage reporting when the project already uses Maven or JaCoCo.

Example display names:

@Test
@DisplayName("Should calculate discount for VIP customer")
void shouldCalculateDiscountForVip() { }

@Test
@DisplayName("Should reject order when customer has insufficient credit")
void shouldRejectOrderForInsufficientCredit() { }

Dependencies

Use only what the project needs:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-webmvc-test</artifactId>
  <scope>test</scope>
</dependency>

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-testcontainers</artifactId>
  <scope>test</scope>
</dependency>

Progressive disclosure and bundled resources

• references/test-slices-overview.md: slice decision matrix and comparisons.
• references/webmvctest.md: web layer testing with MockMvc.
• references/datajpatest.md: data layer tests with Testcontainers.
• references/restclienttest.md: REST client testing.
• references/mockmvc-tester.md: AssertJ-style MockMvc usage.
• references/mockmvc-classic.md: traditional MockMvc for older projects.
• references/resttestclient.md: Spring Boot 4 REST client testing.
• references/mockitobean.md: dependency mocking with @MockitoBean.
• references/assertj-basics.md: scalar, string, boolean, and date assertions.
• references/assertj-collections.md: list, set, map, and array assertions.
• references/testcontainers-jdbc.md: PostgreSQL, MySQL, and JDBC containers.
• references/instancio.md: generating complex test objects with 3+ properties.
• references/context-caching.md: speeding up test suites.
• references/sb4-migration.md: Spring Boot 4 migration notes.

Compatibility vocabulary

Preserve these legacy terms, API names, command placeholders, and literal phrases when applying or migrating this skill:

• Exceptions/Errors**
• OrderRequest
• WebMvc

Output template

## Spring Boot testing result

**Status:** tests-added | guidance-only | blocked
**Test strategy:** <unit/slice/integration and why>

| Target | Test type | Files changed | Main scenarios | Validation |
| --- | --- | --- | --- | --- |
| <class or endpoint> | <@WebMvcTest/@DataJpaTest/plain JUnit/etc.> | <test file> | <happy path, edge, error> | <command and result> |

### Notes
- <coverage, dependency, migration, or refactoring recommendation>

Quality gate

• The narrowest useful test slice was selected and justified.
• Tests cover happy path, alternate valid paths, and error conditions where relevant.
• Assertions verify behavior, not just execution.
• Existing project conventions, fixtures, and build tools were preserved.
• @MockBean was not introduced for Spring Boot 4 code; use @MockitoBean where needed.
• Coverage goals focus on business-critical paths, complex algorithms, error handling, and integration points.
• Bundled references were consulted for detailed API usage when necessary.