Spring Boot 4 Testing

Comprehensive testing patterns including slice tests, Testcontainers, security testing, and Modulith Scenario API.

Critical Breaking Change

Old (Boot 3.x)	New (Boot 4.x)	Notes
`@MockBean`	`@MockitoBean`	Required migration
`@SpyBean`	`@MockitoSpyBean`	Required migration
`MockMvc` (procedural)	`MockMvcTester` (fluent)	New AssertJ-style API
Implicit `@AutoConfigureMockMvc`	Explicit annotation required	Add to `@SpringBootTest`

MockMvcTester (Spring Boot 4)

New fluent, AssertJ-style API for controller testing:

java

@WebMvcTest(UserController.class)
class UserControllerTest {

    @Autowired
    private MockMvcTester mvc;  // NEW: Fluent API

    @Test
    void getUser_returnsUser() {
        mvc.get().uri("/users/{id}", 1)
            .exchange()
            .assertThat()
            .hasStatusOk()
            .bodyJson()
            .extractingPath("$.name")
            .isEqualTo("John");
    }
}

Key Benefits: Fluent assertions, better error messages, AssertJ integration.

Test Annotation Selection

Test Type	Annotation	Use When
Controller	`@WebMvcTest`	Testing request/response, validation
Repository	`@DataJpaTest`	Testing queries, entity mapping
JSON	`@JsonTest`	Testing serialization/deserialization
REST Client	`@RestClientTest`	Testing external API clients
Full Integration	`@SpringBootTest`	End-to-end, with real dependencies
Module	`@ApplicationModuleTest`	Testing bounded context in isolation

Core Workflow

•Choose test slice → Minimal context for fast tests
•Mock dependencies → @MockitoBean for external services
•Use Testcontainers → @ServiceConnection for databases
•Assert thoroughly → Use AssertJ, MockMvcTester (new), RestTestClient (new), WebTestClient
•Test security → @WithMockUser, JWT mocking

Quick Patterns

See EXAMPLES.md for complete working examples including:

•@WebMvcTest with MockMvcTester and @MockitoBean (Java + Kotlin)
•@DataJpaTest with TestEntityManager for lazy loading verification
•Testcontainers with @ServiceConnection for PostgreSQL/Redis
•Security Testing with @WithMockUser for role-based access
•Modulith Event Testing with Scenario API

Detailed References

•Examples: See EXAMPLES.md for complete working code examples
•Troubleshooting: See TROUBLESHOOTING.md for common issues and Boot 4 migration
•Slice Tests: See references/SLICE-TESTS.md for @WebMvcTest, @DataJpaTest, @JsonTest patterns
•Testcontainers: See references/TESTCONTAINERS.md for @ServiceConnection, container reuse
•Security Testing: See references/SECURITY-TESTING.md for @WithMockUser, JWT mocking
•Modulith Testing: See references/MODULITH-TESTING.md for Scenario API, event verification

Anti-Pattern Checklist

Anti-Pattern	Fix
Using `@MockBean` in Boot 4	Replace with `@MockitoBean`
`@SpringBootTest` for unit tests	Use appropriate slice annotation
Missing `entityManager.clear()`	Add to verify lazy loading
High-cardinality test data	Use minimal, focused fixtures
Shared mutable test state	Use `@DirtiesContext` or fresh containers
No security tests	Add `@WithMockUser` tests for endpoints

Related Skills

Need	Skill
Security configuration	`spring-boot-security`
Module boundaries	`spring-boot-modulith`
Data layer patterns	`spring-boot-data-ddd`
Controller patterns	`spring-boot-web-api`

Critical Reminders

•@MockitoBean is mandatory — @MockBean removed in Boot 4
•Slice tests are fast — Use them for focused testing
•Clear EntityManager — Required to test lazy loading behavior
•@ServiceConnection simplifies Testcontainers — No more @DynamicPropertySource
•Test security explicitly — Don't rely on disabled security