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:

@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

1. Choose test slice → Minimal context for fast tests
2. Mock dependencies → @MockitoBean for external services
3. Use Testcontainers → @ServiceConnection for databases
4. Assert thoroughly → Use AssertJ, MockMvcTester (new), RestTestClient (new), WebTestClient
5. 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

• spring-boot-security — For authentication/authorization configuration patterns
• spring-boot-modulith — For module boundary and event-driven architecture patterns

Critical Reminders

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