Spring Boot 4 Testing Skill

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

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 |

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

Agent Skills: Spring Boot 4 Testing

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