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name: spring-boot-testing description: Spring Boot 4 testing strategies and patterns. Use when writing unit tests, slice tests (@WebMvcTest, @DataJpaTest), integration tests, Testcontainers with @ServiceConnection, security testing (@WithMockUser, JWT), or Modulith event testing with Scenario API. Covers the critical @MockitoBean migration from @MockBean.

Spring Boot 4 Testing

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

Critical Breaking Change

MockMvcTester (Spring Boot 4)

New fluent, AssertJ-style API for controller testing:

Copy@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

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

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