16.4 Context Slicing: @WebMvcTest and @DataJpaTest

@SpringBootTest loads the full application context and is very slow. Slice Tests selectively load only the beans needed for a specific layer, dramatically improving test speed.

1. @WebMvcTest — Load Controllers Only

Validates Controller-layer HTTP request/response logic without services or repositories.

@WebMvcTest(UserController.class)
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean  // Registers a Mock bean in the Spring context (different from @Mock!)
    private UserService userService;

    @Autowired
    private ObjectMapper objectMapper;

    @Test
    @DisplayName("POST /api/users — valid DTO returns 201 Created")
    void create_user_returns_201() throws Exception {
        CreateUserRequest request = new CreateUserRequest("John Doe", "john@test.com");
        UserDto createdUser = new UserDto(1L, "John Doe", "john@test.com");
        given(userService.createUser(any())).willReturn(createdUser);

        mockMvc.perform(post("/api/users")
                .contentType(MediaType.APPLICATION_JSON)
                .content(objectMapper.writeValueAsString(request)))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.id").value(1L))
            .andExpect(jsonPath("$.name").value("John Doe"))
            .andDo(print());
    }
}

2. @DataJpaTest — Load JPA Beans Only

Validates repository query logic with only JPA repositories and EntityManager loaded. Uses in-memory H2 by default.

@DataJpaTest
@AutoConfigureTestDatabase(replace = Replace.NONE) // Use real DB (Testcontainers) instead of H2
class UserRepositoryTest {

    @Autowired
    private UserRepository userRepository;

    @Autowired
    private TestEntityManager entityManager;

    @Test
    @DisplayName("User can be found by email.")
    void find_user_by_email() {
        entityManager.persist(new User("Alice", "alice@test.com"));
        entityManager.flush();
        entityManager.clear(); // Clear 1st-level cache to force real DB query

        Optional<User> result = userRepository.findByEmail("alice@test.com");

        assertThat(result).isPresent();
        assertThat(result.get().getName()).isEqualTo("Alice");
    }
}