Spring Boot and JUnit 5: integration tests?

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**Integration tests** verify the work of components **together** (with a real database, external services). **Simple analogy:** A unit test is like checking an individual engine part (piston, valve). An integration test is like starting the engine and verifying the wheels spin. You are testing how parts work together, not in isolation. **Why it matters:** Unit tests prove individual methods work correctly. But bugs often hide at the boundaries — between the controller and the service, between the service and the repository, between the application and the database. Integration tests catch these boundary issues: wrong SQL, missing transaction boundaries, incorrect JSON serialization, and failed dependency injection. **@SpringBootTest (full context):** ```java @SpringBootTest @AutoConfigureMockMvc class UserControllerIntegrationTest { @Autowired MockMvc mockMvc; @Autowired UserRepository userRepository; @BeforeEach void setup() { userRepository.deleteAll(); } @Test @Transactional // Rollback after test void shouldCreateUser() throws Exception { mockMvc.perform(post("/api/users") .contentType(MediaType.APPLICATION_JSON) .content("{\"name\":\"Alice\",\"email\":\"a@b.com\"}")) .andExpect(status().isCreated()) .andExpect(jsonPath("$.name").value("Alice")) .andExpect(jsonPath("$.id").isNumber()); } @Test void shouldReturn404ForNonExistentUser() throws Exception { mockMvc.perform(get("/api/users/9999")) .andExpect(status().isNotFound()); } } ``` **@WebMvcTest (web layer only — fast):** ```java @WebMvcTest(UserController.class) class UserControllerWebTest { @Autowired MockMvc mockMvc; @MockBean UserService userService; // Only the service is mocked @Test void shouldReturnUser() throws Exception { when(userService.findById(1L)).thenReturn(new UserDto(1L, "Alice")); mockMvc.perform(get("/api/users/1")) .andExpect(status().isOk()) .andExpect(jsonPath("$.name").value("Alice")); } @Test void shouldReturn400ForInvalidInput() throws Exception { mockMvc.perform(post("/api/users") .contentType(MediaType.APPLICATION_JSON) .content("{\"name\":\"\",\"email\":\"bad\"}")) .andExpect(status().isBadRequest()); } } ``` **@DataJpaTest (JPA layer only — with test database):** ```java @DataJpaTest class UserRepositoryTest { @Autowired TestEntityManager entityManager; @Autowired UserRepository userRepository; @Test void shouldFindByEmail() { entityManager.persistAndFlush(new User("Alice", "a@b.com")); Optional found = userRepository.findByEmail("a@b.com"); assertTrue(found.isPresent()); assertEquals("Alice", found.get().getName()); } @Test void shouldReturnEmptyForUnknownEmail() { Optional found = userRepository.findByEmail("nobody@test.com"); assertTrue(found.isEmpty()); } } ``` **@TestConfiguration — additional beans for tests:** ```java @TestConfiguration public class TestConfig { @Bean public Clock clock() { return Clock.fixed(Instant.parse("2024-01-15T12:00:00Z"), ZoneId.of("UTC")); } } ``` **Testcontainers (real databases in Docker):** ```java @Testcontainers @SpringBootTest class UserRepositoryIntegrationTest { @Container static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:15"); @DynamicPropertySource static void configure(DynamicPropertyRegistry registry) { registry.add("spring.datasource.url", postgres::getJdbcUrl); registry.add("spring.datasource.username", postgres::getUsername); registry.add("spring.datasource.password", postgres::getPassword); } } ``` **Common mistakes:** - **Using `@SpringBootTest` when `@WebMvcTest` suffices** — full context tests are 5-10x slower. - **Not cleaning up between tests** — test data leaks into other tests. Use `@Transactional` or `@BeforeEach` cleanup. - **Testing against production databases** — always use Testcontainers or embedded H2 for test isolation.

Java: Spring Boot and JUnit 5: integration tests?