Testing Interview Questions

@SpringBootTest vs @WebMvcTest vs @DataJpaTest?

Annotation	Loads	Use for
`@SpringBootTest`	the full application context	integration tests (slow)
`@WebMvcTest(ProductController.class)`	only the web layer + MockMvc	controller tests; services are mocked
`@DataJpaTest`	only JPA, repositories, embedded DB, rolls back each test	repository / query tests
plain JUnit + Mockito	no Spring at all	service unit tests (fastest)

How do you test a controller with MockMvc?

@WebMvcTest(ProductController.class)
class ProductControllerTest {
    @Autowired MockMvc mockMvc;
    @MockitoBean ProductService productService;   // Boot 3.4+; older: @MockBean

    @Test
    void returnsProduct() throws Exception {
        when(productService.findById(1L))
            .thenReturn(new ProductResponse(1L, "Redmi Note 13", new BigDecimal("15999")));

        mockMvc.perform(get("/api/products/1"))
               .andExpect(status().isOk())
               .andExpect(jsonPath("$.name").value("Redmi Note 13"));
    }
}

@Mock vs @MockBean (@MockitoBean)?

@Mock (Mockito) creates a mock in a plain unit test, no Spring involved; inject it with @InjectMocks or a constructor. @MockBean (replaced by @MockitoBean in Spring Boot 3.4) replaces a bean in the Spring context with a mock, used in @WebMvcTest/@SpringBootTest. Changing mocked beans creates a new context, which slows the suite.

How do you write a unit test for a service?

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock ProductRepository productRepository;
    @InjectMocks OrderService orderService;

    @Test
    void failsWhenStockIsLow() {
        when(productRepository.findById(1L))
            .thenReturn(Optional.of(new Product(1L, "Boat Airdopes", new BigDecimal("1299"), 2)));

        assertThrows(InsufficientStockException.class, () -> orderService.placeOrder(1L, 5));
        verify(productRepository, never()).save(any());
    }
}

How do you test against a real database?

Use Testcontainers: it starts a real MySQL/PostgreSQL in Docker for the test run. In Spring Boot 3.1+ annotate the container field with @ServiceConnection and Boot wires the datasource automatically. This catches SQL dialect differences that H2 hides.

@SpringBootTest
@Testcontainers
class OrderFlowIT {
    @Container @ServiceConnection
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine");
}

How do you make @SpringBootTest faster?

Prefer slice tests (@WebMvcTest, @DataJpaTest) and plain unit tests.
Keep test configuration identical across classes so Spring reuses the cached context (avoid many different @MockBean sets).
Use webEnvironment = MOCK unless you really need a running server.
Reuse Testcontainers across classes (static containers).