Unaipimaje (test) Spring Boot app: @SpringBootTest, MockMvc, slices?

Question

IT Interview Practice · Accepted Answer

Spring Boot hukupa **testing pyramid**: unit tests za haraka bila Spring, *slice* tests zinazolenga zinazopakia sehemu ya context, na integration tests chache za context kamili. Kuchagua ngazi sahihi kwa kila test ndio ujuzi wa senior hapa.

## Unit test — bila Spring kabisa

Constructor injection humaanisha service logic nyingi haihitaji Spring hata kidogo; mock tu washirika:

```java
@Test
void appliesDiscount() {
    var repo = mock(PriceRepository.class);
    when(repo.base("A")).thenReturn(BigDecimal.TEN);
    var svc = new PricingService(repo);          // plain constructor, no context
    assertThat(svc.priceOf("A")).isEqualByComparingTo("9.00");
}
```

## Slice test — @WebMvcTest + MockMvc

Hupakia tabaka la web pekee (controllers, JSON, validation) — bila database, bila services zaidi ya mocks. Haraka:

```java
@WebMvcTest(UserController.class)
class UserControllerTest {
    @Autowired MockMvc mvc;
    @MockBean UserService service; // service is mocked, not real

@Test
    void returns404WhenMissing() throws Exception {
        when(service.findById(9L)).thenThrow(new EntityNotFoundException());
        mvc.perform(get("/api/users/9"))
           .andExpect(status().isNotFound()); // verifies controller + advice wiring
    }
}
```

## @DataJpaTest — slice ya persistence

Hupakia JPA/repositories dhidi ya DB ya in-memory (au Testcontainers) na hu-rollback baada ya kila test:

```java
@DataJpaTest
class UserRepositoryTest {
    @Autowired UserRepository repo;
    @Test void findsByEmail() {
        repo.save(new User("a@x.com"));
        assertThat(repo.findByEmail("a@x.com")).isPresent();
    }
}
```

Tumia `@SpringBootTest` (context kamili, `webEnvironment = RANDOM_PORT`) kwa uchache kwa ukaguzi halisi wa mwisho-hadi-mwisho (end-to-end) — ndio polepole zaidi.

## Kwa nini ni muhimu

Wahoji wanataka kuona unaepuka anti-pattern ya `@SpringBootTest` kwa *kila kitu*, ambayo hufanya suites kuwa polepole na dhaifu. Kutaja slice sahihi kwa kila jambo, ku-mock kwa `@MockBean`, na kutumia **Testcontainers** kwa database halisi badala ya H2 kunaashiria mtu anayeweka test suite kubwa kuwa ya haraka na ya kuaminika.

Unit test — bila Spring kabisa

Slice test — @WebMvcTest + MockMvc

@DataJpaTest — slice ya persistence

Kwa nini ni muhimu

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