Kif tittestja app Spring Boot: @SpringBootTest, MockMvc, slices?

Question

IT Interview Practice · Accepted Answer

Spring Boot jagħtik **piramida tat-testing**: unit tests mgħaġġla mingħajr Spring, *slice* tests iffokati li jgħabbu parti mill-context, u ftit integration tests bil-context sħiħ. Li tagħżel il-livell it-tajjeb għal kull test hija l-ħila senior hawnhekk.

## Unit test — ebda Spring

Il-constructor injection tfisser li l-biċċa l-kbira tal-loġika tas-service ma teħtieġ ebda Spring; sempliċement immokkja l-kollaboraturi:

```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

Jgħabbi biss il-web layer (controllers, JSON, validazzjoni) — ebda database, ebda services ħlief mocks. Mgħaġġel:

```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 — is-slice tal-persistenza

Jgħabbi JPA/repositories kontra DB in-memory (jew Testcontainers) u jagħmel rollback wara kull 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();
    }
}
```

Uża `@SpringBootTest` (context sħiħ, `webEnvironment = RANDOM_PORT`) bil-galbu għal verifiki end-to-end veri — huwa l-aktar bil-mod.

## Għaliex huwa importanti

L-intervistaturi jridu jaraw li tevita l-anti-pattern ta' `@SpringBootTest` għal *kollox*, li jagħmel is-suites bil-mod u fraġli. Li ssemmi s-slice t-tajjeb għal kull kwistjoni, l-immokkjar b'`@MockBean`, u l-użu ta' **Testcontainers** għal database reali minflok H2 juri xi ħadd li jżomm test suite kbir mgħaġġel u affidabbli.

Unit test — ebda Spring

Slice test — @WebMvcTest + MockMvc

@DataJpaTest — is-slice tal-persistenza

Għaliex huwa importanti

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