Slice tests: @WebMvcTest, @DataJpaTest, @JsonTest, @RestClientTest. When to use each.

Slice tests boot a deliberately narrow slice of the context — only the auto-configurations relevant to one layer — so you test that layer with real Spring wiring but a fraction of the startup cost. Each @...Test annotation disables full auto-configuration and turns on just the pieces it needs. The skill is matching the slice to what you're verifying.

@WebMvcTest — the web layer

Loads MVC infrastructure (DispatcherServlet, your @Controller/@RestController, @ControllerAdvice, converters, validators, security config) and an auto-configured MockMvc. It does not load @Service, @Repository, or @Component beans — you supply collaborators with @MockitoBean.

@WebMvcTest(OrderController.class)
class OrderControllerTest {
    @Autowired MockMvc mvc;
    @MockitoBean OrderService service;

    @Test void returns404() throws Exception {
        given(service.find(1L)).willReturn(Optional.empty());
        mvc.perform(get("/orders/1")).andExpect(status().isNotFound());
    }
}

Use it for request mapping, status codes, JSON (de)serialization, validation, and exception handling — everything web about a controller.

@DataJpaTest — the persistence layer

Configures JPA, Hibernate, a DataSource, TestEntityManager, and your repositories. Two key behaviours: each test runs in a transaction that rolls back at the end (clean DB between tests), and by default it replaces your DataSource with an embedded in-memory DB (H2) — though you should override that with Testcontainers for fidelity (@AutoConfigureTestDatabase(replace = NONE)).

@DataJpaTest
class OrderRepositoryTest {
    @Autowired OrderRepository repo;
    @Test void findsByStatus() {
        repo.save(new Order(Status.PAID));
        assertThat(repo.findByStatus(Status.PAID)).hasSize(1);
    }
}

Use it for custom @Query methods, derived queries, and mapping correctness — not for service logic.

@JsonTest — serialization only

Loads just Jackson/Gson configuration and gives you JacksonTester/JsonContent to assert that an object serializes to the exact JSON you expect (and back). Cheap and focused — ideal for verifying @JsonProperty, date formats, and custom serializers without an HTTP layer.

@RestClientTest — outbound HTTP clients

For testing your client code that calls other services. It configures RestTemplate/RestClient builders and a MockRestServiceServer so you can stub the remote responses and assert your client parses them correctly — no real network.

@RestClientTest(WeatherClient.class)
class WeatherClientTest {
    @Autowired WeatherClient client;
    @Autowired MockRestServiceServer server;
}