Testing slices: @WebMvcTest vs @SpringBootTest

Pick the smallest harness that proves the thing

Spring offers several test slice annotations that load a narrow context. Use the narrowest slice that actually exercises the behavior you are testing — broader slices are slower, more flaky, and obscure the unit under test.

• @WebMvcTest: load only the MVC layer. Inject MockMvc; mock services.
• @DataJpaTest: load JPA + an in-memory DB; test repositories and queries.
• @SpringBootTest: full context. Good for end-to-end smoke tests; slow.

@WebMvcTest(InvoiceController.class)
class InvoiceControllerTest {

    @Autowired MockMvc mvc;
    @MockBean  InvoiceService service;

    @Test
    void returns_404_for_unknown_invoice() throws Exception {
        when(service.load(99L)).thenThrow(new InvoiceNotFoundException(99L));
        mvc.perform(get("/api/invoices/99"))
           .andExpect(status().isNotFound());
    }
}