Testing: @SpringBootTest vs @WebMvcTest

Key points

• Plain unit tests need no Spring. If a class has no annotations in its logic, just call new.
• @WebMvcTest loads only the web layer: one controller, JSON, validation, error handling. It is fast.
• In a slice, collaborators are absent — supply them with @MockitoBean (this replaced the deprecated @MockBean).
• @SpringBootTest starts the whole context. Use it sparingly, for wiring and end-to-end checks.
• Add webEnvironment = RANDOM_PORT and inject TestRestTemplate for a real HTTP round trip.
• @DataJpaTest slices to JPA and rolls back each test. Point it at Testcontainers, not H2, to test real SQL.

Example

@WebMvcTest(OrderController.class)      // web layer only
class OrderControllerTest {

    @Autowired MockMvc mvc;
    @MockitoBean OrderService service;   // the service is faked

    @Test
    void returnsOrder() throws Exception {
        given(service.find(7L))
            .willReturn(new OrderResponse(7L, "PAID", new BigDecimal("42.00")));

        mvc.perform(get("/api/orders/7"))
           .andExpect(status().isOk())
           .andExpect(jsonPath("$.status").value("PAID"));
    }

    @Test
    void rejectsInvalidBody() throws Exception {
        mvc.perform(post("/api/orders")
                .contentType(MediaType.APPLICATION_JSON)
                .content("{\"customerEmail\":\"not-an-email\"}"))
           .andExpect(status().isBadRequest());
    }
}

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class ShopApplicationIT {

    @Autowired TestRestTemplate rest;

    @Test
    void contextLoadsAndServes() {
        assertThat(rest.getForEntity("/actuator/health", String.class)
                .getStatusCode()).isEqualTo(HttpStatus.OK);
    }
}

Slice tests for speed, one full-context test for confidence — not the other way round.