How does Spring Boot 4 testing work with @SpringBootTest and test slices?

// Integration test: full context
@SpringBootTest
@AutoConfigureMockMvc
class OrderControllerIntegrationTest {

    @Autowired MockMvc mockMvc;
    @Autowired ObjectMapper objectMapper;

    @Test
    void createOrder_returns201() throws Exception {
        mockMvc.perform(post("/api/orders")
            .contentType(MediaType.APPLICATION_JSON)
            .content(objectMapper.writeValueAsString(
                new CreateOrderRequest("product-1", 2))))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.id").isNotEmpty());
    }
}

// Slice test: only MVC layer (no database)
@WebMvcTest(OrderController.class)
class OrderControllerTest {

    @Autowired MockMvc mockMvc;
    @MockBean   OrderService service;  // mock the service

    @Test
    void getOrder_returns200() throws Exception {
        when(service.find("O-1")).thenReturn(new OrderDto("O-1", "PENDING"));
        mockMvc.perform(get("/api/orders/O-1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.status").value("PENDING"));
    }
}

// Spring Boot 4: RestTestClient replaces the choice between
// MockMvc and WebTestClient for most scenarios
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class OrderApiTest {
    @Autowired RestTestClient client;  // new in Spring Boot 4

    @Test
    void getOrders() {
        client.get().uri("/api/orders")
            .exchange()
            .expectStatus().isOk()
            .expectBodyList(OrderDto.class).hasSize(0);
    }
}