What Are Test Slices

Direct Answer

A test slice limits Spring Boot component scanning and auto-configuration to a supported technical area. For example, @WebMvcTest focuses on MVC types and @DataJpaTest focuses on JPA. Imports, security, properties, and bean overrides can still change the context, so a slice is a focused boundary rather than a promise of one exact layer or speed.

Code Example

@WebMvcTest(OrderController.class)
class OrderControllerTest {
    @Autowired private MockMvc mockMvc;

    @MockBean // mock the service layer — slices don't load it
    private OrderService orderService;

    @Test
    void shouldReturnOrderDetails() throws Exception {
        var order = new Order(1L, "Laptop", 1500.0);
        when(orderService.getOrder(1L)).thenReturn(order);

        mockMvc.perform(get("/orders/1"))
               .andExpect(status().isOk())               // verifies HTTP 200
               .andExpect(jsonPath("$.id").value(1))
               .andExpect(jsonPath("$.item").value("Laptop"));
    }
}

Common Mistakes & Pitfalls

• Treating a slice as one exact bean list: Imports, properties, security, and test overrides can change the context.
• Combining unrelated slice annotations: Each slice selects its own bootstrap and auto-configuration. Choose one focused boundary or define a broader test deliberately.
• Assuming every absent collaborator needs a mock: Use @MockitoBean or older @MockBean for a controlled double, @Import for a small real component, or broader configuration when its behavior is required.
• Promising a fixed speed: A slice usually avoids unrelated setup, but measure the real suite.