Testing POST Endpoint with MockMvc

Direct Answer

For an endpoint that consumes JSON, perform the real POST route with contentType(MediaType.APPLICATION_JSON) and a JSON body, then assert the status, headers, and response fields defined by that endpoint. ObjectMapper.writeValueAsString(request) keeps the body consistent with Java serialization. A creation endpoint commonly returns 201, but the API contract decides the expected status.

Code Example

@WebMvcTest(OrderController.class)
class OrderControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Autowired
    private ObjectMapper objectMapper; // Injected by Spring

    @MockBean
    private OrderService orderService;

    @Test
    void shouldCreateOrderAndReturn201() throws Exception {
        var newOrder = new Order(null, "Gaming PC", 1500.00);
        var savedOrder = new Order(42L, "Gaming PC", 1500.00);
        
        when(orderService.createOrder(any(Order.class))).thenReturn(savedOrder);

        mockMvc.perform(post("/api/orders")
               .contentType(MediaType.APPLICATION_JSON) // Required for JSON body
               .content(objectMapper.writeValueAsString(newOrder))) // Serialize to JSON
               .andExpect(status().isCreated())
               .andExpect(jsonPath("$.id").value(42))
               .andExpect(jsonPath("$.item").value("Gaming PC"));
    }
}

Common Mistakes & Pitfalls

• Using the wrong media type: A JSON-consuming mapping may return 415 when the request omits or changes application/json; assert the behavior defined by that mapping.
• Writing fragile JSON by hand: Use ObjectMapper for normal request objects, and write raw JSON only when malformed input is the test case.
• Matching the exact pre-serialization object: MVC creates a different request instance. Match meaningful arguments or capture and inspect them rather than depending on object identity.
• Assuming every POST returns 201: Creation commonly uses 201, while commands, searches, and asynchronous work can use other contracts. Assert the endpoint specification.