我尝试在Spring Boot应用程序中测试我的控制器的一个方法。这是一个post端点，它在一个请求中获取一个id，并将此id传递给一个服务：

@Slf4j
@Controller
public class AdministrationController {

    private final AdministrationService administrationService;

    @Autowired
    public AdministrationController(AdministrationService administrationService) {
        this.administrationService = administrationService;
    }

    @PostMapping("/administration")
    public @ResponseBody ResponseEntity<String> deleteByMessageId(String id) {
        return new ResponseEntity<>(administrationService.deleteMessageById(id), HttpStatus.OK);
    }
}

该方法对控制器的测试：

RunWith(SpringRunner.class)
@WebMvcTest(AdministrationController.class)
public class AdministrationControllerTest {

    @Autowired
    private MockMvc mvc;

    @MockBean
    private AdministrationService service;

    @Test
    public void 
    deleteByMessageId_whenCalled_thenServiceMethodIsCalledWithRequestParameters() throws Exception {

        Object randomObj = new Object() {
            public final String id = "1234";
        };

        ObjectMapper objectMapper = new ObjectMapper();
        String json = objectMapper.writeValueAsString(randomObj);

        MvcResult result = mvc.perform(
            post("/administration")
                    .contentType(MediaType.APPLICATION_JSON)
                    .content(json))
            .andExpect(status().isOk())
            .andReturn();

        verify(service, times(1)).deleteMessageById("1234");
}

}
当我运行这个测试时，post请求被执行，但是主体是空的：

MockHttpServletRequest:
  HTTP Method = POST
  Request URI = /administration
   Parameters = {}
      Headers = {Content-Type=[application/json]}
         Body = <no character encoding set>
Session Attrs = {}

看起来，即使我在测试中设置了内容，它也没有出现在我发送的请求中。而且，确实，测试失败了：

Argument(s) are different! Wanted: "1234"
Actual invocation has different arguments: null

我在这里遗漏了什么？我如何用MockMvc设置请求主体？