When I use run the follwoing code in my test class:
Mockito.when(accountHelper.convertAccountToJson(account)).thenReturn(jsonObject); 
it is returning the jsonObject. However, in my AccountController, the follwoing code is returning null:
JSONObject accountJson = accountHelper.convertAccountToJson(account);
Can someone please explain why the mock response is not being reflected? Thanks in advance.
Here's is my code: TestClass
 @RunWith(MockitoJUnitRunner.class)
    public class AccountControllerTest {
        @InjectMocks private AccountController accountController;
        @Mock private AccountHelper accountHelper;
        MockMvc mockMvc;
        @Before
        public void setup() throws Exception {
            MockitoAnnotations.initMocks(this);
            mockMvc = MockMvcBuilders.standaloneSetup(accountController).build();
        }
        @Test
        public void createAccountEmptyMessage() throws Exception{
            String jsonString = "{}";
            String expectedReturnBody = "{\"name\":"test"}";
            Account account = new Account(test);
            JSONObject jsonObject = new JSONObject("{\"name\":"test"}");
            Mockito.when(accountHelper.convertAccountToJson(account)).thenReturn(jsonObject);
            mockMvc.perform(post("/createAccount").contentType(MediaType.APPLICATION_JSON).characterEncoding("UTF-8")
                    .accept(MediaType.APPLICATION_JSON).content(jsonString))
                    .andExpect(content().string(expectedReturnBody));
        }
    }
HelperClass
@Component
public class AccountHelper {
    public JSONObject convertAccountToJson(Account account) {
    String jsonString;
    JSONObject json;
    Gson gson = new Gson();
    try{
        jsonString = gson.toJson(account);
        json = new JSONObject(jsonString);
        logger.info(jsonString);
    } catch (Exception e) {
        return null;
    }
    return json;
}
Controller
@RestController
public class AccountController {
    @Autowired private AccountService accountService;
    @Autowired private AccountHelper accountHelper;
    @RequestMapping(method = RequestMethod.POST, value = "/createAccount")
    public ResponseEntity<Object> createAccount(@RequestBody(required= false) Account account) {
        try {
            JSONObject accountJson = accountHelper.convertAccountToJson(account); //Return null
            //Create Account
            return new ResponseEntity(accountJson.toString(), HttpStatus.OK);
        } catch (Exception e) {
            return new ResponseEntity(e.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
}
