The problem is I cannot unit test/mock this because the messageByLocaleService is null in the Controller Advice. Please suggest any way to inject the mock of messageByLocaleSerI have a set up like this:

ExceptionControllerAdvice class:

@Autowired
private MessageByLocaleService messageByLocaleService; //CANNOT MOCK THIS

@ControllerAdvice
public class ExceptionControllerAdvice {

    @ExceptionHandler(UserNotFoundException.class)
    public ResponseEntity<String> handleUnexpectedException(HttpServletRequest request, Exception e) {

        //build response as below
        String s = messageByLocaleService.buildMessage(HttpStatus.ERROR,"User Not Valid"); // GETTING NULL HERE
        //and send the response back to the client here
    }
}

Controller method:

@RequestMapping(value = "${foo.controller.requestMappingUrl.login}",
                                 method = RequestMethod.POST)
public ResponseMessage<String> loginUser(
            @RequestParam("username") String username,
               HttpServletRequest httpServletRequest,
               HttpServletResponse httpServletResponse) throws Exception {

       return fooService.login(username);
}

JUnit setup & test methods:

@InjectMocks
private ProjectController projectController;

@Mock
private FooService fooService;


@Mock
private MessageByLocaleService messageByLocaleService;

@Before
    public void setUp() throws Exception {
        MockitoAnnotations.initMocks(this);
    mockMvc = MockMvcBuilders.standaloneSetup(projectController)
                .setMessageConverters(new MappingJackson2HttpMessageConverter())
                .setControllerAdvice(new ExceptionControllerAdvice()).build();
}


@SuppressWarnings("unchecked")
@Test
    public void testControllerUserNotFoundException() throws Exception {
        Response resp = new Response();
        resp.setStatusCode(StatusCode.UserNotFoundErrorCode);
        when(fooService.login(any(String.class)).
        thenThrow(UserNotFoundException.class);

        mockMvc.perform(post("/service-user/1.0/auth/login?&username=test")
                        .contentType(contentType)).
     andExpect(status().isNotAcceptable())
                .andExpect(jsonPath("$.statusCode", is("ERRORCODE144")));
}