Me enfrento a un error al escribir un caso de prueba para el despachador de solicitudes. Mi clase

@Override
        public void doFilter(ServletRequest request, ServletResponse resp, FilterChain chain)
            throws IOException, ServletException
        {
            if(isMockAccountEnabled())
            {
                HttpServletRequest req = (HttpServletRequest)request;
                String reqUrl = req.getRequestURI();
                ApiUserDetails userDetails = userBean.getUserDetails();
                HttpSession session = req.getSession();
                if(isThisTestAccount(reqUrl, session))
                {
                    log.info(userDetails);
                    log.debug("Entering Test acount flow for the request "+reqUrl);
                    RequestDispatcher dispatcher = req.getRequestDispatcher("/mock/" + EnumService.returnMockService(reqUrl));
                    dispatcher.forward(request, resp);
                }
            }
        }

Caso de prueba escrito

@Mock
private FilterChain chain;


@InjectMocks
private MockAccountFilter mockAccountFilter = new MockAccountFilter();


MockHttpServletRequest request = new MockHttpServletRequest();
MockHttpServletResponse response = new MockHttpServletResponse();
MockHttpSession session = new MockHttpSession();
@Test
public void filterRequestMockFirst()
    throws Exception
{
    MockRequestDispatcher dispatcher =new MockRequestDispatcher("/mock/ABCTEST");
    when(request.getRequestDispatcher("/mock/ABCTEST")).thenReturn(dispatcher);
    request.setRequestURI("/check/employee/123456/false");
    mockAccountFilter.doFilter(request, response, chain);  
    Assert.assertTrue(request.getRequestURI().contains("/mock/ABCTEST"));

}

Error

when() requires an argument which has to be 'a method call on a mock'.

¿Alguien puede decirme la forma exacta de escribir este caso de prueba?