<c:choose>
            <c:when test="${empty login}">
                <li><a href="../index/loginUI.do"><i class="fa fa-user"></i>/</a>
                </li>
            </c:when>
            <c:otherwise>
                <li><a href="../index/mycenter.do"><i class="fa fa-user"></i></a></li>
                <li><a href="../index/logout.do"><i class="fa fa-user"></i></a>
                </li>
            </c:otherwise>
        </c:choose>

this is what I wrote in JSTL, and it can run successfully without any exception

 <li><a th:href="@{/login/loginUI}" th:if="${-sharphttpSession.login}eq null"><i class="fa fa-user-md"></i></a>
            </li>
            <li><a th:href="@{/index/loginUI}" th:if="${-sharphttpSession.login}eq null"><i class="fa fa-user"></i>/</a>
            </li>
            <li><a th:href="@{/index/mycenter}" th:unless="${-sharphttpSession.login} eq null"><i class="fa fa-user"></i></a></li>
            <li><a th:href="@{/index/logout}" th:if="${-sharphttpSession.login} ne null"><i class="fa fa-user"></i></a>
            </li>

but I can never find login when I use it in thymleaf

then my java code

 @RequestMapping("/index")
    public ModelAndView index(HttpServletRequest request, HttpServletResponse response) {
        Page page = new Page("filter_form");
        page.setPageSize(8);
        String currentPage = request.getParameter("page.currentPage");
        if (StringUitl.IsNotNull(currentPage)) {
            page.setCurrentPage(Integer.parseInt(currentPage));
        }
        //map
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("page", page);
        List list = changguanService.getForPage(map);
        //
        request.setAttribute("list", list);
        request.setAttribute("paging", page.getPageStr());
        System.out.println(page.getPageStr());
        return new ModelAndView("index/index");
    }
 @RequestMapping("/login")
    public void login(HttpServletRequest request, HttpServletResponse response, HttpSession httpSession) throws IOException {
        String pwd = request.getParameter("pwd");
        String no = request.getParameter("no");
        response.setCharacterEncoding("utf-8");
        Login login = (Login) httpSession.getAttribute("login");
        if (login == null) {
            User user = userService.checkUserNo(no, pwd);
            if (user == null) {
                response.setContentType("text/html;charset=utf-8");
                response.getWriter().write("<script>alert("");</script>");
                response.getWriter().write("<script> window.location="../index/index" ;window.close();</script>");
                response.getWriter().flush();
            }
            login = new Login(user.getId(), user.getName(), "user", "");
            httpSession.setAttribute("login", login);
        }
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().write("<script>alert("");</script>");
        response.getWriter().write("<script> window.location="../index/index" ;window.close();</script>");
        response.getWriter().flush();
    }

how can thymleaf realize the problem of directly judging the null value of an uncreated object by jstl