实际上，我正在用jsp和servlet构建这个项目。下面是我遇到错误的代码：-
userdetails user=（userdetails）session.getattribute（“userd”）*
navbar.jsp

<nav class="navbar navbar-expand-lg navbar-dark bg-custom navbar-custom">
        <a class="navbar-brand" href="#"><i class="fa fa-book"
            aria-hidden="true"></i>ENotes</a>
        <button class="navbar-toggler" type="button" data-toggle="collapse"
            data-target="#navbarSupportedContent"
            aria-controls="navbarSupportedContent" aria-expanded="false"
            aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
                <li class="nav-item active"><a class="nav-link" href="index.jsp"><i
                        class="fa fa-home" aria-hidden="true"></i>Home <span
                        class="sr-only">(current)</span></a></li>

                <li class="nav-item"><a class="nav-link" href="#"><i
                        class="fa fa-plus-circle" aria-hidden="true"></i>Add Notes</a></li>

                <li class="nav-item"><a class="nav-link" href="#"><i
                        class="fa fa-address-book-o" aria-hidden="true"></i>Show Notes</a></li>

            </ul>

            <%
            UserDetails  user=(UserDetails)session.getAttribute("userD"); /*This is part where i'm getting error*/
            if(user!=null)
            {%>

            <%}
            else
            {

            }

            %>

            <a href="login.jsp" class="btn btn-light my-2 my-sm-0 mr-2"
                type="submit"><i class="fa fa-sign-in" aria-hidden="true"></i>Login</a>
            <a href="register.jsp" class="btn btn-light my-2 my-sm-0"
                type="submit"><i class="fa fa-user-circle-o" aria-hidden="true"></i>Register</a>

            <a href="login.jsp" class="btn btn-light my-2 my-sm-0 mr-2"
                type="submit"><i class="fa fa-sign-in" aria-hidden="true"></i>Login</a>
            <a href="register.jsp" class="btn btn-light my-2 my-sm-0"
                type="submit"><i class="fa fa-user-circle-o" aria-hidden="true"></i>Register</a>

        </div>
    </nav>

如何解决这个问题？