displaying error message in login page after login failure

Question

here is some part of code from login.jsp

login.jsp

         <h2 class="swd-postheader">Login</h2>
              <form method="post" action="Login.select">
                 <p><input id="username" name= "username" type="text" placeholder="Username" style="margin:10px" autofocus required></p>
                 <p> <input id="password" name="password" type="password" placeholder="Password" style="margin:10px" required></p>
                 <p><input class="swd-button" type="submit" value="Sign In">
                    <a href="register.html"><button class="swd-button" type="button">New User</button></a>
                 </p>
              </form>
              <h2><%=request.getAttribute("errorMessage") %></h2>
           </div>
        </div>

in the servlet code if the user does't exist i'm using requestdispatcher with error msg

servlet

if(res.next())
        {
                if ((thisname.equals(res.getString("username"))) && (thispwd.equals(res.getString("password"))))
                {
                    session.setAttribute("username", request.getParameter("username"));
                    response.sendRedirect("login-success.html");
                }
                else{
                    session.invalidate();
                    request.setAttribute("errorMessage", "Invalid user or password");
                    RequestDispatcher rd = request.getRequestDispatcher("/login.jsp");
                    rd.forward(request, response);          
                }

but thing is <h2><%=request.getAttribute("errorMessage") %></h2> runs whenever login jsp is opened which returns null on the page when there is no failure.only when the failure occurs it displays "invalid user or password" .so i want to check if request.getAttribute("errorMessage") is null . is its not then invoke <h2><%=request.getAttribute("errorMessage") %></h2>. how do i do it? or is there a better way

Answer 1

Since you are already using scriptlet in your jsp, you can use:

<%
    if(null!=request.getAttribute("errorMessage"))
    {
        out.println(request.getAttribute("errorMessage"));
    }
%>

It would be better if you go for JSTL:

<c:if test="${not empty errorMessage}">
   <c:out value="${errorMessage}"/>
</c:if>

To learn more about those ${} things (the Expression Language, which is a separate subject from JSTL), check here.