jsp – 登录失败后在登录页面中显示错误消息

login.jsp的

<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>

在servlet代码中,如果用户不存在,我正在使用带有错误消息的requestdispatcher

的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);          
                }

但事情是< h2><％= request.getAttribute(“errorMessage”)％>< / h2>每次打开登录jsp时都会运行,当没有失败时会在页面上返回null.只有在发生故障时它才显示“无效的用户或密码”.所以我想检查request.getAttribute(“errorMessage”)是否为空.然后它不会调用< h2><％= request.getAttribute(“errorMessage”)％>< / h2>.我该怎么做？或者,还有更好的方法

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

如果你去JSTL会更好：

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

要了解有关这些${}事物的更多信息(Expression Language,这是JSTL的单独主题),check here.