Servlet 重定向到空白页面

Question

每当我点击 submit 它会重定向到一个空白页面，但 url 工作正常我对我的回复标签是否错误感到困惑

代码：

protected void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {             
    Connection con=connector.getConnection();
    String username = request.getParameter("username");  
    String password = request.getParameter("password");
    String message = null;
    String url = null;
   try { 
        PreparedStatement ps = con.prepareStatement( "Select * from data Where username=?'" + username + "' and password=?'" + password + "';");
        ps.setString(1, request.getParameter("username"));
        ps.setString(2, request.getParameter("password"));
        ResultSet rs = ps.executeQuery();
               if(rs.next()) {
               RequestDispatcher rd=request.getRequestDispatcher("practice1.jsp");
               rd.forward(request,response);

               }
               else {
               message="Invalid Input Please Try Again";
             request.setAttribute("message", message);
             request.setAttribute("url", url);

           }
   }


   catch(SQLException ex){
    message="ERROR"+ex.getMessage();
    ex.printStackTrace();
}

请让我知道我缺少什么。

Answer 1

尝试通过更改以下代码来修复它：

之前:

PreparedStatement ps = con.prepareStatement( "Select * from data Where username=?'" + username + "' and password=?'" + password + "';");
ps.setString(1, request.getParameter("username"));
ps.setString(2, request.getParameter("password"));

之后：

PreparedStatement ps = con.prepareStatement( "Select * from data Where username=? and password=?");
ps.setString(1, username);
ps.setString(2, password);

Answer 2

在 if 块中，只要有数据，请求调度程序就会将请求转发到 practice1.jsp 页面。

if(rs.next()) {
               RequestDispatcher rd=request.getRequestDispatcher("practice1.jsp");
               rd.forward(request,response);
               }

但在 else 块的情况下，仅请求设置如下：

request.setAttribute("message", message);
request.setAttribute("url", url);

但如果要在UI中显示，我们应该指定response而不是request。

else {
         message="Invalid Input Please Try Again";
         response.getWriter().print("<html><head><title>
                                 Oops an error happened!</title> </head>");
         response.getWriter().print("<body>"+message+"</body>");
         response.getWriter().println("</html>");
         //request.setAttribute("message", message);
         //request.setAttribute("url", url);

         }

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