Servlet使用request.setAttribute()向Jsp页面传送信息，获取值为NULL

这是servlet//////////////////////////////////////////////////////////////////////////////////////
package Servlet;
import java.io.*;
import Javabean.*;
import java.sql.*;
import javax.servlet.http.*;
import javax.servlet.*;
public class setMess_servlet extends HttpServlet
{
public void setMess_servlet(ServletConfig config) throws ServletException
{
super.init(config);
try{
Class.forName("com.mysql.jdbc.Driver");
}
catch(Exception e){}
}
public void doPost(HttpServletRequest request,HttpServletResponse response) throws

ServletException,IOException
{
Connection con;
Statement ste;
String url="jdbc:mysql://localhost/messagepane";
String user="root";
String pass="root";
String backnews=new String("hhaha");
HttpSession session=request.getSession();
String logoname=(String)session.getAttribute("logoname");
String mess_content=request.getParameter("mess_content");
try{
con=DriverManager.getConnection(url,user,pass);
ste=con.createStatement();
int i=ste.executeUpdate("insert into mess_table(logoname,message) values ("+logoname

+","+mess_content+")");
if(i!=0)
{
backnews="发表成功！";
}
else
{
backnews="发表失败！";
}
<!--//////////////////////////////////////////////////////////////////////////////////下面是要传送的值！/-->
request.setAttribute("backnews",backnews);
request.setAttribute("logoname",logoname);
request.setAttribute("mess_content",mess_content);
////////////////////////////////////////////////////////////////////////////////////////////////////////////////
con.close();
}
catch(SQLException e)
{
backnews=""+e;
}
RequestDispatcher dispatcher=request.getRequestDispatcher("mess_back.jsp");
dispatcher.forward(request,response);
}
public void doGet(HttpServletRequest request,HttpServletResponse response) throws

ServletException,IOException
{
doPost(request,response);
}
}

以下是jsp页面的
<% String backnews=(String)request.getAttribute("backnews"); %>
<h2><%= backnews %></h2><br>
<% String logoname=(String)request.getAttribute("logoname"); %>
<h2><%= logoname %></h2><br>
<% String message=(String)request.getAttribute("message"); %>
<h2><%= message %></h2>

这是一个发表留言的MVC，请大家帮忙看看哪里错了！
为什么数据库没有更新？为什么servlet传的值，最后获取却是空？？？