数据库连接出错?

这是我的代码:
<%@ page contentType="text/html;charset=GBK" language="java" import="java.sql.*,java.util.Date" errorPage="" %>
<jsp:useBean id="mdb" class="ckstudio.db.faq" scope="page"/>
    <%
boolean foundeer=false;
String user_name,user_pass;
if(!foundeer){    if (session.getAttribute("user_name")==null){
    user_name=request.getParameter("user_name");
    user_pass=request.getParameter("user_pass");    if(user_name==null || user_pass==null){
        out.print("<font size=4 color=red>用户名和用户密码不能为空!</font>");
    foundeer=true;
}    String sql="select * from member where user_name='"+user_name+"'";
    ResultSet rs;
    rs=mdb.executeQuery(sql); if(!rs.next()){     //这是报错的地方,
  out.print("<font size=4 color=red>你输入的用户不存在!</font>");
foundeer=true;
}
else{
if(user_pass.equals(rs.getString("user_pass"))){
           session.setAttribute("user_name",""+rs.getString("user_name")+"");
       session.setAttribute("user_type","yes");
String sql1="update member set user_lasttime='"+((new java.util.Date()).toLocaleString())+"',user_lastip='"+(String)request.getRemoteHost()+"' where user_name='"+user_name+"'";
        mdb.executeUpdate(sql1);
response.sendRedirect("index.jsp");
}
    else{
        out.print("<font size=4 color=red>你输入的密码错误!</font>");
        foundeer=true;
}
}
}else response.sendRedirect("index.jsp"); }if(foundeer){
          %><br><a href="javascript:history.go(-1)">重新登陆</a><%
}%>出错的报告:
18:     ResultSet rs;
19:     rs=mdb.executeQuery(sql);
20: 
21:  if(!rs.next()){
22:  out.print("<font size=4 color=red>你输入的用户不存在!</font>");
23:  foundeer=true;
24:  }
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:505)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
org.apache.jsp.login_jsp._jspService(login_jsp.java:85)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.18 logs.
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