java.lang.NullPointerException

<%@ page contentType="text/html;charset=gb2312" import="java.sql.*"%>
<%@ page pageEncoding="gb2312" %>
<% request.setCharacterEncoding("gb2312"); %>
<jsp:useBean id="sqlBean" class="BYSJ.sqldata">
</jsp:useBean><html>
<html><head><title></title></head>
<body>
<% String username=request.getParameter("username");
   String pwd=request.getParameter("pwd"); //smt=conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
    String sql="select * from user where username='"+username+"' and password='"+pwd+"'";
    ResultSet rs=sqlBean.executeQuery(sql);
    if(!rs.next())
    {
    String errmsg="error1";
    response.sendRedirect("login.jsp?errmsg="+errmsg);
    return;
    }
    request.getSession(true);
    String chk="true";
    session.putValue("Enter",chk);
    session.putValue("username",rs.getString("username"));
    response.sendRedirect("manage.jsp");
%>
</body></html>
这段代码怎么出现java.lang.NullPointerException
        org.apache.jsp.BYSJ.logincheck_jsp._jspService(logincheck_jsp.java:65)
        org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
        org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
        org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:802)