An exception occurred processing

HTTP Status 500 -type Exception reportmessagedescription The server encountered an internal error () that prevented it from fulfilling this request.exceptionorg.apache.jasper.JasperException: An exception occurred processing JSP page /Modpassword.jsp at line 9996:     {
97:        //检测旧密码输入是否正确
98:   out.println("<script>alert('test success!');</script>"); 
99:    oldPas=new String(request.getParameter("Oldpassword").getBytes("ISO8859_1"), "GBK");
100:    Statement smt=con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
101:    sql="select * from admin where username='"+ session.getAttribute("username")+"'";
102:           ResultSet st=smt.executeQuery(sql);
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:510)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)root causejava.lang.NullPointerException
org.apache.jsp.Modpassword_jsp._jspService(Modpassword_jsp.java:174)
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:377)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)note The full stack trace of the root cause is available in the Apache Tomcat/6.0.26 logs.