please help me solve this problem when i am create database connection using servlecontext

hi...

I have create a database connection using servletcontext . in this code when i login first time it will exceute sucessfully but when i use again the same page it will throw sql exception but i don't understand what is the problem ....

the code are as follows. the servlet context classss.

public void contextInitialized(ServletContextEvent arg0) {
ServletContext sc = arg0.getServletContext();
String url=sc.getInitParameter("url");
String user=sc.getInitParameter("username");
String pass= sc.getInitParameter("password");
database db = new database(url, user, pass);
System.out.println("hello*******************************************************************************");
sc.setAttribute("db", db);
    }

//database connection class

Connection con = null;
   public database(String url,String user, String pass) {

               try {
                   Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

                   this.con = DriverManager.getConnection(url,user,pass);

               } catch (Exception e) {
                   // TODO Auto-generated catch block
                   System.out.println("sorry********************"+e);
                   e.printStackTrace();
               }
   }


//the servlet class.

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String user= request.getParameter("username");
        String password= request.getParameter("password");

    database db = (database)getServletContext().getAttribute("db");
    try {
        PreparedStatement ps = db.con.prepareStatement("select * from admintable where vusername=? and vpassword=?");
        ps.setString(1,user);
        ps.setString(2, password);
        ResultSet rs= ps.executeQuery();
        if(rs.next())
        {
            System.out.println("************login sucessfully*******************");
            PrintWriter out=response.getWriter();
            out.println("hello arunnarwal");
            db.con.close();


        }
    } catch (Exception e) {
        // TODO Auto-generated catch block
        System.out.println("sorry**************************");
        e.printStackTrace();
    }