please help me to solve this problem database connection using servletcontext

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JavaSE

J2EE

Hi
I have created a database connection using servletcontext. In this code when I login for the first time, it will exceute sucessfully but when I use it again, it will throw a sql exception. I don't understand what the problem is

The code is 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();
}