public class ControlIndex extends HttpServlet {
    private List errors = new Vector();
    private HttpSession session;
    
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        out.close();
    }

 protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        doPost(request, response);
    }

  protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        session = request.getSession();
        errors.clear();                

        boolean statusLogin = false;
        String aksi = request.getParameter("aksi");
        
        if (aksi.equals("login")){        
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            User user = null;
            // input validation
            if ((username == null) || !username.trim().matches("^[a-zA-Z]+$")){
                errors.add("Username cannot contains white space or non alphabetic characters.");
            } 
            // execute
            if (errors.size()==0){
                user = User.getUser(username);
                if (user == null){
                    errors.add("Cannot find those username.");
                } else{ 
                    if (user.validatePassword(password)){
                        statusLogin = true;
                    } else{
                        errors.add("Wrong password.");
                    }                
                }
            }
            if (statusLogin){
                //set session
                session.setAttribute("user", user);
                RequestDispatcher view = request.getRequestDispatcher("mgmtActivity.jsp");
                try{
                    view.forward(request, response);
                }catch(ServletException e){
                    e.printStackTrace();
                }  
            }
        }else if (aksi.equals("logout")){
            //delete session
            session.setAttribute("user", null);
        }
        if (errors.size()!=0){
            request.setAttribute("errors", errors);
        }

        RequestDispatcher view = request.getRequestDispatcher("main.jsp");
        try{
            view.forward(request, response);
        }catch(ServletException e){
            e.printStackTrace();
        }
    }
}