questa è la servlet

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import M3_package.*;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 *
 * @author Simone Balloccu
 */
@WebServlet(name = "login_servlet", urlPatterns = {"/login_servlet"})
public class login_servlet extends HttpServlet {

    /**
     * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
     * methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");

        HttpSession session = request.getSession(true);

        if(request.getParameter("button") != null)
        {
            String user = request.getParameter("user");
            String password = request.getParameter("password");

            ArrayList<Utente> lista_Utenti = M3_Factory.getInstance().getListautenti();
            for(Utente u : lista_Utenti)
            {
                if(u.getUsername().equals(user) &&
                   u.getPassword().equals(password))
                {
                    session.setAttribute("loggato", true);

                    if(u instanceof Venditore)
                    {
                        request.setAttribute("Venditore", u);
                        request.setAttribute("cliente", M3_Factory.getInstance().getListaClienti());
                        request.getRequestDispatcher("venditore.jsp").forward(request, response);
                    }
                    else
                    {
                        request.setAttribute("Cliente", u);
                        request.getRequestDispatcher("cliente.jsp").forward(request, response);
                    }
                }
            }
        }
        request.getRequestDispatcher("login.jsp").forward(request, response);

    }

    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Handles the HTTP <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>

}