CLASSE JAVA CONEXAO
public class ConexaoBD {
    static String url = "jdbc:mysql://localhost:3306/helpdesk";
    static String userdb = "root";
    static String passdb = "";

    private ConexaoBD() {}

    public static Connection getConnection() throws SQLException {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            return (Connection) DriverManager.getConnection(url, userdb, passdb);
        } catch (ClassNotFoundException e) {
            throw new SQLException(e.getMessage());
        }
    }

}

SERVLET

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        if ((request.getParameter("acao") != null && request.getParameter("acao").equals("autenticar"))) {
            boolean status;
            try {
                UsuarioDAO usuario = UsuarioDAO.getInstancia();
                status = usuario.Autenticar(request.getParameter("login"), request.getParameter("senha"));
                if (status = true) {
                    response.sendRedirect("index.jsp");
                } else {
                    response.sendRedirect("login.jsp");
                }
            } catch (SQLException ex) {
                Logger.getLogger(UsuarioServlet.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

DAO
    public boolean Autenticar(String login, String senha) throws SQLException {
        String sql = "select senha from usuario where login = ?";
        PreparedStatement stmt = (PreparedStatement) connection.prepareStatement(sql);
        stmt.setString(1, login);
        ResultSet rs = stmt.executeQuery();
        UsuarioBean usuario = new UsuarioBean();
        while(rs.next()) {
            usuario.setSenha(rs.getString("senha"));
        }
        rs.close();
        stmt.close();

        if (senha.equals(usuario.getSenha())) {
            return true;
        } else {
            return false;
        }
    }