public void insertarLibro()
    {
        String url = "jdbc:sqlserver://localhost:1433;databaseName=Biblioteca";
        String user = "mi_usuario";
        String password = "mi_contraseña";
        String fk_dni = "SELECT dni FROM Autores WHERE nombre = '?'";


        try
        {
            Connection cn = DriverManager.getConnection(url, user, password);
            PreparedStatement ps = cn.prepareStatement(fk_dni);
            ResultSet rs = ps.executeQuery();
            String nombre = (String)cboAutor.getSelectedItem();

            while(rs.next())
            {
                ps.setString(1, nombre);
            }

            System.out.println("DNI : " + rs.getString(1));
        }
        catch(SQLException e)
        {
            System.out.println("ERROR : ");
            e.printStackTrace(System.out);
        }

    }