private Boolean find_driver()
  {
    try
    {
      if (driver_class == null)
          driver_class = Class.forName(driver_name);
      if (driver_class == null)
          driver_class = Class.forName(driver_name, true, ClassLoader.getSystemClassLoader());
      return true;
    }
    catch (Exception ex)
    {
      throw new InvalidQueryException("Unable to find database driver");
    }
  }