获取客户端操作系统名称

Question

我正在尝试使用以下代码获取servlet中客户端的操作系统名称：

public class SomeServlet extends HttpServlet {
 public void doGet(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {
  String userAgentString = request.getHeader("User-Agent");
  UserAgent userAgent = UserAgent.parseUserAgentString(userAgentString);
  OperatingSystem os = userAgent.getOperatingSystem();
  }
}

我收到错误UserAgent无法解析为某种类型。我该如何解决这个问题？