code: public static String getIpAddress(HttpServletRequest request) {
          String ip = request.getHeader("x-forwarded-for");
          if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
              ip = request.getHeader("Proxy-Client-IP");
          }
          if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
              ip = request.getHeader("WL-Proxy-Client-IP");
          }
          if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
              ip = request.getHeader("HTTP_CLIENT_IP");
          }
          if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
              ip = request.getHeader("HTTP_X_FORWARDED_FOR");
          }
          if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
              ip = request.getRemoteAddr();
          }
          return ip;
      }
docs: 获取用户真实IP地址，不使用request.getRemoteAddr();的原因是有可能用户使用了代理软件方式避免真实IP地址,
  <p>
  可是，如果通过了多级反向代理的话，X-Forwarded-For的值并不止一个，而是一串IP值，究竟哪个才是真正的用户端的真实IP呢？
  答案是取X-Forwarded-For中第一个非unknown的有效IP字符串。
  <p>
  如：X-Forwarded-For：192.168.1.110, 192.168.1.120, 192.168.1.130,
  192.168.1.100
  <p>
  用户真实IP为： 192.168.1.110
  
  @param request
  @return