獲取使用者真實IP地址，不使用request.getRemoteAddr();的原因是有可能使用者使用了代理軟體方式避免真實IP地址, 參考文章：
 http://developer.51cto.com/art/201111/305181.htm
  
  可是，如果通過了多級反向代理的話，X-Forwarded-For的值並不止一個，而是一串IP值，究竟哪個才是真正的使用者端的真實IP呢？
 答案是取X-Forwarded-For中第一個非unknown的有效IP字串。
  
  如：X-Forwarded-For：192.168.1.110, 192.168.1.120, 192.168.1.130,
 192.168.1.100
  

 使用者真實IP為： 192.168.1.110

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;
	}