取得客户端真实IP

 通常情况下request.getRemoteAddr()就可以获得IP地址，可是当apache,nginx等服务器做了负载之后，这个就不怎么灵了

为了方便使用，统一的获取IP方法归集如下：

public static String getIp(HttpServletRequest request) {
		if (request == null)
			return "";
		String ip = request.getHeader("X-Requested-For");
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getHeader("X-Forwarded-For");
		}
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getHeader("Proxy-Client-IP");
		}
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getHeader("WL-Proxy-Client-IP");
		}
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getHeader("HTTP_CLIENT_IP");
		}
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getHeader("HTTP_X_FORWARDED_FOR");
		}
		if (StringUtils.isEmpty(ip) || "unknown".equalsIgnoreCase(ip)) {
			ip = request.getRemoteAddr();
		}
		return ip;
	}