java获取HttpServletRequest 的客户端ip

　　常见的问题，记录操作日志中发现，getRemoteAddr取到的客户端的ip地址不是真实的ip，需要解析请求头中的信息获取正确ip。

public static String getIpAddress(HttpServletRequest request) {

        String ip = request.getHeader("X-Forwarded-For");

        if (ip == null || ip.length() == 0 || UNKNOWN.equalsIgnoreCase(ip)) {
            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.getHeader("X-Real-IP");
            }
            if (ip == null || ip.length() == 0 || UNKNOWN.equalsIgnoreCase(ip)) {
                ip = request.getRemoteAddr();
            }
        } else {
            String[] ips = ip.split(",");
            for (int index = 0; index < ips.length; index++) {
                String strIp = ips[index];
                if (!(UNKNOWN.equalsIgnoreCase(strIp))) {
                    ip = strIp;
                    break;
                }
            }
        }
        return ip;
    }

　　查了下，对应的header头解释如下。

　　x-forwarded-for只是代理的其中一种，最早Squid 引入，还有很多其他的字段

　　Proxy-Client-IP：使用apache代理

　　WL-Proxy-Client-IP：使用weblogic时

　　HTTP_CLIENT_IP和HTTP_X_FORWARDED_FOR：有一些代理

　　X-Real-IP：Nginx代理

　　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，取X-Forwarded-For中第一个非unknown的有效IP字符串。 

　　每经过一个代理做一次转发，X-Forwarded_For就会在后面追加一个代理IP，并且以转发顺序排序。