一般来说，客户端IP通过request.getRemoteAddr()即可得到。但如果客户端通过代理访问后端时，request.getRemoteAddr()可能就不是客户端实际IP，此时需要通过一系列处理来得到客户端实际IP。

package com.origin.util;

import lombok.extern.slf4j.Slf4j;

import javax.servlet.http.HttpServletRequest;
import java.net.InetAddress;
import java.net.UnknownHostException;

/**
 * @Author xiye
 * @File HttpClientUtil
 * @Date 2020/12/10 16:11
 * @Desc 网络处理工具
 */
@Slf4j
public class HttpClientUtil {
    // 代理请求头名称集合。注意顺序
    public static final String[] IP_HEADER = new String[] {"X-Real-IP", "X-Forwarded-For", "Proxy-Client-IP", "WL-Proxy-Client-IP"};

    /* ========================================================================================= */
    /**
     * @author  xiye
     * @date    2020/12/11  16:39
     * @param	request
     * @desc    获取客户端IP
     * @return  java.lang.String
     */
    public static String getRemoteAddress (HttpServletRequest request) {
        /*String client_ip = request.getHeader("x-forwarded-for");
        if(client_ip == null || client_ip.length() == 0 || "unknown".equalsIgnoreCase(client_ip)) {
            client_ip = request.getHeader("Proxy-Client-IP");
        }
        if(client_ip == null || client_ip.length() == 0 || "unknown".equalsIgnoreCase(client_ip)) {
            client_ip = request.getHeader("WL-Proxy-Client-IP");
        }*/
        String client_ip = null;
        int tag = 0;
        for (String header : IP_HEADER) {
            client_ip = request.getHeader(header);
            if (invalidIp(client_ip)) { tag ++; continue; }
            break;
        }
        if (tag < IP_HEADER.length) log.info("【{}代理IP】{}", IP_HEADER[tag], client_ip);

        if(invalidIp(client_ip)) {
            client_ip = request.getRemoteAddr();
            log.info("【无代理IP】{}", client_ip);
        }
        // 对于通过多个代理的情况，第一个IP为客户端真实IP,多个IP按照','分割
        if(client_ip != null && client_ip.length() > 15){ //"***.***.***.***".length() = 15
            if(client_ip.indexOf(",") > 0){
                client_ip = client_ip.substring(0,client_ip.indexOf(","));
            }
        }
        return client_ip;
    }

    /**
     * @author  xiye
     * @date    2020/12/11  15:25
     * @desc    获取内网IP
     * @return  java.lang.String
     */
    public static String getInnerIp () {
        try {
            return InetAddress.getLocalHost().getHostAddress();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        return null;
    }


    /* ========================================= 公共 ================================================= */
    private static boolean invalidIp (String ip) {
        return ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip);
    }
}

【参考1】nginx代理转发客户端IP：“X-Real-IP”、“X-Forwarded-For”