这个题的有两个难点：
1、读取资源文件，下载文件；
2、spring boot controller service的写法。

一、Controller Service代码如下：

package com.qianqiangongzi.controller;

import javax.servlet.http.HttpServletResponse;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import com.qianqiangongzi.utils.DownloadResourceFileUtils;

/**
 * 下载资源文件接口
 * 
 * @author 谦谦公子爱编程
 *
 */
@RestController
@RequestMapping(value = "/api/DownloadResourceFile")
public class DownloadResourceFileController {

    /**
     * 下载资源文件
     * 
     * @param response
     * @throws Exception
     */
    @RequestMapping(value = "/downloadFile", method = RequestMethod.GET)
    public void downloadFile(HttpServletResponse response) throws Exception {
        DownloadResourceFileUtils.downloadFile(response);
    }
}

二、下载文件工具类

package com.qianqiangongzi.utils;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.io.OutputStream;

import javax.servlet.http.HttpServletResponse;

import org.apache.commons.lang3.exception.ExceptionUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.util.ResourceUtils;

/**
 * 下载资源文件
 * 
 * @author 谦谦公子爱编程
 *
 */
public class DownloadResourceFileUtils {
    public static final String INLINE = "inline"; // 在线打开
    public static final String ATTACHMENT = "attachment"; // 附件下载

    public static final Logger logger = LoggerFactory.getLogger(DownloadResourceFileUtils.class);

    public static void downloadFile(HttpServletResponse response) throws Exception {
        // 下载文件部分路径
        String fileName = "upload\\load.txt";
        // 获取文件根路径
        String basePath = getResourceBasePath();
        // 获取文件路径
        File downloadFile = new File(basePath, fileName);
        // 下载问价
        downloadFile(response, downloadFile.getAbsolutePath(), downloadFile.getName(), ATTACHMENT);
    }

    /**
     * 获取项目根路径
     * 
     * @return
     */
    private static String getResourceBasePath() {
        // 获取跟目录
        File path = null;
        try {
            path = new File(ResourceUtils.getURL("classpath:").getPath());
        } catch (FileNotFoundException e) {
            // nothing to do
        }
        if (path == null || !path.exists()) {
            path = new File("");
        }

        String pathStr = path.getAbsolutePath();
        // 如果是在eclipse中运行，则和target同级目录,如果是jar部署到服务器，则默认和jar包同级
        pathStr = pathStr.replace("\\target\\classes", "");
        pathStr += "\\src\\main\\resources";

        return pathStr;
    }

    /*
     * 下载文件，需要文件绝对路径
     * 
     * @param response 写回流对象
     * 
     * @param filePath 文件的绝对路径
     * 
     * @param fileName 下载时的文件名称
     * 
     * @param downloadWay 下载方式INLINE（在线打开） or ATTACHMENT（附件下载）
     * 
     * @throws Exception
     */
    private static void downloadFile(HttpServletResponse response, String filePath, String fileName, String downloadWay)
            throws Exception {
        try {
            response.addHeader("Content-Disposition", downloadWay + "; filename=" + fileName);
            InputStream is = new FileInputStream(filePath);
            byte[] b = new byte[8 * 1024];
            int length = -1;
            OutputStream out = response.getOutputStream();
            while ((length = is.read(b)) != -1) {
                out.write(b, 0, length);// 输出
            }
            out.close();
            is.close();
        } catch (Exception e) {
            if (logger.isErrorEnabled()) {
                logger.error(ExceptionUtils.getMessage(e));
            }
            throw e;
        }
    }
}

经过测试，没有任何问题：
浏览器输入下载地址：
http://localhost:8088/api/DownloadResourceFile/downloadFile

编码不易，望采纳！

https://blog.csdn.net/Dreamhai/article/details/80631821