前臺（ftl）

<a href="/file/down?url='html路徑'&name='檔名'"  download>下載</a>

後臺（controller）

@GetMapping("/file/down")
public void Down(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    req.setCharacterEncoding("UTF-8");
    String name = req.getParameter("name");//獲取要下載的檔名
    //第一步：設定響應型別
    name = URLEncoder.encode(name, "UTF-8");
    resp.setHeader("Content-Disposition", "attachment;filename="+name);   
    resp.setContentType("application/force-download");//應用程式強制下載
    //第二讀取檔案
    String url = req.getParameter("url");
    OutputStream out = resp.getOutputStream();
    CloseableHttpClient httpclient = HttpClients.createDefault();
    HttpGet httpGet = new HttpGet(url);
    CloseableHttpResponse httpResp = httpclient.execute(httpGet);
            try {
                HttpEntity httpEntity = httpResp.getEntity();
        resp.setContentLength((int)httpEntity.getContentLength());
                IOUtils.copy(httpEntity.getContent(), out);
            } catch (Exception ex) {
            httpclient.close();
            }
    out.flush();
    out.close();    
}