public HttpServletResponse download(String path, HttpServletResponse response) {
        try {
            // path是指欲下載的檔案的路徑。
            File file = new File(path);
            // 取得檔名。
            String filename = file.getName();
            // 取得檔案的字尾名。
            String ext = filename.substring(filename.lastIndexOf(".") + 1).toUpperCase();
 
            // 以流的形式下載檔案。
            InputStream fis = new BufferedInputStream(new FileInputStream(path));
            byte[] buffer = new byte[fis.available()];
            fis.read(buffer);
            fis.close();
            // 清空response
            response.reset();
            // 設定response的Header
            response.addHeader("Content-Disposition", "attachment;filename=" + new String(filename.getBytes()));
            response.addHeader("Content-Length", "" + file.length());
            OutputStream toClient = new BufferedOutputStream(response.getOutputStream());
            response.setContentType("application/octet-stream");
            toClient.write(buffer);
            toClient.flush();
            toClient.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
        return response;
    }
 
    public void downloadLocal(HttpServletResponse response) throws FileNotFoundException {
        // 下載本地檔案
        String fileName = "Operator.doc".toString(); // 檔案的預設儲存名
        // 讀到流中
        InputStream inStream = new FileInputStream("c:/Operator.doc");// 檔案的存放路徑
        // 設定輸出的格式
        response.reset();
        response.setContentType("bin");
        response.addHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
        // 迴圈取出流中的資料
        byte[] b = new byte[100];
        int len;
        try {
            while ((len = inStream.read(b)) > 0)
                response.getOutputStream().write(b, 0, len);
            inStream.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
 
    public void downloadNet(HttpServletResponse response) throws MalformedURLException {
        // 下載網路檔案
        int bytesum = 0;
        int byteread = 0;
 
        URL url = new URL("windine.blogdriver.com/logo.gif");
 
        try {
            URLConnection conn = url.openConnection();
            InputStream inStream = conn.getInputStream();
            FileOutputStream fs = new FileOutputStream("c:/abc.gif");
 
            byte[] buffer = new byte[1204];
            int length;
            while ((byteread = inStream.read(buffer)) != -1) {
                bytesum += byteread;
                System.out.println(bytesum);
                fs.write(buffer, 0, byteread);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

參考地址：https://blog.csdn.net/pangqiandou/article/details/53234586

https://blog.csdn.net/wangfeng8720/article/details/6588679