简单封装下：

/**
     * 从网络Url中下载文件
     * @param response 响应对象
     * @param URL      文件网络地址
     * @param fileName 下载到本地的文件名称
     * @param fileExt  文件后缀
     * @param contentType 文件的contentType
     */
    protected void downloadFromUrl(HttpServletResponse response, String URL, String fileName, String fileExt, String contentType) {
        try {
            URL url = new URL(URL);
            URLConnection conn = url.openConnection();
            InputStream is = conn.getInputStream();
            download(response, is, fileName, fileExt, contentType);
        } catch (Exception e) {
            log.error(e.getMessage(), e);
        }
    }

    /**
     * 下载文件
     * @param response 响应对象
     * @param is       输入流
     * @param fileName 下载到本地的文件名称
     * @param fileExt  文件后缀
     * @param contentType 文件的contentType
     */
    protected void download(HttpServletResponse response, InputStream is, String fileName, String fileExt, String contentType) {
        if (null != is) {
            response.reset(); // 必要地清除response中的缓存信息
            response.setContentType(contentType);
            try {
                String encodeFileName = java.net.URLEncoder.encode(fileName + "." + fileExt, "UTF-8");
                encodeFileName = encodeFileName.replaceAll("%28", "(");
                encodeFileName = encodeFileName.replaceAll("%29", ")");
                response.setHeader("Content-Disposition",
                    "attachment; filename=" + encodeFileName);
            } catch (UnsupportedEncodingException e1) {
                e1.printStackTrace();
            }

            javax.servlet.ServletOutputStream out;
            try {
                out = response.getOutputStream();
                byte[] content = new byte[1024]; // is.available()
                int length = 0;
                while ((length = is.read(content)) != -1) {
                    out.write(content, 0, length);
                }
                out.flush();
                out.close();
                is.close();
            } catch (IOException e) {
                log.error(e.getMessage(), e);
            }
        }
    }