Java实现网络文件下载以及下载到指定目录

	public InputStream getDownloadInputStreamByUrl(String downloadUrl) {
		if (ObjectUtils.isEmpty(downloadUrl)) {
            throw new RuntimeException("无效参数：fileUrl 不能为空");
        }
		
		InputStream in = null;
		try {
			URL url = new URL(downloadUrl);

			HttpURLConnection conn = (HttpURLConnection) url.openConnection();
			// 设置超时时间为3秒
			conn.setConnectTimeout(3 * 1000);
			// 防止屏蔽程序抓取而返回403错误
			conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 5.0; Windows NT; DigExt)");
			// 得到输入流
			in = conn.getInputStream();
		} catch (Exception e) {
			throw new RuntimeException(e);
		}
		return in;
	}

	@GetMapping("/fileDownload/{filename}")
    public void downloadPdfFile(HttpServletResponse response, @PathVariable(name="filename") String filename) {
        try {
            if (StringUtils.isEmpty(filename)) {
            	throw new RuntimeException("文件名不可为空！");
            }
            //通过文件名查找文件对应的url
            String downloadUrl = fileDownloadService.getUrlByFiname(filename);
            if (StringUtils.isEmpty(filename)) {
            	throw new RuntimeException("文件对应地址不存在");
            }
            // 此处异常方法中已处理
            InputStream in = fileDownloadService.getDownloadInputStreamByUrl(downloadUrl);
   			// 设置编码格式
            response.setCharacterEncoding("UTF-8");
            /* 若要下载指定目录，以下两行需注释，否则会下载两份，一份到指定目录，一份到浏览器下载目录 */
            // 设置下载格式
            response.setHeader("Content-Type", "application/pdf");
            // 注释这行代码，可实现 预览 功能(只限pdf文件)
            response.setHeader("Content-Disposition","attachment;filename=" + URLEncoder.encode(filename, "UTF-8"));
			//下载到指定目录
			/*
			File file = new File("C:\\tmp");
            if (!file.exists()) {
                //如果文件夹不存在，则创建新的的文件夹
                file.mkdirs();
            }

            File file1 = new File(file.getPath() + File.separator + filename);
           	OutputStream outputStream = new FileOutputStream(file1);
           	FileCopyUtils.copy(in, outputStream);
           	*/

            StreamUtils.copy(in, response.getOutputStream());
        } catch (Exception e) {
        	throw new RuntimeException(e);
        } 
    }