code: protected static InputStream downloadMaterial(String url, String params) throws IOException {
  		URL _url = new URL(url);
  		HttpURLConnection conn = (HttpURLConnection) _url.openConnection();
  		// 连接超时
  		conn.setConnectTimeout(25000);
  		// 读取超时 --服务器响应比较慢，增大时间
  		conn.setReadTimeout(25000);
  		conn.setRequestMethod("POST");
  		conn.setRequestProperty("Content-Type", "Keep-Alive");
  		conn.setRequestProperty("User-Agent", DEFAULT_USER_AGENT);
  		conn.setDoOutput(true);
  		conn.setDoInput(true);
  		conn.connect();
  		if (StrKit.notBlank(params)) {
  			OutputStream out = conn.getOutputStream();
  			out.write(params.getBytes(Charsets.UTF_8));
  			out.flush();
  			IOUtils.closeQuietly(out);
  		}
  		return conn.getInputStream();
  	}
docs: 获取永久素材
  
  @param url
  素材地址
  @return params post参数
  @return InputStream 流，考虑到这里可能返回json或file
  @throws IOException