apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/util/ConfigToFileUtils.java/ConfigToFileUtils/fileToString
class ConfigToFileUtils:  public static String fileToString(InputStream inputStream) {
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
    return bufferedReader.lines().collect(Collectors.joining(System.lineSeparator()));
  }