apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/service/ConfigsImportService.java/ConfigsImportService/readContent
class ConfigsImportService:  private String readContent(ZipInputStream zipInputStream) {
    try (ByteArrayOutputStream out = new ByteArrayOutputStream()) {
      byte[] buffer = new byte[1024];
      int offset;
      while ((offset = zipInputStream.read(buffer)) != -1) {
        out.write(buffer, 0, offset);
      }
      return out.toString("UTF-8");
    } catch (IOException e) {
      LOGGER.error("Read file content from zip error.", e);
      return null;
    }
  }