code: @SuppressWarnings("unchecked")
    protected boolean beforeExport() {
      if (null == propertyExtractor) {
        propertyExtractor = context.get("extractor", PropertyExtractor.class);
      }
      if (null == metadatas) {
        metadatas = (List<Metadata>) context.get("metadatas");
      }
      if (null == metadatas || null == propertyExtractor) {
        logger.error("without metadatas or propertyExtractor,exporter stopped!");
        return false;
      }
      return true;
    }
docs: <p>
  beforeExport.
  </p>
  
  @return a boolean.