apollo-client/src/main/java/com/ctrip/framework/apollo/spi/DefaultConfigFactory.java/DefaultConfigFactory/createConfigFile
class DefaultConfigFactory:  @Override
  public ConfigFile createConfigFile(String namespace, ConfigFileFormat configFileFormat) {
    ConfigRepository configRepository = createConfigRepository(namespace);
    switch (configFileFormat) {
      case Properties:
        return new PropertiesConfigFile(namespace, configRepository);
      case XML:
        return new XmlConfigFile(namespace, configRepository);
      case JSON:
        return new JsonConfigFile(namespace, configRepository);
      case YAML:
        return new YamlConfigFile(namespace, configRepository);
      case YML:
        return new YmlConfigFile(namespace, configRepository);
      case TXT:
        return new TxtConfigFile(namespace, configRepository);
    }

    return null;
  }