apollo-client/src/main/java/com/ctrip/framework/apollo/internals/DefaultConfigManager.java/DefaultConfigManager/getConfigFile
class DefaultConfigManager:  @Override
  public ConfigFile getConfigFile(String namespace, ConfigFileFormat configFileFormat) {
    String namespaceFileName = String.format("%s.%s", namespace, configFileFormat.getValue());
    ConfigFile configFile = m_configFiles.get(namespaceFileName);

    if (configFile == null) {
      synchronized (this) {
        configFile = m_configFiles.get(namespaceFileName);

        if (configFile == null) {
          ConfigFactory factory = m_factoryManager.getFactory(namespaceFileName);

          configFile = factory.createConfigFile(namespaceFileName, configFileFormat);
          m_configFiles.put(namespaceFileName, configFile);
        }
      }
    }

    return configFile;
  }