apollo-core/src/main/java/com/ctrip/framework/foundation/internals/DefaultProviderManager.java/DefaultProviderManager/provider
class DefaultProviderManager:  @Override
  @SuppressWarnings("unchecked")
  public <T extends Provider> T provider(Class<T> clazz) {
    Provider provider = m_providers.get(clazz);

    if (provider != null) {
      return (T) provider;
    }
    logger.error("No provider [{}] found in DefaultProviderManager, please make sure it is registered in DefaultProviderManager ",
        clazz.getName());
    return (T) NullProviderManager.provider;
  }