code: public <T> T getBean(Class<T> beanType) throws BeansException {
          return beanType.cast(BeanFactoryUtils.beanOfTypeIncludingAncestors(this, beanType));
      }
docs: 返回对应类型的唯一 Bean, 包括可能的祖先 {@link ApplicationContext} 中对应类型的 Bean.
  
  @param beanType - Bean 的类型
  
  @throws BeansException