controller返回值为空

<li>If a {@link View} or view name is not supplied by the user, then the configured {@link
* RequestToViewNameTranslator} will translate the current request into a view name. The corresponding bean name is
* "viewNameTranslator"; the default is {@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}.

<li>如果用户未提供{@link View}或视图名称，则配置{@link
  * RequestToViewNameTranslator}会将当前请求转换为视图名称。 相应的bean名称是
  *“viewNameTranslator”; 默认值为{@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}。