UrlBasedViewResolvers should not override custom requestContextAttribute with null

[sbrannen]

Overview

org.springframework.web.servlet.view.UrlBasedViewResolver.buildView(String) and org.springframework.web.reactive.result.view.UrlBasedViewResolver.createView(String) both override the requestContextAttribute in the created View even if the requestContextAttribute in the UrlBasedViewResolver is null.

Consequently if a custom subclass of org.springframework.web.servlet.view.AbstractView or org.springframework.web.reactive.result.view.AbstractUrlBasedView configures the requestContextAttribute, it will be overwritten with null if the view is dynamically instantiated by a UrlBasedViewResolver. On the other hand, the custom configured requestContextAttribute will remain in tact if the view is used outside of a UrlBasedViewResolver, and this subtle difference can lead to unexpected results.

If the user explicitly configures the requestContextAttribute for a UrlBasedViewResolver, the user is probably (hopefully) aware of the effect. However, if the user does not explicitly configure the requestContextAttribute for a UrlBasedViewResolver we should not overwrite any custom configuration with null.

Deliverables

• Only propagate a non-null requestContextAttribute to a dynamically instantiated View in UrlBasedViewResolver in spring-webmvc.
• Only propagate a non-null requestContextAttribute to a dynamically instantiated View in UrlBasedViewResolver in spring-webflux.

[sbrannen]

This work has already been completed in the following feature branch.

https://github.com/spring-projects/spring-framework/commits/issues/gh-23129