This page is for documenting the usage of Spring in uPortal. There a general Spring page for discussion of Spring itself and its use across JA-SIG.
Why does uPortal use the Spring framework?
How?
How does uPortal 2.5 use Spring?
How does uPortal 3 use Spring?
Spring for channels and portlets
The Alerts Portlet is built using the Spring PortletMVC framework.
All of the Spring documentation is applicable to understanding Spring PortletMVC except for some of the WebMVC stuff, but even that will be helpful as the PortletMVC follows the same overall pattern with a slight twist to support the additional constraints/features of the Portlet API. I would suggest starting with the Spring MVC Tutorial, and tracking PortletMVC development.