The Portlet Specification introduces PortletFilter as a standard approach to extend the behaviors of portlet objects. For example, a filter can transform the content of portlet requests and portlet responses. According to the Portlet Specification, normally there are three steps in setting up a portlet filter:
Implement a PortletFilter object
Define the filter in portlet application deployment descriptor
Define the filter mapping in portlet definitions
Two first steps are quite simple and easy to be done, however, at the step 3, developers/administrators need to replicate the filter mapping in many portlet definitions, that makes work error and tedious in several use cases. The global portlet feature is designed to compensate such limitation.