In a J2EE application, how do I create a global Filter that can access other webapps installed on that app server?
Some application servers/servlet containers (Tomcat, JBoss) provide the cross context feature. It does depend on what you're running your application within however, it's not a J2EE feature.
http://tomcat.apache.org/tomcat-5.5-doc/config/context.html
I'm not sure whether this will help you in what you want to do. What is it exactly? : )
Take a look at this response as well:
What does the crossContext attribute do in Tomcat? Does it enable session sharing?