Web Application referencing another web application

How would you go about referencing one WEB application inside another WEB application. I have Java Bean classes built using XSDs in Project A. I want to use them in Project B (and other future projects - hence, I don't want to copy those classes into Project B). I've put Project A into the Build Path and Deployment Assembly, but any time I run the B's Servlet using A's classes, I get the NoClassDefFoundError exception.

Why is that?