Sun Java System Portal Server 7 Configuration Guide

Session Information

If any struts application, deployed as a portlet, is invalidating the session using session.invalidate(), the session obtained by the struts-portlet bridge becomes the invalid one. Because of this, the bridge is unable to store rendering related information. In application server, struts application, deployed as a portlet, must not use session.invalidate() as the same session is used by struts portlet bridge.