Http Session and PortletSession

When the user initially accesses a portlet, a PortletSession is created. The portlet
session stores transient data associated with an individual use of the portlet.

The PortletSession object extends from HttpSession and serves much the same
purpose. The PortletSession is intended to represent an ongoing conversation between the client and the portlet. To this end, the PortletSession can be used to store information needed between requests. The PortletSession is intended to store data between requests, not between portlets. As such, data stored in the session by one portlet is not accessible by another. The PortletSession is retrieved from the request object.

Well both Portlets and Servlets are loaded using the same classloader.So it's a plain fact that session can be made available in both the contexts.

As far as the portlets are concerned,
The PortletSession is namespaced :
1) PORTLET_SCOPE : Only available to the particular portlet.
2) APPLICATION_SCOPE: Available to all Portlets and also available in Servlets/JSP etc

So, if you really want to share sessions among Servlets and Portlets...
1) Define the scope of the session attributes as APPLICATION_SCOPE in Portlets....
2) Access the session attributes defined or set in servlets by specifying APPLICATION_SCOPE.. else you will get null.

As far as retrieving the PortletSession from HttpRequest is concerned.. You can get them via the APIs available..
For JSR168 Struts Portlets we have WpsStrutsUtil package that will return you the PortletSession...