Session Expiry Check

[arpita kale]

My project is based on java,jsp,servlet.
I am working on enhancemenmt of this project.
In this project,in any action class of jsp,session expiry check is not present.Everwhere just session is taken without cheking wheather it is expired or not.Is such check is reaaly required?I saw this type of session expiry check in most of the webapplication projects.If we do not apply it then what will happen at the session expiry?
And also how to check session is expired or not in java?