hi sanjay & avad,
valueUnbound() is called when the session expires , but closing the browser does not let the session expire.
If the browser is closed , the session object stays at the server for quite sometime (the idle time specified for the
servlet specified in the web.xml file) , if the instance of the servlet idle upto the time then the server removed the instance from container thus causing the session to expire.
try specifying short time in web.xml file
manas