undeploy then redeploy, sessionId valid?

[Dave]

Undeploy a web application, then redeploy it.
   
  Click a link in an old page(page before undeploy), making the first request 
after redeploy.
   
  In a filter:
   
  String sessionId = httpRequest.getRequestedSessionId();
boolean valid = httpRequest.isRequestedSessionIdValid();
   
  sessionId is " AD890098123763728AD80"
  valid: true.
   
  This means that the old sessionId is still valid.
   
  what is the criteria to determine whether a session id is valid or not?
  How to detect redeploy in a filter?  
   
  Thanks!
  David
   
   

                
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