Dear Reddy,
This question that you have asked is more generic. There are n reasons for when to go for Vector and when for array list. But, it all depends on the application.
Servlet runs on server side. It is instantiated once (but depending on webserver, there can be more than 1 instance for load balancing) and whenever a user request for its service, ONE NEW
THREAD for this servlet is created for you.
Thus the entire service method should be synchronized. I mean it should handle concurrent access of the member variables that your servlet possess. So, if you have a ArrayList in your servlet, you will have to take care of its synchronization.
When multiple threads of the same servlet access this ArrayList, it should lead to consistent data read/store and prevent dirty reads.
Hope you got me. Vector might be good I believe.
All those experts there ... correct me If I am wrong!
Thanks!
Mani