toplink entity in servlet session
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What are the best practices of putting objects in session?
What should be the nature of those objects?
Can ORM entities be used in session or is it a bad practice?

We are using Toplink entities in session. They are working fine in non clustered environment. But when we take them to clustered environment and test the fail over scenario then we are having problems. 

All the ValueHandlerInterface properties in user class gets detached serialization/Deserialization. 

We are using Spring MVC.

Thanks in advance