If a server restarts, then data stored in session scope may be lost. The behaviour of a web container in this case is not defined by the servlet specification, and varies between implementations. Some web containers provide "failover", in which session data is not lost during a restart, while others do not.

In those that do implement failover, a common requirement is that only Serializable objects can be restored.

Implementing Serializable
Session-scope objects often Serializable

Yes   No   Undecided