Spring Webflow has some interesting feature called Flow Managed Persistence. In short: it allows you to change some persistent entity during the flow, and merge the state of entity into persistence context at the end of the flow.
Sounds interesting, and is worth trying for sure :) - but you have to be aware, that Spring Webflow guys have overlooked something very important. The JpaFlowExecutionListener, which is involved in the Flow Managed Persistence handling, binds an Entity Manager instance to the flow scope, when a flow execution starts.
So what? - you may say :) - hm, let's look at the Spring Webflow documentation: "any objects stored in flow scope need to be Serializable" - and now back to the Entity Manager interface - nope - it doesn't extend Serializable, so we have no guarantee that it will be.
Indeed when you use the EclipseLink as JPA provider you'll get beautiful "SnapshotCreationException: Could not serialize flow execution; make sure all objects stored in flow or flash scope are serializable".
I've reported this problem in Spring Webflow JIRA - see JpaFlowExecutionListener shouldn't assume that EntityManager is Serializable
You may also find this problem in Community Forum - EclipseLink, Toplink NotSerializableException - mentioned first on August 28th, 2008.
I'm waiting eagerly for the fix :)