EntityManagers cannot be serialized
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Key: OPENJPA-126
URL: https://issues.apache.org/jira/browse/OPENJPA-126
Project: OpenJPA
Issue Type: New Feature
Components: kernel, jpa
Reporter: Patrick Linskey
EntityManagers are not serializable or externalizable. This makes passivation
of an EntityManager a difficult task. We should investigate how to externalize
an EntityManager in a meaningful way. This could mean just writing out a stub
that contains configuration information (potentially even just the persistence
unit name, or the Configuration's ID), or it could mean actually serializing
some or all of the local transactional cache to disk. The implications for the
functionality available after deserialization would differ depending on the
approach taken.
I would like to see an implementation that efficiently wrote all the unflushed,
dirty objects to disk. This would probably be best implemented via a
writeReplace() strategy, to avoid handling all the transient fields in a
Broker. Deserialization would then turn into a factory lookup plus some sort of
in-place reattachment of the deserialized unflushed instances.
Of course, if the entity instances themselves were not serializable, it would
be difficult to write them to disk. Theoretically, we could just write out the
corresponding StateManagers, and track the changed fields ourselves. I do not
think that this is a good approach, however, since it would cause the
deserialized objects to lose any non-persistent state after deserialization. I
think that it is fair to require that instances be declared Serializable in
order to use this feature.
(We could optimize this a tad by detecting if an instance has only persistent
fields, and if so, do our own serialization work.)
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