Enhance PDX serialization support to delegate back to Geode for Session attribute value serialization

[jxblum]

Currently, Spring Session for Apache Geode/Pivotal GemFire (SSDG) supports delegation when using Data Serialization to serialize both the Session and the Session Attributes along with the corresponding attribute values.

This gives users the ability to apply GemFire serialization semantics to their own application domain object types. However, the same strategy is not applied when using PDX Serialization to de/serialize the Session and its contents.

This ticket sets out to enhance SSDG's PDX Serialization to support delegation.

[jxblum]

It turns out, Apache Geode (and by extension, Pivotal GemFire & Pivotal Cloud Cache (PCC)) may very well delegate to the PDX serialization mechanics to serialize objects passed to PdxWriter.writeObject(..).

I initially thought this was not the case because the Session Attributes were 1) first, encapsulated in a java.util.Map object (as seen in the custom PdxSerializer, the PdxSerializableSessionSerializer, used to serialize the Session object using PDX) and 2) the moment Geode processes a java.io.Serializable object (e.g. like java.util.Map), and the Serializable object's type (e.g. again, Map) is not PdxSerializable (which a java.util.Map would not be), nor is there a custom PdxSerializer registered with the cache that knows how to serialize the object (e.g. again, a Map), then Geode would uses Java Serialization to serialize the Map and its contents.

However, I was informed that Geode identifies Collection objects (like Map) and inspects their contents. So, it may be the case that Geode will serialize objects (values) in the Map (which would be the case for Session Attributes in the Spring Session object) when the Map object is passed to PdxWriter.writeObject("fieldName", mapObject).

I need to investigate the logic in PdxWriter.writeObject(..) further (and again) to verify.