To be sure to talk about the same thing as you :
I was not doing this with the Thingworx java sdk comming from the market.
I was attempting to do it with a Thingworx java extension using the thingorx java api (downloadable from the support) and an external caching library.
When you look at the api, none of the objects are serializable.
And when you try to use the JSON methods that could have been used, these classes return null...