Spring transactions with Objectify and Appengine

Question

I am using appengine with Objectify to access my datasource. I use Spring for my business layer. In order to play with data I use the objectify-appengine-spring factory.

I would like to use annotation based local transactions. Do you know about an existing implementation which I could plug directly as a spring bean?

I'd really like to avoid the pain of implementing my own transaction provider with thread locals.