Barrapunto

https://developers.google.com/web-toolkit/articles /using_gwt_with_hibernate [google.com]

When you take an object and turn it into a Hibernate object, the object is now enhanced to be persistent. That persistence does not come without some type of instrumentation of the object. In the case of Hibernate, the Javassist library actually replaces and rewrites the bytecode for these objects by persistent entities to make the Hibernate magic work. What this means for GWT RPC is that by the time the object is ready to be transferred over the wire, it actually isn't the same object that the compiler thought was going to be transferred, so when trying to deserialize, the GWT RPC mechanism no longer knows what the type is and refuses to deserialize it.