You are not the only one! With your suggestion the behavior changed:

Without context-param:
The VRaptor instances only one of the CalendarSerializers or CalendarDeserializer from <code>gson.adapters</code> package. Fine!

With context-param (weird):
The VRaptor instances duplicate instances of <code>gson.adapters.CalendarSerializer</code> when invokes serialization (oh noes!), but duplicate instances of <code>iso8601.gson.CalendarDeserializer</code> when invokes consumes (ok, fine!).

This weird behavior is caused by guice provider? I think I am not able to fix this without help =(