Understanding how to allow POSTing of JSON to RESTful service

I have a RESTful service that I want to post an Object to, using JSON from
the client. However, I keep getting the error: No message body reader has
been found for request class xyz

I have <bean id="jsonProvider"
class="org.apache.cxf.jaxrs.provider.json.JSONProvider"/> configured, and
have @XmlRoolElement configured on my class.

Can someone explain how CXF handles the conversion of JSON received to the
object specified?

What else do I need?

cheers,

David