We're having some trouble using the RESTEasy Atom provider to create atom feeds that contain additional (extension) elements. It appears that although the atom spec allows extension elements within most of the atom elements, the RESTEasy API (JAXB annotated objects such as org.jboss.resteasy.plugins.providers.atom.Source and org.jboss.resteasy.plugins.providers.atom.Entry) does not offer any way to provide them.
I've studied RFC 4287 and I believe the relevant parts are e.g. 4.1.1 and 4.1.2. You can see that 'extensionElement' is allowed as part of atom:feed and atom:entry.
An example of what the schema document for Atom would look like is here:
We can see that feedType allows additional elements:
<xs:any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
In general we'd much prefer to use the RESTEasy API for Atom, since it has many convenience features that make it easier to interact with than our own types (generated from the Atom XSD). However we need to include various OpenSearch elements in our feed.