Resteasy provides way to specify alternate content types by using file extensions using resteasy.media.type.mappings. However, once mapping is done, to call a resource having query parameter in the URI, the URI needs to be specified as

<HOST>/content-negotiation/rest/user/.xml?email=user1@abc.com (for xml)

and

<HOST>/content-negotiation/rest/user/.json?email=user1@abc.com (for json).

This is non-intuitive. It would be better if mapping works with URI such as the one given below
<HOST>/content-negotiation/rest/user.json/?email=user1@abc.com

I am attaching the source code for the sample I created for content-negotiation