I figured it out on my own. The problem was a server side issue. Once I removed the @FormParam
annotation on the server, everything worked as expected. The combined question and answer will provide a rudimentary tutorial for others. My server now looks like:
@POST
@Path("apath")
@Consumes(MediaType.APPLICATION_JSON)
public String postAPath(InstanceWithXmlRootElementAnnotation instanceWithXmlRootElementAnnotation) {