RESTEasy considers every JAX-RS provider listed in a service loader-like to be a built-in provider. RESTEasy take care of loading of these providers early in the startup. Once loaded, these providers are registered with "built-in" flag, which is important to guarantee that any custom provider is given priority over a built-in one at runtime. The registration is handled by the RegisterBuiltin class https://resteasy.svn.sourceforge.net/svnroot/resteasy/trunk/jaxrs/resteasy-jaxrs/src/main/java/org/jboss/resteasy/plugins/providers/RegisterBuiltin.java

However, it seems that AS7 scans resteasy jars for providers and registers the built-in providers again - as application-provided providers. As a result, every built-in provider is registered twice. Once as built-in and once as application-provided provider. This has several consequences. Once of them is that the priority of application-provided providers is not guaranteed any longer.