"Rest is the sweet sauce of labor." (Plutarch – a long time ago!)
is JAX-RS 2.0 Reference Implementation and as such it's fully compliant to JSR 339
. The Jersey Team goes beyond the specification by adding features and functionalities to their JAX-RS implementation. Overtime, some of those features might be good candidates for standardization. The Client API is a good example of that. In JAX-RS 1.x, Jersey and some other implementations has their own proprietary Client API. In JAX-RS 2.0, the Client API has been standardized and is now part of the specification. So it is not uncommon to have, in an implementation, features that are (not yet?) part of the specification. Those extra capabilities are also a way for an implementation to differentiate itself from the other implementations.
Tracing is a feature that was available in Jersey 1.x, this feature has now been ported to Jersey 2.x (2.3 and above). The Jersey Tracing capabilities allows to easily diagnose server-side request processing pipelines. Libor Kramoliš, a member of the Jersey Team, has written a detailed post on how to use the Jersey's Tracing capability.