...
CXF 3.1.2 Provider Sorting Changes
Starting from CXF 3.1.2 custom MessageBodyReader customMessageBodyReader (MBR) and , MessageBodyWriter (MBW) and ExceptionMapper providers are sorted together with default providers.
...
For example, suppose you have a custom StringReader which is not typed by String but by Object. In this case the default provider which is typed by String wins. To have the custom String provider winning one needs to type it by String.
Check the specification or ask at the users list for more details.
From CXF 2.6.x to CXF 2.7.x
...