What I learned today:

I had the same problem and it finally turned out that the class I was trying to convert JSON to cannot be an inner class.

What helped me to find the problem was turning on debug logs (log4j jar and properties file in the build path) and tracing attached Jackson JSON source in Eclipse.

 

http://forum.springsource.org/showthread.php?85532-HttpMediaTypeNotSupportedException-amp-JSON

Spring/Jackson JSON un/marshalling does not work with inner-classes.