How do I get JSON data from my RESTful service using JEE 5?

Question

I have a simple RESTful web service written in JEE 5. It works fine but I've decided to output JSON from the GET method. As an initial step I've only add one line of code:

ObjectMapper mapper = new ObjectMapper();

That one line results in this error:

48d5-9878-2ef7c0a948c4;|StandardWrapperValve[Jersey Web Application]: PWC1406: Servlet.service() for servlet Jersey Web Application threw exception
java.lang.VerifyError: Cannot inherit from final class

What am I missing?

I've read the Jackson documentation here and here plus a most of the JAXB/Jackson questions on this site. It makes sense how this should work, but I can't touch ObjectMapper without getting an error. Should I utilize Jackson at all? I'm using jackson-core-asl/jax