@Provider
@Produces(MediaType.APPLICATION_JSON)
public class MyObjWriter implements MessageBodyWriter<MyObject> {

    @Override
    public boolean isWriteable(Class<?> type, Type genericType,
            Annotation[] annotations, MediaType mediaType) {

        return MyObject.class.isAssignableFrom(type);
    }

    ...
}

@javax.interceptor.Interceptor
@MyAnn
public class MyAnnInterceptor {

    @Context
    private Providers providers;

    @AroundInvoke
    public Object aroundServiceMethod(InvocationContext ctx) throws Exception {
        Object myObj = ctx.proceed();

        MessageBodyWriter writer = providers.getMessageBodyWriter(
                myObj.getClass(), myObj.getClass(),
                new Annotation[0], MediaType.APPLICATION_JSON_TYPE);

        // writer is an instance of MyObjWriter only if MyObjWriter
        // is not in a jar (and I need it to work while in a jar).

        ...
    }
}