When creating a client for HornetQ 2.3.0 beta, I was getting a NoClassDefFoundError for org/hornetq/api/core/HornetQIllegalStateException. I had to add hornetq-commons.jar to my classpath even though the manual doesn't mention it. It seems like either the manual needs to be updated or the required classes need to be bundled in the hornetq-core-client jar.
The docs are out of date, we will fix before Final