Hi,
There not yet lot's of code but you can take a look in the examples provided by the jbpm official distribution.
It should be enought to start plyaing with jbpm 4.
If you want to integrate Spring and Jbpm you will found the demo made by Andries Inzé (http://www.slideshare.net/ainze/spring-integration-with-jbpm4) It is a little old (using jbpm 4.1) but it is a good start.
Regards