In my opinion using script task to perform logic is not best practice - mainly because you embed your java code inside the process definition (bpmn2). You could give a try with following approaches:
- use Service task that executes (with default work item handler) your java class
- use domain specific nodes that allows you to do whatever you like
For details about domain specific nodes please refer to the documentation http://docs.jboss.org/jbpm/v5.2/userguide/ch13.html and for ServiceTask take a look at example shipped with jBPM.
HTH