OK, first of all, i am assuming you are using activiti as a dependency in your Spring project,
How are you calling these service task Java delegates? Using an expression like ${mySpringBean.myMethod} ? Because the activiti:class won't work, it will do as advertised and create a instance of the class when needed.
>The documentation is really light here (afaics).
Cause there is not much to it, except using expressions. Which is in the docs.