You can't replace the out of the box logic with an automation script, but you can add your own processing with automation scripts. For example, it's common for people to do an attribute launch point validation on the STATUS field. When this is changed (whether via integration, escalation, or UI), this script would fire and any errors you throw would prevent the status change. For example, some people will throw an error on WO completion if no labor is reported.
Generally speaking, this is what you want anyways. The java class should have its own set of rules and you do additional processing on top of it with automation scripts. If you have a more specific question, I can try and expand upon that.
------------------------------
Steven Shull
IBM
------------------------------