Thanks for the reply. I did upgrade to 4.4 and after a little head-scratching I realised I was encountering JBPM-2830. Your patch restored my application built on 4.3 to the point that I can start and list process instances so I'm hoping it is agreed to restore the previous behaviour.
I tried again to call a Java Service Task with an arg that is not populated at the time the service task is invoked:
<itemDefinition id="registerUserItemDef" >
<jbpm:arg>
<jbpm:object expr="#{user}" />
</jbpm:arg>
<jbpm:arg>
<jbpm:object expr="#{groups}" />
</jbpm:arg>
</itemDefinition>
...
<operation id="registerUser" name="registerUser">
and this time I get a different error:
org.jbpm.pvm.internal.wire.WireException: couldn't invoke method registerUser: couldn't create argument 1: Cannot resolve identifier 'groups'
at org.jbpm.bpmn.flownodes.JavaServiceTaskActivity.perform(JavaServiceTaskActivity.java:92)
Once again, creating a variable named groups allows the service task to be invoked successfully.
Tim