I think in your case not comboBox should call the list according to some components values. But the components which influence the combo should update it via ajax after changed.
If you want to make decision in the moment when list called - you should use suggestionBox.
ok , i could do that. but how can i ensure the user only selects an entry from the list and does not type any "stupid" text?
so how can i validate this?
just create custom validator which will check this. In general comboBox not support this out of the box because it's input component and not a select.
I think you can do that without using custom validator, if you use the h:selectOneMenu which is the standard Combo Box from JSF.
By this, you will have two selectOneMenu components, add to the first one the a4j:support to trigger event onchange....