Ah, I see why this compiles... because it's not actually being used :)

The semantics of the exception map is
  'class_name' : { 'member_name': 'type' }
So to make the member auxiliary_id of message of of_features_reply be of JType 'aux_id', you'd have to do this 
```python
   'of_features_reply' : { 'auxiliary_id' : aux_id },
```
The do a make java and check the generated class OFFeaturesReply - its method getAuxiliaryId should now return the java type AuxId, not short.