XOM API for Processing XML with Java

[Elliotte Rusty Harold <elharo AT metalab.unc.edu>]

Last night I started looking at XOM's exception hierarchy, and 
decided it might benefit form some rejiggering, though I seem to be 
changing my mind every hour or so about exactly how to reorganize it, 
so commennts are appreciated.

The specific incentive was the desire to add extra information to 
four exceptions: IllegalDataException, IllegalNameException, 
IllegalTargetException, and MalformedURIException. Specifically, I 
wanted to give each of them a method that returned the dpecific 
illegal string, the malformed URI, the illegal name, etc. This seems 
useful for debugging and error reporting, and should have negligible 
if any impact in the non-exceptional case.

However, in the process of doing this it became tricky because of 
inheritance issues. For instance, getTarget and setTarget seem like 
obvious methods for IllegalTargetException. However, that's a 
subclass of IllegalDataException, so what do I to with getData and 
setData inherited from IllegalDataException?

I've also noticed problems with issues between IllegalNameException 
and NamespaceException. It's not always obvious which one is thrown 
when. This has been a consistent and occasional thorn in my side for 
the last year. It's not a big issue, but I often find myself 
surprised by an IllegalNameException when I expect a 
NamespaceException and vice versa, and that isn't good. Perhaps the 
problem here is that NamespaceException is used for two different 
things: namespace illegal content such as a prefix that contains a 
colon and namespace conflicts such as an atttribute that maps a 
prefix to a different URI than another attribute on the same element 
does.

Thus my current thinking is to rearrange the exception hierarchy like this:


WellformednessException
   |
   +--NamespaceConflictException
   |
   +--IllegalDataException
        |
        +--IllegalContentException
        |
        +--MalformedURIException
        |
        +--IllegalNameException
             |
             +--IllegalTargetException
             |
             +--IllegalNamespaceException


This would mean IllegalDataException  becomes a common superclass for 
pretty much anything that could go wrong as a result of verification 
issues. What was IllegalDataException in the past would now be 
IllegalContentException. IllegalDataException would have setData and 
getData methods that return the specific illegal string. These 
methods would of course be inherited by all the subclasses.

One of the things I'm least certain of is whether 
IllegalTargetException and IllegalNamespaceException should be direct 
subclasses of IllegalNameException or of IllegalDataException.

Thoughts? Comments? Does anyone have a better idea? This is very much 
up in the air, but I would like to resolve it soon, since I'm very 
close to a "last Call" on the API.

--

  Elliotte Rusty Harold
  elharo AT metalab.unc.edu
  Effective XML (Addison-Wesley, 2003)
  http://www.cafeconleche.org/books/effectivexml
  http://www.amazon.com/exec/obidos/ISBN%3D0321150406/ref%3Dnosim/cafeaulaitA