XOM API for Processing XML with Java

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

The second major feature for XOM 1.1 is XPath. To some extent this 
already available through NUX, Jaxen, SAXON, etc. However, I'd like to 
build in basic XPath 1.0 support to XOM. However, there are some 
mismatches between XPath and XOM that make this tricky:

1. XPath has namespace nodes. XOM doesn't.

I could either create a special Namespace class, to be used just for 
XPath; or I could simply rule out queries that return namespace nodes, 
and throw an exception if one is encountered. I'm leaning toward the 
latter, but not strongly. I might be convinced otherwise.


2. XPath functions tend to return numbers, booleans, strings, and 
node-sets. In XOM this would need to be expressed as returning a list of 
objects, or some such, which I really don't like for reasons of type safety.

My thought here is to simply restrict all queries to location paths and 
multiple location paths separated by |. (I wish there were a name for 
this.) Is there an alternative to this? Would it be useful to allow 
other XPath expressions that return node-sets, even if they're not full 
location paths?


3. How are XPath variables bound?

My current thinking is that variables simply will not be bound. XOM will 
treat an XPath expression that contains a variable exactly as any XPath 
engine would treat an XPath expression that contains an unbound variable.


4. How are namespace prefixes bound? (especially important for the 
default namespace)

This one's tougher because, unlike the variable binding, XOM really 
can't just punt on this. It has to provide some sort of binding 
mechanism. Possibilities:

1. A Map
2. A custom type-safe map of namespace bindings
3. An element that carries all the relevant prefix bindings, not 
necessarily the element being queried. This is sort of ugly, but does 
perhaps allow for more reuse of code.

Also a possibility is that the query will simply inherit the namespace 
bindings of the context node, but even if XOM does this, it still 
wouldn't handle the default namespace. We need some way to pass in a 
list of bindings with the query for queries that use namespace prefixes.

The final question is what the API will look like. My current thinking 
is that this will be two methods added to the Node class that look like 
this:

public Nodes query(String xpath) throws ToBeDeterminedException
public Nodes query(String xpath, Map namespaceBindings)
  throws ToBeDeterminedException

The context node for each query would simply be the node on which the 
query method was invoked.

Also worth exploring: what happens if this is invoked on a disconnected 
node? e.g. what happens if we try to use XPath to get the parent node of 
an Attribute object with no parent? For this case, I can probably just 
return an empty Nodes. However, is the possibility of nodes that do not 
belong to any document likely to break some implicit assumption in XPath 
somewhere?

There are probably other issues, but those are the big ones I see. 
Thoughts? Ideas? Comments? Anything else I'm missing?

--
Elliotte Rusty Harold  elharo AT metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim