XOM API for Processing XML with Java

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

John Cowan asked me to look at implementing xml:id in XOM. herewith are 
some initial thoughts.

IDness intersects XOM in three places:

1. Shorthand XPointers used in XInclude
2. The XPath id() function
3. The type of the xml:id attribute returned by getType()

1 is straightforward to implement. 2 is not hard, though it's arguably 
incorrect with respect to the XPath spec.

3 is where matters get tricky. To really implement xml:id in this 
context some behavioral changes need to be made:

1. xml:id attribute values must be normalized by XOMHandler, even if the 
parser doesn't do it.

2. Should we also autonormalize when the attribute value is set?

3. Should the type be reported as ID even if the schema declares a 
different type such as CDATA or NMTOKEN?

4. Error handling. The spec states,

An xml:id processor must assure that the following constraints hold for 
all xml:id attributes:

    * The normalized value of the attribute is a valid NCName.
    * The declared type of the attribute, if it has one, is “ID”. All 
declarations for xml:id attributes must specify “ID” as the type of the 
attribute.

An xml:id processor should assure that the following constraints hold:

    * The values of all xml:id attributes and all attributes of type 
“ID” within a document are unique.

An xml:id error occurs for any xml:id attribute that does not satisfy 
the constraints.

An xml:id error is a non-fatal error, but that conflicts with the *must* 
requirements.

Do we really want to reject documents on parse that do not satisfy the 
first two constraints? Maybe. There's not a lot of legacy here to worry 
about, and it's in keeping with XOM's authoritarian bent. If so, XOM 
should also reject setting the xml:id value or type to an illegal value. 
If we do this, it answers question 3 as well.




--
Elliotte Rusty Harold  elharo AT metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
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