XOM API for Processing XML with Java

[Johan Strandberg <johan AT acm.org>]

On Thu, 2003-09-04 at 07:22, Elliotte Rusty Harold wrote:

> I understand the use case, but right now that's still on the other 
> side of 80/20 for me. First of all, it's extremely tricky because 
> documents can have both the encoding specified in the XML declaration 
> and the one actually used, and these are not necessarily the same 
> thing. 


I assume you are talking about the case in XML1.0 section 4.3.3
[XML-433] where they discuss encoding declarations:

        
        In the absence of information provided by an external transport
        protocol (e.g. HTTP or MIME), it is an error for an entity
        including an encoding declaration to be presented to the XML
        processor in an encoding other than that named in the
        declaration, or for an entity which begins with neither a Byte
        Order Mark nor an encoding declaration to use an encoding other
        than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary
        ASCII entities do not strictly need an encoding declaration.
        

As far as I can tell, you have exactly four cases:


     1. Explicit external information provided by the transport
        protocol.
     2. An explicit XML declaration (EBNF rule #23 and #77) is present
        and it contains an encoding declaration (EBNF rule #80).
     3. An explicit XML declaration (EBNF rule #23 and #77) without an
        encoding declaration
     4. No information about encoding is available. (I.e., none of the
        above).


In each case there is a simple strategy, so the algorithm could be
something like this (in pseudo-code):


        IF there is an external encoding provided by the transport
        
                RETURN the provided encoding (case1)
        
        ELSE
        
                apply table in Appendix F [XML-F]
                compute tentative encoding from table
                IF table indicates presence of ?xml declaration
                
                        IF xml declaration contains encoding
                        
                                RETURN specified encoding (case2)
                
                RETURN tentative encoding from table (case3&4)


Algorithmically, the only tricky one appears to be case #2, where you
have a tentative encoding provided by the tables in Appendix-F, and have
to try to extract the encoding declaration.  I suspect one or several of
the open source XML parsers out there have a code snippet to borrow.

As to an API for XOM, case #1 isn't very interesting. If the user knows
the encoding, XOM doesn't have to do anything. Otherwise just provide a
method that returns an encoding given an input source (I.e., the same
set of arguments provided by the various declarations of
nu.xom.Build.build(...) ).


--j


[XML-433] Extensible Markup Language (XML) 1.0 (Second Edition), 
    Section 4.3.3 - Character Encoding in Entities
    http://www.w3.org/TR/REC-xml#charencoding

[XML-F] Extensible Markup Language (XML) 1.0 (Second Edition), 
    Appendix F -  Autodetection of Character Encodings (Non-Normative)
    http://www.w3.org/TR/REC-xml#sec-guessing