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From John Bollinger <thinma...@yahoo.com>
Subject Re: [LANG] Wanted - spec lawyer.
Date Tue, 30 Jun 2009 14:16:35 GMT



Jörg Schaible wrote:
> As pointed out http://www.w3.org/TR/2006/REC-xml11-20060816/#charsets and
> http://www.w3.org/TR/2006/REC-xml11-20060816/#charsets define the valid
> characters for XML 1.0 and 1.1.
> 
> However, the escape functionality is actually different. If you transport
> XML (or HTML) in a UTF-8 encoded text file or one encoded by ASCII-7 is a
> big difference. In the former you don't have to encode anything, while you
> have to encode anything above 0x7f in the latter case. And this applies to
> XML, HTML or Java source files at equal level.
> 
> The character set definition of the two XML versions is a vertical condition
> set. An attempt to encode a character outside the XML definition is
> actually a situation that cannot be handled and should raise an exception
> (like every XML parser will do anyway).
> 
> Therefore the question is, whether (Un)EscapeUtils should actually be an
> instance initialized with the target character encoding. And that raises
> the question how close we're actually at reimplementing
> java.nio.Charset.encode.

As I understand it, the basic idea of StringEscapeUtils.escapeXml() is to convert arbitrary
character data from memory (a String) into a character sequence that has the same meaning
when it appears literally in XML character data.  This is a conversion from character data
to character data, so character encoding is not directly relevant for this use (and this is
a fundamental difference from Charset.encode()).  The characters that must be escaped for
this purpose are well defined by the XML specifications.

The appearance of an encoding attribute in the xml declaration
notwithstanding, the character encoding of an XML document is a
property of a representation of the document, not a property of the
document itself.  There is therefore a *separate*, albeit related, consideration of escaping
characters that cannot be expressed in a particular character encoding, so as to be able to
encode the document to a byte sequence without data loss. This is a useful thing to do, and
it is compatible with the main objective, but I think it would be well to avoid conflating
the two as an indivisible task.  They can be performed in one pass by one method, but they
are logically distinct behaviors.

If StringEscapeUtils wants to support the second use, then it needs a way for the user to
tell it which additional characters to escape.  One possibility would be to pass it a Charset
which the user intends to apply (later) to encode the characters.  StringEscapeUtils could
then escape those input characters for which Charset.canEncode() returns false.

Yet another separate question has arisen as to how to handle input characters which cannot
appear in any way in a well formed XML (1.0 / 1.1) document, even as character references
(e.g. U+0000).  I'm not so certain that StringEscapeUtils needs to be concerned about that,
and it would simplify things immensely if it considered that out of scope.  Among other effects,
I believe that would moot the distinction between XML 1.0 and XML 1.1 (and future versions)
for this class.  In addition, I strongly suspect that there are multiple production applications
that (mis)use XML in a way that would be broken if character references to characters outside
the XML character set were flagged as application errors; it would be considerate for StringEscapeUtils
to be compatible with such (mis)use.


Best Regards,

John

--
John Bollinger
thinman42@yahoo.com


      


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