XOM API for Processing XML with Java

[Wolfgang Hoschek <wolfgang.hoschek AT mac.com>]

On Oct 21, 2006, at 4:26 PM, John Cowan wrote:

> Steve Loughran scripsit:
>
> > > > Also, ASCII 0 is a forbidden character in an XML document. So you
> > > > could write something that streams out a document and sticks a  
> > > > NUL at
> > > > the end.  The reciepient relays everything to an XML parser (via a
> > > > special Reader), until that NUL gets received, at which point an
> > > > end-of-file should be signalled. Result: no need to determine  
> > > > the size
> > > > of the message.
> > >
> > > Might be a non-issue, but to be precise: NUL may occur for encodings
> > > other than ASCII and UTF-8.
> > > In contrast, Consistent Overhead Byte Stuffing (COBS) provides
> > > efficient, reliable, unambigous packet framing regardless of packet
> > > content.
> >
> > that's a good point. While NUL may be illegal character in an XML
> > document, it may not be illegal in an encoding. I duly corrected :)
>
> Actually, you had it right the first time.  The byte 0x00 may  
> appear in
> in an encoding such as UTF-16, but the character U+0000 cannot appear
> in an XML document.  So it would be straightforward to layer a  
> specialized
> Reader over a standard InputStreamReader that detects U+0000 and  
> returns
> end-of-input.

I, too, should've replaced "NUL" with "zero byte" :-).

Nonetheless, a zero *byte* as an unambigous end-of-document marker  
would work as long as only ASCII or UTF-8 are used. This would be  
much more straightforward, practical and efficient than to append,  
and scan for, an (encoded) *unicode character* U+0000, because, in  
the general case, the custom Reader would somehow have to implement  
the (non-trivial) XML rules for character encoding detection, also to  
find the encoded U+0000 marker.

In addition, as recently discussed in another thread on this mailing  
list, an XML parser/serializer tends to be more reliable and  
efficient if being passed a byte-oriented InputStream/OutputStream,  
rather than a character-oriented Reader/Writer.

But to put the discussion into perspective: Any reasonable higher  
level protocol defines a generic framing mechanism, thereby omitting  
the need to think about XML specific framing anyway.

Wolfgang.