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I have another one of those "is it done in XML" questions (my last one about xml comments hasn't been answered if anyone has a good explanation)

I was just wondering if anyone, anywhere would:

  1. Use multiple root elements in an XML document
  2. Put text content outside of a root element

W3C discourages these practices, Javascript's DOMParser doesn't even allow these cases, and I can't think of one sane reason to do either of these things. However, I know how bizarre some implementations of XML have been, so I wouldn't be surprised.

Does anyone have any real world examples where this would be done? I will also accept an answer that specifies if other mainstream parsers allow doing either of these.

Community
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Jeffrey Sweeney
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3 Answers3

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As per the standard, an XML document must have exactly one root element in order to be well formed XML. Any parser that accepts otherwise is wrong, and any XML document that is structured any different is not in fact an XML document.

It is not merely discouraged by W3C, but simply against the standard.

Pete
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tdammers
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  • I've seen plenty of XML that doesn't follow the standard, from not using quotes correctly to closing tags inaccurately. Ticks me off too ;) I just need to know if you have personally ever seen code that goes against these two particular standards, and what it was for. – Jeffrey Sweeney Apr 07 '12 at 16:02
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    @JeffreySweeney then the "XML" gets rejected by any compliant XML parser. –  Apr 07 '12 at 18:09
  • @JeffreySweeney a very simple test is to open the XML "document" in Internet Explorer. If it complains, it is an invalid XML document. This also has the benefit of being available to everyone and having a good reputation. –  Jun 01 '12 at 08:11
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    @ThorbjørnRavnAndersen: I don't have IE available, and it has a pretty bad reputation among those I socialize with. – tdammers Jun 01 '12 at 10:05
  • @tdammers In that case find another browser with a validating XML parser and use it instead to test the XML files with. Regarding IE, this particular functionality is very useful. –  Jun 01 '12 at 10:23
  • @ThorbjørnRavnAndersen: I get your point. Just calling 'biased' on your comment ;-) Firefox works fine for this purpose. There's also xmllint and similar tools. – tdammers Jun 01 '12 at 10:39
  • @tdammers No need for calling me biased. This is just one of the things that IE do well (I did not know about Firefox) and IE is almost guaranteed to be available to a Windows user. –  Jun 01 '12 at 10:46
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    Not everyone is a Windows user. – tdammers Jun 01 '12 at 10:50
  • I modified you answer from "valid XML" to "well formed XML". Whether XML is valid or not refers to if it corresponds to a specific schema or DTD. XML that follows the basic rules of xml is merely said to be "well formed". – Pete Jun 01 '12 at 12:57
  • @Pete: I know the difference, and yes, you are right. However, "well-formed" is much less intuitive than "valid", and the following sentence clearly explains what I mean, so I chose to go with "valid" - if it's not well-formed, it can't be valid either. – tdammers Jun 01 '12 at 13:07
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As a "real world" example of multiple roots and intervening text code, consider the XML produced by "mysql -xml".

A compound query (or stored procedure) can return multiple result sets, each starting with the root ... . If one of the subqueries has an error, then mysql produces a single non-XML line "ERROR xxxx - message" instead of XML.

Parsing mysql's XML output involves both multiple roots as well as textual messages.

I'd like to say mysql is messed up and should be fixed, but the reality is, I need to parse mysql output as it currently exists.

John
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  • That's very good to know. I can only hope that that's a fringe case... clients wouldn't likely need to edit MySQL XML output I don't think. Thank you very much! – Jeffrey Sweeney Jun 01 '12 at 14:48
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This can be handled by specifying the snippet in question as an external system entity.

<xml version="1.0">

<!DOCTYPE root [
<!ENTITY about SYSTEM "path/to/about.xml">
]>

<root>
&about;
</root>

The W3C recommendation use the term "Parameter-entity" for this. See http://www.w3.org/TR/2004/REC-xml-20040204/#dt-PERef for the formal definition.

  • So, it is "kind of" allowed in the DOCTYPE assuming it follows the correct syntax. My concern was more for those who hire inexperienced XML coders that write XML their own way. My code does handle a variable DOCTYPE definition too ;) – Jeffrey Sweeney Apr 07 '12 at 18:18
  • You can read it. Thought that was your concern. –  Apr 07 '12 at 19:27