Xerxes/SAX2 reports wrong element - xsd

Question

One of the things I've written down for the airing-of-grievances for Festivus this year is how Xerces/SAX2 reports parsing errors.
Take this bit of XSD:
<xs:sequence>
<xs:element ref="element1" />
<xs:element ref="element2" />
<xs:element ref="element3" />
<xs:element ref="element4" minOccurs="0" />
<xs:element ref="element5" />
<xs:element ref="element6" minOccurs="1" />
<xs:element ref="element7" minOccurs="0" />
<xs:element ref="element8" minOccurs="0" />
<xs:choice minOccurs="0">
<xs:element ref="choiceElement1" />
<xs:element ref="choiceElement2" />
</xs:choice>
<xs:element ref="element9" minOccurs="0" />
</xs:sequence>
and sample XML
<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<xmldocument xmlns="http://www.somewebsite.com/xsd/xmldocument" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.somewebsite.com/xsd/xmldocument xmldocument.xsd">
<transaction msgId="MESSAGE-ID">
<element1>KS0003</element1>
<element2>2016-05-09</element2>
<element3>10:20:50</element3>
<element5>99433</element5>
<element8>jesse</element8>
</transaction>
</xmldocument>
I get this error:
RAW SAX2 ERROR: Error at file "/tmp/QACXV0Z346", line=10, column=17,
XML element=element8, Element 'element8' is not valid for content
model:
'((element1,element2,element3,element4,element5,element6,element7,element8,(choiceElement1|choiceElement1)),element9)'
Seems to me the problem here isn't element8, it's element6, which is set to required but is the one actually missing in the XML.
I have some code that attempts to parse out this string and figure out what the real problem is, but the error string doesn't contain any information about optional elements, etc. I may not be setting things up correctly - maybe. I have a problem in general with SAXException - it's nearly useless - so what I need is more information from something that tells me what the real problem is.
We're using Xerces 2.6 or 2.8 because we're running on an IBM i and they don't provide updates to stuff like this unless you upgrade the OS.

Answer 1

Xerces error messages are actually quite good.
You might argue that in this particular case, it would be better to say something along the lines of
Encountered element8 but element6 was expected.
That's fine for this simple case, but realize that in the general case, there can be an arbitrarily complex expression covering what possibly could have been expected. Be prepared to introduce a whole lot of complexity to concisely explain what all would be allowed at a given point where parsing goes awry. Citing the first point of contradiction along with the violated parent content model requirement is not a bad diagnostic in general.