Ruminating on Schematron and XML Validation

Schematron is a rule-based XML validation language. It can be used for making assertions on patterns in XML nodes. For e.g. if a person (element) has "Mr" as prefix (attribute), then gender (child element) should be "male". If the assertion fails, an error message that is supplied by the author of the schema can be displayed.

Thus Schematron is capable of expressing complex constraints that cannot be expressed using XML Schema or DTD. Using XML schemas, you can only put constraints on the document structure and basic datatype validation.  The following sites give good intro about this language.

 http://www.schematron.com/
 http://www.ldodds.com/papers/schematron_xsltuk.html

So how do we apply Schematron schema to validate XML document instances? Given below are the simple steps one should follow:

1. Create a schematron schema file
2. Use a meta-stylesheet to convert this schema file into a XSLT stylesheet. ( A meta-stylesheet is a stylesheet which generates other stylesheets)
3. The above generated XSLT stylesheet can be used as a XML validator against the XML instance document (using a XSLT transformation engine)
4. The output of the transformation would be a XML document with validation errors.

Since the fundamental technologies used as XML Schema, XSLT and XPath; most of the XML processing APIs of Java/.NET can be used for the same.

Difference between XSL, XSL-FO and XSLT

The XSL (Extensible StyleSheet Language) specification has a long history and has gone through multiple revisions. Many folks are confused between the difference between XSL, XSL-FO and XSLT.

Found a good link that explains the history of XSL - http://www.dpawson.co.uk/xsl/sect1/history.html
W3Schools has a good explanation - snippet below:

XSL-FO is Formally Named XSL !!  Why this confusion? Is XSL-FO and XSL the same thing?
Yes it is, but we will give you an explanation:
Styling is both about transforming and formatting information. When the World Wide Web Consortium (W3C) made their first XSL Working Draft, it contained the language syntax for both transforming and formatting XML documents.
Later, the Working Group at W3C split the original draft into separate Recommendations:

•     XSLT, a language for transforming XML documents
•     XSL or XSL-FO, a language for formatting XML documents
•     XPath, a language for navigating in XML documents

Troubleshooting XML namespaces binding in SOAP request using JAXB

Recently helped a team resolve an issue regarding namespace handling in JAX-WS / CXF. Jotting down the solution as it might help other folks breaking their heads on this issue :)

Our Java application needed to consume a .NET webservice and were facing challenges in SOA interoperability. We created the client stubs using the WSDL2Java tool of CXF. The WSDL had - elementFormDefault="qualified"

The problem we ran across was that the complex types were all being returned as 'null'. Enabling the network sniffer, we checked the raw SOAP reponse reaching the client. The response was OK with all fields populated. Hence the real issue was in the data-binding that was happening to the Java objects.

A quick google search revealed that the default behavior of JAXB when it encounters marshelling/unmarshalling errors is to ingore the exception and put it as a warning messages !!. This was a shock, as there was no way to debug the issue on the server.

We then wrote a sample JAXB application and tried to un-marshall the XML/SOAP message. It is here that we started getting the following error stack:
org.apache.cxf.interceptor.Fault: Unmarshalling Error: unexpected element

Hence, it was proved that the real culprit was the JAXB unmarshaller that was not generating the Java classes with the correct namespace.

First we checked the generated Java classes and found a file called as "package-info.java" that had a XML namespace 'annotated' on the package name - essentially a package annotation. So this was supposed to work, but why is the unmarshaller throwing an exception?

We tried adding the following attributes to the annotation and then the unmarshaller started working !!!
@javax.xml.bind.annotation.XmlSchema (
    namespace = "http://com.mypackage",
    elementFormDefault = javax.xml.bind.annotation.XmlNsForm.QUALIFIED,
    attributeFormDefault = javax.xml.bind.annotation.XmlNsForm.UNQUALIFIED
  ) 

Not sure why the WSDL2Java tool did not add these automatically based on the elementFormDefault="qualified" present in the WSDL.  But this trick worked and we could consume the .NET webservice. We had to modify the build script to replace the "package-info.java" file everytime it was recreated.

Another option is to manually add the "namespace" attribute to all XMLTypes in the generated Java classes; but this is very tedious.

Alternatives to XML Serialization

Today, there are a lot of alternatives for XML serialization of data structures. These data interchange formats are smaller and faster than processing XML.
Most popular are Google Protocol Buffers, Thrift (from FaceBook),  Avro and MessagePack. A good article comparing these alternatives is available here -
http://www.igvita.com/2011/08/01/protocol-buffers-avro-thrift-messagepack/

Wikipedia also has an interesting article comparing various data serialization. 

XSD to XML and vice versa

I was looking for a quick and free tool that could convert XSD schema files to sample XML files and also generate a XSD file from a sample XML file. Commerical tools such as XML Spy and OxygenXML were powerful tools that provided these features, but I was looking for a free one.
First I checkout the open source Java IDEs.

• Netbeans had a beautiful editor to visualize and edit a XSD schema in a graphical tree structure. But unfortunately it did not have the ability to generate sample XML file or vice versa.
• Eclipse too had a visual editor and allowed visual editing of schema elements. It could generate a sample XML file from a XSD file, but had no option for the reverse; i.e. generating a schema file from a sample XML.
• VS 2008 SP1 had both the options - coversion between XML and XSD. For schema files, right click on a node in XSD Explorer view and select "Generate XML". For XML files, select "Tools -> Generate schema" to create the XSD file. Both these operations are very quick in Visual Studio.
• VS 2010 has extensive support for XML tooling. You have 3 different views for schema designing that should suffice for most complex schema definition.

Besides these free tools, there are other command line tools that can be used. For e.g.

XSD.exe tool can be schema to XML and vice versa transformation.

There was another .NET tool that I found on MSDN for generating XML documents from XSD.

Another cool Java desktop tool that supported these features was XMLSpear.