XML Document Validator is an implementation of CLiXML - Constraint Language in XML - that has been designed to express complex constraints about XML content using a mixture of first order logic and XPath.

CLiXML is complementary to XML-Schema or RelaxNG. Document structure, data types and references should be expressed in schema, while more complex contraints regarding XML document content should be reserved for CLiXML.
CLiXML plays the same role in XML validation as Schematron. While Schematron relies only on functionality offered by XPath, CLiXML has additional adventage of first order logic which makes rules simpler to express and understand.

Key features

• Free!
  Open source, distributed on Apache License 2.0, compatible with Java 1.5.
• Flexible XPath module - more.
  Implementation is not bound to any XPath engine. XPath functionality has been extracted into a separate, independent, reusable, well defined API. Project includes connectors to Jaxen and Saxon 6.5.5.
• Many errors in one run.
  Validation does not stop at first constraint violation but reports as many errors in one run as possible.
• Dynamic error messages.
  An error message can include information extracted from XML node being tested by the rule.

Download

• Sourceforge SVN: source code - XDV source code at Sourceforge.net SVN repository.
  
• Source code - more information.
  

More information

• Source code - how to download and build the project.
  
• Implementation - architecture, modules' description.
  
• XPath module - XPath module description with example usage.
• CLiXML module - CLiXML module description with example usage.
• CLiXML examples - examples, sample rules expressed in CLiXML.
• Future plans
• FAQ
• External:
  
• CLiXML - Constraint Language in XML - language used to constrain the content of XML documents.
• Schematron - language for making assertions about patterns found in XML document.
• Relax NG - schema language for XML.
  
• XML-Schema - W3C recommendation.
• XPath 1.0 - W3C recommendation.
  
• XSLT 1.0 and XPath 1.0 quick reference (PDF) - great XPath 1.0 reference guide from Mulberry Technologies, Inc.
• Jaxen - universal Java XPath engine.
• Saxon - XSLT and XQuery processor.
  

Origins

XDV is being developed and maintined by Tomasz Kokoszka.
XDV has been originally created in year 2006 as a validation module for project FESA developed at CERN. FESA extensively uses XML as a modeling language and quickly reached the limits of XML-Schema validation abilities.