Apache Xerces2/J 2.11.0 Released

The Apache Xerces project team is pleased to announce that version 2.11.0 of Apache Xerces-J is now available.

Xerces-J 2.11.0 can be downloaded at: http://xerces.apache.org/mirrors.cgi

This release expands on Xerces' experimental support for XML Schema 1.1 (see Xerces-J-bin.2.11.0-xml-schema-1.1-beta.zip) by providing implementations for the simplified complex type restriction rules (also known as subsumption), xs:override and a few other XML Schema 1.1 features. Xerces-J 2.11.0 also introduces experimental support for XML Schema Component Designators (SCD). It fixes several bugs which were present in Xerces-J 2.10.0 and also includes a few other minor enhancements.

Specifically, the changes introduced in this release are:

• Implemented XML Schema 1.1's simplified complex type restriction rules (also known as subsumption)
• Added support for XML Schema 1.1's overriding component definitions (<xs:override>)
• Added experimental support for a parser and evaluator for XML Schema Component Designators (SCD)
• Implemented support for the vc:typeAvailable, vc:typeUnavailable, vc:facetAvailable and vc:facetUnavailable attributes for conditional inclusion processing in XML Schema 1.1 documents
• Implemented support for allowing xs:group as a child of xs:all in XML Schema 1.1 documents
• Made several enhancements to the support for assertions in the XML Schema 1.1 implementation
• Improved the ways in which the XML Schema API exposes values and identity constraint information
• Fixed a bug where XMLSchemaValidator.findSchemaGrammar() was not getting called for substitution groups
• Fixed a bug in the regular expression support which was introduced in the previous release. See JIRA Issue XERCESJ-1456 for details
• Fixed multiple issues with decimal precision in the processing of xs:precisionDecimal value
• Fixed various bugs and made various improvements

A complete list of JIRA issues resolved in this release is available here: http://s.apache.org/pI

For more information please visit: http://xerces.apache.org/xerces2-j/

Committed to Apache Xerces

The Apache Xerces community granted me committer status for Apache Xerces2-J. The vote was called in just a few days back and it was officially closed on 8th of April (yesterday). Xerces2-J is an open source library for parsing, validating and manipulating XML documents. It is the most matured and most standard complaint XML parser available for Java applications today. 

Apache Xerces used to be a subproject of the Apache XML project but due to the immense popularity and high adoption rate that Xerces continued to exhibit, it became a top level project in year 2004. Presently Apache Xerces is a collection of four sub projects.

• Apache Xerces C++
  
• Apache Xerces2 Java
  
• Apache Xerces Perl
  
• Apache XML Commons
  

The original code base which formed the foundation of Xerces C++ and Xerces-J was developed at IBM and donated to ASF in year 1999. Therefore Apache Xerces happens to be one of the oldest projects of the ASF as well. In fact Xerces is nearly as old as the ASF itself and will be celebrating its 10th anniversary this November. Given all the facts and figures, it is indeed a great honour to be a part of such a majestic open source project :)

Talking about Apache Xerces2-J, it supports a large number of XML and XML related standards and APIs. The latest released version (2.9.1) supports the following specs.

• XML 1.0 (4th Edition)
  
• Namespaces in XML 1.0 (2nd Edition)
  
• XML 1.1 (2nd Edition)
  
• Namespaces in XML 1.1 (2nd Edition)
  
• W3C XML Schema 1.0 (2nd Edition)
  
• XInclude 1.0 (2nd Edition)
  
• OASIS XML Catalogs 1.1
  
• SAX 2.0.2
  
• DOM Level 3 Core, Load and Save
  
• DOM Level 2 Core, Events, Traversal and Range
  
• JAXP 1.3
  

Due to the performance levels achievable through Xerces and its high level of standard compliancy, hundreds of open source projects and other commercial software applications make use of Xerces2-J. It is fair enough to say that Xerces2-J is the most widely used XML parser in the Java world. If you have a Java application which does some XML related stuff it's very likely that Xerces2-J is somewhere in your application's classpath. People sometimes use it even without knowing that they are actually using Xerces2-J. Sun Microsystems ships Xerces2-J with their JDK. This enables Sun JDK users to develop XML parsing applications powered by Xerces2-J without having to explicitly put the Xerces2-J jars in the project classpaths. However folks at Sun Microsystems have changed the Xerces2-J package name from org.apache.xerces.* to com.sun.org.apache.xerces.* which in my opinion is being a little ungrateful :(

Nowadays we are working on getting Xerces2-J to support the new XML Schema 1.1 specification from W3C. In fact I started contributing to Xerces2-J by implementing a XML Schema 1.1 feature as a Google Summer of Code participant, last year. I've been taking part in mailing list discussions and fixing bugs ever since. The Xerces2-J community is rather small but there's a very helpful and friendly bunch of experienced developers there, who have dedicated their lives and careers to Xerces. That encourages young developers like me to do more and more towards the betterment of the project, as and when the time permits.

GSoC 2008 ....... Done

You may have noticed that my blog has gone rather silent in the last couple of months. Well, the reason was my Google Summer of Code (GSoC) project. The final deadline was on 18th of August so I worked really hard to deliver the goods on time and finally I can proudly state that I managed to successfully complete my project on time.

The objective of my GSoC project was to implement XML schema type alternatives support for Apache Xerces2/J, the legendary open source XML parser for Java applications. Type alternatives is the answer from W3C XML schema working group, for conditional type assignment problem. This feature was first introduced in the XML schema 1.1 structures specification. Type alternatives allow a type to be assigned to an element dynamically at validation time based on one or more conditions. Conditions are expressed as XPath 2.0 expressions. Here is an example element declaration which uses XML schema type alternatives.

As stated in the above code snippet the declared type of the message element is the complex type called messageType. However the element declaration also contains a few type alternatives. These type alternatives allow the type of the message elements to be determined at the validation time. Based on the conditions stated as the test attribute values the schema validator will assign a type for the message element prior to validating its content. The expressions '@kind' and '@code' refer to two attributes of the message element. The first type alternative will assign the type called messageTypeString if the kind attribute has the value 'string' and the code attribute has a value greater than 1000.

When the schema validator encounters an element declaration with one or more type alternatives it will evaluate the test expressions one by one until an expression which evaluates to true is found. When such a matching type alternative is found the corresponding type will be assigned to the element. If none of the type alternatives match then a default type will be assigned.

My project mainly consisted of two main sections. First section of the project was to implement the type alternatives traversal support so that Xerces2/J can properly traverse an XML schema document which contains type alternatives and add the corresponding information to the schema grammar. Implementing this was fairly easy and I managed to complete it prior to th GSoC mid term evaluations. The second part of the project was to implement type alternatives validation. This was fairly difficult since I had to develop a bare minimal XPath 2.0 implementation for Xerces2/J. Developing the XPath processor actually covered a significant portion of the entire project.

My workings will be fully available in the Xerces2/J code base (even now the code related to traversal part is in one of the SVN branches) very soon. All in all it was a great learning experience as it was a great opportunity for me to learn a whole bunch of cool technologies like XML, XML schema, and XPath 2.0. I also got the opportunity to put some of my knowledge on theory of computing into action and sharpen my programming skills. I would like to give my heartiest gratitude to the Google, the Apache community and very specially to my mentor Khaled Noaman for being a very supportive guide right from the start of my project.

GSoC 2008 is On

The all important initial phase of the Google Summer of Code 2008 (GSoC) came to an end last week. The selected project proposals were published on the GsoC website on 21st April. I too applied for this year's contest and my project proposal to implement type alternatives for Apache Xerces2-J was among the qualified proposals. The Apache Software Foundation has received around 30 slots this year and one of them is allocated to my proposal (yipeee!!!). I consider this as an great opportunity to learn and master XML and XML schema while contributing to a world renowned open source project.

Apache Xerces2-J is a high performance, standard compliant XML parser. It currently supports a number of XML related standards like XML 1.0, XML 1.1, DOM, SAX, JAXP and XML schema 1.0. A variety of open source and proprietary software projects make use of Apache Xerces2-J as the core XML parsing and processing mechanism. The reason for this immense popularity of Apache Xerces2-J is probably the high number of standards it supports and the way it supports them. Nowadays Apache Xerces2-J is even distributed along with popular Sun's JDK.

Xerces development team is currently involved in getting Xerces2-J to support the XML schema 1.1 standard which is the latest XML schema specification. XML schema 1.1 specification like its predecessor is comprised of three main parts namely the primer, structures and data types. Type alternatives is a feature that falls under the XML schema 1.1: structures spec. This is one of the most significant additions to the XML schema standard and it provides a well organized mechanism to implement conditional type assignment which has been in the XML schema feature wish lists for years.

With type alternatives XML elements can be assigned types based on one or more conditions (thus the name conditional type assignment). The conditions are specified as Xpath 2.0 expressions and the relationship between a condition and the corresponding type can be expressed using the 'alternative' element as in the following example.

The Xpath expression which specifies the condition is expressed as the value of the 'test' attribute and the corresponding type is expressed as the value of the 'type' attribute. Alternatively one could use 'simpleType' or 'complexType' child elements to specify the type instead of using the 'type' attribute. A complete example illustrating XML schema type alternatives would be as follows.
Here we have defined an element named 'value' which is of declared type 'valueType'. But based on the actual value of the 'kind' attribute the 'value' elements can have a different governing type. When XML schema validations are performed the elements will be validated against their governing types.

Type alternatives can add lot more flexibility to the way XML schema documents are used and it gives more freedom and power to the XML schema author. With type alternatives the XML elements having the same name can be of different governing types. In the above example different 'value' elements can take one of three types (integer, short or byte). Also the same element can have a governing type that is different of the declared type. This wouldn't have been possible if not for the type alternatives.

All in all type alternatives is a very interesting and useful feature for XML schema authors. That makes it very important for Xerces2-J to support it. I have worked with XML and XML parsers like DOM and SAX a lot in the past. I have used Apache Xerces2-J in a number of occasions too. But to be honest I haven't really worked with XML schema much. So this really is a big learning opportunity to me. I have been studying the XML schema specs for the last few weeks and I have already collected a whole bunch of stuff on XML schema to my knowledge base.

My heart is itching to start with the coding part but I know that there are lot of things to be studied, analyzed and clarified before I get to that point. Wish me luck!!