How To Develop And Test Custom Xpath Function For Xalan.

Introduction

  XSLT 2.0 offers a rich set of transformation tools for xml transformation. But still in some cases we may encounter very specific business data transformations which can not be handled by generic transformation function. To handle those cases we may have to use the extension capability of the underling processing engine.

Problem

 Lets imagine a legacy backend system responding with following xml payload.

 It follows the upper camel case format to construct the xml payload.

 And the mobile client which consumes the system payload expects lower camel case format to process the payload.

Solution

  We are going to implement extension function using Xalan XSLT processor. Let's divide the solution in to few steps.

   1.Create the Java implementation of the logic.

       Google guvava api has a rich set of string manipulation tools. We are going to reuse one of those for converting UpperCamelCase string to lowerCamelCase string.

   2. Plug the custom function to XSLT.

       
       We have define new namespace with following pattern.

       xmlns: prefix ="xalan:// packagename.classname "

       In our example we have 

          xmlns:custom = "xalan://custom.xpath.CustomXpath"

       And we can call the function as follows

         {custom:toLowerCamelCase(local-name())}

    3. Test the custom Xpath function.

       As the final step we are creating a Junit test for test the custom Xpath function. One thing to note here is we are not using the default xslt processor instead we set the transformerfactory as "org.apache.xalan.processor.TransformerFactoryImpl" so that transformation will be done using Xalan apis.

Conclusion
Developing a custom Xpath function and plugin it to your code is not that hard but always look for existing Xpath function or existing extension function set like EXSLT. After all its good old Java , open for you to play with.  

You can find the eclipse project with the sample code at  https://github.com/KalpaD/xalan_custom_xpath_sample