This post will illustrate WSO2 ESB XSLT Transform Mediator with sample code.

When we need XSLT Mediator is need in wso2 ESB?

When we have to deal with a dynamic request for an ESB proxy. (If the proxy request is dynamic (not static/not predefined) we will have to use the XSLT mediator to mediate the message)

What is XSLT Mediator ?

The XSLT Mediator applies a specified XSLT transformation to a selected element of the current message payload. The source attribute specifies which element to have XSLT transformation. The feature element defines in to TransformerFactory.

If source element is not specified then soap body as the selected element. Parameters can be passed into the transformations through the property elements and it is optional and those can be accessed during transformation by <xsl:param name="the name of the property"/>.

What is the WSO2 ESB XSLT Mediator Syntax?

<xslt key="string" [source="xpath"]>
     <property name="string" (value="literal" | expression="xpath")/>*
     <feature name="string" value="true| false" />*
     <resource location="string" key="string"/>*
</xslt>

• Source - Specifies which element to be selected to apply the given XSLT transformation.
• Property - Allows optional parameters to be passed into the transformations.
• Feature - Defines any features which should be explicitly set to the TransformerFactory.
• Resource - Resolves XSLT imports and includes from the repository.

Here are some thing that I will be using in XSLT script

The <xsl:apply-templates> element applies a template to the current element or to the current element's child nodes. If we add a select attribute to the <xsl:apply-templates> element it will process only the child element that matches the value of the attribute. We can use the select attribute to specify the order in which the child nodes are processed.[1]

omit-xml-declaration 

values : yes OR no  Optional.

"yes" specifies that the XML declaration (<?xml...?>) should be omitted in the output. "no" specifies that the XML declaration should be included in the output. The default is "no"

indent 

values:  yes OR no  Optional.

"yes" indicates that the output should be indented according to its hierarchic structure. "no" indicates that the output should not be indented according to its hierarchic structure.
This attribute is not supported by Netscape 6[2]

Let do a sample

We can try our Transport Services (BusServices)[3]

Here Is the Expecting request

<sample:getBusNo xmlns:sample="http://transport.org">
      <xs:rootId xmlns:xs="http://transport.org">2</xs:rootId>
   </sample:getBusNo>

Back end Serive expecting request

<p:getBusNo xmlns:p="http://transport.org">
<xs:rootId xmlns:xs="http://transport.org">2</xs:rootId>
</p:getBusNo>

1. Start BusServices in WSO2 AS
2. Start WSO2 ESB in offset 1
3. Go to Home > Manage > Service Bus > Local Entries in ESB
4. Then add Add In-lined XML Entry
5. Five name as "in_xslt" and Value as below (transform.xsl file contains)

<?xml version="1.0" encoding="ISO-8859-1"?>

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:fn="http://www.w3.org/2005/02/xpath-functions"
                xmlns:xs="http://transport.org"
                xmlns:sample="http://transport.org"
                exclude-result-prefixes="sample fn">
    <xsl:output method="xml" omit-xml-declaration="yes" indent="yes"/>

    <xsl:template match="/">
        <xsl:apply-templates select="//sample:getBusNo"/>
    </xsl:template>

    <xsl:template match="sample:getBusNo">
<p:getBusNo xmlns:p="http://transport.org">
      <xs:rootId xmlns:xs="http://transport.org">
<xsl:value-of select="xs:rootId"/>
</xs:rootId>
   </p:getBusNo>

    </xsl:template>
</xsl:stylesheet>

Now I need to change my respond 

from:

<ns:getBusNoResponse xmlns:ns="http://transport.org">
   <ns:return>4Colombo</ns:return>
   <ns:return>Negombo</ns:return>
   <ns:return>Galle</ns:return>
</ns:getBusNoResponse>

to:

<ns:getBusNoResponse xmlns:ns="http://transport.org">
<ns:rootName>2Colombo</ns:rootName>
<ns:rootName>Negombo</ns:rootName>
<ns:rootName>Galle</ns:rootName>
</ns:getBusNoResponse>

Here is XSLT Script for that

<?xml version="1.0" encoding="ISO-8859-1"?>

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:fn="http://www.w3.org/2005/02/xpath-functions"
                xmlns:ns="http://transport.org"
                exclude-result-prefixes="ns fn">
    <xsl:output method="xml" omit-xml-declaration="yes" indent="yes"/>

    <xsl:template match="/">
        <xsl:apply-templates select="//ns:getBusNoResponse"/>
    </xsl:template>

    <xsl:template match="ns:getBusNoResponse">
<ns:getBusNoResponse xmlns:ns="http://transport.org">
 <xsl:for-each select="ns:return">
   <ns:rootName> <xsl:value-of select="."/> </ns:rootName>

</xsl:for-each>
</ns:getBusNoResponse>
    </xsl:template>
</xsl:stylesheet>

Add it to the out sequences in proxy

Here is Final testing

NOTE
If any case If I get empty String in RootID I am adding new RootName

<xsl:choose>
    <xsl:when test=".!= ''">
         <rootNmae><xsl:value-of select="."/></rootNmae>
       
    </xsl:when>
    <xsl:otherwise>
       <rootNmae>Root<xsl:text> </xsl:text>text</rootNmae>
    </xsl:otherwise>
</xsl:choose>