XSLT, like XQuery, uses XPath expressions to address specific nodes within the source document and perform a calculation. XSLT can use a range of functions that XPath provides to further augment itself.

Originally an acronym for XML Path Language, XPath is used for selecting nodes from an XML document. It models XML as a tree of nodes. XPath is named after its use of a path notation for navigating through the hierarchical structure of an XML document. XPath uses a compact, non-XML syntax to form expressions for use in Uniform Resource Identifier (URI) and XML attribute values.

XPath models an XML document as a tree of nodes. The following figure depicts the different types of nodes, as seen by XPath, in an XML document:

The elements in the preceding diagram are:

In the XPath model, the document root is not the same as the root element node. XPath defines a way to compute a string value for each type of node, some of which have names. Because XPath fully supports the XML Namespaces Recommendation, the node name is modeled as a pair known as the expanded name, which consists of a local part and a namespace URL, which may be null.

XPath allows various kinds of expressions to be nested with full generality (that is, using wildcards). It is a strongly typed language, such that the operands of various expressions, operators, and functions must conform to designated types. XPath is a case-sensitive expression language.

The following are some examples of XPath expressions:

Get the store_name node of the bookstore identified with number 1:

/bookstores/bookstore[@num="1"]/store_name 

Get the text value of the store_name node of the last bookstore:

/bookstores/bookstore[last()]/store_name/text() 

Check if a bookstore exists in the source document (prefixing // means a wildcard search):

count(//bookstore[store_name="Packt Publishing"]) = 1 

With XSLT it is possible to create really dynamic output documents using XSLT constructs together with XPath expressions. XLST construct are specific elements that are defined by the XSLT standard. XSLT defines the following construct elements: output, template, value-of, for-each, if, and choose.

The xsl:output element

The output element specifies the output format of the result tree and it must be a direct child of the style sheet root element. The element has a few attributes. The method attribute defines which processor needs to be used, such as xml, html, or text, and the media-type attribute defines the target type, for example, application/json:

<xsl:output method="html" media-type="text/xml"/> 

The xsl:template element

The template element defines a template rule that matches a specific node in the source document using an XPath pattern value. The output expression contains the formatting instructions to produce a result tree:

<xsl:template match="pattern"> 
  output-expressions 
</xsl:template>  

The match pattern determines the context node within the template. The most common match is on the root element of the source document tree:

<xsl:template match="/"> 
  A simple text string 
</xsl:template> 

A template can also be given a name and called by its name within another template by passing a parameter:

<xsl:template name="minValue"> 
  <xsl:param name="values"/> 
  ... 
</xsl:template> 
<xsl:template match="/"> 
  <xsl:call-template name="minValue"> 
    <xsl:with-param name="values" 
                    select="bookstore/book/year"/> 
  </xsl:call-template> 
</xsl:template> 

The xsl:value-of element

The value-of element is used to insert the text value of the expression. This element defines a select attribute, which contains an expression and is used inside a template element:

<xsl:template match="name"> 
  Name: 
  <xsl:value-of select="name"/> 
</xsl:template> 

The xsl:for-each element

The for-each element is used to loop over node-sets. This element defines a select attribute, which instructs the XSLT processor to loop over the node set returned by the given XPath expression:

<xsl:template match="/"> 
  <xsl:for-each select="catalog/book"> 
    <p><xsl:value-of select="title"/></p> 
  </xsl:for-each> 
</xsl:template> 

Inside the for-each element, the current node in the set is the context. The position() function returns the index in the loop, that is, the iteration counter. It is also possible to sort the order in which the nodes are looped over using the xsl:sort element. It defines a select attribute that contains an expression whose value is sorted on:

<xsl:for-each select="catalog/book">
<xsl:sort select="year" data-type="number" order="descending"/>
  <p>
    <xsl:value-of select="title"/>
  </p>
</xsl:for-each> 

To apply multiple criteria, we can use several xsl:sort elements after each other.

The xsl:if element

Sometimes a section of the XSLT tree should only be processed under certain conditions. With the if element we can build that conditionally. The if element defines a test attribute, which contains an XPath expression that should evaluate to a Boolean value:

<xsl:for-each select="catalog/book"> 
  <xsl:if test="year = 2016" > 
    <p><xsl:value-of select="title"/></p> 
  </xsl:if> 
</xsl:for-each> 

There is no else statement, but for this XSLT defines the xsl:choose element.

The xsl:choose element

XSLT supports multiple mutually exclusive branches with the choose element. The xsl:choose element contains xsl:when tags that define parallel branches, one of which is executed. The xsl:otherwise tag can be used to define a default branch.

We use the choose element for if-the-else constructions:

<xsl:for-each select="catalog/book"> 
  <xsl:choose> 
    <xsl:when test="year = 2016" > 
      <xsl:text>New Arrivals</xsl:text> 
    </xsl:when> 
    <xsl:when test="year < 2015" > 
      <xsl:text>Our Classics</xsl:text> 
    </xsl:when> 
    <xsl:otherwise> 
      <xsl:text>Top Picks</xsl:text> 
    </xsl:otherwise> 
  </xsl:choose> 
</xsl:for-each> 

When implementing integrations with ICS we will use XSLT for creating mappings between source and target connections. The major advantage of ICS is that we can build these mappings with a visual editor.