What is XML, XPATH, XSL and XSLT? Explain with an example.

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eXtensible Markup Language (XML)

XML is developed by the World Wide Web Consortium (W3C), which is used as a format to store and transfer data over the Web.
XML is a markup language based on simple, platform-independent rules for processing and displaying textual information in a structured way.
XML was designed to carry data, not to display data. In XML tags are not predefined. User defines his own meaningful tags.
An XML document can be converted into another format, such as HTML and PDF, to make it more readable.
XML has been used to implement the following various operations, like
- Configuration information
- Object Serialization
Example:

<?xml version="1.0" encoding="UTF-8"?>
<bookstore>  
<book>  
    <title>Harry Potter</title>  
    <author>J K. Rowling</author>  
    <year>2005</year>    
  </book>  
  <book>  
    <title>Internet Programming</title>  
    <author>Binita Mayekar</author>  
    <year>2018</year>    
  </book>  
</bookstore>

Extensible Stylesheet Language (XSL)

XSL is a language for expressing style sheets. An XSL style sheet is, like with CSS, a file that describes how to display an XML document of a given type.
W3C developed XSL because an XML based stylesheet language was needed to transform XML documents.
XSL act as an intermediary between the XML elements and Web browser. It tells the browser how to display the various elements.
XSL used to performs following operations, like
- Support for browsing, printing, and rendering.
- Formatting highly structured documents (XML).
- Performing complex publishing tasks: tables of contents, indexes, reports.
- Addressing accessibility and internationalization issues.
- Written in XML.
XSL began as a single language to serve the purpose, but further, it was divided into three parts, such as
- XSLT – For transforming XML document.
- XPATH – For navigating in XML document.
- XSL-FO – For formatting XML document.
XSL has two attributes
- ’type’ - The type of file being linked to like ‘text/xsl’.
- ’href’ - Specify location of the file.
Following code structure is used to add XSL file into XML document <?xml-stylesheet type='text/xsl' href='style.xsl'?>
Example:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="https://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<html>
<body>
<table border="1">
<tr style="background-color:lightblue;color:black">
<th align="center">Book Name</th>
<th align="center">Author</th>
<th align="center">Year</th>
</tr>
<xsl:for-each select="bookstore/book">
<tr><td><xsl:value-of select="title"/></td>
<td><xsl:value-of select="author"/></td>
<td><xsl:value-of select="year"/></td></tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>

XSL Transformations (XSLT)

XSLT is an XML-based language that transforms an XML documents and generates output into a different format such as HTML, XHTML or PDF.
XSLT is an extension of XSL, which is a stylesheet definition language for XML. XSLT is the most important part of XSL.
XSLT helps to add or remove elements and attributes from the final output. It also re-arranges and sort elements, performs tests and makes decisions about which elements to hide and display.
It allows an XML author to write content only once and put it in many different formats.
We simply say like XSLT transforms an XML source-tree into an XML result-tree.
XSLT contains many elements that can be used to manipulate, iterate and select XML, for output.
- <xsl:value-of>- Extracts the value of a selected node.
- <xsl:for-each> - Select every XML element of a specified node-set.
- <xsl:if>- Put a conditional if test against the content of the XML file.
- <xsl:sort> - Sort the output.
- <xsl:choose> - Used along with <xsl:when> and <xsl:otherwise> to express multiple conditional tests.
- <xsl:template> - Defines a template.
- <xsl:apply-template>- Applies a template to the current element or to the current element's child node.

XSLT Working:

In the transformation process, XSLT uses XPath to define parts of the source document that should match one or more predefined templates.
When a match is found, XSLT will transform the matching part of the source document into the result document.
Example: Following example only display the records for book published in year 2018.

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="https://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<html>
<body>
<table border="1">
<tr style="background-color:lightblue;color:black">
<th align="center">Book Name</th>
<th align="center">Author</th>
<th align="center">Year</th>
</tr>
<xsl:for-each select="bookstore/book">
<xsl:if test="year=2018">
      <tr>
        <td><xsl:value-of select="title"/></td>
        <td><xsl:value-of select="author"/></td>
        <td><xsl:value-of select="year"/></td>        
      </tr>
    </xsl:if>
    </xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>

XML Path Language (XPATH)

XPath can be used to navigate through elements and attributes in an XML document. It allows XML authors to link to very specific locations within an XML document.
XPath uses "path like" syntax to identify and navigate nodes in an XML document.
XPath contains over 200 built-in functions.
XPath is a major element used in the XSLT standard.
XPath uses path expressions to select nodes or node-sets in an XML document. These path expressions look very much like the path expressions used in traditional computer file systems.
The most useful path expressions are as follows:
- nodename - Selects all nodes with the name "nodename".
- / - Selects from the root node.
- // - Selects nodes in the document from the current node that match the selection no matter where they are.
- . - Selects the current node.
- .. - Selects the parent of the current node.
- @ - Selects attributes.
Predicates are also used to find a specific node or a node that contains a specific value. Predicates are always embedded in square brackets.
XPath wildcards can be used to select unknown XML nodes and also the use of | operator in an XPath expression select several paths.

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