XML

stands for

Extensible Markup Language

application/xml

is the official Internet media type for XML.

.xml

is the XML filename extension.

XML is extensible

− XML allows us to create our own self-descriptive tags.

XML carries the data, does not present it

− XML allows us to store the data irrespective of how it will be presented.

XML is a public standard

- XML was developed by an organization called the World Wide Web Consortium. Any type of data can be expressed as an

XML document

XML can be used to

exchange

the information between companies and systems

An XML file contains XML-elements, also called XML-nodes or XML-tags

<?xml version="1.0" encoding = "UTF-8"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

The names of XML-elements are enclosed by triangular brackets

< >

<element>

Each XML-element

needs to be closed

either with start or with end elements as shown below

<element>...</element>

XML allows self-closing elements, for example if the tag empty

<?xml version="1.0"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <address/>
   <phone>(011) 123-4567</phone>
</student>

Children elements must not overlap parent elements. i.e., an element end tag must follow all of its children's end tags.

<company>

is closed after the

</contact-info>

tag but it's opened after

</contact-info>

tag, which is wrong!

<?xml version = "1.0"?>
<contact-info>
<company>Learn Practice Teach
</contact-info>
</company>

The following example shows the correct nested tags

<?xml version = "1.0"?>
<contact-info>
   <company>Applications.ge</company>
<contact-info>

One

root element

is necessary for an XML document. In this example below, both

<x>

and

<y>

elements are at the top level and they don't have one parent element, which is wrong:

<?xml version = "1.0"?>
<x>...</x>
<y>...</y>

XML-elements are

case-sensitive

, which means that the start and the end elements names need to be exactly in the same case. This example is not correct XML document, because

case sensitivity

is not correctly applied

<?xml version="1.0"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <address/>
   <phone>(011) 123-4567</Phone>
</student>

In this case

<phone>

and its close tag

</phone>

is not in the same case

XML declaration example

<?xml version = "1.0" encoding = "UTF-8" standalone = "yes"?>

XML document can optionally have an XML declaration. XML document without declaration is also valid

<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

If the XML declaration is included, it must contain version number attribute

<?xml encoding="UTF-8" standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

It will generate the following error:

error on line 1 at column 7: Malformed declaration expecting version

The names are always in lower case

<?xml Version="1.0" encoding="UTF-8" standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

It will generate the following error:

error on line 1 at column 7: Malformed declaration expecting version

The XML declaration must begin with

<?xml

<? version="1.0" encoding="UTF-8" standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

The following error will be generated:

error on line 1 at column 3: xmlParsePI : no target name

If document contains XML declaration, it must be the

first statement

<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>
<?xml version="1.0" encoding="UTF-8" standalone="no"?>

The error will be the following:

error on line 6 at column 6: XML declaration allowed only at the start of the document

The order of placing the parameters is important. The correct order is:

version

,

encoding

and

standalone

<?xml encoding="UTF-8" standalone="no" version="1.0"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

This will generate the following error:

error on line 1 at column 7: Malformed declaration expecting version

Either single or double quotes may be used. Here is valid XML document

<?xml version='1.0' encoding='UTF-8' standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
</student>

An HTTP protocol can override the value of encoding that we put in the declaration.

XML Schema Definition (XSD) is used to validate and describe an XML data structure and content . It defines the

elements

,

attributes

and

data types

. Here is how XML schema looks like

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="company">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="id" type="xs:unsignedInt" />
        <xs:element name="name" type="xs:string" />
        <xs:element name="phone" type="xs:string" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

The XML schema is derived from the following XML:

<company>
    <id>2859385</id>
    <name>Tanmay Patil</name>
    <phone>(011) 123-4567</phone>
</company>

The XML schema is generated using the following online tool:

https://www.liquid-technologies.com/online-xml-to-xsd-converter

XML documents form a tree structure that starts at

the root

An

XML tree

starts at a

root element

and branches from the root to

child elements

<root>
  <child>
    <subchild>.....</subchild>
  </child>
</root>

The terms

parent

,

child

, and

sibling

are used to describe the relationships between elements. ...

Parents

have children.

Children

have parents.

Siblings

are children on the same level (brothers and sisters). ... We can use any of the online XML tree viewer tools to see the document's tree structure, like this one:

https://www.xmlviewer.org/

Books

XML document to see on tree-viewer

<books>
  <book category="cooking">
    <title lang="en">The goals</title>
    <author>Giada De Laurentiis</author>
    <year>2007</year>
    <price>30.00</price>
  </book>
  <book category="children">
    <title lang="en">12 rules for life</title>
    <author>J K. Rowling</author>
    <year>2002</year>
    <price>29.99</price>
  </book>
  <book category="web">
    <title lang="en">The richest man in Babilon</title>
    <author>Erik T. Ray</author>
    <year>2001</year>
    <price>39.95</price>
  </book>
</books>

XML Attribute

specifies a single property for the element, using a

name/value

pair. An XML-element can have

one or more

attributes

<a href = "http://www.applications.ge/">Applications.ge</a>

In the example

href

is the

attribute name

and

http://www.applications.ge

is the

attribute value

... XML attribute names (unlike HTML) are case sensitive. Which means that

HREF

and

href

are considered two different XML attributes

<?xml version="1.0"?>
<student>
    <a href="http://www.applications.ge/" hreF="HTTP://applications.ge/">Applications.ge</a>
</student>

Several different values for the same attribute is not allowed. An attribute name must not appear more than once in the same tags:

<?xml version="1.0"?>
<student>
    <a href="http://www.applications.ge/" href="HTTP://applications.ge/">Applications.ge</a>
</student>

The following error will appear on the browser

error on line 3 at column 73: Attribute 
href
 redefined

Attribute names must always be defined without quotation marks, whereas attribute values must always appear in single or double quotation marks

<?xml version="1.0"?>
<student>
    <a "href"="http://www.applications.ge/">Applications.ge</a>
</student>

The following error will appear:

error on line 3 at column 8: error parsing 
attribute name
 

Attribute values must always be in quotation marks (single

'

or double

"

quotes)

<?xml version="1.0"?>
<student>
    <a href=http://www.applications.ge/>Applications.ge</a>
</student>

This incorrect syntax will generate the following error:

error on line 3 at column 13: AttValue: " or ' expected

The symbols: hyphen

-

, under-score

_

and period

.

are allowed in element name. The XML example is valid

<?xml version="1.0" encoding="UTF-8"?>
<student>
   <first-name>George</first-name>
   <phone.mobile>(011) 123-4567</phone.mobile>
   <native_language>English</native_language>
   <city />
</student>

- An element name can contain any alphanumeric characters. Allowed

symbols

in names are the hyphen

-

, under-score

_

, period

.

and digits

0-9

- Names are

case sensitive

, Address, address, and ADDRESS are different names. - Start and end tags of an element must be

the same

. - An element, which is a container, can contain

text

or

elements

<?xml version="1.0" encoding="UTF-8"?>
<student>
   <first-name>George</first-name>
   <phone.mobile>(011) 123-4567</phone.mobile>
   <native_language>English</native_language>
   <city />
</student>

Note: XML element name must not start with

.

,

-

,

digit

The term

CDATA

means,

Character Data

. CDATA is defined as blocks of text that

are not parsed

by the parser, but are otherwise recognized as markup

<?xml version="1.0" encoding="UTF-8"?>
<student>
    <!-- Some comment about the student -->
    <first-name>George</first-name>
    <phone.mobile>(011) 123-4567</phone.mobile>
    <city />
    <description>
        <![CDATA[
            <p>
            <a href="/mylink/article1"><img style="float: left; margin-right: 5px;" height="80" src="/mylink/image" alt=""/></a>
            Author Names
            <br/><em>Date</em>
            <br/>Paragraph of text describing the article to be displayed</p>
        ]]>        
    </description>
</student>

CDATA Start section

- CDATA begins with the nine-character delimiter

<![CDATA[

CDATA End section

- CDATA section ends with

]]>

delimiter

CData section

- Characters inside

CData

section are interpreted as characters, and not as markup. It may contain markup characters

<

,

>

, and

&

, but they are ignored by the XML processor

1. CDATA is still

part of the document

, while a comment is not 2. In CDATA we cannot include the string

]]>

, while in a comment

--

3. CDATA content is visible on the web if we specify

xmlns

attribute as

http://www.w3.org/1999/xhtml

, even if the file is saved as

.xml

<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>CDATA Example</title>
    </head>
    <body>

        <h2>Using a Comment</h2>
        <div id="commentExample">
            <!--
            You won't see this in the document
            and can use reserved characters like
            < > & "
            -->
        </div>

        <h2>Using a CDATA Section</h2>
        <div id="cdataExample">
            <![CDATA[
            You will see this in the document
            and can use reserved characters like
            < > & "
            ]]>
        </div>

    </body>
</html>