XML Tutorial

XML stands for eXtensible Markup Language.
XML is designed to transport and store data.
XML is important to know, and very easy to lear

What is XML?

XML stands for EXtensible Markup Language
XML is a markup language much like HTML
XML was designed to carry data, not to display data
XML tags are not predefined. You must define your own tags
XML is designed to be self-descriptive
XML is a W3C Recommendation

The Difference Between XML and HTML

XML is not a replacement for HTML.
XML and HTML were designed with different goals:

XML was designed to transport and store data, with focus on what data is
HTML was designed to display data, with focus on how data looks

HTML is about displaying information, while XML is about carrying information.

XML Does Not DO Anything

Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store, and transport information.
The following example is a note to Tove, from Jani, stored as XML:

<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The note above is quite self descriptive. It has sender and receiver information, it also has a heading and a message body.
But still, this XML document does not DO anything. It is just information wrapped in tags. Someone must write a piece of software to send, receive or display it.

With XML You Invent Your Own Tags

The tags in the example above (like <to> and <from>) are not defined in any XML standard. These tags are "invented" by the author of the XML document.
That is because the XML language has no predefined tags.
The tags used in HTML are predefined. HTML documents can only use tags defined in the HTML standard (like <p>, <h1>, etc.).
XML allows the author to define his/her own tags and his/her own document structure.

XML is a W3C Recommendation

XML became a W3C Recommendation on February 10, 1998.

XML is Everywhere

XML is now as important for the Web as HTML was to the foundation of the Web.
XML is the most common tool for data transmissions between all sorts of applications.

XML Separates Data from HTML

If you need to display dynamic data in your HTML document, it will take a lot of work to edit the HTML each time the data changes.
With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for layout and display, and be sure that changes in the underlying data will not require any changes to the HTML.
With a few lines of JavaScript code, you can read an external XML file and update the data content of your web page.

XML Simplifies Data Sharing

In the real world, computer systems and databases contain data in incompatible formats.
XML data is stored in plain text format. This provides a software- and hardware-independent way of storing data.
This makes it much easier to create data that can be shared by different applications.

XML Simplifies Data Transport

One of the most time-consuming challenges for developers is to exchange data between incompatible systems over the Internet.
Exchanging data as XML greatly reduces this complexity, since the data can be read by different incompatible applications.

XML Simplifies Platform Changes

Upgrading to new systems (hardware or software platforms), is always time consuming. Large amounts of data must be converted and incompatible data is often lost.
XML data is stored in text format. This makes it easier to expand or upgrade to new operating systems, new applications, or new browsers, without losing data.

XML Makes Your Data More Available

Different applications can access your data, not only in HTML pages, but also from XML data sources.
With XML, your data can be available to all kinds of "reading machines" (Handheld computers, voice machines, news feeds, etc), and make it more available for blind people, or people with other disabilities.

XML is Used to Create New Internet Languages

A lot of new Internet languages are created with XML.
Here are some examples:

XHTML
WSDL for describing available web services
WAP and WML as markup languages for handheld devices
RSS languages for news feeds
RDF and OWL for describing resources and ontology
SMIL for describing multimedia for the web

XML documents form a tree structure that starts at "the root" and branches to "the leaves".

Example:

The image above represents one book in the XML below:

<bookstore>
<book category="COOKING">
    <title lang="en">Everyday Italian</title>
    <author>Giada De Laurentiis</author>
    <year>2005</year>
    <price>30.00</price>
</book>
<book category="CHILDREN">
    <title lang="en">Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <price>29.99</price>
</book>
<book category="WEB">
    <title lang="en">Learning XML</title>
    <author>Erik T. Ray</author>
    <year>2003</year>
    <price>39.95</price>
</book>
</bookstore>

The root element in the example is <bookstore>. All <book> elements in the document are contained within <bookstore>.
The <book> element has 4 children: <title>,< author>, <year>, <price>

XML Syntax Rules

All XML Elements Must Have a Closing Tag

In XML, it is illegal to omit the closing tag. All elements must have a closing tag:

Note: You might have noticed from the previous example that the XML declaration did not have a closing tag. This is not an error. The declaration is not a part of the XML document itself, and it has no closing tag.

XML Tags are Case Sensitive

XML tags are case sensitive. The tag <Letter> is different from the tag <letter>.
Opening and closing tags must be written with the same case:
Note: "Opening and closing tags" are often referred to as "Start and end tags". Use whatever you prefer. It is exactly the same thing.

XML Elements Must be Properly Nested

In HTML, you might see improperly nested elements:

XML Documents Must Have a Root Element

XML documents must contain one element that is the parent of all other elements. This element is called the root element.

XML Attribute Values Must be Quoted

XML elements can have attributes in name/value pairs just like in HTML.
In XML, the attribute values must always be quoted.

Entity References

Some characters have a special meaning in XML.
If you place a character like "<" inside an XML element, it will generate an error because the parser interprets it as the start of a new element.
This will generate an XML error:

<message>if salary < 1000 then</message>

To avoid this error, replace the "<" character with an entity reference:

<message>if salary < 1000 then</message>

There are 5 predefined entity references in XML:

<	<	less than
>	>	greater than
&	&	ampersand
'	'	apostrophe
"	"	quotation mark

XML Stores New Line as LF

In Windows applications, a new line is normally stored as a pair of characters: carriage return (CR) and line feed (LF). In Unix applications, a new line is normally stored as an LF character. Macintosh applications also use an LF to store a new line.
XML stores a new line as LF.

XML Elements

An XML element is everything from (including) the element's start tag to (including) the element's end tag.

An element can contain:

other elements
text
attributes
or a mix of all of the above...

<bookstore>
<book category="CHILDREN">
    <title>Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <price>29.99</price>
</book>
<book category="WEB">
    <title>Learning XML</title>
    <author>Erik T. Ray</author>
    <year>2003</year>
    <price>39.95</price>
</book>
</bookstore>

In the example above, <bookstore> and <book> have element contents, because they contain other elements. <book> also has an attribute (category="CHILDREN"). <title>, <author>, <year>, and <price> have text content because they contain text.

XML Attributes

In HTML, attributes provide additional information about elements:

Attributes often provide information that is not a part of the data. In the example below, the file type is irrelevant to the data, but can be important to the software that wants to manipulate the element:

<file type="gif">computer.gif</file>

XML Attributes Must be Quoted

Attribute values must always be quoted. Either single or double quotes can be used. For a person's sex, the person element can be written like this:

or like this:

If the attribute value itself contains double quotes you can use single quotes, like in this example:

or you can use character entities:

XML Elements vs. Attributes

Take a look at these examples:

<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>

<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>

In the first example sex is an attribute. In the last, sex is an element. Both examples provide the same information.
There are no rules about when to use attributes or when to use elements. Attributes are handy in HTML. In XML my advice is to avoid them. Use elements instead.

My Favorite Way

The following three XML documents contain exactly the same information:
A date attribute is used in the first example:

<note date="10/01/2008">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

A date element is used in the second example:

<note>
<date>10/01/2008</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

An expanded date element is used in the third: (THIS IS MY FAVORITE):

<note>
<date>
    <day>10</day>
    <month>01</month>
    <year>2008</year>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

Avoid XML Attributes?

Some of the problems with using attributes are:

attributes cannot contain multiple values (elements can)
attributes cannot contain tree structures (elements can)
attributes are not easily expandable (for future changes)

Attributes are difficult to read and maintain. Use elements for data. Use attributes for information that is not relevant to the data.
Don't end up like this:

XML Attributes for Metadata

Sometimes ID references are assigned to elements. These IDs can be used to identify XML elements in much the same way as the id attribute in HTML. This example demonstrates this:

<messages>
<note id="501">
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!</body>
</note>
<note id="502">
    <to>Jani</to>
    <from>Tove</from>
    <heading>Re: Reminder</heading>
    <body>I will not</body>
</note>
</messages>

The id attributes above are for identifying the different notes. It is not a part of the note itself.
What I'm trying to say here is that metadata (data about data) should be stored as attributes, and the data itself should be stored as elements.

Well Formed XML Documents

A "Well Formed" XML document has correct XML syntax.
The syntax rules were described in the previous chapters:

XML documents must have a root element
XML elements must have a closing tag
XML tags are case sensitive
XML elements must be properly nested
XML attribute values must be quoted

Valid XML Documents

A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE note SYSTEM "Note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The DOCTYPE declaration in the example above, is a reference to an external DTD file. The content of the file is shown in the paragraph below.

XML DTD

The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:

<!DOCTYPE note
[
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>

Raw XML files can be viewed in all major browsers.

Don't expect XML files to be displayed as HTML pages.

Viewing XML Files

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

- <note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

Look at this XML file: note.xml
The XML document will be displayed with color-coded root and child elements. A plus (+) or minus sign (-) to the left of the elements can be clicked to expand or collapse the element structure. To view the raw XML source (without the + and - signs), select "View Page Source" or "View Source" from the browser menu.

CS - IT Study Material

Sunday, 30 December 2012

XML Tutorial

What is XML?

The Difference Between XML and HTML

XML Does Not DO Anything

With XML You Invent Your Own Tags

XML is a W3C Recommendation

XML is Everywhere

XML Separates Data from HTML

XML Simplifies Data Sharing

XML Simplifies Data Transport

XML Simplifies Platform Changes

XML Makes Your Data More Available

XML is Used to Create New Internet Languages

Example:

XML Syntax Rules

All XML Elements Must Have a Closing Tag

XML Tags are Case Sensitive

XML Elements Must be Properly Nested

XML Documents Must Have a Root Element

XML Attribute Values Must be Quoted

Entity References

XML Stores New Line as LF

XML Elements

XML Attributes

XML Attributes

XML Attributes Must be Quoted

XML Elements vs. Attributes

My Favorite Way

Avoid XML Attributes?

XML Attributes for Metadata

Well Formed XML Documents

Valid XML Documents

XML DTD

Viewing XML Files

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