XML Tutorial ~ Appychip

Introduction to XML

It is a format and was designed to transport and store data. Its file extension is .xml.

HTML was designed to display data.

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. In XML every tag must be closed.
HTML was designed to display data, with focus on how data looks. In HTML, there are some tag which will not closed (i.e. , <hr> etc.)

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 Mahesh from Ramesh, stored as XML:

<note>

<to>Mahesh</to>

<from>Ramesh</from>
<heading>Reminder</heading>
<body>Hi, It’s good to see you.</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 pure information wrapped in tags. Someone must write a piece of software to send, receive or display it.

XML is Just Plain Text

XML is nothing special. It is just plain text. Software that can handle plain text can also handle XML.

However, XML-aware applications can handle the XML tags specially. The functional meaning of the tags depends on the nature of the application.

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 (and the structure of HTML) are predefined. HTML documents can only use tags defined in the HTML standard (like , <h1>, etc.).XML allows the author to define his own tags and his own document structure.

XML is Not a Replacement for HTML

XML is a complement to HTML.

It is important to understand that XML is not a replacement for HTML. In most web applications, XML is used to transport data, while HTML is used to format and display the data.

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, you can read an external XML file and update the data content of your HTML.

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 different applications can share.

XML Simplifies Data Transport

With XML, data can easily be exchanged between incompatible systems. 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 very 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.

An Example XML Document

XML documents use a self-describing and simple syntax:

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

<to>Mahesh</to>
 <from>Ramesh</from>
 <heading>Reminder</heading>
 <body> Hi, It’s good to see you!</body>
</note>

The first line is the XML declaration. It defines the XML version (1.0) and the encoding used (UTF-8 = 8-bit UCS/Unicode Transformation Format is a variable-length character encoding for Unicode).

The next line describes the root element of the document (like saying: "this document is a note"):

<note>

The next 4 lines describe 4 child elements of the root (to, from, heading, and body):

<to>Mahesh</to>
<from>Rameshi</from>
<heading>Reminder</heading>
<body> Hi, It’s good to see you!</body>

And finally the last line defines the end of the root element:

</note>

XML Syntax Rules

The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.

All XML Elements Must Have a Closing Tag

In HTML, you will often see elements that don't have a closing tag:

This is a paragraph
This is another paragraph

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

This is a paragraph
This is another paragraph

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 elements are defined using XML tags.

XML tags are case sensitive. With XML, the tag <Letter> is different from the tag <letter>.

Opening and closing tags must be written with the same case:

<Message>This is incorrect</message>
<message>This is correct</message>

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:

This text is bold and italic

In XML, all elements must be properly nested within each other:

This text is bold and italic

In the example above, "Properly nested" simply means that since the element is opened inside the element, it must be closed inside the element.

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 value must always be quoted. Study the two XML documents below. The first one is incorrect, the second is correct:

<note date=12/11/2007>
<to>Mahesh</to>
<from>Ramesh</from>
</note>

<note date="12/11/2007">
<to>Mahesh</to>
<from>Ramesh</from>
</note>

The error in the first document is that the date attribute in the note element is not 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

Note: Only the characters "<" and "&" are strictly illegal in XML. The greater than character is legal, but it is a good habit to replace it.

Comments in XML

The syntax for writing comments in XML is similar to that of HTML.

What is an XML Element?

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, simple text or a mixture of both. Elements can also have attributes.

<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. <author> has text content because it contains text.

In the example above only <book> has an attribute (category="CHILDREN").

XML Naming Rules

XML elements must follow these naming rules:

Names can contain letters, numbers, and other characters
Names cannot start with a number or punctuation character
Names cannot start with the letters xml (or XML, or Xml, etc)
Names cannot contain spaces

Any name can be used, no words are reserved.

XML Attributes

XML elements can have attributes in the start tag, just like HTML.

Attributes provide additional information about elements.

XML Attributes

From HTML you will remember this: <img src="computer.gif">. The "src" attribute provides additional information about the <img> element.

In HTML (and in XML) 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 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 enclosed in quotes, but either single or double quotes can be used. For a person's sex, the person tag can be written like this:

<person sex="female">

or like this:

<person sex='female'>

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

<dept name='IIPS "DAVV" Indore'>

or you can use character entities:

<dept name="IIPs "DAVV" Indore">

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 and when to use elements. Attributes are handy in HTML. In XML my advice is to avoid them. Use elements instead.

XML Validation

XML with correct syntax is "Well Formed" XML.

XML validated against a DTD is "Valid" XML.

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

<?xml version="1.0" encoding="UTF-8"?>
<note>
<to>Mahesh</to>
<from>Ramesh</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

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="UTF-8"?>
<!DOCTYPE note SYSTEM "Note.dtd">
<note>
<to>Mahesh</to>
<from>Ramesh</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 document structure with a list of legal elements. A DTD can be declared inline in your XML document, or as an external reference.

Internal DTD

<?xml version="1.0"?>

<!DOCTYPE note [

  <!ELEMENT note    (to,from,heading,body)>

  <!ELEMENT to      (#PCDATA)>

  <!ELEMENT from    (#PCDATA)>

  <!ELEMENT heading (#PCDATA)>

  <!ELEMENT body    (#PCDATA)>

]>

<note>

<to>Mahesh</to>

<from>Ramesh</from>

<heading>Reminder</heading>

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

</note>

External DTD

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

PCDATA

PCDATA means parsed character data. Think of character data as the text found between the start tag and the end tag of an XML element.

PCDATA is text that will be parsed by a parser. Tags inside the text will be treated as markup and entities will be expanded.

CDATA

CDATA also means character data. CDATA is text that will NOT be parsed by a parser. Tags inside the text will NOT be treated as markup and entities will not be expanded.

Displaying your XML Files with CSS?

It is possible to use CSS to format an XML document. Below is an example of how to use a CSS style sheet to format an XML document: Look at this style sheet:

cd_catalog.css file coding is

CATALOG

{

background-color: #ffffff;

width: 100%;

}

{

display: block;

margin-bottom: 30pt;

margin-left: 0;

}

TITLE

{

color: #FF0000;

font-size: 20pt;

}

ARTIST

{

color: #0000FF;

font-size: 20pt;

}

COUNTRY, PRICE, YEAR, COMPANY

{

display: block;

color: #000000;

margin-left: 20pt;

}

Below is a fraction of the XML file. The second line links the XML file to the CSS file:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="cd_catalog.css"?>
<CATALOG>
<CD>
 <TITLE>Empire Burlesque</TITLE>
 <ARTIST>Bob Dylan</ARTIST>
 <COUNTRY>USA</COUNTRY>
 <COMPANY>Columbia</COMPANY>
 <PRICE>10.90</PRICE>
 <YEAR>1985</YEAR>
</CD>
<CD>
 <TITLE>Hide your heart</TITLE>
 <ARTIST>Bonnie Tyler</ARTIST>
 <COUNTRY>UK</COUNTRY>
 <COMPANY>CBS Records</COMPANY>
 <PRICE>9.90</PRICE>
 <YEAR>1988</YEAR>
</CD>
</CATALOG>

Appychip

Friday, 18 November 2011

XML Tutorial

Introduction to XML

What is XML?

The Difference Between XML and HTML

XML Does not DO Anything

XML is Just Plain Text

With XML You Invent Your Own Tags

XML is Not a Replacement for HTML

XML is a complement to HTML.

XML Separates Data from HTML

XML Simplifies Data Sharing

XML Simplifies Data Transport

XML Simplifies Platform Changes

An Example XML Document

XML Syntax Rules

All XML Elements Must Have a Closing Tag

XML Tags are Case Sensitive

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

XML Documents Must Have a Root Element

XML Attribute Values Must be Quoted

Entity References

Comments in XML

What is an XML Element?

XML Naming Rules

XML Attributes

XML Attributes

XML Attributes Must be Quoted

XML Elements vs. Attributes

XML Validation

Well Formed XML Documents

Valid XML Documents

XML DTD

PCDATA

CDATA

Displaying your XML Files with CSS?

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