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 | HTML |
|---|
| designed to transport and store data, with focus on what data is | designed to display data, with focus on how data looks |
| carrying information | displaying information |
Invent Your Own Tags With XML
- 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 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.
What XML can do ?
1) 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/CSS for display and layout, and be sure that changes in the underlying data will not require any changes to the HTML
2) 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.
3)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.
4)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.
5)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 Tree
DOCUMENTATION
<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>
| to | from | heading | body |
|---|
| Tove | Jani | Reminder | Don't forget me this weekend! |
XML Syntax Rules
All XML Elements Must Have a Closing Tag
In HTML, some elements do not have to have a closing tag:
<p>This is a paragraph.
<br>
In XML, it is illegal to omit the closing tag. All elements must have a closing tag:
<p>This is a paragraph.</p>
<br />
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:
<Message>This is incorrect</message>
<message>This is correct</message>
XML Elements Must be Properly Nested
In HTML, you might see improperly nested elements:
<b><i>This text is bold and italic</b></i>
In XML, all elements must be properly nested within each other:
<b><i>This text is bold and italic</i></b>
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 rootelement.
<root>
<child>
<subchild>.....</subchild>
</child>
</root>
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.
Study the two XML documents below. The first one is incorrect, the second is correct:
<note date=12/11/2007>
<to>Tove</to>
<from>Jani</from>
</note>
<note date="12/11/2007">
<to>Tove</to>
<from>Jani</from>
</note>
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 Attributes
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:
<gangster name='George "Shotgun" Ziegler'>
or you can use character entities:
<gangster name="George "Shotgun" Ziegler">
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.
Thank You !!
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