Introduction to XML

XML was designed to transport and store data.
HTML was designed to display data.

What You Should Already Know
Before you continue you should have a basic understanding of the following:
  • HTML
  • JavaScript
If you want to study these subjects first, find the tutorials on our Home page.

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:
Don't forget me this weekend!
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 and ) 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

, 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.
My best description of XML is this:
XML is a software and hardware independent tool for carrying information.

XML is Everywhere
We have been participating in XML development since its creation. It has been amazing to see how quickly the XML standard has developed, and how quickly a large number of software vendors have adopted the standard.
XML is now as important for the Web as HTML was to the foundation of the Web.
XML is everywhere. It is the most common tool for data transmissions between all sorts of applications, and is becoming more and more popular in the area of storing and describing information.

Using XML in ASP.NET

XML is a cross-platform, hardware and software independent, text based markup language, which enables you to store data in a structured format by using meaningful tags. XML stores structured data in XML documents that are similar to databases. Notice that unlike Databases, XML documents store data in the form of plain text, which can be used across platforms.
The World Wide Web (WWW) is the biggest infrastructure for information publishing and exchange. In this context XML as standard format for documents becomes more and more important. In most cases XML documents are generated automatically from databases. To exploit the content of any XML structured file, it is important to find an efficient representation. One common representation is the Document Object Model
(DOM) (W3C,1998), which has the advantage that every XML document can be gathered. On the contrary, accessing single elements is very inefficient, because knowledge about the XML document’s specific structure is not supported by the DOM. Thus an XPath query selecting child elements with a certain name can only be evaluated by testing all siblings, which results in linear access time. Since most XML documents
are structured according to a given XML language description like XML Schema (W3C,2001) or DTD (W3C,2004), it is more efficient to use so called class generators to produce classes which are capable to represent XML documents of a specific XML language much more efficiently. In this paper we will introduce a new XML Class Generator for Java (XCG), which offers efficient access to sub elements in constant time, provides a simple class structure and moreover guarantees universality. In this context universality means that every correct DTD or XML Schema can be mapped onto (Java) classes.
Xml Code Generator (XCG)

What is XCG?

XCG is a technology that allows you to create classes using an xml syntax. You can declaratively specify a class' members and their initial values, as well as a number of additional things that make certain common programming tasks easier.
Above all else, XCG is designed to be flexible. This means that everything it does is customisable. XCG stands as a framework around which you can build your own code-generation system. Basically, this means that if XCG does not do what you want, you can make it do what you want. Virtually any part of programming you find tedious can be automated by XCG, and instead of having to build your own code generation system from scratch, you can hook into XCG, which also has the added benefit that your extensions can cooperate with XCG's intrinsic functionality as well as with other extensions written by you or others. Of course you don't have to learn how to write extensions if you just want to use XCG; you can use XCG and any existing extensions without even knowing how the extension system works.
Although XCG and similar systems are ideal for situations where you are designing a graphical UI, it should be remarked that XCG is not limited to this. It is a general purpose method for hooking up .Net objects.

An XCG primer

Rather than trying to tell you what XCG is, it would be far simpler to give examples of what it can do. All code examples, unless otherwise specified, use C# and Visual Basic .Net syntax. Even if you're not familiar with C# per se this should make the samples reasonably straight-forward to follow if you're familiar with C++ or Java. 

No comments: