What is XML?

  Ø  What is XML?

·         XML is the eXtensible Markup Language.

XML

·         Became a W3C Recommendation in 1998.

·         Tag-based syntax, very much like HTML.

·         We get to make up our own tags (or we can use an existing tag set to solve a particular problem).

·         Foundation for several next-gen Web Technologies like XHTML, RSS (Blogs), AJAX, Web Services.

·         XML does not replace HTML.

  Ø  What does XML do?

·         XML is used to structure and describe information.

·         Intended to be used with the Internet.

·         XML can be used as a way to interchange data between disparate systems.

  Ø  Related XML Technologies:

·         XPath:

ü  eXtensible Path Language.

ü  Used to extract data from inside an XML file.

ü  Uses a path-like syntax, similar to directory or folder paths like “drive:/folder/folder/file”

·         XSLT:

ü  eXtensible Stylesheet Language Transformations.

ü  Styling language that takes an XML file and “transforms” it into  something else, like HTML, PDF, ASCII, or even another XML file.

·         XQuery:

ü  Used to perform query functions on XML data, similar to SQL for databases.

·         XPointer and XLink:

ü  Used for creating hyperlinks to XML documents and arbitrary points within XML documents.

  Ø  Example: Describing Information

·         Typical Information found on a business card:

Santosh Tewari

(415) 555-4567 (mobile)

(800) 555-9876 (work)

(510) 555-1234 (fax)

imstkgp@gmail.com

·         Same business card data, expressed in XML:

<BusinessCard>

            <Name>Santosh Tewari</Name>

<phone type=”mobile”>(415) 555-4567</phone>

<phone type=”work”>(800) 555-9876</phone>

<phone type=”fax”>(510) 555-1234</phone>

<email>imstkgp@gmail.com</email>

                        </BusinessCard>        

  Ø  Advantages Of XML:

·         Content is kept separately from any notion of presentation.

·         Information can be easily read and understood.

·         Specific tag sets that target specific problems can be easily created.

·         XML is an open format that can be processed by any XML aware application, like a browser, word processor, spreadsheet, etc.

·         XML data can be exchanged between systems that were designed to do so.

  Ø  Disadvantages Of XML:

·         XML is not especially good at handling very large amount of data.

·         XML can quickly become more difficult to read if a lot of information is included in one file.

·         Certain types of data (images, other binary data) are not represented well in XML.

  Ø  Proper XML Syntax:

·         All XML documents have a single root tag.

·         XML documents must be “well-formed”

ü  Empty tags, i.e. tags that do not contain any content themselves, must be closed with />

i.e. Instead of <elem></elem> use <elem />

ü  Attributes cannot be minimized

i.e. <element attr> is wrong – must do <element attr=”atrr”>

Attributes values must always be inside quotes, either single or double:

i.e. <element attr=val> is wrong – must do <element attr=”val”>

ü  Tags must br properly nested inside each other

<elem1><elem2></elem1></elem2> is wrong – always do <elem1><elem2></elem2></elem1>

  Ø  Contents of XML Files:

·         XML Declaration:

<?xml version=”1.0” encoding=”utf-8” standalone=”yes” ?>

·         Elements (Sometimes called tags)

<element>, just like we’ve used in HTML.

·         Attributes

<element attr=”val”>, again like HTML.

·         Comments

<!-- XML Comment -->

·         Character Data Sections

<! [CDATA [This is some text & data]] >

·         Processing Instructions

<?SpellCheckMode mode=”us-english” ?>

·         Entity References

Character(&#60;) and General (&copyright”;)

  Ø  The XML Declaration:

<?xml version=”1.0” encoding=”utf-8” standalone=”yes” ?>

·         Optional, though the W3C recommends including it

§  Identifies the file as being XML.

§  Provides a place for the encoding and standalone declarations.

§  Must be at very beginning of file - not even whitespace before it.

·         The encoding declaration identifies the encoding of the document

§  UTF-8 is assumed if we don’t state otherwise.

·         The standalone declaration states whether the document stands by itself or references some external markup.

  Ø  Elements (Tags):

·         Elements must have a valid name

§  Begin with underscore _ or letter, then have zero or more letters, digits, periods, hyphens or underscore.

§  Can’t use string “xml” in any case combination.

·         Valid Names:

<_Element1>

<My.Element>

<My.Ele_ment>

·         Invalid Names:

<1Tag> (Wrong: can’t begin with numbers)

<#Element> (Wrong: invalid character in name)

<Element&Name> (Wrong: invalid character in name)

<XmL> (Wrong: xml is reserved)

  Ø  Attributes:

·         Attributes are specified on the start tag of an element.

·         Must start with underscore ( _ ) or letter, then have zero or more letters, digits, periods, hyphens or underscore.

·         Attributes that starts with “xml” are reserved.

·         Attributes can only appear one time on a given element. For example, <element attr1=”a” attr1=”b”> is wrong.

  Ø  Comments:

·         Comments begin with <!-- and end with -->.

·         They can contain any characters except a double hyphen (&, <, etc. are allowed)

·         Comments can appear pretty much anywhere in an XML file as long as they are not inside another markup element and are not before the xml declaration.

<element <!-- comment -->> is wrong.

<element>

            <!-- Comment -->

</element> is correct.

  Ø  Character Data Section:

·         Used to contain character data that we want to be part of the document content but don’t want the parser to try and parse.

·         Have the form <! [CDATA[Some text here]]>

·         Typically used when the character data contains a lot of characters (like & or <) that would otherwise be illegal in XML markup.

·         They are not nest like elements- we can’t have a CDATA section inside another CDATA section. 

  Ø  Processing Instructions:

·         Processing instructions are special instructions that are only of interest  to the application that is processing the XML.

·         Have the form <?target instruction ?>

§  The “xml” target name is reserved.

§  Target names can start with underscore ( _ ) or letter, then have zero or more letters, digits, periods, hyphens or underscore.

·         Example: Our application has different spell checking modes, and we want to be able to specify the mode in the document:

<?SpellCheckMode mode=”us-english”?>

  Ø  Entities:

·         Provides a way to help shorten and modularize our XML document.

·         Provide a way to include characters that would otherwise be illegal to type in markup.

·         General Entities:

We can define these to be whatever we want, and they will be replaced by the parser with a full string.

&copyright; or &author;

·         Character Entities:

&#060;

&amp; or &quot;

  Ø  Valid XML Documents:

·         A valid XML Document is one that has been tested against a set of rules.

·         These rules are specified as either DTD (Document Type Definition) or XML Schema files.

·         DTDs are simpler but less powerful and don’t use the same syntax as XML itself.

·         Schema ismore powerful but more complex and is written using XML syntax.