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Introduction to XML
Extensible Markup Language

What is XML
• XML stands for eXtensible Markup Language.
• A markup language is used to provide
information about a document.
• Tags are added to the document to provide the
extra information.
• HTML tags tell a browser how to display the
document.
• XML tags give a reader some idea what some of
the data means.

What is XML Used For?
• XML documents are used to transfer data from one place
to another often over the Internet.
• XML subsets are designed for particular applications.
• One is RSS (Rich Site Summary or Really Simple
Syndication ). It is used to send breaking news bulletins
from one web site to another.
• A number of fields have their own subsets. These
include chemistry, mathematics, and books publishing.
• Most of these subsets are registered with the
W3Consortium and are available for anyone’s use.

Advantages of XML
• XML is text (Unicode) based.
– Takes up less space.
– Can be transmitted efficiently.
• One XML document can be displayed differently
in different media.
– Html, video, CD, DVD,
– You only have to change the XML document in order
to change all the rest.
• XML documents can be modularized. Parts can
be reused.

Example of an HTML Document
<html>
<head><title>Example</title></head.
<body>
<h1>This is an example of a page.</h1>
<h2>Some information goes here.</h2>
</body>
</html>

Example of an XML Document
<?xml version=“1.0”/>
<address>
<name>Alice Lee</name>
<email>alee@aol.com</email>
<phone>212-346-1234</phone>
<birthday>1985-03-22</birthday>
</address>

Difference Between HTML and XML
• HTML tags have a fixed meaning and
browsers know what it is.
• XML tags are different for different
applications, and users know what they
mean.
• HTML tags are used for display.
• XML tags are used to describe documents
and data.

XML Rules
• Tags are enclosed in angle brackets.
• Tags come in pairs with start-tags and
end-tags.
• Tags must be properly nested.
– <name><email>…</name></email> is not allowed.
– <name><email>…</email><name> is.
• Tags that do not have end-tags must be
terminated by a ‘/’.
– <br /> is an html example.

More XML Rules
• Tags are case sensitive.
– <address> is not the same as <Address>
• XML in any combination of cases is not allowed
as part of a tag.
• Tags may not contain ‘<‘ or ‘&’.
• Tags follow Java naming conventions, except
that a single colon and other characters are
allowed. They must begin with a letter and may
not contain white space.
• Documents must have a single root tag that
begins the document.

Encoding
• XML (like Java) uses Unicode to encode characters.
• Unicode comes in many flavors. The most common one
used in the West is UTF-8.
• UTF-8 is a variable length code. Characters are
encoded in 1 byte, 2 bytes, or 4 bytes.
• The first 128 characters in Unicode are ASCII.
• In UTF-8, the numbers between 128 and 255 code for
some of the more common characters used in western
Europe, such as ã, á, å, or ç.
• Two byte codes are used for some characters not listed
in the first 256 and some Asian ideographs.
• Four byte codes can handle any ideographs that are left.
• Those using non-western languages should investigate
other versions of Unicode.

Well-Formed Documents
• An XML document is said to be well-formed if it
follows all the rules.
• An XML parser is used to check that all the rules
have been obeyed.
• Recent browsers such as Internet Explorer 5
and Netscape 7 come with XML parsers.
• Parsers are also available for free download
over the Internet. One is Xerces, from the
Apache open-source project.
• Java 1.4 also supports an open-source parser.

XML Example Revisited
<?xml version=“1.0”/>
<address>
<name>Alice Lee</name>
<phone>212-346-1234</phone>
<birthday>1985-03-22</birthday>
</address>
• Markup for the data aids understanding of its purpose.
• A flat text file is not nearly so clear.
Alice Lee
alee@aol.com
212-346-1234
1985-03-22
• The last line looks like a date, but what is it for?

Expanded Example
<?xml version = “1.0” ?>
<address>
<name>
<first>Alice</first>
<last>Lee</last>
</name>
<phone>123-45-6789</phone>
<birthday>
<year>1983</year>
<month>07</month>
<day>15</day>
</birthday>
</address>

XML Files are Trees
address
name email phone birthday
first last year month day

XML Trees
• An XML document has a single root node.
• The tree is a general ordered tree.
– A parent node may have any number of
children.
– Child nodes are ordered, and may have
siblings.
• Preorder traversals are usually used for
getting information out of the tree.

Validity
• A well-formed document has a tree structure and
obeys all the XML rules.
• A particular application may add more rules in
either a DTD (document type definition) or in a
schema.
• Many specialized DTDs and schemas have
been created to describe particular areas.
• These range from disseminating news bulletins
(RSS) to chemical formulas.
• DTDs were developed first, so they are not as
comprehensive as schema.

Document Type Definitions
• A DTD describes the tree structure of a
document and something about its data.
• There are two data types, PCDATA and
CDATA.
– PCDATA is parsed character data.
– CDATA is character data, not usually parsed.
• A DTD determines how many times a
node may appear, and how child nodes
are ordered.

DTD for address Example
<!ELEMENT address (name, email, phone, birthday)>
<!ELEMENT name (first, last)>
<!ELEMENT first (#PCDATA)>
<!ELEMENT last (#PCDATA)>
<!ELEMENT email (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
<!ELEMENT birthday (year, month, day)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT month (#PCDATA)>
<!ELEMENT day (#PCDATA)>

Schemas
• Schemas are themselves XML documents.
• They were standardized after DTDs and provide
more information about the document.
• They have a number of data types including
string, decimal, integer, boolean, date, and time.
• They divide elements into simple and complex
types.
• They also determine the tree structure and how
many children a node may have.

Schema for First address Example
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="address">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string"/>
<xs:element name="email" type="xs:string"/>
<xs:element name="phone" type="xs:string"/>
<xs:element name="birthday" type="xs:date"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

Explanation of Example Schema
<?xml version="1.0" encoding="ISO-8859-1" ?>
• ISO-8859-1, Latin-1, is the same as UTF-8 in the first 128 characters.
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
• www.w3.org/2001/XMLSchema contains the schema standards.
<xs:element name="address">
<xs:complexType>
• This states that address is a complex type element.
<xs:sequence>
• This states that the following elements form a sequence and must
come in the order shown.
<xs:element name="name" type="xs:string"/>
• This says that the element, name, must be a string.
<xs:element name="birthday" type="xs:date"/>
• This states that the element, birthday, is a date. Dates are always of
the form yyyy-mm-dd.

XSLT
Extensible Stylesheet Language Transformations
• XSLT is used to transform one xml document
into another, often an html document.
• The Transform classes are now part of Java 1.4.
• A program is used that takes as input one xml
document and produces as output another.
• If the resulting document is in html, it can be
viewed by a web browser.
• This is a good way to display xml data.

A Style Sheet to Transform address.xml
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="address">
<html><head><title>Address Book</title></head>
<body>
<xsl:value-of select="name"/>
<br/><xsl:value-of select="email"/>
<br/><xsl:value-of select="phone"/>
<br/><xsl:value-of select="birthday"/>
</body>
</html>
</xsl:template>
</xsl:stylesheet>

The Result of the Transformation
Alice Lee
alee@aol.com
123-45-6789
1983-7-15

Parsers
• There are two principal models for
parsers.
• SAX – Simple API for XML
– Uses a call-back method
– Similar to javax listeners
• DOM – Document Object Model
– Creates a parse tree
– Requires a tree traversal

References
• Elliotte Rusty Harold, Processing XML with
Java, Addison Wesley, 2002.
• Elliotte Rusty Harold and Scott Means,
XML Programming, O’Reilly & Associates,
Inc., 2002.
• W3Schools Online Web Tutorials,
http://www.w3schools.com.

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Xml unit1

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