Xml

The eXtensible Markup Language
(XML)
An Applied Tutorial
Kevin Thomas

Presentation Outline
 Part 1: The basics of creating an XML
document
 Part 2: Developing constraints for a well
formed XML document
 Part 3: XML and supplementary technologies

Part 1: Background for XML
 An Extensible Markup Language (XML)
document describes the structure of data
 XML and HTML have a similar syntax …
both derived from SGML
 XML has no mechanism to specify the format
for presenting data to the user
 An XML document resides in its own file with
an ‘.xml’ extension

Main Components of an XML
Document
 Elements: <hello>
 Attributes: <item id=“33905”>
 Entities: < (<)
 Advanced Components
– CData Sections
– Processing Instructions

An Example XML Document
 An example of an well-commented XML document

The Basic Rules
 XML is case sensitive
 All start tags must have end tags
 Elements must be properly nested
 XML declaration is the first statement
 Every document must contain a root element
 Attribute values must have quotation marks
 Certain characters are reserved for parsing

Common Errors for Element Naming
 Do not use white space when creating
names for elements
 Element names cannot begin with a digit,
although names can contain digits
 Only certain punctuation allowed – periods,
colons, and hyphens

Walking through an Example
 Modify the computer.xml document
– Add a new element named “software” with an attribute
named “language”
– The attribute’s value should be the name of a programming
language
– Create another XML element called “IFStatment”
– Use the IFStatment element to tag the following data:
if (a = 0)
– Close the “software” tag
 After you have added these new items into the XML
document, parse it again to ensure that it is still well
formed. Use the feedback to correct any errors.

Part 2: Legal Building Blocks of XML
 A Document Type Definition (DTD) allows the
developer to create a set of rules to specify legal
content and place restrictions on an XML file
 If the XML document does not follow the rules
contained within the DTD, a parser generates an
error
 An XML document that conforms to the rules within a
DTD is said to be valid

Why Use a DTD?
– A single DTD ensures a common format for each
XML document that references it
– An application can use a standard DTD to verify
that data that it receives from the outside world is
valid
– A description of legal, valid data further
contributes to the interoperability and efficiency of
using XML

Some Example DTD Declarations
 Example 1: The Empty Element
<!ELEMENT Bool (EMPTY)> 
<Bool Value="True"></Bool> 
 Example 2: Elements with Data
<!ELEMENT Month (#PCDATA)> 
<Month>This is a month</Month> <!—Valid usage within XML file-->
<Month> <!—Invalid usage within XML file, can’t have children!-->
<January>Jan</January>
<March>March</March>
</Month>

Some Example DTD Declarations
 Example 3: Elements with Children
To specify that an element must have a single child element, include the element name within the
parenthesis.
<!ELEMENT House (Address)> <!—A house has a single address-->
<House> <!—Valid usage within XML file-->
<Address>1345 Preston Ave Charlottesville Va 22903</Address>
</House>
An element can have multiple children. A DTD describes multiple children using a sequence, or a
list of elements separated by commas. The XML file must contain one of each element in the
specified order.

<!ELEMENT address (person,street,city, zip)>
<!ELEMENT person (#PCDATA)>
<!ELEMENT street (#PCDATA)>
<!ELEMENT city (#PCDATA)>
<!ELEMENT zip (#PCDATA)>
<!—Valid usage within XML file-->
<address>
<person>John Doe</person>
<street>1234 Preston Ave.</street>
<city>Charlottesville, Va</city>
<zip>22903</zip>
</address>

Cautions concerning DTDs
 All element declarations begin with <!ELEMENT
and end with >
 The ELEMENT declaration is case sensitive
 The programmer must declare all elements within
an XML file
 Elements declared with the #PCDATA content
model can not have children
 When describing sequences, the XML document
must contain exactly those elements in exactly that
order.

Walking Through an Example
 Using the file “music.xml” contained in the extras
folder, create a Document Type Definition that
describes all of the elements. The goals for this
exercise are to:
– map out the elements
– define each element
– pick the best content model for each element
– correctly order the elements using sequences
 Internally embed the DTD within the XML document

Part 3: XML and Supplementary
Technologies
 The W3C Document Object Model (DOM)
– an API that allows developers to programmatically manage
and access XML nodes
– allows programmers to update and change XML documents
within an application
– reads the whole XML file and then stores a hierarchical tree
structure containing all elements within the document
– This tree has a single root node, which is the root element,
and may contain many children, each of which represents
an XML element

W3C DOM with JavaScript
 Example 1: Loading the XML document: DOMDocument
• The programmer can use a Microsoft Active X object to
parse an XML file
//Instantiate DOMDocument object
var XMLfile = new ActiveXObject("Msxml2.DOMDocument");
XMLfile.load("newspaper.xml");
var rootElement = XMLfile.documentElement;
document.write("The root node of the XML file is: ");
document.writeln("" + rootElement.nodeName
+"");

 Example 2: Accessing the Children Elements
• The childNodes member of any element node gives the
programmer access to all of the sibling nodes of that
element
//traverse through each child of the root element
//and print out its name
for (i=0; i<rootElement.childNodes.length; i++) {
var node = rootElement.childNodes.item(i);
document.write("The name of the node is ");
document.write("" + node.nodeName + "");
}

 Example 3: Getting Element Attributes
//traverse through each child of the root element
//and print out its name
for (i=0; i<rootElement.childNodes.length; i++) {
//get the current element
var elementNode = rootElement.childNodes.item(i);
document.writeln("Processing Node: " +
elementNode.nodeName + " ");
var attributeValue;
//get an attribute value by specific name
attributeValue = elementNode.getAttribute("articleID");
//print it out
document.writeln("Attribute value: " + attributeValue +
" ");
}

Cautions with DOM
 Make sure that the XML file resides in the same directory as
the html file with the JavaScript code
 The Attribute node does not appear as the child node of any
other node type; it is not considered a child node of an
element
 Use caution when outputting raw XML to Internet Explorer.If
the programmer uses the document.writeln method, IE
attempts to interpret the XML tags and jumbles the text.
Instead, use an alert box when debugging.

Walking through an Example
1. Create an HTML file with notepad. Insert
some JavaScript code that will parse
newspaper.xml into a DOM tree. Print out
the attribute values for each articleID.

Xml

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