Extensible Markup Language (XML)

CSE 190 M (Web Programming), Spring 2007

University of Washington

Reading: Sebesta Ch. X sections ***

What is XML?

a specification for creating markup languages to store hierarchical data
used to facilitate sharing data between different systems
XHTML is a subset of XML
- a language created using XML specifications
- an adaptation of old HTML to fit XML's syntax requirements

Structure of an XML document

a header, then a single document tag that can contain other tags (tree)
```
<?xml version="1.0" encoding="UTF-8"?>
document tag
```
tag syntax:
```
<element attributes>
    text or tags
</element>
```
- or a tag with no inner tags/content can end with />
attribute syntax:

name="value"

comments:

example XML file

<?xml version="1.0" encoding="UTF-8"?>
<note>
    <to>Tove</to>
    <from>Jani</from>
    <subject>Reminder</subject>
    <body>
        Don't forget me this weekend!
    </body>
</note>

other examples: menu, CD catalog, plant catalog

What tags are legal in XML?

any tag you want; you can make up your own structure
schema: description of which tags are legal with your data
a schema can be formally defined in different languages:
- Document Type Definition (DTD)
- W3C XML Schema
if you define a formal schema for your XML files, you can use tools to validate XML files to make sure that they match your schema
- XHTML is an XML language that has a schema
- allows W3C validator to check HTML files to see if they follow it

Where does XML data come from?

many web servers store data as XML files
databases sometimes return query results as XML
web services: applications on servers that use XML to communicate
more examples

XML pros and cons

pro:
- human- and machine-readable format
- self-documenting
- can represent almost any general structure of data (record, list, tree)
- strict syntax allows standardized tools
- international, platform-independent
con:
- bulky syntax makes files large
- bulky structure causes possible performance issues when parsing
- doesn't natively support some features such as data types (string, integer)

Displaying XML data

XML is pure data; doesn't specify how it should be shown on a screen
we can transform XML data into HTML and display it on a web page
ways to specify how to display XML on a web page:
- CSS
- XSLT (Extensible Stylesheet Language Transformation)
- Javascript's XML DOM
  - uses same DOM properties we've already seen to process HTML tags
  - we'll use the JS XML DOM to process XML in this course

DOM node properties/methods

properties:
- nodeName, nodeType, nodeValue, attributes
- firstChild, lastChild, childNodes, nextSibling, previousSibling, parentNode
methods:
- getElementsByTagName, hasAttributes, hasChildNodes
- appendChild, insertBefore, removeChild, replaceChild

Details about XML node properties

nodeType : what kind of node it is

Kind of node nodeType value

element 1

attribute 2

text 3

comment 8

document 9
nodeName : uppercase version of tag such as "DIV" or "ARTICLE"
- an attribute node's name is the attribute's name
- all text nodes have name "#text"
- document node has name "#document"
nodeValue : text inside a text node, or value of an attribute node

Kind of node	`nodeType` value
element	1
attribute	2
text	3
comment	8
document	9

Reading an XML document using AJAX

var ajax = new XMLHttpRequest();
ajax.open("GET", url, false);
ajax.send(null);
var xmlDoc = ajax.responseXML;
do something with xmlDoc;

if the response received is in XML, responseXML will contain an XML DOM object
- responseText used for all other types of data
- frequent issue: MIME type received must be application/xml or won't be parsed as XML
can use all DOM properties and methods on the object returned
passing false makes this example synchronous — the script will pause until data is loaded
- infrequently used, but another option

document.implementation.createDocument accepts some parameters related to XML namespaces, but we don't need them so we just pass empty strings and null