• 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 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.
What is an XML Element?
An XML element is everything from (including) the element's start tag to (including) the element's end tag.
An element can contain other elements, simple text or a mixture of both. Elements can also have attributes
Attributes provide additional information about elements.
XML Attributes Must be Quoted
Attribute values must always be enclosed in quotes, but either single or double quotes can be used. For a person's sex, the person tag can be written like this:
or like this:
XML Elements vs. Attributes
Take a look at these examples:
In the first example sex is an attribute. In the last, sex is an element. Both examples provide the same information.
There are no rules about when to use attributes and when to use elements. Attributes are handy in HTML. In XML my advice is to avoid them. Use elements instead
Avoid XML Attributes?
Some of the problems with using attributes are:
• attributes cannot contain multiple values (elements can)
• attributes cannot contain tree structures (elements can)
• attributes are not easily expandable (for future changes)
Attributes are difficult to read and maintain. Use elements for data. Use attributes for information that is not relevant to the data.
With XSLT you can transform an XML document into HTML.
The XMLHttpRequest Object
With an XMLHttpRequest you can communicate with your server from inside a web page
What is the XMLHttpRequest Object?
The XMLHttpRequest object is the developer’s dream, because you can:
• Update a web page with new data without reloading the page
• Request and receive new data from a server after the page has loaded
• Communicate with a server in the background
Creating an XMLHttpRequest Object
Creating an XMLHttpRequest object is done with one single line of JavaScript.
In all modern browsers:
var xmlhttp=new XMLHttpRequest()
In older Microsoft browsers (IE 5 and 6):
var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")
In the next chapter, we will use the XMLHttpRequest object to retrieve XML information from a server.
The XMLHttpRequest object is supported in all modern browsers
XML Parser
Most browsers have a built-in XML parser to read and manipulate XML.
The parser converts XML into a JavaScript accessible object (the XML DOM).
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XML Parser
The XML DOM contains methods (functions) to traverse XML trees, access, insert, and delete nodes.
However, before an XML document can be accessed and manipulated, it must be loaded into an XML DOM object.
An XML parser reads XML, and converts it into an XML DOM object that can be accessed with JavaScript.
Most browsers have a built-in XML parser.
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Load an XML Document
The following JavaScript fragment loads an XML document ("books.xml"):
Example
if (window.XMLHttpRequest)
{
xhttp=new XMLHttpRequest();
}
else // Internet Explorer 5/6
{
xhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xhttp.open("GET","books.xml",false);
xhttp.send("");
xmlDoc=xhttp.responseXML;
XML DOM
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The DOM (Document Object Model) defines a standard way for accessing and manipulating documents.
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The XML DOM
The XML DOM (XML Document Object Model) defines a standard way for accessing and manipulating XML documents.
The DOM views XML documents as a tree-structure. All elements can be accessed through the DOM tree. Their content (text and attributes) can be modified or deleted, and new elements can be created. The elements, their text, and their attributes are all known as nodes.
In the examples below we use the following DOM reference to get the text from the
xmlDoc.getElementsByTagName("to")[0].childNodes[0].nodeValue
• xmlDoc - the XML document created by the parser.
• getElementsByTagName("to")[0] - the first
• childNodes[0] - the first child of the
• nodeValue - the value of the node (the text itself)
You can learn more about the XML DOM in our XML DOM tutorial.
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The HTML DOM
The HTML DOM (HTML Document Object Model) defines a standard way for accessing and manipulating HTML documents.
All HTML elements can be accessed through the HTML DOM.
In the examples below we use the following DOM reference to change the text of the HTML element where id="to":
document.getElementById("to").innerHTML=
• document - the HTML document
• getElementById("to") - the HTML element where id="to"
• innerHTML - the inner text of the HTML element
You can learn more about the HTML DOM in our HTML DOM tutorial.
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Load an XML File - A Cross browser Example
The following code loads an XML document ("note.xml") into the XML parser:
Example
W3Schools Internal Note
To:
From:
Message:
XML Namespaces provide a method to avoid element name conflicts.
Solving the Name Conflict Using a Prefix
Uniform Resource Identifier (URI)
A Uniform Resource Identifier (URI) is a string of characters which identifies an Internet Resource.
The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address. Another, not so common type of URI is the Universal Resource Name (URN).
All text in an XML document will be parsed by the parser.
But text inside a CDATA section will be ignored by the parser
PCDATA - Parsed Character Data
XML parsers normally parse all the text in an XML document.
CDATA - (Unparsed) Character Data
The term CDATA is used about text data that should not be parsed by the XML parser.
Characters like "<" and "&" are illegal in XML elements.
"<" will generate an error because the parser interprets it as the start of a new element.
"&" will generate an error because the parser interprets it as the start of an character entity.
Some text, like JavaScript code, contains a lot of "<" or "&" characters. To avoid errors script code can be defined as CDATA.
Everything inside a CDATA section is ignored by the parser.
A CDATA section starts with "":
In the example above, everything inside the CDATA section is ignored by the parser.
Notes on CDATA sections:
A CDATA section cannot contain the string "]]>". Nested CDATA sections are not allowed.
The "]]>" that marks the end of the CDATA section cannot contain spaces or line breaks.
Introduction to DTD
Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.
A DTD can be declared inline inside an XML document, or as an external reference.
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Internal DTD Declaration
If the DTD is declared inside the XML file, it should be wrapped in a DOCTYPE definition with the following syntax:
External DTD Declaration
If the DTD is declared in an external file, it should be wrapped in a DOCTYPE definition with the following syntax:
Why Use a DTD?
With a DTD, each of your XML files can carry a description of its own format.
With a DTD, independent groups of people can agree to use a standard DTD for interchanging data.
Your application can use a standard DTD to verify that the data you receive from the outside world is valid.
You can also use a DTD to verify your own data.
What is an XML Schema?
The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD.
An XML Schema:
• defines elements that can appear in a document
• defines attributes that can appear in a document
• defines which elements are child elements
• defines the order of child elements
• defines the number of child elements
• defines whether an element is empty or can include text
• defines data types for elements and attributes
• defines default and fixed values for elements and attributes
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XML Schemas are the Successors of DTDs
We think that very soon XML Schemas will be used in most Web applications as a replacement for DTDs. Here are some reasons:
• XML Schemas are extensible to future additions
• XML Schemas are richer and more powerful than DTDs
• XML Schemas are written in XML
• XML Schemas support data types
• XML Schemas support namespaces
What are Web Services?
• Web services are application components
• Web services communicate using open protocols
• Web services are self-contained and self-describing
• Web services can be discovered using UDDI
• Web services can be used by other applications
• XML is the basis for Web services
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How Does it Work?
The basic Web services platform is XML + HTTP.
XML provides a language which can be used between different platforms and programming languages and still express complex messages and functions.
The HTTP protocol is the most used Internet protocol.
Web services platform elements:
• SOAP (Simple Object Access Protocol)
• UDDI (Universal Description, Discovery and Integration)
• WSDL (Web Services Description Language)
We will explain these topics later in the tutorial.
Web Services have three basic platform elements: SOAP, WSDL and UDDI.
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What is SOAP?
SOAP is an XML-based protocol to let applications exchange information over HTTP.
Or more simple: SOAP is a protocol for accessing a Web Service.
• SOAP stands for Simple Object Access Protocol
• SOAP is a communication protocol
• SOAP is a format for sending messages
• SOAP is designed to communicate via Internet
• SOAP is platform independent
• SOAP is language independent
• SOAP is based on XML
• SOAP is simple and extensible
• SOAP allows you to get around firewalls
• SOAP is a W3C standard
Read more about SOAP on our Home page.
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What is WSDL?
WSDL is an XML-based language for locating and describing Web services.
• WSDL stands for Web Services Description Language
• WSDL is based on XML
• WSDL is used to describe Web services
• WSDL is used to locate Web services
• WSDL is a W3C standard
Read more about WSDL on our Home page.
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What is UDDI?
UDDI is a directory service where companies can register and search for Web services.
• UDDI stands for Universal Description, Discovery and Integration
• UDDI is a directory for storing information about web services
• UDDI is a directory of web service interfaces described by WSDL
• UDDI communicates via SOAP
What is WSDL?
• WSDL stands for Web Services Description Language
• WSDL is written in XML
• WSDL is an XML document
• WSDL is used to describe Web services
• WSDL is also used to locate Web services
• WSDL is a W3C recommendation
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WSDL Describes Web Services
WSDL stands for Web Services Description Language.
WSDL is a document written in XML. The document describes a Web service. It specifies the location of the service and the operations (or methods) the service exposes.
A WSDL document is just a simple XML document.
It contains set of definitions to describe a web service.
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The WSDL Document Structure
A WSDL document describes a web service using these major elements:
Element Defines
A WSDL document can also contain other elements, like extension elements, and a service element that makes it possible to group together the definitions of several web services in one single WSDL document.
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WSDL Ports
The
It describes a web service, the operations that can be performed, and the messages that are involved.
The
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WSDL Messages
The
Each message can consist of one or more parts. The parts can be compared to the parameters of a function call in a traditional programming language.
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WSDL Types
The
For maximum platform neutrality, WSDL uses XML Schema syntax to define data types.
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WSDL Bindings
The
A WSDL port describes the interfaces (legal operations) exposed by a web service.
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WSDL Ports
The
It defines a web service, the operations that can be performed, and the messages that are involved.
The binding element has two attributes - name and type.
The name attribute (you can use any name you want) defines the name of the binding, and the type attribute points to the port for the binding, in this case the "glossaryTerms" port.
The soap:binding element has two attributes - style and transport.
The style attribute can be "rpc" or "document". In this case we use document. The transport attribute defines the SOAP protocol to use. In this case we use HTTP.
The operation element defines each operation that the port exposes.
For each operation the corresponding SOAP action has to be defined. You must also specify how the input and output are encoded. In this case we use "literal".
What is SOAP?
• SOAP stands for Simple Object Access Protocol
• SOAP is a communication protocol
• SOAP is for communication between applications
• SOAP is a format for sending messages
• SOAP communicates via Internet
• SOAP is platform independent
• SOAP is language independent
• SOAP is based on XML
• SOAP is simple and extensible
• SOAP allows you to get around firewalls
• SOAP is a W3C recommendation
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Why SOAP?
It is important for application development to allow Internet communication between programs.
Today's applications communicate using Remote Procedure Calls (RPC) between objects like DCOM and CORBA, but HTTP was not designed for this. RPC represents a compatibility and security problem; firewalls and proxy servers will normally block this kind of traffic.
A better way to communicate between applications is over HTTP, because HTTP is supported by all Internet browsers and servers. SOAP was created to accomplish this.
SOAP provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.
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XSL Languages
It Started with XSL
XSL stands for EXtensible Stylesheet Language.
The World Wide Web Consortium (W3C) started to develop XSL because there was a need for an XML-based Stylesheet Language.
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CSS = Style Sheets for HTML
HTML uses predefined tags, and the meaning of each tag is well understood.
The tag in HTML defines a table - and a browser knows how to display it.
Adding styles to HTML elements are simple. Telling a browser to display an element in a special font or color, is easy with CSS.
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XSL = Style Sheets for XML
XML does not use predefined tags (we can use any tag-names we like), and therefore the meaning of each tag is not well understood.
A tag could mean an HTML table, a piece of furniture, or something else - and a browser does not know how to display it.
XSL describes how the XML document should be displayed!
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XSL - More Than a Style Sheet Language
XSL consists of three parts:
• XSLT - a language for transforming XML documents
• XPath - a language for navigating in XML documents
• XSL-FO - a language for formatting XML documents
What is XSLT?
• XSLT stands for XSL Transformations
• XSLT is the most important part of XSL
• XSLT transforms an XML document into another XML document
• XSLT uses XPath to navigate in XML documents
• XHTML elements must be properly nested
• XHTML elements must always be closed
• XHTML elements must be in lowercase
• XHTML documents must have one root element
Some More XHTML Syntax Rules
• Attribute names must be in lower case
• Attribute values must be quoted
• Attribute minimization is forbidden
• The id attribute replaces the name attribute
• The XHTML DTD defines mandatory elements
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