Like DHTML and LAMP, Ajax is not a technology in itself, but a group of technologies. Ajax uses a combination of HTML and CSS to mark up and style information. The DOM is accessed with JavaScript to dynamically display, and to allow the user to interact with, the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.
Examples of applications using AJAX:
Google Maps, Gmail, Youtube, and Facebook tabs.
How AJAX Works
AJAX is Based on Internet Standards
AJAX is based on internet standards, and uses a combination of:- XMLHttpRequest object (to exchange data asynchronously with a server)
- JavaScript/DOM (to display/interact with the information)
- CSS (to style the data)
- XML (often used as the format for transferring data)
1. XMLHttpRequest
AJAX (Asynchronous Javascript and XML) is like DHTML in that it is a combination of several existing technologies rather than being a single technology. In this case the technologies involved are Javascript, a server side language to handle the request, and a markup language to return the requested data with. The only common part of this technology is Javascript since different server side languages and markups can be used. Even with Javascript though we don't have a single standard way to use this technology because Internet Explorer (which introduced this concept before the standards were developed) has its own way of handling things.The very first thing that we need to be able to do in order to retrieve something from the server to update the current web page (without reloading the whole page) is to create the object that will perform the processing for us. Modern browsers have a predefined class called XMLHttpRequest that we can use to define our object directly. Internet Explorer has provided five different interfaces to provide us with the functionality that we need each of which is accessed via AxtiveX. As each new version has improved the functioning of the object we want to grab the most recent version that is installed in the browser.
There are only a few different ways that we can code the creation of our new AJAX object. The following version is slightly more efficient than the alternatives that I have seen.
function createXMLHttp() {
if (typeof XMLHttpRequest != 'undefined')
return new XMLHttpRequest();
else if (window.ActiveXObject) {
var avers = ["Microsoft.XmlHttp", "MSXML2.XmlHttp",
"MSXML2.XmlHttp.3.0", "MSXML2.XmlHttp.4.0",
"MSXML2.XmlHttp.5.0"];
for (var i = avers.length -1; i >= 0; i--) {
try {
httpObj = new ActiveXObject(avers[i]);
return httpObj;
} catch(e) {}
}
}
throw new Error('XMLHttp (AJAX) not supported');
}
var ajaxObj = createXMLHttp();
This code starts by checking if the XMLHttpRequest class exists. If it does then we use that to create our object. If it doesn't then we check if ActiveX is supported (which means that the browser is running Internet Explorer on Windows with ActiveX enabled). If it is then we try to create our object using the latest XMLHttp version (version 5.0). If that fails then we work our way back through the array trying each earlier version in turn until we find one that is installed in the browser and which therefore allows us to create our object. If all of those attempts fail or if neither XMLHttpRequest or ActiveX are supported then we throw an error to indicate that AJAX is not supported in this browser. if (typeof XMLHttpRequest != 'undefined')
return new XMLHttpRequest();
else if (window.ActiveXObject) {
var avers = ["Microsoft.XmlHttp", "MSXML2.XmlHttp",
"MSXML2.XmlHttp.3.0", "MSXML2.XmlHttp.4.0",
"MSXML2.XmlHttp.5.0"];
for (var i = avers.length -1; i >= 0; i--) {
try {
httpObj = new ActiveXObject(avers[i]);
return httpObj;
} catch(e) {}
}
}
throw new Error('XMLHttp (AJAX) not supported');
}
var ajaxObj = createXMLHttp();
Technologies
The term Ajax has come to represent a broad group of web technologies that can be used to implement a web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:
- HTML or XHTML and CSS for presentation
- the Document Object Model (DOM) for dynamic display of and interaction with data
- XML for the interchange of data, and XSLT for its manipulation
- the XMLHttpRequest object for asynchronous communication
- JavaScript to bring these technologies together
- JavaScript is not the only client-side scripting language that can be used for implementing an Ajax application. Other languages such as VBScript are also capable of the required functionality.However JavaScript is the most popular language for Ajax programming due to its inclusion in and compatibility with the majority of modern web browsers.
- XML is not required for data interchange and therefore XSLT is not required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange,[ although other formats such as preformatted HTML or plain text can also be used.
Introducing XMLHttpRequest and pseudomultithreading, it is possible to swap the role of client and server (web browser may start behaving as a server and web server may start behaving as a client) in Client-Server models.
Drawbacks
- Pages dynamically created using successive Ajax requests do not automatically register themselves with the browser's history engine, so clicking the browser's "back" button may not return the user to an earlier state of the Ajax-enabled page, but may instead return them to the last full page visited before it. Workarounds include the use of invisible IFrames to trigger changes in the browser's history and changing the URL fragment identifier (the portion of a URL after the '#') when Ajax is run and monitoring it for changes.
- Dynamic web page updates also make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the '#') to keep track of, and allow users to return to, the application in a given state.
- Depending on the nature of the Ajax application, dynamic page updates may interfere disruptively with user interactions, especially if working on an unstable internet connection. For instance, editing a search field may trigger a query to the server for search completions, but the user may not know that a search completion popup is forthcoming, and if the internet connection is slow, the popup list may show up at an inconvenient time, when the user has already proceeded to do something else.
- Because most web crawlers do not execute JavaScript code,[ publicly indexable web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, thereby allowing search engines to index it.
- Any user whose browser does not support JavaScript or XMLHttpRequest, or simply has this functionality disabled, will not be able to properly use pages which depend on Ajax. Similarly, devices such as mobile phones, PDAs, and screen readers may not have support for the required technologies. Screen readers that are able to use Ajax may still not be able to properly read the dynamically generated content. The only way to let the user carry out functionality is to fall back to non-JavaScript methods. This can be achieved by making sure links and forms can be resolved properly and do not rely solely on Ajax.
- The same origin policy prevents some Ajax techniques from being used across domains, although the W3C has a draft of the XMLHttpRequest object that would enable this functionality. Techniques exist to sidestep this limitation by using a special Cross Domain Communications channel embedded as an iframe within a page.
- Like other web technologies, Ajax has its own set of vulnerabilities[which?] that developers must address. Developers familiar with other web technologies may have to learn new testing and coding methods to write secure Ajax applications.[
- Ajax-powered interfaces may dramatically increase the number of user-generated requests to web servers and their back-ends (databases, or other). This can lead to longer response times and/or additional hardware needs.
- The asynchronous, callback-style of programming required can lead to complex code that is hard to maintain or debug. [
- Ajax cannot easily be read by screen-reading technologies, such as JAWS, without hints built into the corresponding HTML based on WAI-ARIA standards. Screen-reading technologies are used by individuals with an impairment that hinders or prevents the visually reading of content on a screen
http://www.ibm.com/developerworks/web/library/wa-ajaxintro1.html
http://www.javapassion.com/ajax/AJAXBasics.pdf
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