AJAX - Quick Guide

What is AJAX?

• AJAX stands for Asynchronous JavaScript and XML. AJAX is a new technique for creating better, faster, and more interactive web applications with the help of XML, HTML, CSS, and Java Script.
  
• Ajax uses XHTML for content, CSS for presentation, along with Document Object Model and JavaScript for dynamic content display.
• Conventional web applications transmit information to and from the sever using synchronous requests. It means you fill out a form, hit submit, and get directed to a new page with new information from the server.
• With AJAX, when you hit submit, JavaScript will make a request to the server, interpret the results, and update the current screen. In the purest sense, the user would never know that anything was even transmitted to the server.
• XML is commonly used as the format for receiving server data, although any format, including plain text, can be used.
• AJAX is a web browser technology independent of web server software.
• A user can continue to use the application while the client program requests information from the server in the background.
• Intuitive and natural user interaction. Clicking is not required, mouse movement is a sufficient event trigger.
• Data-driven as opposed to page-driven.

Rich Internet Application Technology

AJAX is the most viable Rich Internet Application (RIA) technology so far. It is getting tremendous industry momentum and several tool kit and frameworks are emerging. But at the same time, AJAX has browser incompatibility and it is supported by JavaScript, which is hard to maintain and debug.

AJAX is Based on Open Standards

AJAX is based on the following open standards:

• Browser-based presentation using HTML and Cascading Style Sheets (CSS).
• Data is stored in XML format and fetched from the server.
• Behind-the-scenes data fetches using XMLHttpRequest objects in the browser.
• JavaScript to make everything happen.

AJAX - Technologies

AJAX cannot work independently. It is used in combination with other technologies to create interactive webpages.

JavaScript

• Loosely typed scripting language.
• JavaScript function is called when an event occurs in a page.
• Glue for the whole AJAX operation.

DOM

• API for accessing and manipulating structured documents.
• Represents the structure of XML and HTML documents.

CSS

• Allows for a clear separation of the presentation style from the content and may be changed programmatically by JavaScript.

XMLHttpRequest

• JavaScript object that performs asynchronous interaction with the server.

AJAX - Examples

Here is a list of some famous web applications that make use of AJAX.

Google Maps

A user can drag an entire map by using the mouse, rather than clicking on a button.

• http://maps.google.com/

Google Suggest

As you type, Google will offer suggestions. Use the arrow keys to navigate the results.

• http://www.google.com/webhp?complete=1&hl=en

Gmail

Gmail is a webmail, built on the idea that email can be more intuitive, efficient and useful.

• http://gmail.com/

Yahoo Maps (new)

Now it's even easier and more fun to get where you're going!

• http://maps.yahoo.com/

Difference in AJAX and Conventional CGI Program

Try these two examples one by one and you will feel the difference. While trying AJAX example, there is not discontinuity and you get the response very quickly, but when you try the standard GCI example, you would have to wait for the response and your page also gets refreshed.
AJAX Example:

* =

Standard Example:

* =

NOTE: We have given a more complex example in AJAX Database.

AJAX - Browser Support

All the available browsers cannot support AJAX. Here is a list of major browsers, that support AJAX.

• Mozilla Firefox 1.0 and above.
• Netscape version 7.1 and above.
• Apple Safari 1.2 and above.
• Microsoft Internet Explorer 5 and above.
• Konqueror.
• Opera 7.6 and above.

When you write your next application, do consider the browsers that do not support AJAX.
NOTE: When we say that a browser does not support AJAX, it simply means that the browser does not support creation of Javascript object XMLHttpRequest object.

Writing Browser Specific Code

The Simplest way to make your source code compatible with a browser is to use try...catch blocks in your JavaScript.

<html>
<body>
<script language="javascript" type="text/javascript">
<!-- 
//Browser Support Code
function ajaxFunction(){
   var ajaxRequest;  // The variable that makes Ajax possible!
   
   try{
      // Opera 8.0+, Firefox, Safari 
      ajaxRequest = new XMLHttpRequest();
   }catch (e){
   
      // Internet Explorer Browsers
      try{
         ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
         try{
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
      
            // Something went wrong
            alert("Your browser broke!");
            return false;
         }
      }
   }
}
//-->
</script>
<form name='myForm'>
   Name: <input type='text' name='username' /> <br />
   Time: <input type='text' name='time' />
</form>
</body>
</html>

In the above JavaScript code, we try three times to make our XMLHttpRequest object. Our first attempt:

• ajaxRequest = new XMLHttpRequest();

It is for Opera 8.0+, Firefox, and Safari browsers. If it fails, we try two more times to make the correct object for an Internet Explorer browser with:

• ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
• ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");

If it doesn't work, then we can use a very outdated browser that doesn't support XMLHttpRequest, which also means it doesn't support Ajax.
Most likely though, our variable ajaxRequest will now be set to whatever XMLHttpRequest standard the browser uses and we can start sending data to the server. The step-wise AJAX workflow is explained in the next chapter.

AJAX - Action

This chapter gives you a clear picture of the exact steps of AJAX operation.

Steps of AJAX Operation

• A client event occurs.
• An XMLHttpRequest object is created.
• The XMLHttpRequest object is configured.
• The XMLHttpRequest object makes an asynchronous request to the Webserver.
• The Webserver returns the result containing XML document.
• The XMLHttpRequest object calls the callback() function and processes the result.
• The HTML DOM is updated.

Let us take these steps one by one.

A Client Event Occurs

• A JavaScript function is called as the result of an event.
• Example: validateUserId() JavaScript function is mapped as an event handler to an onkeyup event on input form field whose id is set to "userid"
• <input type="text" size="20" id="userid" name="id" onkeyup="validateUserId();">.

The XMLHttpRequest Object is Created

var ajaxRequest;  // The variable that makes Ajax possible!
function ajaxFunction(){
   try{
      
      // Opera 8.0+, Firefox, Safari
      ajaxRequest = new XMLHttpRequest();
   }catch (e){
   
      // Internet Explorer Browsers
      try{
         ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
      
         try{
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
      
            // Something went wrong
            alert("Your browser broke!");
            return false;
         }
      }
   }
}

The XMLHttpRequest Object is Configured

In this step, we will write a function that will be triggered by the client event and a callback function processRequest() will be registered.

function validateUserId() {
   ajaxFunction();
   
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;
   
   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);
}

Making Asynchronous Request to the Webserver

Source code is available in the above piece of code. Code written in bold typeface is responsible to make a request to the webserver. This is all being done using the XMLHttpRequest object ajaxRequest.

function validateUserId() {
   ajaxFunction();
   
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;
   
   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);
}

Assume you enter Zara in the userid box, then in the above request, the URL is set to "validate?id=Zara".

Webserver Returns the Result Containing XML Document

You can implement your server-side script in any language, however its logic should be as follows.

• Get a request from the client.
• Parse the input from the client.
• Do required processing.
• Send the output to the client.

If we assume that you are going to write a servlet, then here is the piece of code.

public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException 
{
   String targetId = request.getParameter("id");
   
   if ((targetId != null) && !accounts.containsKey(targetId.trim()))
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("true");
   }
   else
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("false");
   }
}

Callback Function processRequest() is Called

The XMLHttpRequest object was configured to call the processRequest() function when there is a state change to the readyState of the XMLHttpRequest object. Now this function will receive the result from the server and will do the required processing. As in the following example, it sets a variable message on true or false based on the returned value from the Webserver.

 
function processRequest() {
   if (req.readyState == 4) {
      if (req.status == 200) {
         var message = ...;
...
}

The HTML DOM is Updated

This is the final step and in this step, your HTML page will be updated. It happens in the following way:

• JavaScript gets a reference to any element in a page using DOM API.
• The recommended way to gain a reference to an element is to call.

document.getElementById("userIdMessage"), 
// where "userIdMessage" is the ID attribute 
// of an element appearing in the HTML document

• JavaScript may now be used to modify the element's attributes; modify the element's style properties; or add, remove, or modify the child elements. Here is an example:

<script type="text/javascript">
<!--
function setMessageUsingDOM(message) {
   var userMessageElement = document.getElementById("userIdMessage");
   var messageText;
   
   if (message == "false") {
      userMessageElement.style.color = "red";
      messageText = "Invalid User Id";
   }
   else 
   {
      userMessageElement.style.color = "green";
      messageText = "Valid User Id";
   }
   var messageBody = document.createTextNode(messageText);
   
   // if the messageBody element has been created simple 
   // replace it otherwise append the new element
   if (userMessageElement.childNodes[0]) {
      userMessageElement.replaceChild(messageBody, userMessageElement.childNodes[0]);
   } 
   else
   {
      userMessageElement.appendChild(messageBody);
   }
}
-->
</script>
<body>
<div id="userIdMessage"><div>
</body>

If you have understood the above-mentioned seven steps, then you are almost done with AJAX. In the next chapter, we will see XMLHttpRequest object in more detail.