• AJAX is not a new programming language, but simply a new technique for creating better, faster, and more interactive web applications.
• AJAX uses JavaScript to send and receive data between a web browser and a web server.
• makes web pages more responsive by exchanging data with the web server behind the scenes, instead of reloading an entire web page each time a user makes a change.
• AJAX is a technology that runs in your browser as it uses JavaScript
• It uses asynchronous data transfer (HTTP requests) between the browser and the web server, allowing web pages to request small bits of information from the server instead of whole pages. The technology makes Internet applications smaller, faster and more user friendly. lamp AJAX is a web browser technology independent of web server software.
• Why AJAX? More stuff is done on the client side. In some instances an Ajax based site will load quicker than a comparable traditional Web site. Uses callback function that manipulates the page using DOM.
• Note: Asynchronous meansthe ability to handle processes independently from other processes
• Uses:
  • JavaScript
  • XML
  • HTML
  • CSS
    

Advantages and Disadvantages?

Whats really NEW HERE? These techniques have been available to developers targeting Internet Explorer on the Windows platform for many years. Until recently, the technology was known as web remoting or remote scripting. Web developers have also used a combination of plug-ins, Java applets, and hidden frames to emulate this interaction model for some time. What has changed is that the inclusion of support for the XMLHttpRequest object has became ubiquitous in the mainstream browsers across all platforms. The real magic is the result of the JavaScript technology's XMLHttpRequest object. Although this object is not specified in the formal JavaScript technology specification, all of today's mainstream browsers support it. The subtle differences with the JavaScript technology and CSS support among current generation browsers such as Firefox, Internet Explorer, and Safari are manageable. If you are required to support older browsers, AJAX may not be the answer for you.

Example: AJAX calling a Java Servelet as the server program.

• Applications Include:
  • Real-Time Form Data Validation: Form data such as user IDs, serial numbers, postal codes, or even special coupon codes that require server-side validation can be validated in a form before the user submits a form.
  • Autocompletion: A specific portion of form data such as an email address, name, or city name may be autocompleted as the user types.
  • Master Details Operations: Based on a client event, an HTML page can fetch more detailed information on data such as a product listing that enables the client to view the individual product information without refreshing the page.
  • Sophisticated User Interface Controls: Controls such as tree controls, menus, and progress bars may be provided that do not require page refreshes.
  • Refreshing Data on the Page: HTML pages may poll data from a server for up-to-date data such as scores, stock quotes, weather, or application-specific data.

  • Server-side Notifications: An HTML page may simulate a server-side push by polling the server for event notifications that may notify the client with a message, refresh page data, or redirect the client to another page.

    
    

• Possible Problems (exist for other technologies too!)
  • Complexity: Server-side developers will need to understand that presentation logic will be required in the HTML client pages as well as in the server-side logic to generate the XML content needed by the client HTML pages. HTML page developers must have JavaScript technology skills. Creating AJAX-enabled applications will become easier as new frameworks are created and existing frameworks evolve to support the interaction model.
  • Standardization of the XMLHttpRequest Object: The XMLHttpRequest object is a newer part of the JavaScript technology specification, which means that the behavior may vary depending on the client and its uspport of JavaScript.

  • JavaScript Technology Implementations: AJAX interactions depend heavily on JavaScript technology, which has subtle differences depending on the client. See QuirksMode.org for more details on browser-specific differences.

  • Debugging: AJAX applications are also difficult to debug because the processing logic is embedded both in the client and on the server.

  • Security & Viewable Source: The client-side JavaScript technology may be viewed simply by selecting View Source from an AJAX-enabled HTML page. A poorly designed AJAX-based application could open itself up to hackers or plagiarism.

Resources:

• can get JSP tag libraries that let you implement AJAX by writing JSP tags only. Example 1 or find the most current by searching for "JSP AJAX"