The Java EE 7 Tutorial
Ajax enables asynchronous and partial updating of web applications. Such functionality allows for highly responsive web pages that are rendered in near real time. Ajax-based web applications can access server and process information and can also retrieve data without interfering with the display and rendering of the current web page on a client (such as a browser).
Some of the advantages of using Ajax are as follows:
Form data validation in real time, eliminating the need to submit the form for verification
Enhanced functionality for web pages, such as user name and password prompts
Partial update of the web content, avoiding complete page reloads