Java Enterprise in a Nutshell, Second Edition

Session Tracking

Very few web applications are confined to a single page, so having a mechanism for tracking users through a site can often simplify application development. The Web, however, is an inherently stateless environment. A client makes a request, the server fulfills it, and both promptly forget about each other. In the past, applications that needed to deal with a user through multiple pages (for instance, a shopping cart) had to resort to complicated dodges to hold onto state information, such as hidden fields in forms, setting and reading cookies, or rewriting URLs to contain state information.

The Servlet API provides classes and methods specifically designed to handle session tracking. A servlet can use the session-tracking API to delegate most of the user-tracking functions to the server. The first time a user connects to a session-enabled servlet, the servlet simply creates a javax.servlet.http.HttpSession object. The servlet can then bind data to this object, so subsequent requests can read the data. After a certain amount of inactive time, the session object is destroyed.

A servlet uses the getSession( ) method of HttpServletRequest to retrieve the current session object. This method takes a single boolean argument. If you pass true, and there is no current session object, the method creates and returns a new HttpSession object. If you pass false, the method returns null if there is no current session object. For example:

HttpSession thisUser = req.getSession(true); ...