The Session Object

As we all know there are three commonly used 'ways' of maintaining 'state' with the stateless HTTP protocol. Cookies, URL re-writing and Hidden form fields in the HTML form. Session object or the session tracking API isolates the programmer from these implementation details. Although the actual implementation may be server dependant theoretically:

1. The client request a particular page
   
2. The server 'creates a session' with this client (by either sending a cookie with the session id or URL re-writing etc.).
   
3. Every time the same client (identified by the cookie or URL) comes back to the server, the server uses this session id to provide a facility to 'put' and 'get' some session dependant information.
   
4. This information can be accessed till the session is not: invalidated or timed out.
   

Servlet API makes the session tracking very simple.

In your code you can 'get' a session object associated with every user by :

HttpSession currSession = request.getSession(flag);

Where request is the HttpServletRequest object and flag is a boolean variable. 

• When flag = true - A new session object is created for this user request.
• When flag = false - This method returns the already created session object.

To add any session variables we call:

	
currSession.putValue("userName",  Name);

This call will put the Name variable in the userName session value. (Note: it actually will put a reference and hence you can 'put' objects in the 'session').

To access a previously stored session variable:

	
String storedName =   currSession.getValue("userName");

We will write a simple web application which uses session values. But before that let's look at one more class in this package: RequestDispatcher.

Java Server Pages
Building Web Applications Using Servlets and JSP
Request Dispatcher