Creating a Session | Java Servlets Tutorial pdf

Creating a Session

A session is considered “new” when it is only a prospective session and has not been established. Because HTTP is a request-response based protocol, an HTTP session is considered to be new until a client “joins” it. A client joins a session when session tracking information has been returned to the server indicating that a session has been established. Until the client joins a session, it cannot be assumed that the next request from the client will be recognized as part of a session.
The session is considered to be “new” if either of the following is true:
=> The client does not yet know about the session
 => The client chooses not to join a session.
These conditions define the situation where the servlet container has no mechanism by which to associate a request with a previous request.

A Servlet Developer must design his application to handle a situation where a client has not, can not, or will not join a session.