Servlet Session Tracking API
Internal
Overview
The servlet API provides facilities that help the developer to track the servlet's session. Servlets have built-in session tracking. The level of support, however, depends on the web server. Most servers support session tracking through the use of persistent cookies, and when the client doesn't accept cookies, the servers could revert to URL rewriting.
JBossWeb and Tomcat manage HTTP sessions for web applications.
Example
Establishing a Session
- To ensure that a session is properly maintained, the getSession() method must be called at least once before committing the response. Sessions are not created automatically, unless we invoke HttpServletRequest.getSession(), which is equivalent with HttpServletRequest.getSession(true).
Authentication and Session
Both unauthenticated users and authenticated users can maintain sessions.
Session Lifecycle
When a user first accesses a web application, that user is assigned a new javax.servlet.http.HttpSession object and a unique session ID. The session ID identifies the user and it is used to match the user with HttpSession object in subsequent requests. HttpSession provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user. The servlet container uses this interface to create a session between an HTTP client and an HTTP server. The session persists for a specified time period, across more than one connection or page request from the user. A session usually corresponds to one user, who may visit a site many times.
Behind the scenes, the session ID is usually saved on the client in a cookie called JSESSIONID. For clients that don't support cookies, the session ID can be sent as part of a rewritten URL, encoded using a jsessionid parameter, e.g. http://server.com/servlet/MyServlet__;jsessionid=123__?item=1234. Other implementations using SSL are also possible.
The programmer can get the HttpSession associated with an user from inside the servlet using getSession() method.
The programmer then can save any set of arbitrary Java objects in a session object (the only limitation is that the saved objects must be Serializable). To add data to the session, use
public void HttpSession.setAttribute(String name, Object value)
To retrieve data from a session use
public Object HttpSession.getAttribute(String name)
ServletContext
The ServletContext defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.
There is one context per "web application" per Java Virtual Machine. In the case of a web application marked "distributed" in its deployment descriptor, there will be one context instance for each virtual machine. In this situation, the context cannot be used as a location to share global information (because the information won't be truly global). Use an external resource like a database instead.
The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized.
To allow web applications to operate independently, session are scoped at the web application level. This means each ServletContext maintains its own pool of HttpSession instances, and a servlet running inside one context cannot access session information saved by a servlet in another context.