Web Application Basic Scopes Application Session Request Page 1

Sessions … HttpSession session = request.getSession(); … session.getAttribute(String name) session.setAttribute(String name) session.removeAttribute(String name) Session.invalidate() Remember there are resources costs… 2

Controlling Session Timeouts Programmatically:  session.setMaxInactiveInterval(int seconds) Declaratively (web.xml) 180 Negative value means the session will never expire (watch out!). Same result if 0 is set declaratively. 3

Application-Wide Initialization Parameters web.xml element: context-param support- Read with the getInitParameter method of ServletContext (not ServletConfig which is used for servlet/jsp read of element) You may also use attributes for handling objects 4

5 Listeners A listener is an event handler that the server invokes when certain events occur (e.g. web application initialization/shutdown, session created/timed out…) In design pattern terms – observer pattern: An observer (in this case the listener) is notified when an event occurs in the subject(server). Typical uses:  Application-wide initialization routines  Managing dependencies between data stored in context or session attributes  Monitoring the running application (e.g. number of current sessions)

6 Listeners – Cont. There are different kinds of listener, each corresponding to an interface and a group of events. Some of them are: ServletContextListener  Web application initialized / shut down ServletRequestListener  request handler starting / finishing HttpSessionListener  session created / invalidated ServletContextAttributeListener  context attribute added / removed / replaced HttpSessionAttributeListener  session attribute added / removed / replaced

7 Listeners – Cont. To use a listener one simply implements the appropriate interface and registers the listener in the deployment descriptor. As an example, the following listener monitors the current and maximum number of active sessions.  This class implements both HttpSessionListener and ServletContextListener such that it is notified when the application starts, to initialize the context attributes, and when a session is created or invalidated.

8 Example: SessionMonitor (1/2) import javax.servlet.*; import javax.servlet.http.*; public class SessionMonitor implements HttpSessionListener, ServletContextListener { private int active = 0, max = 0; public void contextInitialized(ServletContextEvent sce) { store(sce.getServletContext()); } public void contextDestroyed(ServletContextEvent sce) {} public void sessionCreated(HttpSessionEvent se) { active++; if (active>max) max = active; store(se.getSession().getServletContext()); } Next Slide (Update Context Attributes)

9 Example: SessionMonitor (2/2) public void sessionDestroyed(HttpSessionEvent se) { active--; store(se.getSession().getServletContext()); } private void store(ServletContext c) { c.setAttribute("sessions_active", new Integer(active)); c.setAttribute("sessions_max", new Integer(max)); } Registration in web.xml : SessionMonitor Now that we have the class, what must we do? Context Attributes

Filters Code being executed before / after the servlet Can intercept and redirect processing  security  auditing Can modify requests and responses  data conversion (XSLT, gzip,...)  specialized caching – all without changing the existing servlet code! 10

11 Resources An Introduction to XML and Web Technologies / Anders Møller and Michael I. Schwartzbach – course literature Coreservlets.com