1. 1 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)

2. 2 Listeners – Cont. There are different kinds of listener, each corresponding to an interface and a group pf 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

3. 3 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.

4. 4 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)

5. 5 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