Web Application Basic Scopes

1. Web Application Basic Scopes • Application • Session • Request • Page

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

3. Controlling Session Timeouts • Programmatically: • session.setMaxInactiveInterval(int seconds) • Declaratively (web.xml) <session-config> <session-timeout>180</session-timeout><!--minutes--> </session-config> • Negative value means the session will never expire (watch out!). Same result if 0 is set declaratively.

4. Application-Wide Initialization Parameters • web.xml element: context-param <context-param> <param-name>support-email</param-name> <param-value>blackhole@mycompany.com</param-value> </context-param> • Read with the getInitParameter method of ServletContext (not ServletConfig which is used for servlet/jsp read of <init-param> element) • You may also use attributes for handling objects

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: <listener> <listener-class>SessionMonitor</listener-class> <listener> Context Attributes Now that we have the class, what must we do?

10. 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!

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