| Scenario | Listener interface | Event type |
| You want to know if an attribute in a web app context has been added, removed, or replaced. | javax.servlet.ServletContextAttributeListener attributeAdded attributeRemoved attributeReplaced |
ServletContextAttributeEvent |
| You want to know how many concurrent users there are. In other words, you want to track the active sessions. (We cover sessions in detail in the next chapter). | javax.servlet.http.HttpSessionListener sessionCreated sessionDestroyed |
HttpSessionEvent |
| You want to know each time a request comes in, so that you can log it. | javax.servlet.ServletRequestListener requestInitialized requestDestroyed |
ServletRequestEvent |
| You want to know when a request attribute has been added, removed, or replaced. | javax.servlet.ServletRequestAttributeListener attributeAdded attributeRemoved attributeReplaced |
ServletRequestAttributeEvent |
| You have an attribute class (a class for an object that will be stored as an attribute) and you want objects of this type to be notified when they are bound to or removed from a session. | javax.servlet.http.HttpSessionBindingListener valueBound valueUnbound |
HttpSessionBindingEvent |
| You want to know when a session attribute has been added, removed, or replaced. | javax.servlet.http.HttpSessionAttributeListener attributeAdded attributeRemoved attributeReplaced |
HttpSessionBindingEvent |
| You want to know if a context has been created or destroyed. | javax.servlet.ServletContextListener contextInitialized contextDestroyed |
ServletContextEvent |
| You have an attribute class, and you want objects of this type to be notified when the session to which they’re bound is migrating to and from another JVM. | javax.servlet.http.HttpSessionActivationListener sessionDidActivate sessionWillPassivate |
HttpSessionEvent |