Binding Notification with HttpSessionBindingListener

Some objects require you to know when they are placed in or removed from a session. To obtain this information, implement the HttpSessionBindingListener interface in those objects. When your application stores or removes data with the session, the servlet engine checks whether the object being bound or unbound implements HttpSessionBindingListener. If it does, the Web Server notifies the object under consideration, through the HttpSessionBindingListener interface, that it is being bound into or unbound from the session.