Practical Usage of HttpSessionBindingListener And HttpSessionAttributeListener

up vote5down votefavorite 4

I am reading through head first JSP and servlets. Going through different type of listeners, I came acrossHttpSessionBindingListener and HttpSessionAttributeListener. I was thinking about the difference between the two - I want to see the practical usages in real world examples of those two listeners. I tested HttpSessionBindingListener by implementingvalueBound() and valueUnBound() - why would an object need to know whether it has been added or not? I am pretty confused about the practical usages. Please help in clarifying this. session servlets listener share|improve this question edited Aug 23 '13 at 11:49 Beryllium 4,4956728 asked Jul 21 '13 at 20:42 benz 1,182318

add comment

1 Answer

activeoldestvotes

up vote6down voteaccepted

The HttpSessionBindingListener is to be implemented on the class whose instances may be stored in the session, such as the logged-in user. E.g. public class ActiveUser implements HttpSessionBindingListener { @Override public void valueBound(HttpSessionBindingEvent event) { logins.add(this); } @Override public void valueUnbound(HttpSessionBindingEvent event) { logins.remove(this); } } When an instance of this ActiveUser get set as a session attribute byHttpSession#setAttribute(), then the valueBound() will be invoked. When it get removed by either HttpSession#removeAttribute(), or an invalidate of the session, or get replaced by anotherHttpSession#setAttribute(), then the valueUnbound() will be invoked. Here are some real world use cases: Getting SessionScoped bean from HttpSessionListener? How to call a method before the session object is destroyed? Call an action when closing a JSP implementing HttpSessionListener How to access HTTP sessions in Java The HttpSessionAttributeListener is to be implemented as an application wide @WebListenerwhich get invoked when any attribute is added, removed or replaced in the HttpSession. Continuing with the above ActiveUser example, this is particularly useful if you can't modify the ActiveUserclass to implement HttpSessionBindingListener (because it's 3rd party or so), or when you want to make use of a "marker interface" on an arbitrary amount of classes so that you can do the listening job in a single central place. @WebListener public class ActiveUserListener implements HttpSessionAttributeListener { @Override public void attributeAdded(HttpSessionBindingEvent event) { if (event.getValue() instanceof ActiveUser) { logins.add(event.getValue()); } } @Override public void attributeRemoved(HttpSessionBindingEvent event) { if (event.getValue() instanceof ActiveUser) { logins.remove(event.getValue()); } } @Override public void attributeReplaced(HttpSessionBindingEvent event) { if (event.getValue() instanceof ActiveUser) { logins.add(event.getValue()); } } } Here's a real world use case: Getting notification when bounded/unbounded to a HTTP session share|improve this answer edited Jul 22 '13 at 11:52 answered Jul 21 '13 at 20:52 BalusC 450k8211361472

Thanks so much @BalusC. I understood the difference. Thanks so much for the detailed answer. –  benz Jul 22 '13 at 5:06 This is a great answer. @BalusC how do you manage to get this all information and present in such nice way.–  Makky Jul 22 '13 at 9:35 You're welcome. –  BalusC Jul 22 '13 at 11:52 add comment

Your Answer