Getting notification when bounded/unbounded to a HTTP session

notifications, servlets, session

Solution

Let the object's class implement `HttpSessionBindingListener`.

public class YourObject implements HttpSessionBindingListener {

    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        // The current instance has been bound to the HttpSession.
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        // The current instance has been unbound from the HttpSession.
    }

}

If you have no control over the object's class code and thus you can't change its code, then an alternative is to implement `HttpSessionAttributeListener`.

@WebListener
public class YourObjectSessionAttributeListener implements HttpSessionAttributeListener {

    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        if (event.getValue() instanceof YourObject) {
            // An instance of YourObject has been bound to the session.
        }
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent event) {
        if (event.getValue() instanceof YourObject) {
            // An instance of YourObject has been unbound from the session.
        }
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent event) {
        if (event.getValue() instanceof YourObject) {
            // An instance of YourObject has been replaced in the session.
        }
    }

}

Note: when you're still on Servlet 2.5 or older, replace `@WebListener` by a `<listener>` configuration entry in `web.xml`.

Problem

Original source