Practical Usage of HttpSessionBindingListener And HttpSessionAttributeListener

listener, servlets, session

Solution

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 by `HttpSession#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 another `HttpSession#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 `@WebListener` which 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 `ActiveUser` class 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

Problem

Original source

Related problems

• How to call a method before the session object is destroyed?
• Getting notification when bounded/unbounded to a HTTP session
• How to access HTTP sessions in Java
• Call an action when closing a JSP
• implementing HttpSessionListener