Presents your 
JAVA E-NEWSLETTER for September 23, 2002

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BIND AN OBJECT TO THE HTTPSESSION

Sometimes we want to create an object, maybe as a resource or cache,
that will last for the lifetime of a user's servlet or jsp session. 
This can
be a problem if we also want that object to be available from another
location. We don't want the object to get garbage collected when and if 
the
session is terminated. We can prevent this problem by implementing the
HttpSessionBindingListener.

Here's a simple example (try/catches are ignored):

public class SomeObject implements HttpSessionBindingListener {

    private Connection someDbConnection = null;

    .... various bits of code ....

    public void valueBound(HttpSessionBindingEvent event) {
        // open the Connection
        this.someDbConnection = ....
    }

    public void valueUnbound(HttpSessionBindingEvent event) {
        // close the Connection
        this.someDbConnection.close();
    }

}

Binding this object to the session is so easy it requires no work. All
we have to do is place the object in the session. If an object placed 
in
the session implements HttpSessionBindingListener, then the session
automatically makes it a listener and will notify it when bound and 
unbound.

By implementing HttpSessionBindingListener, we can fully place our
objects into the session's scope, knowing that even their creation and
destruction in that scope can be monitored.

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