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13.2. Event system

If you have to react to particular events in your persistence layer, you can also use the Hibernate3 event architecture. The event system can be used in addition, or as a replacement, for interceptors.
All the methods of the Session interface correlate to an event. You have a LoadEvent, a FlushEvent, etc. Consult the XML configuration-file DTD or the org.hibernate.event package for the full list of defined event types. When a request is made of one of these methods, the Hibernate Session generates an appropriate event and passes it to the configured event listeners for that type. Out-of-the-box, these listeners implement the same processing in which those methods always resulted. However, you are free to implement a customization of one of the listener interfaces (i.e., the LoadEvent is processed by the registered implementation of the LoadEventListener interface), in which case their implementation would be responsible for processing any load() requests made of the Session.
The listeners should be considered singletons. This means they are shared between requests, and should not save any state as instance variables.
A custom listener implements the appropriate interface for the event it wants to process and/or extend one of the convenience base classes (or even the default event listeners used by Hibernate out-of-the-box as these are declared non-final for this purpose). Custom listeners can either be registered programmatically through the Configuration object, or specified in the Hibernate configuration XML. Declarative configuration through the properties file is not supported. Here is an example of a custom load event listener: 
public class MyLoadListener implements LoadEventListener {
    // this is the single method defined by the LoadEventListener interface
    public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType)
            throws HibernateException {
        if ( !MySecurity.isAuthorized( event.getEntityClassName(), event.getEntityId() ) ) {
            throw new MySecurityException("Unauthorized access");
        }
    }
}
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You also need a configuration entry telling Hibernate to use the listener in addition to the default listener: 
<hibernate-configuration>
    <session-factory>
        ...
        <event type="load">
            <listener class="com.eg.MyLoadListener"/>
            <listener class="org.hibernate.event.def.DefaultLoadEventListener"/>
        </event>
    </session-factory>
</hibernate-configuration>
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Instead, you can register it programmatically: 
Configuration cfg = new Configuration();
LoadEventListener[] stack = { new MyLoadListener(), new DefaultLoadEventListener() };
cfg.getEventListeners().setLoadEventListeners(stack);
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Listeners registered declaratively cannot share instances. If the same class name is used in multiple <listener/> elements, each reference will result in a separate instance of that class. If you need to share listener instances between listener types you must use the programmatic registration approach.
Why implement an interface and define the specific type during configuration? A listener implementation could implement multiple event listener interfaces. Having the type additionally defined during registration makes it easier to turn custom listeners on or off during configuration.
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