Chapter 12. Interceptors and events

It is often useful for the application to react to certain events that occur inside Hibernate. This allows implementation of certain kinds of generic functionality, and extension of Hibernate functionality.

12.1. Interceptors

The Interceptor interface provides callbacks from the session to the application allowing the application to inspect and/or manipulate properties of a persistent object before it is saved, updated, deleted or loaded. One possible use for this is to track auditing information. For example, the following Interceptor automatically sets the createTimestamp when an Auditable is created and updates the lastUpdateTimestamp property when an Auditable is updated.
You may either implement Interceptor directly or (better) extend EmptyInterceptor.
package org.hibernate.test;

import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;

import org.hibernate.EmptyInterceptor;
import org.hibernate.Transaction;
import org.hibernate.type.Type;

public class AuditInterceptor extends EmptyInterceptor {

    private int updates;
    private int creates;
    private int loads;

    public void onDelete(Object entity,
                         Serializable id,
                         Object[] state,
                         String[] propertyNames,
                         Type[] types) {
        // do nothing
    }

    public boolean onFlushDirty(Object entity,
                                Serializable id,
                                Object[] currentState,
                                Object[] previousState,
                                String[] propertyNames,
                                Type[] types) {

        if ( entity instanceof Auditable ) {
            updates++;
            for ( int i=0; i < propertyNames.length; i++ ) {
                if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) {
                    currentState[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public boolean onLoad(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {
        if ( entity instanceof Auditable ) {
            loads++;
        }
        return false;
    }

    public boolean onSave(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {

        if ( entity instanceof Auditable ) {
            creates++;
            for ( int i=0; i<propertyNames.length; i++ ) {
                if ( "createTimestamp".equals( propertyNames[i] ) ) {
                    state[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public void afterTransactionCompletion(Transaction tx) {
        if ( tx.wasCommitted() ) {
            System.out.println("Creations: " + creates + ", Updates: " + updates,
	        "Loads: " + loads);
        }
        updates=0;
        creates=0;
        loads=0;
    }

}
Interceptors come in two flavors: Session-scoped and SessionFactory-scoped.
A Session-scoped interceptor is specified when a session is opened using one of the overloaded SessionFactory.openSession() methods accepting an Interceptor.
Session session = sf.openSession( new AuditInterceptor() );
A SessionFactory-scoped interceptor is registered with the Configuration object prior to building the SessionFactory. In this case, the supplied interceptor will be applied to all sessions opened from that SessionFactory; this is true unless a session is opened explicitly specifying the interceptor to use. SessionFactory-scoped interceptors must be thread safe, taking care to not store session-specific state since multiple sessions will use this interceptor (potentially) concurrently.
new Configuration().setInterceptor( new AuditInterceptor() );