You can easily add **Callback classes** in order to complete <wrap adicode>persist</wrap>, <wrap adicode>merge</wrap> or <wrap adicode>remove</wrap> processes in the EJB.
For example, how to automatically set value for <wrap adicode>lastUpdate</wrap> property in <wrap adicode>Film</wrap> class when a record is created or updated.
\\ \\
A **callback class** must extends class <wrap adicode>org.adichatz.common.ejb.AEntityCallback</wrap>. You can override <wrap adicode> preMerge</wrap>, <wrap adicode> prePersist</wrap>, <wrap adicode> preRemove</wrap>,
 <wrap adicode> postMerge</wrap>, <wrap adicode> postPersist</wrap> or <wrap adicode> postRemove</wrap> methods to finalize processing. Zero, one or more callback classes can be specified to be executed during saving data changes.
 If you declare several callback classes for an entity,  

=== Step 1: Create the Callback class ===
Create class <wrap adicode>org.mycompany.myproject.model.callback.FilmCallback</wrap> as below:

<sxh java; first-line: 1; highlight: [15,16,17,19,20,21]; title: excerpt from 'FilmCallback.java' file (new version).>
package org.mycompany.myproject.model.callback;

import java.util.Date;

import javax.persistence.EntityManager;

import org.adichatz.common.ejb.AEntityCallback;
import org.mycompany.myproject.model.Film;

public class FilmCallback extends AEntityCallback<Film> {
	public FilmCallback(EntityManager entityManager) {
		super(entityManager);
	}
	@Override
	public void preMerge(Film bean) {
		bean.setLastUpdate(new Date());
	}
	@Override
	public void prePersist(Film bean) {
		bean.setLastUpdate(new Date());
	}
}
</sxh>
<WRAP indic><wrap adititle>Remarks</wrap>:\\
  * Line 15-17: Sets value for <wrap adicode>lastUpdate</wrap> property when bean is updated.
  * Line 19-21: Sets value for <wrap adicode>lastUpdate</wrap> property when bean is inserted.
</WRAP>
\\ \\
=== Step 2: Change 'FilmMM.axml' file ===
Open file <wrap adicode>$projectDirectory/resources/xml/model/film/FilmDIMM.xml</wrap>.\\ \\
**Following XML elements:**
<sxh xml; first-line: 1; title: excerpt from 'FilmMMGENERATED.axml' file.>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<entityTree xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            entityURI="adi://myproject/model.film/FilmMM" idFieldName="filmId"
            xsi:noNamespaceSchemaLocation="http://www.adichatz.org/xsd/v0.8.7/generator/entityTree.xsd">
    <propertyField mandatory="true" id="filmId">
    ...
    ...
</sxh>\\
\\
**Change XML elements**
  * Copy <wrap adicode>FilmMMGENERATED.axml</wrap> file to <wrap adicode>FilmMM.axml</wrap> which will become the reference for generated code.
  * Replace above XML lines with the following ones:
<sxh xml; first-line: 1; highlight: [4]; title: excerpt from 'FilmMM.axml' file (new version).>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<entityTree xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            entityURI="adi://myproject/model.film/FilmMM" idFieldName="filmId"
            callbackClassNames="org.mycompany.myproject.model.callback.FilmCallback"
            xsi:noNamespaceSchemaLocation="http://www.adichatz.org/xsd/v0.8.7/generator/entityTree.xsd">
    <propertyField mandatory="true" id="filmId">
    ...
</sxh>
<WRAP indic><wrap adititle>Remark</wrap>:\\
Line 4: a Callback class is defined.\\ \\
//if you want to set muliple **callback classes** set **callbackClassNames** property with class names separated by comma:// \\
<wrap adicode>callbackClassNames="org.mycompany.myproject.model.callback.FilmCallback, org.mycompany.myproject.model.callback.CommonCallback"</wrap>
</WRAP>
\\ \\
=== Step 3: Change scenario.xml file for adding callback resources when building EJB ===
Open file $projectDirectory/resources/xml/scenario.xml.
<sxh xml; first-line: 1; highlight: [3]; title: excerpt from 'scenario.xml' file.>
    <actionResources>
    	...
        <copyResource sourceURI="#PLUGINHOME()/src/org/mycompany/myproject/call" targetURI="#EJBJARFILE()!org/mycompany/myproject/call" actionWhen="WHEN_BUILDING_EJB_JAR" relative="false" throwError="false"/>
    </actionResources>
</sxh>
<WRAP indic><wrap adititle>Remark</wrap>:\\
Line 3: package <wrap adicode>org.mycompany.myproject.call</wrap>, containing callback classes is added to **EJB**.</WRAP>
\\ \\
=== Step 4: Build and deploy EJB ===
<columns 100% l 450px middle>
{{tutorial:generate_ejb.png?450| Generate EJB}}
<newcolumn left>
  * Open file <wrap adicode>$projectDirectory/resources/xml/Scenario.xml</wrap> with Adichatz editor.
  * {{img_generate_scenario.png| Generate scenario}} Select <wrap adicode>Generate scenario</wrap> at top right of the editor.
  * Check '<wrap adicode>Generate EJB</wrap>' and '<wrap adicode>DeployEJB on application server</wrap>' options and select '<wrap adicode>OK</wrap>' button.
</columns>
\\ \\
=== Step 5: Execute ===
  * Open editor for <wrap adicode>Film 1</wrap>.
  * Change a value and save editor.
  * Check that <wrap adicode>lastUpdate</wrap> property has changed.
\\ \\
<WRAP adihi>Callback classes are implemented on the server side. The same principle can be used on the client side using extensions of the class org.adichatz.common.ejb.AEntityForeback and setting class names in **callforeClassNames** property</WRAP>