Liferay: Working with model listeners

What 

Model Listener - as the name specifies that there is some class that in continuously looking to a entity model in liferay. More technically its a call back class which is called when some operation happens on the model (operation can be add/edit/delete).

Model Listener

public interface ModelListener<T> {
 public void onAfterAddAssociation(Object classPK, String associationClassName, Object associationClassPK) throws ModelListenerException;

public void onAfterCreate(T model) throws ModelListenerException;

public void onAfterRemove(T model) throws ModelListenerException;

public void onAfterRemoveAssociation(Object classPK, String associationClassName,  Object associationClassPK) throws ModelListenerException;

public void onAfterUpdate(T model) throws ModelListenerException;

public void onBeforeAddAssociation(Object classPK, String associationClassName, Object associationClassPK) throws ModelListenerException;

public void onBeforeCreate(T model) throws ModelListenerException;

public void onBeforeRemove(T model) throws ModelListenerException;

public void onBeforeRemoveAssociation(Object classPK, String associationClassName, Object associationClassPK) throws ModelListenerException;

public void onBeforeUpdate(T model) throws ModelListenerException;
}

How

To implement own listener on some Model (for example User) yin your portlet you need:

<hook>
 <portal-properties>portal.properties</portal-properties>
</hook>

Make an entry in file WEB-INF/src/portal.properties to define hook for specific model:
value.object.listener.com.liferay.portal.model.User=my.hook.UserListener 

Implement class by inheriting it from BaseModelListener<T> in our case BaseModelListener<User> and implementing only
  method we want to call-back:

    public class GroupListener extends BaseModelListener<Group> {
@Override public void    onAfterUpdate(User u) throws ModelListenerException { // do something here } }