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Chapter 5. Entity listeners and Callback methods

5.1. Definition
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It is often useful for the application to react to certain events that occur inside the persistence mechanism. This allows the implementation of certain kinds of generic functionality, and extension of built-in functionality. The EJB3 specification provides two related mechanisms for this purpose.
A method of the entity may be designated as a callback method to receive notification of a particular entity life cycle event. Callbacks methods are annotated by a callback annotation. You can also define an entity listener class to be used instead of the callback methods defined directly inside the entity class. An entity listener is a stateless class with a no-arg constructor. An entity listener is defined by annotating the entity class with the @EntityListeners annotation: 
@Entity 
@EntityListeners(class=Audit.value)
public class Cat {
    @Id private Integer id;
    private String name;
    private Date dateOfBirth;
    @Transient private int age;
    private Date lastUpdate;
    //getters and setters

    /**
     * Set my transient property at load time based on a calculation,
     * note that a native Hibernate formula mapping is better for this purpose.
     */
    @PostLoad
    public void calculateAge() {
        Calendar birth = new GregorianCalendar();
        birth.setTime(dateOfBirth);
        Calendar now = new GregorianCalendar();
        now.setTime( new Date() );
        int adjust = 0;
        if ( now.get(Calendar.DAY_OF_YEAR) - birth.get(Calendar.DAY_OF_YEAR) < 0) {
            adjust = -1;
        }
        age = now.get(Calendar.YEAR) - birth.get(Calendar.YEAR) + adjust;
    }
}

public class LastUpdateListener {
    /**
     * automatic property set before any database persistence
     */
    @PreUpdate
    @PrePersist
    public void setLastUpdate(Cat o) {
        o.setLastUpdate( new Date() );
    }
}
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The same callback method or entity listener method can be annotated with more than one callback annotation. For a given entity, you cannot have two methods being annotated by the same callback annotation whether it is a callback method or an entity listener method. A callback method is a no-arg method with no return type and any arbitrary name. An entity listener has the signature void <METHOD>(Object) where Object is of the actual entity type (note that Hibernate Entity Manager relaxed this constraint and allows Object of java.lang.Object type (allowing sharing of listeners accross several entities.)
A callback method can raise a RuntimeException. The current transaction, if any, must be rolled back. The following callbacks are defined:
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Table 5.1. Callbacks
Type Description
@PrePersist Executed before the entity manager persist operation is actually executed or cascaded. This call is synchronous with the persist operation.
@PreRemove Executed before the entity manager remove operation is actually executed or cascaded. This call is synchronous with the remove operation.
@PostPersist Executed after the entity manager persist operation is actually executed or cascaded. This call is invoked after the database INSERT is executed.
@PostRemove Executed after the entity manager remove operation is actually executed or cascaded. This call is synchronous with the remove operation.
@PreUpdate Executed before the database UPDATE operation.
@PostUpdate Executed after the database UPDATE operation.
@PostLoad Eexecuted after an entity has been loaded into the current persistence context or an entity has been refreshed.
A callback method must not invoke EntityManager or Query methods!
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