Hibernate JPA @Transient Example

@Transient

The @Transient annotation is used to specify that a given entity attribute should not be persisted.

Transient Specifies that the property or field is not persistent. It is used to annotate a property or field of an entity class, mapped superclass, or embeddable class.
    Example:

    @Entity
    public class Employee {
        @Id int id;
        @Transient User currentUser;
        ...
    }

Example of specifying JPA callbacks

@Entity
@EntityListeners( LastUpdateListener.class )
public static class Person {

@Id
private Long id;

private String name;

private Date dateOfBirth;

@Transient
private long age;

private Date lastUpdate;

public void setLastUpdate(Date lastUpdate) {
this.lastUpdate = lastUpdate;
}

/**
* Set the 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() {
age = ChronoUnit.YEARS.between( LocalDateTime.ofInstant(
Instant.ofEpochMilli( dateOfBirth.getTime()), ZoneOffset.UTC),
LocalDateTime.now()
);
}
}

public static class LastUpdateListener {

@PreUpdate
@PrePersist
public void setLastUpdate( Person p ) {
p.setLastUpdate( new Date() );
}
}
These approaches can be mixed, meaning you can use both together.

Regardless of whether the callback method is defined on the entity or on an entity listener, it must have a void-return signature. The name of the method is irrelevant as it is the placement of the callback annotations that makes the method a callback. In the case of callback methods defined on the entity class, the method must additionally have a no-argument signature. For callback methods defined on an entity listener class, the method must have a single argument signature; the type of that argument can be either java.lang.Object (to facilitate attachment to multiple entities) or the specific entity type.

A callback method can throw a RuntimeException. If the callback method does throw a RuntimeException, then the current transaction, if any, must be rolled back.

A callback method must not invoke EntityManager or Query methods!

It is possible that multiple callback methods are defined for a particular lifecycle event. When that is the case, the defined order of execution is well defined by the JPA spec (specifically section 3.5.4):

Any default listeners associated with the entity are invoked first, in the order they were specified in the XML. See the javax.persistence.ExcludeDefaultListeners annotation.

Next, entity listener class callbacks associated with the entity hierarchy are invoked, in the order they are defined in the EntityListeners. If multiple classes in the entity hierarchy define entity listeners, the listeners defined for a superclass are invoked before the listeners defined for its subclasses. See the `javax.persistence.ExcludeSuperclassListener`s annotation.

Lastly, callback methods defined on the entity hierarchy are invoked. If a callback type is annotated on both an entity and one or more of its superclasses without method overriding, both would be called, the most general superclass first. An entity class is also allowed to override a callback method defined in a superclass in which case the super callback would not get invoked; the overriding method would get invoked provided it is annotated.