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3.2.4.4. Inherit properties from superclasses

This is sometimes useful to share common properties through a technical or a business superclass without including it as a regular mapped entity (ie no specific table for this entity). For that purpose you can map them as @MappedSuperclass. 
@MappedSuperclass
public class BaseEntity {
    @Basic
    @Temporal(TemporalType.TIMESTAMP)
    public Date getLastUpdate() { ... }
    public String getLastUpdater() { ... }
    ...
}

@Entity class Order extends BaseEntity {
    @Id public Integer getId() { ... }
    ...
}
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In database, this hierarchy will be represented as an Order table having the id, lastUpdate and lastUpdater columns. The embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddable superclass is not the root of the hierarchy though.

Note

Properties from superclasses not mapped as @MappedSuperclass are ignored.

Note

The access type (field or methods), is inherited from the root entity, unless you use the Hibernate annotation @AccessType

Note

The same notion can be applied to @Embeddable objects to persist properties from their superclasses. You also need to use @MappedSuperclass to do that (this should not be considered as a standard EJB3 feature though)

Note

It is allowed to mark a class as @MappedSuperclass in the middle of the mapped inheritance hierarchy.

Note

Any class in the hierarchy non annotated with @MappedSuperclass nor @Entity will be ignored.
You can override columns defined in entity superclasses at the root entity level using the @AttributeOverride annotation. 
@MappedSuperclass
public class FlyingObject implements Serializable {

    public int getAltitude() {
        return altitude;
    }

    @Transient
    public int getMetricAltitude() {
        return metricAltitude;
    }

    @ManyToOne
    public PropulsionType getPropulsion() {
        return metricAltitude;
    }
    ...
}

@Entity
@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )
@AssociationOverride( name="propulsion", joinColumns = @JoinColumn(name="fld_propulsion_fk") )
public class Plane extends FlyingObject {
    ...
}
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The altitude property will be persisted in an fld_altitude column of table Plane and the propulsion association will be materialized in a fld_propulsion_fk foreign key column.
You can define @AttributeOverride(s) and @AssociationOverride(s) on @Entity classes, @MappedSuperclass classes and properties pointing to an @Embeddable object.
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