Entity Inheritance Using Mapped Superclass

When I engaged myself into code re-factoring of our VELACORE project, I found that almost all Entities have four common properties createdBy, creationDate, modifiedBy and modificationDate.  I wanted to reduce this code duplication and found that JPA is providing Mapped Superclass for this type of requirement where state and mapping information are common to multiple Entity classes. It contains persistent state and mapping information, but is not Entitity. That is, it is not decorated with the @Entity annotation and is not mapped as an entity by JPA.

Mapped Superclasses do not have any corresponding tables in the underlying Datastore (that was also my requirement). Entities that inherit from the mapped Superclass define the table mappings. Mapped superclasses are specified by decorating the class with the javax.persistence.MappedSuperclass annotation.

Here is an example of using Mapped Superclass for entity inheritance

@MappedSuperclass
public class BaseEntity
{
    @Column(name = "created_by")
    private String createdBy;
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "creation_date")
    private Calendar creationDate;
    
   //getter setter of the properties ....
}

@Entity
public class Customer extends BaseEntity 
{
    @Column(name = "first_name")
    private String firstName;

    ...
}

@Entity
public class Invoice extends BaseEntity {
    @Column(name = "invoice_no")
    private String invoiceNo;

    ...
}

In the above code snippet BaseEntity is the MappedSuperclass which contains the mapping information of the common properties. Customer and Invoice are two Entities extending the BaseEntity. In this scenario only CUSTOMER and INVOICE tables will be created in the database not the BASEENTITY. "created_by" and "creation_date" columns will be present in both CUSTOMER and INVOICE tables.

Limitations of using Mapped Superclass :
-Mapped Superclasses are not queryable, and can’t be used in EntityManager or Query operations.
-Mapped Superclasses can’t be targets of entity relationships.