SpringJPA: map Java inheritance hierarchies to database tables

example-1-InheritanceType.JOINED

// @Note: Sample code from Broadleaf

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "BLC_CUSTOMER", uniqueConstraints = @UniqueConstraint(columnNames = { "USER_NAME" }))
public class CustomerImpl implements Customer, AdminMainEntity {

    @Id
    @Column(name = "CUSTOMER_ID")
    protected Long id;

    @Column(name = "USER_NAME")
    protected String username;

    @Column(name = "PASSWORD")
    protected String password;
    
    // ........
    
}    
    
// ================================================================= 

// @Note: in the new table `WEBA_CUSTOMER_EXT`, we must add a column `CUSTOMER_ID` to play
// as a foreign key to reference back to the table `BLC_CUSTOMER` of supperclass CustomerImpl
@Entity
@Table(name = "WEBA_CUSTOMER_EXT")
public class WebaCustomerEntity extends CustomerImpl {
	
	public WebaCustomerEntity() {
		System.out.println("TKT - constructor of WebaCustomerEntity()");
	}
}

example-2-MappedSuperclass.java

// Is it possible to build a JPA entity by extending a POJO?
// - http://stackoverflow.com/questions/2516329/is-it-possible-to-build-a-jpa-entity-by-extending-a-pojo

JPA specification states
- Entities may extend non-entity classes as well as entity classes, 
and non-entity classes may extend entity classes.

@javax.persistence.MappedSuperclass annotation allows you to define 
this kind of mapping

// =================================================================

@MappedSuperclass
public class MyThing implements Serializable {
    private int myNumber;
    private String myData;

    // getter's and setter's
}

And

@Entity
@Table(name="MY_THING")
public class MyThingEntity extends MyThing {


}

// =================================================================

As said by JPA specification
- The MappedSuperclass annotation designates a class whose mapping 
information is applied to the entities that inherit from it.

And

+ A class designated with the MappedSuperclass annotation can be 
mapped in the same way as an entity except that the mappings 
will apply only to its subclasses since no table exists for 
the mapped superclass itself.
+ If you need to override some property defined by MyThing, use 
@AttibuteOverride (when you want to override a single property) 
or @AttibuteOverrides (when you want to override more than one 
property)

// =================================================================

@Entity
@Table(name="MY_THING")
@AttributeOverride(name="myData", column=@Column(name="MY_DATA"))
public class MyThingEntity extends MyThing {


}

spring-jpa-inheritance.java

// JPA Implementation Patterns: Mapping Inheritance Hierarchies
// - http://java.dzone.com/articles/jpa-implementation-patterns-mapping
// - http://stackoverflow.com/questions/2516329/is-it-possible-to-build-a-jpa-entity-by-extending-a-pojo

JPA provides three ways to map Java inheritance hierarchies to
database tables:

- InheritanceType.SINGLE_TABLE - The whole inheritance hierarchy is
  mapped to one table. An object is stored in exactly one row in that
  table and the discriminator value stored in the discriminator column
  specifies the type of the object.

- InheritanceType.TABLE_PER_CLASS - Every concrete entity class in the
  hierarchy is mapped to a separate table. An object is stored in
  exactly one row in the specific table for its type.

- InheritanceType.JOINED - Every class in the hierarchy is represented
  as a separate table, causing no field duplication to occur. An
  object is stored spread out over multiple tables ; one row in each
  of the tables that make up its class inheritance hierarchy.