Java inheritance

We have looked at Java inheritance but let’s look at how it is applied to JPA

good reference: https://www.baeldung.com/hibernate-inheritance

Inheritance in JPA

There are multiple strategies to choose from

Single table strategy

The Single Table strategy creates one table for each class hierarchy. JPA also chooses this strategy by default if we don’t specify one explicitly.

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class MyProduct {
    @Id
    private long productId;
    private String name;

    // constructor, getters, setters
}

Note that the The identifier (id) of the entities is also defined in the superclass.

Then for the subclasses,

@Entity
public class Book extends MyProduct {
    private String author;
}

@Entity
public class Pen extends MyProduct {
    private String color;
}

We also need discriminator values because the records for all entities will be in the same table, Hibernate needs a way to differentiate between them. By default, this is done through a discriminator column called DTYPE that has the name of the entity as a value.

To customize the discriminator column, we can use the @DiscriminatorColumn annotation:

@Entity(name="products")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="product_type", 
  discriminatorType = DiscriminatorType.STRING)
public class MyProduct {
    // ...
}

@Entity
@DiscriminatorValue("book")
public class Book extends MyProduct {
    // ...
}

If @DiscriminatorValue is not explicitly set in the subclass, the class name of subclass is taken as the value by default.