Inherited abstract class with JPA (+Hibernate)

Question

How would you configure annotations in the following example code? I'd like to stick with JPA annotations only and avoid Hibernate specific dependencies. Is the code below correct?

@Entity
public class RefExample extends RefData {

}

(There will be multiple versions of these classes, RefSomeOtherExample, etc, and one db table per class. Some may add additional fields (columns) but most will simply make use of the basic fields inherited from the "RefData" base class.)

Base class:

@Entity
public abstract class RefData {

    private long id;
    private String code;
    private String desc;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(unique = true, nullable = false)
    public long getId() {

        return id;
    }

    public void setId(long id) {

        this.id = id;
    }

    @Column(unique = true, nullable = false, length=8)
    public String getCode() {

        return code;
    }

    public void setCode(String code) {

        this.code = code;
    }

    @Column(unique = true, nullable = false, length=80)
    public String getDesc() {

        return desc;
    }

    public void setDesc(String desc) {

        this.desc = desc;
    }
}

Ultimately I'd like to generate schema creation scripts from this using Hibernate's SchemaExport class. In the case above these two classes should only result in the creation of a single table named "RefExample" with the three columns from "RefData". Will this work?

Solution 1

From JPA 1.0 specification:

Both abstract and concrete classes can be entities. Both abstract and concrete classes can be annotated with the Entity annotation, mapped as entities, and queried for as entities.

Entities can extend non-entity classes and non-entity classes can extend entity classes.

As you want a single table, you should use Single Table inheritance.

Just define a discriminator column as follows:

@Entity
@DiscriminatorColumn(name="REF_TYPE")
public abstract class RefData {

But if you do not want to rely on JPA inheritance strategies, you can use MappedSuperclass instead:

@MappedSuperclass
public abstract class RefData {

JPA specification

An entity may inherit from a superclass that provides persistent entity state and mapping information, but which is not itself an entity. Typically, the purpose of such a mapped superclass is to define state and mapping information that is common to multiple entity classes.

Keep in mind you can not use @Entity and @MappedSuperclass at the same time.

Solution 2

@MappedSuperclass is worked for me. I was struggling to map a view to 2 objects which are Parent and child classes. My view is joined from 2 tables. Primary keys from both the tables are present in the view. @DiscriminatorColumn is not worked for me since it requires a column exclusively allotted to data type of the object and also it is throwing 'repeated Column in object exception' which I could not solve.

I read this forum and I tried @MappedSuperclass annotation. It does the trick.

I've put @MappedSuperclass at the superclass and put the @Id and @GeneratedValue in the superclass identifier. In the sublass I've given as

@Entity
@Table(name="view_name")

and used sub class object to fetch the data from view. That's it.

Inheritance in hibernate annotations for Joined table with out using @DiscriminatorColumn worked for me.

Comments

[Manius]

That article has me stumped. I was actually picturing the Table per concrete class strategy described there (one table per concrete "RefExample" class), but it says that approach is "not popular" AND optional in JPA. None of the three options seem to exactly fit my requirements. Rats!

[Arthur Ronald]

@Crusader See here: stackoverflow.com/questions/2700680/… how i workaround perfomance issues when dealing with inheritance. If you want to use TABLE PER CLASS strategy, the target database must support identity generation strategy.

[Manius]

Not 100% sure yet but I think MappedSuperclass may be my best option. All these tables are really for is to store values presented in drop down fields which get assigned (one to many) to actual data records.

[Sunil]

Is is possible to query(HQL or JPQL) on properties present in superclass annotated with @MappedSuperclass

[Stephan]

The link in this post seems dead.