JBoss.org Community Documentation

Chapter 30. Introduction Single Inheritance in EJB3 Entities

The EJB specification allows you to define entities that inherit from one another. The inheritance relationships can be reflected in queries as well. So, if you queried based on the base class, the query is polymorphic.

The tutorial example uses the single table strategy to map an inheritance relationship of org.jboss.tutorial.singleinheritance.bean.Pet, which is the base class for org.jboss.tutorial.singleinheritance.bean.Cat and org.jboss.tutorial.singleinheritance.bean.Dog. With the single table strategy, the entire class hierarchy is persisted in one big single table. A discriminator column is required to differentiate between which class type is persisted in a particular row. This is what the annotations look like for Pet.

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "ANIMAL_TYPE", discriminatorType = DiscriminatorType.STRING)
public class Pet implements java.io.Serializable

The @DiscriminatorColumn specifies the column that will hold the type of the persisted entity. For subclasses, they must define the value of the discriminator column that will identify the class.

Here's the Dog entity which extends Pet:

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(discriminatorType = DiscriminatorType.STRING)
public class Dog extends Pet

Polymorphic queries:

org.jboss.tutorial.singleinheritance.bean.PetDAOBean stateless EJB wraps some polymorphic queries.

public List findByWeight(double weight)
   return manager.createQuery("from Pet p where p.weight < :weight").setParameter("weight", weight).getResultList();


Even though the <listing>findByWeight</listing> method queries on Pet, either Dog or Cat instances can be returned.
Table Mapping :

The table mapping for this example looks like this:

create table PET (
  ID integer primary key,
  ANIMAL_TYPE varchar,
  NAME varchar,
  WEIGHT double,
  LIVES int,


Building and Running

From the command prompt, move to the "singleinheritance" folder under the Section 1.3, “Set the EJB3_TUTORIAL_HOME”

Ant Users:

Make sure your JBossAS-5.x is running

$ ant
$ ant run

     [java] Sox
     [java] Junior


Maven Users: Make sure the AS is not running.

$ mvn clean install -PRunSingleTutorial

View the tables and rows:

You can view the tables created by JBoss by going to the Hypersonic Service, scrolling down to the startDatabaseManager button and clicking it. A Hypersonic SQL window will be minimized, but you can open it up to look at the tables and do queries.