Chapter 33. Table per Class inheritance in EJB3 Entities

The EJB3 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.

This tutorial uses the table per class strategy to map an inheritance relationship of org.jboss.tutorial.tableperinheritance.bean.Pet, which is the base class for org.jboss.tutorial.tableperinheritance.bean.Cat and org.jboss.tutorial.tableperinheritance.bean.Dog.

With the table per class strategy there is a table per class in the hierarchy, and each table has every single property that particular class will persist.

This is what the annotations look like for Pet:

			
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Pet implements java.io.Serializable
{

...

The subclass annotations look like this:

			
@Entity
public class Dog extends Pet
{
...

Polymorphic Queries :

org.jboss.tutorial.tableperinheritance.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 findByWeight 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 CAT (
  ID integer primary key,
  LIVES int
  NAME varchar,
  WEIGHT double
);

create table DOG (
  ID integer primary key,
  NUMBONES int
  NAME varchar,
  WEIGHT double
);

The table per class strategy is less efficient than the single table strategy as the SQL query is more complicated.

Building and Running

Note

To build and run the example, make sure you have installed JBoss 5.x. See the Section 1.1, “JBoss Application Server 5.x” for details.

Important

Because of a bug in HSQL, this tutorial does not work against the HSQL DB (which JBoss ships by default). Till this is fixed, running the client is not possible. Alternately, you can configure a datasource to use some other database and then use that datasource in the persistence.xml

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

Ant Users:

Make sure the "default" server configuration of JBossAS-5.x is running

			
$ ant

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.