Chapter 22. Introduction to merging and querying of entities in EJB3

This example shows a bunch of things. First, it introduces the @Column annotation. It also shows how entities can be detached and re-attached to persistence storage using the EntityManager.merge(). It also shows some basic queries.

@Column :

EJB 3.0 has a complete Object/Relational mapping. You can use the @Column annotation to specify which column in the table your property should map to. Take a look at the org.jboss.tutorial.merge.bean.Customer entity.

				
@Column(name = "FIRST")
public String getFirst()
{
   return first;
}

Find by primary key :

The EntityManager service has a built in find by primary key method:

				
<T> find(Class<T>, Object pk)

The org.jboss.tutorial.merge.bean.CustomerDAOBean stateless EJB's find() method wraps remote calls to the EntityManager.

				
public Customer find(int id)
{
   return manager.find(Customer.class, id);
}

Queries :

EntityManager allows you to create query objects on the fly that can be reused over and over, or just one time. Queries also support named parameters. The org.jboss.tutorial.merge.bean.CustomerDAOBean reflects this usage in the findByLastName method.

				
public List findByLastName(String name)
{
   return manager.createQuery("from Customer c where c.last = :name").setParameter("name", name).getResultList();
}

Merging :

The Value Object pattern is built into EJB 3.0. You can detach an object from persistence storage and send it across the network to the client. The client can make updates locally to the object, send it back to the server and the changes can be merged/synchronized back to the database using the EntityManager.merge() method. This is exactly what the org.jboss.tutorial.merge.client.Client does.

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.

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

Ant Users:

Make sure your JBossAS-5.x is running

			
$ ant
$ ant run

run:
     [java] Create Bill Burke and Monica Smith
     [java] Bill and Monica get married
     [java] Get all the Burkes
     [java] There are now 2 Burkes

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.