JPA using Hibernate - Entity Manager doubts

[Tiago Melo]

Hi dear friends,

I'm learning about JPA and I've wrote a fairy simple application based on the Oreilly's EJB 3.0 Book. First, let me show you my classes and after that I'll tell what's happening.

Cabin entity

package com.titan.domain;
 
import java.io.Serializable;
 
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;
 
@Entity
@Table(name="CABIN")
public class Cabin implements Serializable {
    private int id;
    private String name;
    private int deckLevel;
    private int shipId;
    private int bedCount;
    
    @Id
    @Column(name="ID")
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    @Column(name="NAME")
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    @Column(name="DECK_LEVEL")
    public int getDeckLevel() {
        return deckLevel;
    }
    
    public void setDeckLevel(int deckLevel) {
        this.deckLevel = deckLevel;
    }
    
    @Column(name="SHIP_ID")
    public int getShipId() {
        return shipId;
    }
    
    public void setShipId(int shipId) {
        this.shipId = shipId;
    }
    
    @Column(name="BED_COUNT")
    public int getBedCount() {
        return bedCount;
    }
    
    public void setBedCount(int bedCount) {
        this.bedCount = bedCount;
    }
    
}
 

TravelAgentRemote

package com.titan.travelagent;
 
import javax.ejb.Remote;
import javax.persistence.EntityManager;
 
import com.titan.domain.Cabin;
import com.titan.domain.CreditCard;
import com.titan.domain.Customer;
import com.titan.domain.Phone;
 
@Remote
public interface TravelAgentRemote {
    public void createCabin(Cabin cabin);
    public Cabin findCabin(int id);
}

TravelAgentBean

package com.titan.travelagent;
 
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
 
import com.titan.domain.Cabin;
import com.titan.domain.CreditCard;
import com.titan.domain.Customer;
import com.titan.domain.Phone;
 
@Stateless
public class TravelAgentBean implements TravelAgentRemote{
    @PersistenceContext(unitName="titan")
    private EntityManager manager;
    
    public void createCabin(Cabin cabin) {
        manager.persist(cabin);
    }
 
    public Cabin findCabin(int id) {
        return manager.find(Cabin.class, id);
    }
 
}

persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence>
<persistence-unit name="titan">
      <jta-data-source>java:/TitanDB</jta-data-source>
      <properties>
      <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
      <property name="hibernate.hbm2ddl.auto" value="create"/>
    </properties>
   </persistence-unit>
</persistence>

And now, a fairy simple standalone client to test everything:

Client

package com.titan.clients;
 
import java.util.Collection;
import java.util.Properties;
 
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.rmi.PortableRemoteObject;
 
import com.titan.domain.Cabin;
import com.titan.domain.CreditCard;
import com.titan.domain.Customer;
import com.titan.domain.Phone;
import com.titan.travelagent.TravelAgentRemote;
 
public class Client {
    
    public static void main(String[] args) {
        try {
            Context jndiContext = getInitialContext();
            Object ref = jndiContext.lookup("TravelAgentBean/remote");
            TravelAgentRemote dao = (TravelAgentRemote)PortableRemoteObject.narrow(ref, TravelAgentRemote.class);
            
            // Create cabin
            Cabin cabin1 = new Cabin();
            cabin1.setId(1);
            cabin1.setName("Master Suite");
            cabin1.setDeckLevel(1);
            cabin1.setShipId(1);
            cabin1.setBedCount(3);
            
            // Persist the cabin
            dao.createCabin(cabin1);
            
            // update the name
            cabin1.setName("Basic suite");
            
            // Search the cabin newly persisted
            Cabin cabin2 = dao.findCabin(1);
            System.out.println("=========== Cabin ===========");
            System.out.println(cabin2.getName( ));
            System.out.println(cabin2.getDeckLevel( ));
            System.out.println(cabin2.getShipId( ));
            System.out.println(cabin2.getBedCount( ));
                                 
            
        }
        catch (NamingException e) {
            e.printStackTrace();
        }
        
    }
    
    public static Context getInitialContext() throws NamingException{
        Properties env = new Properties();
        env.put(Context.INITIAL_CONTEXT_FACTORY, "org.jnp.interfaces.NamingContextFactory");
        env.put(Context.URL_PKG_PREFIXES, "org.jnp.interfaces");
        env.put(Context.PROVIDER_URL, "localhost");
        return new InitialContext(env);
    }
    
}
 

Ok, so let's go to the problem. The JPA's spec says that once an entity bean is managed by the EntityManager (after calling EntityManager.persist() or calling EntityManager.find()), every change on the entity is tracked and is persisted automatically on the data base.

So, here I'm persisting an instance of the Cabin Entity:

            // Create cabin
            Cabin cabin1 = new Cabin();
            cabin1.setId(1);
            cabin1.setName("Master Suite");
            cabin1.setDeckLevel(1);
            cabin1.setShipId(1);
            cabin1.setBedCount(3);
            
            // Persist the cabin
            dao.createCabin(cabin1);

This works fine. I've checked at the database and a record was successfully created at the CABIN table.

So, according to the spec, since I've called EntityManager.persist passin in the cabin entity instance, this instance is managed, therefore if I do this:

            // update the name
            cabin1.setName("Basic suite");
 

the name should be automatically changed from "Master suite" to "Basic suite", and since this instance is managed, this change should be reflected at the database. But it's not what's happening! It don't have any effect, the name "Master Suite" is kept at the database. Why this is happening?

These are the jars I used to compile:

hibernate-entitymanager.jar (from Hibernate)
ejb3-persistence.jar (from Hibernate)
jboss-ejb3x.jar (from JBoss)
jbossall-client.jar (from JBoss)

The spec says that the <provider> element at persistence.xml is optional. If it's not specified, a default value is assumed. But in my case, what's the value that's being assumed?

Thank you all!

[Edvins Reisons]

Do I see it correctly that the entity that you are modifying is on the
client and the EntityManager on the server?

[Tiago Melo]

Well, the entity is at the server as well, inside de jar file along with the other classes

[Edvins Reisons]

The interfaces that the client uses are remote, which may be a bit too
complex for a first example.
The entity is detached; one good reason for this is that the persistence context here is, according to the specification, "container-managed transaction scoped", and for the stateless session bean, a transaction is one method call. When the bean method returns, the transaction ends, and with it the persistence context.

[Tiago Melo]

Right... so how could I turn the Persistence Context extended in this example?

[Mark Spritzler]

your code is in the client, and therefore isn't being run in the context of an EntityManager. The EntityManager is only on the server, and therefore only in the context of the createCabin method.

Also why are you using PortableRemoteObject.narrow()? That isn't needed anymore in EJB 3

Mark