Migrating EJB 2.1 Entity and Session Beans to EJB 3.0

The nuts and bolts

By Deepak Vohra • Get more from this author

Posted in Developer, 25th April 2006 06:02 GMT

Table 1. EJB 3.0 Metadata Annotations

Annotation	Description	Annotation Elements
@Entity	Specifies an entity bean.
@Table	Specifies the entity bean table.	name, schema
@Id	Specifies an identifier property.
@Column	Specifies the database table column for a persistent entity bean property.	name, primaryKey, nullable, length
@NamedQueries	Specifies a group of named queries.
@NamedQuery	Specifies a named query or a query associated with a finder method.	name, queryString
@OneToMany	Specifies a one-to-many CMR relationship.	cascade
@OneToOne	Specifies a one-to-one CMR relationship.	cascade
@ManyToMany	Specifies a many-to-many CMR relationship.	cascade
@ManyToOne	Specifies a many-to-one CMR relationship.	cascade

The EJB 3.0 annotation types are defined in the javax.persistence package. The EJB 3.0 entity bean class, corresponding to the EJB 2.1 entity bean class, is annotated with metadata annotation @Entity. The finder method findByJournal in the EJB 2.1 bean class is specified in the EJB 3.0 POJO class with the @NamedQuery annotation. The CMR relationship Catalog-Editions in the EJB 2.1 entity bean is specified in EJB 3.0 entity bean class with the @OneToMany annotation. The @Id annotation specifies the identifier property catalogId. The @Column annotation specifies the database column corresponding to the identifier property catalogId. If a @Column annotation is not specified for a persistent entity bean property, the column name is same as the entity bean property name. Transient entity bean properties are specified with the @Transient annotation. The EJB 3.0 entity bean POJO class, corresponding to the EJB 2.1 entity bean is listed in Listing 8.

Listing 8. CatalogBean.java. EJB 3.0 Entity Bean POJO Class

import javax.persistence.Entity;
import javax.persistence.NamedQuery;
import javax.persistence.Id;
import javax.persistence.Column;
import javax.persistence.OneToMany;

@Entity
@NamedQuery(name="findByJournal",
    queryString="SELECT DISTINCT OBJECT(obj)  FROM Catalog obj WHERE obj.journal = ?1")

public class CatalogBean{
    public CatalogBean() {}
    public CatalogBean(String catalogId) {
        this.catalogId=catalogId;
    }

    private String catalogId;
    private String journal;
    private String publisher;

    @Id
    @Column(name="CatalogId", primaryKey="true")
    public  String getCatalogId() {return catalogId;}
    public  void setCatalogId() {this.catalogId=catalogId;}

    public  void setJournal(String journal) {this.journal=journal;}
    public String getJournal() {return journal;}

    public  void setPublisher(String publisher) {this.publisher=publisher;}
    public  String getPublisher() {return publisher;}

    private java.util.Collection<Edition> editions;

    @OneToMany
    public  void setEditions(java.util.Collection editions) {
        this.editions=editions;
    }

    public  java.util.Collection getEditions() {return editions;}
}

An EJB 2.1 entity bean is created with the create() method in the entity bean home/local home interface. A client for an EJB 2.1 entity bean obtains a reference for the entity bean with JNDI lookup. An example code snippet to create an instance of the example EJB 2.1 entity bean is:

InitialContext ctx=new InitialContext();
Object objref=ctx.lookup("CatalogLocalHome");
CatalogLocalHome catalogLocalHome=(CatalogLocalHome)objref;

//Create an instance of Entity bean
CatalogLocal catalogLocal=(CatalogLocal)catalogLocalHome.create(catalogId);

The example client class for the EJB 2.1 entity bean class is available in the resources zip file . CatalogLocalHome is the JNDI name of the CatalogBean entity bean.

To access the getter/setter methods of an entity bean, the remote/local object in EJB 2.1 is obtained with the finder methods:

CatalogLocal catalogLocal =
    (CatalogLocal) catalogLocalHome.findByPrimaryKey(catalogId);

A entity bean instance is removed with the remove() method:

catalogLocal.remove();

The example client class for the EJB 2.1 entity bean class is available in the resources zip file.

An EJB 3.0 entity bean class does not include the local/remote and home/local home interfaces. In EJB 3.0, persistence and lookup is provided by the EntityMangerclass.

Some of the methods in the javax.persistence.EntityManager class are listed in Table 2.

Next page: Table 2. EntityManager Class