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Migrating EJB 2.1 Entity and Session Beans to EJB 3.0
The nuts and bolts
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 EntityManger
class.
Some of the methods in the javax.persistence.EntityManager
class are listed in Table 2.