JPA 2 new feature @ElementCollection explained

@ElementCollection is new annotation introduced in JPA 2.0, This will help us get rid of One-Many and Many-One shitty syntax.

Example 1: Stores list of Strings in an Entity

@Entity
public class Users implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @ElementCollection
    private List<String> certifications = new ArrayList<String>();

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public List<String> getCertifications() {
        return certifications;
    }

    public void setCertifications(List<String> certifications) {
        this.certifications = certifications;
    }
..
}

        Users u = new Users();
        u.getCertifications().add("Sun Certified Java Programmer");
        em.persist(u);

Generated Tables

   Users
   Column --> ID
    Row             1

Users_CERTIFICATIONS
  Column  Users_ID      CERTIFICATIONS
  Row           1                 Sun Certified Java Programmer

 @ElementCollection rules

1.    This annotation was introduced to map basic and embedded type objects
2.    You can use any collection of type Collection, List, Set, Map
3.    Its easy to override default values for generated Tables and Columns
4.    JPA 1.0 only supported collection holding entity types using (@ManyToOne, @OneToMany) annotations

Lets quickly look at code snippet for Embedded object based collection


@Embeddable
public class Certification {

    @Column(name = "name")
    private String name;
    @Temporal(TemporalType.DATE)
    @Column(name = "issue_date")
    private Date issueDate;

    public Date getIssueDate() {
        return issueDate;
    }

    public void setIssueDate(Date issueDate) {
        this.issueDate = issueDate;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}


@Entity
public class Users implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @ElementCollection
    @CollectionTable(name = "Certification", joinColumns = {@JoinColumn(name="user_id")})
    private List<Certification> certifications = new ArrayList<Certification>();

   
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public List<Certification> getCertifications() {
        return certifications;
    }

    public void setCertifications(List<Certification> certifications) {
        this.certifications = certifications;
    }
..
}


        Users u = new Users();
        Certification c = new Certification();
        c.setName("Sun Certified Java Programmer");
        c.setIssueDate(new Date());
        u.getCertifications().add(c);
        em.persist(u);

Resulted Table structure

Table --> Users
   Column --> ID
    Row             1

Table --> Users_Certification
  Column  user_id      name                                                   Issue_Date                         
  Row           1                 Sun Certified Java Programmer        2010-09-05

Map based Collection is a rich area it needs a seperate article to describe it some detail.