JPA 2 new feature @ElementCollection explained

最新推荐文章于 2024-04-02 17:08:44 发布

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本文介绍JPA 2.0中新增的@ElementCollection注解，该注解简化了基本类型和嵌入对象集合的映射过程。通过示例展示了如何使用@ElementCollection来存储字符串列表及嵌入式对象集合，并说明了其在数据库中的映射方式。

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@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

This annotation was introduced to map basic and embedded type objects
You can use any collection of type Collection, List, Set, Map
Its easy to override default values for generated Tables and Columns
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