The Top 5 New Annotations in JPA 2

By Nitin KL, Mahalakshmi K.

Enhancements to Existing JPA 1.x Annotations

In this section, we will describe some changes made to the existing JPA 1.x annotations.

Additional Support for Embeddable Classes

Embeddable classes in JPA 2 can contain other embeddable classes and can also contain relationships to other entities, which was not possible in JPA 1.x. The Employee entity has an embedded PersonalInfo type which has a nested embeddable object Address. The PersonalInfo entity also has a bidirectional ManyToOne relationship with the Project entity. The mappedBy attribute of OneToMany determines the field that owns the relationship. In the following example it is determined by the attribute of the Employee entity followed by the dot (.) operator and the attribute of the PersonalInfo entity.

@EntityEmployee{@Embeddedprivate PersonalInfo personalInfo;// Other properties and methods}@EmbeddablePersonalInfo{@JoinColumn(name = "projectId", referencedColumnName = "id")@ManyToOne(optional = false, cascade=CascadeType.ALL)private Project project;      @Embeddableprivate Address address;// Other Properties and methods}   @EntityAddress{      // All the Properties and methods}   @EntityProject {@OneToMany(cascade = CascadeType.ALL, mappedBy = "personalInfo.project")          private List<Employee> employeeCollection;      // Other code}

@JoinTable for OneToOne Relationships

Suppose the Employee entity has a OneToOne relationship with the EmployeeRecord entity. In the EMPLOYEERECORD table a foreign key is maintained which corresponds to the EMPLOYEE table. But when the foreign keys are maintained in a separate table, you can decorate the EmployeeRecord instance with @JoinTable, which specifies the name of the join table. Here's an example:

@Entity@Tablepublic class Employee {// Other Code      @OneToOne(cascade = CascadeType.ALL)          @JoinTable(name = "employee_record", joinColumns = {@JoinColumn(name="employeeId", referencedColumnName = "id")},          inverseJoinColumns = {@JoinColumn(name = "recordId", referencedColumnName = "id")})          private EmployeeRecord employeeRecord;}

@JoinColumn for Unidirectional OneToMany Relationships

JPA 1.x supports unidirectional OneToMany mapping by using a join table that stores the two foreign keys of the participating entities. JPA 2 avoids the usage of this join table by storing the foreign key in the many-to-one side.

Consider the Employee entity, which has a OneToMany unidirectional relationship with the File entity by having a List object. Theoretically the File entity should not have any relationship to the Employee entity, but it is not possible to insert a File object through the Employee entity without having the relationship. Hence, the File entity will have an Employee type that will not be part of the relationship. The @JoinColumn annotation specifies the foreign key in the File table, and this snippet shows very clearly that @JoinTable is not required to achieve the same result:

   @Entity@Tablepublic class Employee {// Other Code@OneToMany(cascade=CascadeType.ALL)          @JoinColumn(name="empId")          private List<File> fileList;}

Conclusion

In this article, we have discussed the new annotations introduced in JPA 2 with code examples. All these features have been implemented in EclipseLink. We also covered some additional capabilities provided by the existing annotations to make JPA perform a better and more convenient ORM.

Acknowledgements

The authors would like to sincerely thank Mr. Subrahmanya SV (VP, ECOM Research Group, E&R) for his constant encouragement and Ms. Sangeetha S. for providing the idea for this article, as well as guidance, valuable comments and reviews.

About the Authors

Mahalakshmi K. works at the E-Commerce Research Labs at Infosys Technologies Ltd. on Java EE technologies. Her core interest is in researching image processing, and she is also involved in the design and development of Java EE applications using Hibernate and JPA.

Nitin KL works at the E-Commerce Research Labs at Infosys Technologies. He is involved in design and development of Java EE applications using Hibernate, iBATIS, and JPA.

Page 2 of 2