Question

我正在通过NetBeans 8.1与Java DB作为数据库和EclipseLink（JPA 2.1）提供程序来探索JPA。我正在尝试OneToOne双向关系。我有2个班级学生和科目。

Student.java

@Entity
public class Student implements Serializable
{
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }

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

    public Student() {}
    public Student(String name) { this.name = name; }

    public static void print(EntityManager entitymanager)
    {
            //Print the database
        List<Student>     list=(List<Student>)entitymanager.createQuery("select e from Student e").getResultList( );
        System.out.println("Student");
        System.out.println("Id\tName\tSubject");
        for( Student e:list ) System.out.println(e.getId()+"\t"+e.getName()+"\t"+e.getSubject().getId());
    }

    @OneToOne(cascade = CascadeType.ALL, orphanRemoval=true)
    Subject subject;
    public Subject getSubject() { return subject; }
    public void setSubject(Subject subject) { this.subject = subject; }
}

Subject.java

@Entity
public class Subject implements Serializable
{
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)     // Increment of 1
    private Long id;
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }

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

    public Subject() {}
    public Subject(String name) { this.name = name; }

    @OneToOne(mappedBy = "subject", cascade = CascadeType.ALL, orphanRemoval=true)
    private Student student;
    public Student getStudent() { return student; }
    public void setStudent(Student student) { this.student = student; }

    public static void print(EntityManager entitymanager)
    {
        //Print the database
        List<Subject> list=(List<Subject>)entitymanager.createQuery("select e from Subject e").getResultList( );
        System.out.println("Subject");
        System.out.println("Id\tName\tStudent");
        for( Subject e:list ) {System.out.println(e.getId()+"\t"+e.getName()+"\t"+(e.student == null ? "null":e.student.getId()));}
    }
}

main（）函数

    public static void main(String[] args)
    {
        // Get entity manager and begin the transaction
        EntityManagerFactory emfactory = javax.persistence.Persistence.createEntityManagerFactory("JPA_TestPU");
        EntityManager entitymanager = emfactory.createEntityManager();
        entitymanager.getTransaction().begin();


        System.out.println("\nREAD : ");
        Student.print(entitymanager); Subject.print(entitymanager);

        Subject maths = new Subject();
        maths.setName("Maths");

        Student archit = new Student();
        archit.setName("Archit");
        archit.setSubject(maths);
        entitymanager.persist(archit);

        entitymanager.flush();

        System.out.println("\nCREATE : Archit selected Maths");

        // I want to print maths.getStudent(), which does not work        
        System.out.println("-----------"+maths.getId()+"---"+maths.getStudent());

        Student.print(entitymanager); Subject.print(entitymanager);

        Subject science = new Subject();
        science.setName("Science");
        archit.setSubject(science);
        entitymanager.persist(archit);

        entitymanager.flush();

        System.out.println("\nUPDATE : Archit selected Science");
        Student.print(entitymanager); Subject.print(entitymanager);

        // Commit and close entity manager
        entitymanager.getTransaction().commit();

        entitymanager.close();
        emfactory.close();
    }

我希望Subject.print（）应该获得成员“student”的良好价值，但它是null。

main（）的输出

READ : 
Student
Id  Name    Subject
Subject
Id  Name    Student

CREATE : Archit selected Maths
-----------1---null
Student
Id  Name    Subject
1   Archit  1
Subject
Id  Name    Student
1   Maths   null

UPDATE : Archit selected Science
Student
Id  Name    Subject
1   Archit  2
Subject
Id  Name    Student
2   Science null

我发现数据库已正确更新且Student表具有SubjectID但Subject表没有StudentID。

有人能解释我为什么我没有在Subject对象中获得学生价值吗？

双向OneToOne不刷新

0 个答案: