双向一对多关系不起作用

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OneToMany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; }

    @OneToMany(mappedBy = "student", cascade = CascadeType.ALL, orphanRemoval=true)
    List<Subject> subject;
    public List<Subject> getSubject() { return subject; }
    public void setSubject(List<Subject> subject) { this.subject = subject; }

    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\tSubjectId");
        for( Student e:list ) { System.out.print(e.getId()+"\t"+e.getName()+"\t"); for(Subject s : e.subject) System.out.print(s.getId()+",");System.out.println();}
    }
}

Subject.java

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

    @ManyToOne
    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);

    LinkedList<Subject> pcm = new LinkedList<Subject>();
    Subject maths = new Subject();
    maths.setName("Maths");
    entitymanager.persist(maths);
    pcm.add(maths);
    Subject science = new Subject();
    science.setName("Science");
    entitymanager.persist(science);
    pcm.add(science);

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

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

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

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

首次执行main（）

READ :
Student
Id  Name    SubjectId
Subject
Id  Name    Student

CREATE : Archit selected Maths Science
Student
Id  Name    SubjectId
1   Archit  2,1,
Subject
Id  Name    Student
1   Science null
2   Maths   null

第二次执行main（）

READ :
Student
Id  Name    SubjectId
1   Archit
Subject
Id  Name    Student
1   Science null
2   Maths   null

CREATE : Archit selected Maths Science
Student
Id  Name    SubjectId
1   Archit
2   Archit  3,4,
Subject
Id  Name    Student
1   Science null
2   Maths   null
3   Maths   null
4   Science null

我看到学生保存后，科目也会被保存。 SUBJECT表包含STUDENT_ID列作为FK。但是此列设置为NULL而不是Student对象的Id值。同时在Subject的Memory对象中不包含Student对象的有效引用，但它被设置为NULL。

任何人都可以帮我确定我是否做错了。