我有两个非常简单的对象，并且一个对象应在一组“一对多”关系中包含另一个对象。对象已正确插入数据库中，但“子项”表中的外键始终为“ null”。

我不知道为什么：

这是测试对象，它将子对象保持在其集合中：

@Entity
@Table(name="test")
public class TestObj {

    public TestObj(){}

    private Long id;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }

    private Set<Children> children = new HashSet<Children>();

    @OneToMany(mappedBy = "testObj", cascade = CascadeType.ALL)
    public synchronized Set<Children> getChildren() {
        return children;
    }
    public synchronized void setChildren(Set<Children> children) {
        this.children = children;
    }
    public void addChildren(Children child){
        children.add(child);
    }
}

这是子对象，它包含指向“ TestObj”的反向链接：

@Entity
@Table(name = "children")
public class Children {

    public Children(){}

    private Long id;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    public Long getId() {
        return id;
    }

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

    private TestObj testObj;

    @ManyToOne
    @JoinColumn
    public TestObj getTestObj() {
        return testObj;
    }

    public void setTestObj(TestObj testObj) {
        this.testObj = testObj;
    }
}

我使用以下代码来持久化此对象：

EntityManagerFactory entityManagerFactory = HibernateEntityMangerSingelton.getEntityManagerFactory();
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();


TestObj user = new TestObj();

Children child = new Children();
user.addChildren(child);
try {

    entityManager.persist(user);

    entityManager.getTransaction().commit();

} catch (Exception e) {
    System.out.println(e);
}finally{
    entityManager.close();
}

有人可以解释一下为什么会这样吗？