JPAHibernate不会更新实体对象，即使从数据库加载它

Question

我有我的问题的简单例子。有两个学生和三个科目。起初，学生1拥有所有科目，学生2没有科目。关系是一个学生和多个学科的关系。

我正在尝试使用jpa hibernate将subject1从学生1交换到学生2。在数据库中出现事务提交更改后：主题表的外键student_id发生了更改，但在java中，即使我从数据库加载学生1，对象仍然具有subject1。

@Entity
@Table(name = "STUDENTS")
public class Students {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "ST_ID")
    private int id;


    @OneToMany(mappedBy = "student", cascade = CascadeType.ALL)
    List<Subject> subjects = new ArrayList<>();
    //getters and setters....
}



@Entity
@Table(name = "SUBJECT")
public class Subject {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "SB_ID")
    private int id;

    @Column(name = "MARK")
    private int mark;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "ST_ID")
    private Students student;
    //getters and setter....
}

主要代码：

    public static void main(String[] args) throws Exception {
    
        EntityManager em = Persistence.createEntityManagerFactory("Test").createEntityManager();
//        em.getTransaction().begin(); //does not work
        Students student1 = getStudentById(1, em); //load student1 from database by id
        Students student2 = getStudentById(2, em); //load student2 from database by id
        
        Subject subject1 = getSubjectById(1, em); //load subject1 from database by id
        Subject subject2 = getSubjectById(2, em); //load subject2 from database by id
        Subject subject3 = getSubjectById(3, em); //load subject3 from database by id
        System.out.println("student1 subjects");
        for(Subject sb : student1.getSubjects()){
            System.out.println(sb.getId()); //prints subjects: 1,2,3
        }
        System.out.println("\nstudent2 subjects");
        for(Subject sb : student2.getSubjects()){
            System.out.println(sb.getId()); //prints no subjects
        }    
        //swap subject1 between student1 and student2
        subject1.setStudent(student2);
        student2.getSubjects().add(subject1);
        
        //save changes
        em.getTransaction().begin();
        em.persist(student2);
        em.persist(student1);
        em.persist(subject2);
        em.persist(subject1);
        //em.merge does't work
        //em.flush(); //doesn't work
        em.getTransaction().commit();
        
        student1 = getStudentById(1, em); //load student1 from database by id
        System.out.println("\nstudent1 subjects");
        for(Subject sb : student1.getSubjects()){
            System.out.println(sb.getId()); //and still student1 has subject1
        }
    
        student2 = getStudentById(2, em); //load student2 from databse by id
        System.out.println("\nstudent2 subjects");
        for(Subject sb : student2.getSubjects()){
            System.out.println(sb.getId()); //now student2 also has subject1
        }
    
        
    }
    
    public static Students getStudentById(int id, EntityManager em){
        Query query = em.createQuery("from entity.Students where ST_ID="+id);
        return (Students)query.getSingleResult();
    }
    public static Subject getSubjectById(int id, EntityManager em){
        Query query = em.createQuery("from entity.Subject where SB_ID="+id);
        return (Subject) query.getSingleResult();
    }

提交后主题表如下所示。它显示id为1的科目现在有id为2的学生。

只有在提交后进行em.refresh时，实体对象student1才会更新，但我不明白为什么从数据库加载student1时需要使用refresh？而且我也不确定，刷新实体是一种好的做法吗？

Answer 1

您必须了解，如果可能的话，Hibernate会从一级缓存或持久性上下文返回对象，以保留对象标识。因此，即使使用查询从数据库中加载学生，它也只会返回之前持久化的对象，因为它与持久化上下文关联。

当您想从数据库中重新加载状态时，您必须使用刷新或分离，然后执行查询。