java - Session.save 抛出 TransientObjectException

Question

我有两个名为学生和类(class)的实体，它们具有多对多关系。正如我在this中读到的那样文章中我们可以调用 save 方法来保存事务中的相关实体，或者一旦 session 被刷新它就会被保存。我也明白typical cascading problem这会导致此异常，但在这里我也处理了它，但仍然在调用 save 方法时遇到 TransientObjectException，而 persist 方法工作得很好。有人可以解释一下这里发生了什么吗？

这是我的代码:

SaveVsPersistTest.java

public class SaveVsPersistTest {


    public static void main(String[] args) {
        // Create session factory object 
        SessionFactory sessionFactory = new Configuration()
                .configure("hibernate.cfg.xml")
                .addAnnotatedClass(Student.class)
                .addAnnotatedClass(Course.class)
                .buildSessionFactory();

        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();

        Student student = new Student("Gopal");

        Course course1 = new Course("Java");
        Course course2 = new Course("C++");

        student.addCourse(course1);
        student.addCourse(course2);

        session.save(student);

        transaction.commit();
        session.flush();
    }

}

Student.java

@Entity
@Table(name="student")
public class Student {

    public Student() {
    }

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

    @Id
    @Column(name="student_id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    @Column(name="student_name")
    private String name;

    @ManyToMany(cascade={CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST,CascadeType.MERGE})
    @JoinTable(name="student_course",
                joinColumns={@JoinColumn(name="student_id")},
                inverseJoinColumns={@JoinColumn(name="course_id")})
    private List<Course> courseList;

    public void addCourse(Course tempcourse) {
        if(courseList == null){
            courseList= new ArrayList<>();
        }
        courseList.add(tempcourse);
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public List<Course> getCourseList() {
        return courseList;
    }

    public void setCourseList(List<Course> courseList) {
        this.courseList = courseList;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", courseList=" + courseList + "]";
    }

}

类(class).java

@Entity
@Table(name="course")
public class Course {

    public Course() {
    }

    public Course(String courseTitle) {
        super();
        this.courseTitle = courseTitle;
    }



    @Id
    @Column(name="course_id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    @Column(name="course_title")
    private String courseTitle;

    @ManyToMany(cascade={CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST,CascadeType.MERGE})
    @JoinTable(name="student_course",
                joinColumns={@JoinColumn(name="course_id")},
                inverseJoinColumns={@JoinColumn(name="student_id")})
    private List<Student> StudentList;

    public int getId() {
        return id;
    }

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

    public String getCourseTitle() {
        return courseTitle;
    }

    public void setCourseTitle(String courseTitle) {
        this.courseTitle = courseTitle;
    }

    public List<Student> getStudentList() {
        return StudentList;
    }

    public void setStudentList(List<Student> studentList) {
        StudentList = studentList;
    }

    @Override
    public String toString() {
        return "Course [id=" + id + ", courseTitle=" + courseTitle + ", StudentList=" + StudentList + "]";
    }

}

异常

Exception in thread "main" org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: manytomany.demo.Course

Answer 1

正如其他人建议的，您可以显式地在所有相关实体上调用 session.save，或者您可以使用 Hibernate Specific @Cascade注释如下所示。

@Cascade(CascadeType.SAVE_UPDATE)

在任何关联上使用 cascade={CascadeType.ALL} 时需要注意的是，一旦对父对象调用 delete，所有子对象(或表中的行)都会被删除。所以要谨慎一点。