如何解决从休眠中定位未映射的类错误？

Question

我正在尝试映射 2 个实体（课程和学生），我有 2 个 Java 类和 2 个 MySQL 表，具有ManyToMany 关系。我创建了联结表和 java 类 Enrollment（因为我想要额外的信息，例如学生注册课程的日期）。

我正在尝试在 MySQL 的这个 Enrollments 表中使用休眠功能插入数据，但我不断收到错误消息。这是我的 POJO 课程：

课程类别：

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


private int id;

@Column(name = "chapter_id")
private int chapterId;;

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

@Column(name = "teacher_user_id")
private int teacherId;

 @OneToMany(targetEntity=Enrolment.class, mappedBy="course", fetch=FetchType.LAZY)
// @JoinTable(name = "enrolment",
         //   joinColumns = @JoinColumn(name = "course_id"),
         //   inverseJoinColumns = @JoinColumn(name = "student_user_id"))
private List<Enrolment> enrolments = new ArrayList<Enrolment>();

public Course(){}

public Course(int id, int chapterId, String title, int teacherId) {
    super();
    this.id = id;
    this.chapterId = chapterId;
    this.title = title;
    this.teacherId = teacherId;
}


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


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


public int getChapterId() {
    return chapterId;
}


public void setChapterId(int chapterId) {
    this.chapterId = chapterId;
}


public String getTitle() {
    return title;
}


public void setTitle(String title) {
    this.title = title;
}


public int getTeacherId() {
    return teacherId;
}


public void setTeacherId(int teacherId) {
    this.teacherId = teacherId;
}

@OneToMany(mappedBy = "course")
public List<Enrolment> getEnrolments() {
    return enrolments;
}

public void setEnrolments(List<Enrolment> courses) {
    this.enrolments = courses;
}

public void addEnrolment(Enrolment enrolment) {
    this.enrolments.add(enrolment);
}
}

Student 类（这个类继承自 User 父类，下面我也会附上 User Class。在数据库中也有不同的表：User，然后 Student 和 Teacher 继承了 User 父实体）：

@Entity
@Table(name = "student")
@PrimaryKeyJoinColumn(name = "user_id")
@OnDelete(action = OnDeleteAction.CASCADE)
public class Student extends User  {

private int grade;


private List<Enrolment> enrolments = new ArrayList<Enrolment>();

public Student(){} 

public Student(String fname, String lname, String email, String password, String address, String phone,
        int userType, int grade, boolean isAdmin) 
{
    super(fname, lname, email, password, address, phone, userType, isAdmin);

    this.grade=grade;

}

public int getGrade() {
    return grade;
}

public void setGrade(int grade) {
    this.grade = grade;
}


public void setEnrolments(List<Enrolment> courses) {
    this.enrolments = courses;
}

@OneToMany(mappedBy = "student")
public List<Enrolment> getEnrolments() {
    return enrolments;
}

public void addCourse(Enrolment course) {
    this.enrolments.add(course);
}

public void addEnrolment(Enrolment enrolment) {
    this.enrolments.add(enrolment);
}

}

用户类别：

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "user")
public class User {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
private String firstname;
private String lastname;
private String email;
private String password;
private String address;
private String phone;

@Column(name = "user_type_id")
private int userType;

@Column(name = "is_admin")
private boolean isAdmin;


public User(String fname, String lname, String email, String password, String address, String phone,
        int userType, boolean isAdmin) {
    //super();

    this.firstname = fname;
    this.lastname = lname;
    this.email = email;
    this.password = password;
    this.address = address;
    this.phone = phone;
    this.userType = userType;
    this.isAdmin = isAdmin;
}

public User() {}
//getters & setters

最后是 Enrollment 类：

@Entity
@Table(name = "enrolment")
public class Enrolment {


private int id;
private Student user;
private Course course;

@Column(name = "enrolment_date")
private Date enrolmentDate;

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

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


@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "student_user_id")
public User getUser() {
    return user;
}

public void setUser(Student user) {
    this.user = user;
}

@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "course_id")
public Course getCourse() {
    return course;
}

public void setCourse(Course course) {
    this.course = course;
}

public Date getEnrolmentDate() {
    return enrolmentDate;
}

public void setEnrolmentDate(Date enrolmentDate) {
    this.enrolmentDate = enrolmentDate;
}

所以我正在尝试从数据库中读取课程和学生并将信息插入此 Enrollment 表中，但由于尝试读取课程而出现错误。这是DAO方法：

 @SuppressWarnings("deprecation")
@Transactional
public List<Course> getCoursesOfChapter(int chapterId) {


    Configuration con = new Configuration().configure("hibernate.cfg.xml").addAnnotatedClass(Course.class);
    SessionFactory sf = con.buildSessionFactory();
    Session session = sf.openSession();
    Transaction tx = session.beginTransaction();


    Query query = session.createQuery("from Course where chapter_id = :chapterId");
      query.setParameter("chapterId",chapterId);

     // List list = query.list();

        tx.commit();
      return (List<Course>) query.list();

它在会话工厂构建时抛出错误：

Exception in thread "main" org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class: models.Course.enrolments[models.Enrolment]
at org.hibernate.cfg.annotations.CollectionBinder.bindManyToManySecondPass(CollectionBinder.java:1255)
at org.hibernate.cfg.annotations.CollectionBinder.bindStarToManySecondPass(CollectionBinder.java:808)
at org.hibernate.cfg.annotations.CollectionBinder$1.secondPass(CollectionBinder.java:733)
at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:54)
at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processSecondPasses(InFlightMetadataCollectorImpl.java:1696)
at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.processSecondPasses(InFlightMetadataCollectorImpl.java:1664)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:287)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:84)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:474)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:85)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:689)
at dao.CourseDAO.getCourse(CourseDAO.java:52)
at webapp.Main.main(Main.java:132)

最后，这是我的电话：

CourseDAO cdao = new CourseDAO();

Course course = cdao.getCourse(1);

我尝试过使用注释，将它们设为多对多而不是多对一。我试图映射用户类而不是学生，但仍然没有工作。我试图在没有 Enrollment 连接类的情况下制作它，并且在没有实际类的情况下生成它，但仍然没有工作（因为我不得不一个接一个地使用 2 个 session.save() 方法，这也给了一些我无法解决的错误）。可能这是我在这里遗漏的一件小事，但我无法弄清楚，抱歉代码太长，但我真的需要快速解决它。如果你读到这里，我真的很感谢你！

所以我的问题是：我是否从这些映射和注释中遗漏了一些东西，或者我应该改变我的类的结构？

Answer 1

将问题归结为最低限度总是有帮助的。以下是您的学生、课程和注册课程的更简单版本，可以轻松进行单元测试。课程和学生之间的多对多关联从 Enrollment 分为两个多对一关联。请注意，关联是双向的，并且 many 端由 one 端映射。学生将持久性操作级联到 Enrollment，这反映了学校的正常工作方式：学生注册课程，而不是相反。

Course.java

@Entity
public class Course {

    @Id
    @GeneratedValue
    private Long id;

    private String title;

    @OneToMany(mappedBy = "course")
    private List<Enrollment> enrollments;

    Course() {
    }

    Course(String title) {
        this.title = title;
        this.enrollments = new ArrayList<>();
    }

    void add(Enrollment enrollment) {
        enrollments.add(enrollment);
    }

    Long getId() {
        return id;
    }

    List<Enrollment> getEnrollments() {
        return enrollments;
    }
}

Student.java

@Entity
public class Student {

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @OneToMany(mappedBy = "student", cascade = ALL, orphanRemoval = true)
    private List<Enrollment> enrollments;

    Student() {
    }

    Student(String name) {
        this.name = name;
        this.enrollments = new ArrayList<>();
    }

    void enroll(Course course) {
        enrollments.add(new Enrollment(course, this));
    }
}

Enrollment.java

@Entity
public class Enrollment {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne
    private Course course;

    @ManyToOne
    private Student student;

    Enrollment() {
    }

    Enrollment(Course course, Student student) {
        this.course = course;
        this.student = student;
        course.add(this);
    }
}

下面的测试用例检查实体是否正确映射和关联。您可以使用 Spring Boot 运行它。

SchoolTest.java

@RunWith(SpringRunner.class)
@SpringBootTest
@Transactional
public class SchoolTest {

    @Autowired
    private CourseRepository courseRepository;
    @Autowired
    private StudentRepository studentRepository;

    @Test
    public void run() {
        Course course = courseRepository.save(new Course("cs_101"));

        int studentCount = 3;
        for (int i = 1; i <= studentCount; i++) {
            Student student = new Student("student_" + i);
            student.enroll(course);
            studentRepository.save(student);
        }

        // push changes to the database and clear the existing entities 
        // to make the subsequent operations load from the database
        entityManager.flush();
        entityManager.clear();

        Optional<Course> savedCourse = courseRepository.findById(course.getId());
        assertTrue(savedCourse.isPresent());
        assertEquals(studentCount, savedCourse.get().getEnrollments().size());
    }
}

Answer 2

正如警告所说，您的 Enrollment 未在 Hibernate 中注册。如果你真的不需要它。请使用瞬态注释。阅读更多here