Java多对多从连接关系中检索数据

Question

我有两个名为 Course 和 Student 的 Bean，它们具有如下所示的多对多关系。

类(class).java

package ca.sheridancollege.beans;

import lombok.*;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.*;

@Data
@Entity
@AllArgsConstructor
@NoArgsConstructor
@NamedQuery(name="Course.byProfID", query="from Course where prof_id=:id")
public class Course implements Serializable {
    @Id @GeneratedValue
    private int id;
    private String courseName;
    private String department;

    @ManyToOne(cascade=CascadeType.ALL)
    private Professor prof;

    @ManyToMany(cascade=CascadeType.ALL, mappedBy="courseList")
    private List<Student> studentList = new ArrayList<Student>();

    public Course(String cName, String dept) {
        super();
        this.courseName = cName;
        this.department = dept;
    }


}

学生.Java

package ca.sheridancollege.beans;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.*;

import lombok.*;

@Data
@Entity
@NoArgsConstructor
@AllArgsConstructor
@NamedQuery(name="Student.byID", query="from Student where id=:id")
public class Student implements Serializable{

    @Id @GeneratedValue
    private int id;
    private String firstName;
    private String lastName;
    private String email;

    @ManyToMany(cascade=CascadeType.ALL)
    private List<Course> courseList = new ArrayList<Course>();

    public Student(String fName,String lName, String e) {
        this.firstName = fName;
        this.lastName = lName;
        this.email = e;

    }


}

我必须为学生分配一些类(class)，这是我通过这种 dao 方法完成的。

public void assignCourse(int cid, int sid) {
    Session session = sessionFactory.openSession();
    session.beginTransaction();

    Student s = (Student) session.get(Student.class, sid);
    Course c = (Course) session.get(Course.class, cid);

    s.getCourseList().add(c);
    c.getStudentList().add(s);




    session.getTransaction().commit(); 
    session.close();



}

此方法会生成一个包含正确数据的连接表，称为 Student_course。 StudentList_id = 学生的 ID，courseList_id = 我分配的类(class) ID。

select * from student_course;
+----------------+---------------+
| studentList_id | courseList_id |
+----------------+---------------+
|              5 |             3 |
|              6 |             4 |
+----------------+---------------+



select * from course;
+----+------------+------------+---------+
| id | courseName | department | prof_id |
+----+------------+------------+---------+
|  3 | Java101    | Technology |       1 |
|  4 | Biology    | Science    |       2 |
+----+------------+------------+---------+

select * from student;
+----+-----------------+-----------+----------+
| id | email           | firstName | lastName |
+----+-----------------+-----------+----------+
|  5 | sabeeh@mail.com | Sabeeh    | Shah     |
|  6 | td@mail.com     | test      | student  |
+----+-----------------+-----------+----------+

现在我试图从数据库中获取每个学生拥有的类(class)并将其显示在jsp中，但我不知道如何从连接表中进行选择，我认为这甚至是不允许的，那么其他方式是什么我能实现这个吗？

Answer 1

要创建您想要的连接表“student_course”，您需要在关系的一侧使用@JoinTable，您不应该使用“mappedBy”，您正在做的是创建两个单独的关系

@ManyToMany(cascade=CascadeType.ALL)
@JoinTable
private List<Student> studentList = new ArrayList<Student>();