java - 使用 JOIN-Table 的 Hibernate 多对多关联

Question

我发现很难理解使用连接表的 hibernate 多对多关联。

我从 net 获得了示例项目。它包括

类(class).java

package com.chad.model;

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;

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

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

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

    @ManyToMany(fetch=FetchType.LAZY,cascade={CascadeType.REFRESH,CascadeType.MERGE,CascadeType.PERSIST})
    @JoinTable(name="course_student",
               joinColumns=@JoinColumn(name="course_id"),
               inverseJoinColumns=@JoinColumn(name="student_id"))
    private List<Student> studentList;

    public Course()
    {

    }

    public Course( String title) {
        this.title = title; 
    }




    public int getId() {
        return id;
    }


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


    public String getTitle() {
        return title;
    }


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

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

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


}

Student.java

package com.chad.model;

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

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

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    @Column(name="FIRST_NAME")
    private String firstName;

    @Column(name="LAST_NAME")
    private String lastName;

    @Column(name="EMAIL_ID")
    private String email;

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

    public Student(){}

    public Student(String firstName, String lastName, String email) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
    }

    public int getId() {
        return id;
    }

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

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }


}

1) My doubt was , The example that i got from internet has

@ManyToMany(fetch=FetchType.LAZY,cascade={CascadeType.REFRESH,CascadeType.MERGE,CascadeType.PERSIST})
        @JoinTable(name="course_student",
                    joinColumns=@JoinColumn(name="student_id"),
                    inverseJoinColumns=@JoinColumn(name="course_id"))

上面的 block 写在两个实体类中。但是在他们写的其他一些文章中一个实体类中的相同 block 。
并且在另一个实体类中只有 @ManyToMany --> 没有 joinColumn

我能知道这两种语法的重要性吗？？

2) After running the program i got the following logs

主.java

Course course = new Course("Photography");

        session.save(course);

        Student student1 = new Student("John", "Dao", "john@gmail.com");
        Student student2 = new Student("John1", "Dao1", "john1@gmail1.com");


        List<Student> studentList = new ArrayList<Student>();
        studentList.add(student1);
        studentList.add(student2);

        course.setStudentList(studentList);

        session.save(student1);
        session.save(student2);

        session.getTransaction().commit();

日志

Hibernate: insert into chad_course (title, instructor_id) values (?, ?)
Hibernate: insert into Chad_student (FIRST_NAME, LAST_NAME, EMAIL_ID) values (?, ?, ?)
Hibernate: insert into Chad_student (FIRST_NAME, LAST_NAME, EMAIL_ID) values (?, ?, ?)
Hibernate: insert into course_student (course_id, student_id) values (?, ?)
Hibernate: insert into course_student (course_id, student_id) values (?, ?)

现在

• 第一次插入到 chad_course 中。 正确

• 第二个和第三个插入到 Chad_student 中。 正确

• 第 4 和第 5 个插入到 course_student --> hibernate 怎么知道要插入什么 course_id 和 student_id ，因为没有选择查询发生。

Answer 1

GenerationType.IDENTITY 用于支持自动增量的数据库，如 MySQL、SQLServer 和其他一些数据库，这不适用于所有数据库，如 Oracle 等。

这就是您没有看到任何 Select SQL 查询被触发的原因

检查 - GenerationType.AUTO vs GenerationType.IDENTITY in hibernate