举一个例子来说明这个多对对的关系。

我们在选课过程中,一个学生可以选多门课。反过来一个课程会被多个人选。两边都会存在多对多的关系。

在JPA中就可以用@ManyToMany去解决,具体如下

pom.xml关键依赖

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
  <groupId>mysql</groupId>
  <artifactId>mysql-connector-java</artifactId>
  <scope>runtime</scope>
</dependency>
<dependency>
  <groupId>org.projectlombok</groupId>
  <artifactId>lombok</artifactId>
  <optional>true</optional>
</dependency>

3个实体类

学生表Student

package com.rumenz.lession16.controller.entity;
import lombok.Getter;
import lombok.Setter;
import javax.persistence.*;
import java.util.Set;
/**
 * @className: Student
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Getter
@Setter
@Entity
@Table(name="student")
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String name;
    @ManyToMany
    @JoinTable(
            name = "student_course",
            joinColumns = @JoinColumn(name = "sid"),
            inverseJoinColumns = @JoinColumn(name = "cid"))
    Set<Course> likedCourses;
}

• @ManyToMany标注存在对对多的关系
• @JoinTable定义数据库关系,joinColumns属性链接自己,inverseJoinColumns属性链接对方
• @JoinColumn注解定义外键

在使用@ManyToMany注解的时候，在输出实体类时，出现了java.lang.StackOverflowError报错，这个是因为在实体类中，使用了@Data注解，@Data注解中的@ToString会导致java.lang.StackOverflowError,需要重写

不能用lombok的@EqualsAndHashCode和@ToString，否则死循环内存溢出

课程表Course

package com.rumenz.lession16.controller.entity;
import com.fasterxml.jackson.annotation.JsonIgnore;
import lombok.*;
import javax.persistence.*;
import java.util.Set;
/**
 * @className: Course
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Getter
@Setter
@Entity
@Table(name="course")
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String name;
    @JsonIgnore
    @ManyToMany(mappedBy = "likedCourses")
    Set<Student> likes;
}

关系表StudentCourse

package com.rumenz.lession16.controller.entity;
import lombok.Data;
import javax.persistence.*;
/**
 * @className: StudentCourse
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Data
@Entity
@Table(name="student_course")
public class StudentCourse {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private Integer sid;
    private Integer cid;
    private String  rating;
}

repository

数据库访问的ORM,也叫持久层,负责访问数据库。一般通过继承JpaRepository,CurdRepository实现，这里声明的接口不必实现,只要遵循Jpa规范就可以自动生成。

StudentRepository

package com.rumenz.lession16.controller.repository;
import com.rumenz.lession16.controller.entity.Student;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
/**
 * @className: StudentRepository
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Repository
public interface StudentRepository extends CrudRepository<Student,Integer> {
}

CourseRepository

package com.rumenz.lession16.controller.repository;
import com.rumenz.lession16.controller.entity.Course;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
/**
 * @className: CourseRepository
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Repository
public interface CourseRepository extends CrudRepository<Course,Integer> {
}

StudentCourseRepository

package com.rumenz.lession16.controller.repository;
import com.rumenz.lession16.controller.entity.StudentCourse;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
/**
 * @className: StudentCourseRepository
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Repository
public interface StudentCourseRepository extends CrudRepository<StudentCourse,Integer> {
}

service

业务层,负责调用Repository返回,处理数据

UserService

package com.rumenz.lession16.controller.service;
import com.rumenz.lession16.controller.entity.Student;
import java.util.List;
/**
 * @className: UserService
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
public interface UserService {
    List<Student> listStudent();
}

UserServiceImpl

UserService接口的实现类

package com.rumenz.lession16.controller.service.Impl;
import com.rumenz.lession16.controller.entity.Student;
import com.rumenz.lession16.controller.repository.StudentRepository;
import com.rumenz.lession16.controller.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.ArrayList;
import java.util.List;
/**
 * @className: UserServiceImpl
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@Service
public class UserServiceImpl implements UserService {
    @Autowired
    StudentRepository studentRepository;
    @Override
    public List<Student> listStudent() {
        List<Student> list=new ArrayList<>();
        Iterable<Student> res = studentRepository.findAll();
        res.forEach(item->{
            System.out.println(item.getLikedCourses().toString());
        });
        res.forEach(list::add);
        return list;
    }
}

Controller

控制器,负责接收前端请求,调用service返回数据。

RumenzController

package com.rumenz.lession16.controller;
import com.rumenz.lession16.controller.entity.Student;
import com.rumenz.lession16.controller.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.List;
/**
 * @className: RumenzController
 * @description: TODO 类描述
 * @author: 入门小站 rumenz.com
 * @date: 2021/12/15
 **/
@RestController
@RequestMapping("/rumenz")
public class RumenzController {
    @Autowired
    UserService userService;
    //多对多查询
    @GetMapping("/listStudent")
    public List<Student> listStudent(){
        return userService.listStudent();
    }
}

本小结源码地址:

• GitHub:https://github.com/mifunc/springboot/tree/main/lession16
• Gitee:https://gitee.com/rumenz/springboot/tree/master/lession16
• https://rumenz.com/rumenbiji/springboot-jpa-base.html

介绍

• 我的博客 https://rumenz.com/ ,
• 我的工具箱 https://tooltt.com/
• 微信公众号:【入门小站】

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