JPA 多对多映射

多对多映射表示集合值关联，其中任意数量的实体可以与其他实体的集合相关联。在关系数据库中，一个实体的任意数量的行都可以引用另一个实体的任意数量的行。

JPA @ManyToMany多对多映射示例

项目结构如下：

在此示例中，我们将在学生和图书馆之间创建多对多关系，以便可以向任意数量的学生分发任何类型的书籍。

pom依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.yiidian</groupId>
    <artifactId>JPAProject</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.eclipse.persistence</groupId>
            <artifactId>javax.persistence</artifactId>
            <version>2.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.persistence</groupId>
            <artifactId>eclipselink</artifactId>
            <version>2.5.0</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
    </dependencies>
</project>

Student实体类

package com.yiidian.pojo;

import javax.persistence.*;
import java.util.List;

@Entity
public class Student {  
  
    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    private int s_id;  
    private String s_name;  
      
    @ManyToMany(targetEntity=Library.class)
    @JoinTable(name = "library_student",joinColumns = @JoinColumn(name = "student_id"),inverseJoinColumns = @JoinColumn(name = "library_id"))
    private List lib;  

    public Student(int s_id, String s_name, List lib) {  
        super();  
        this.s_id = s_id;  
        this.s_name = s_name;  
        this.lib = lib;  
    }  

    public Student() {  
        super();  
    }

    public List getLib() {
        return lib;  
    }  
    public void ListLib(List lib) {  
        this.lib = lib;  
    }  
    public int getS_id() {  
        return s_id;  
    }  
    public void ListS_id(int s_id) {  
        this.s_id = s_id;  
    }  
    public String getS_name() {  
        return s_name;  
    }  
    public void ListS_name(String s_name) {  
        this.s_name = s_name;  
    }  
    } 

Library实体类

package com.yiidian.pojo;

import javax.persistence.*;
import java.util.List;

@Entity
public class Library {  
    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    private int b_id;
    private String b_name;

    @ManyToMany(targetEntity=Student.class)
    @JoinTable(name = "library_student",joinColumns = @JoinColumn(name = "library_id"),inverseJoinColumns = @JoinColumn(name = "student_id"))
    private List stud;

    public Library(int b_id, String b_name, List stud) {
        super();
        this.b_id = b_id;
        this.b_name = b_name;
        this.stud = stud;
    }

    public Library() {
        super();
    }

    public int getB_id() {
        return b_id;
    }

    public void ListB_id(int b_id) {
        this.b_id = b_id;
    }

    public String getB_name() {
        return b_name;
    }

    public void ListB_name(String b_name) {
        this.b_name = b_name;
    }

    public List getStud() {
        return stud;
    }

    public void ListStud(List stud) {
        this.stud = stud;
    }
}  

persistence.xml

在resources目录下创建META-INF目录，建立persistence.xml，内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
             xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="books_issued">
        <class>com.yiidian.pojo.Student</class>
        <class>com.yiidian.pojo.Library</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
                      value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                      value="jdbc:mysql://localhost:3306/jpa?serverTimezone=UTC&amp;useSSL=false" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="root" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

ManyToManyExample测试类

package com.yiidian.test;

import com.yiidian.pojo.Library;
import com.yiidian.pojo.Student;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.util.ArrayList;

public class ManyToManyExample {
      
    public static void main(String[] args) {  
          
        EntityManagerFactory emf= Persistence.createEntityManagerFactory("books_issued");
        EntityManager em=emf.createEntityManager();
          
        em.getTransaction().begin();  
          
        Student st1=new Student(1,"Jack",null);
        Student st2=new Student(2,"Rose",null);
          
        em.persist(st1);  
        em.persist(st2);
          
        ArrayList<Student> al1=new ArrayList<Student>();
        ArrayList<Student> al2=new ArrayList<Student>();
          
        al1.add(st1);  
        al1.add(st2);  
          
        al2.add(st1);  
        al2.add(st2);  
          
        Library lib1=new Library(101,"数据结构",al1);
        Library lib2=new Library(102,"MySQL入门",al2);

        em.persist(lib1);  
        em.persist(lib2);  
          
        em.getTransaction().commit();  
        em.close();  
        emf.close();
    }  
  
}  

程序执行后，在MySQL下生成了三个表。

• student表 ： 此表包含学生详细信息。要获取数据，请在 MySQL 中运行select * from student查询

 

• library表 ： 此表包含图书馆的详细信息。要获取数据，请在 MySQL 中运行select * from library查询

 

• library_student ： 此表包含图书馆的详细信息。要获取数据，请在 MySQL 中运行select * from library_student查询