JPA 多对一映射

多对一映射表示单值关联，其中实体集合可以与相似实体相关联。因此，在关系数据库中，一个实体的任何一行都可以引用另一个实体的相似行。

JPA @ManyToOne多对一映射示例

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一关系，以便多个学生可以发行同一本书。

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.*;

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    private int s_id;  
    private String s_name;  

    @ManyToOne
    private Library lib;  
      
    public int getS_id() {  
        return s_id;  
    }  
    public void setS_id(int s_id) {  
        this.s_id = s_id;  
    }  
    public String getS_name() {  
        return s_name;  
    }  
    public void setS_name(String s_name) {  
        this.s_name = s_name;  
    }  
    public Library getLib() {  
        return lib;  
    }  
    public void setLib(Library lib) {  
        this.lib = lib;  
    }  

}

Library实体类

package com.yiidian.pojo;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

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

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

    }
    public Library() {
        super();
    }
    public int getB_id() {
        return b_id;
    }

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

    public String getB_name() {
        return b_name;
    }

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

}

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="Book_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>

ManyToOneExample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;

public class ManyToOneExample {
  
    public static void main(String[] args) {  
          
        EntityManagerFactory emf= Persistence.createEntityManagerFactory("Book_issued");
          
        EntityManager em=emf.createEntityManager();
          
        em.getTransaction().begin();  
          
        Library lib=new Library();
        lib.setB_id(101);  
        lib.setB_name("Spring入门");
          
        em.persist(lib);  
          
        Student st1=new Student();
        st1.setS_id(1);  
        st1.setS_name("Jack");
        st1.setLib(lib);
          
        Student st2=new Student();  
        st2.setS_id(2);  
        st2.setS_name("Lucy");  
        st2.setLib(lib);  
          
        em.persist(st1);;  
        em.persist(st2);  

        em.getTransaction().commit();  
        em.close();  
        emf.close();  
          
    }  
      
}

程序执行后，在MySQL下生成了两张表。