多对一映射表示实体集合可以与相似实体关联的单值关联。 因此，在关系数据库中，实体的多于一行可以引用另一个实体的相似行。

完整的项目目录结构如下所示 -

@ManyToOne 示例

在这个例子中，我们将创建一个学生和图书库之间的多对一关系，多个学生可以发布同一本书。

这个例子包含以下步骤 -

第1步: 在com.zyiz.mapping包中创建一个实体类Student.java，包含学生ID(s_id)和学生姓名(s_name)，其中包含一个包含库类型对象的@ManyToOne注解。

文件:Student.java 的代码如下所示 -

package com.zyiz.mapping;

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;
    }

}

第2步: 在com.zyiz.mapping包下创建另一个实体类Library.java，它包含book id(b_id)，book name(b_name)。

文件:Library.java 的代码如下所示 -

package com.zyiz.mapping;

import javax.persistence.*;

@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();
        // TODO Auto-generated constructor stub
    }

    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;
    }

}

第3步: 将实体类和其他数据库配置映射到persistence.xml文件中。

文件:persistence.xml 的代码如下所示 -

<?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.zyiz.mapping.Student</class>
        <class>com.zyiz.mapping.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/testdb?serverTimezone=UTC" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password"
                value="123456" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation"
                value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

在com.zyiz.mapping包下创建一个持久化类ManyToOneExample，用于将实体对象与数据保持一致。

文件:ManyToOneExample.java 的代码如下所示 -

package com.zyiz.mapping;

import javax.persistence.*;

import javax.persistence.EntityManagerFactory;

import com.zyiz.mapping.Library;

public class ManyToOneExample {

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("books_issued");

        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();

        Library lib = new Library();
        lib.setB_id(101);
        lib.setB_name("Data Structure");

        em.persist(lib);

        Student st1 = new Student();
        st1.setS_id(1);
        st1.setS_name("Maxsu");
        st1.setLib(lib);

        Student st2 = new Student();
        st2.setS_id(2);
        st2.setS_name("Curry");
        st2.setLib(lib);

        em.persist(st1);
        ;
        em.persist(st2);

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

程序输出结果

程序执行完成后，下面的表格在MySQL工作台下生成。

• 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。结果如下所示 -

mysql> select * from student;
+------+--------+----------+
| S_ID | S_NAME | LIB_B_ID |
+------+--------+----------+
|    2 | Curry  |      101 |
|    1 | Maxsu  |      101 |
+------+--------+----------+
rows in set

• Library表 - 这个表格代表学生和图书库之间的映射。 要获取数据，请在MySQL中运行select * from library查询。结果如下所示 -

mysql> select * from library;
+------+----------------+
| B_ID | B_NAME         |
+------+----------------+
|  101 | Data Structure |
+------+----------------+
row in set