JPA一对多映射

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2023-03-14

一对多映射进入集合值关联的类别，其中实体与其他实体的集合关联。因此，在这种类型的关联中，一个实体的实例可以映射到另一个实体的任意数量的实例。

完整的项目目录结构如下所示 -

@OneToMany 示例

在这个例子中，我们将创建一个学生和图书库之间的一对多系，这样一个学生能发放多种类型的图书。

这个例子包含以下步骤 -

第1步: 在com.javatpoint.mapping包下创建一个实体类Student.java，它包含学生ID(s_id)，学生姓名(s_name)，其中包含包含List类型的Library类对象的@OneToMany注释。

文件:Student.java 的代码如下所示 -

package com.yiibai.mapping;

import java.util.List;

import javax.persistence.*;

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int s_id;
    private String s_name;

    @OneToMany(targetEntity = Library.class)
    private List books_issued;

    public List getBooks_issued() {
        return books_issued;
    }

    public void setBooks_issued(List books_issued) {
        this.books_issued = books_issued;
    }

    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;
    }

}

第2步: 在com.yiibai.mapping包下创建另一个实体类Library.java，其中包含book id(b_id)，book name(b_name)以及标记有@OneToMany注释的学生类型对象。

文件:Library.java 的代码如下所示 -

package com.yiibai.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.yiibai.mapping.Student</class>
        <class>com.yiibai.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.yiibai.onetoone包下创建一个持久化类OneToManyExample，用于将实体对象与数据保持一致。

文件:OneToManyExample.java 的代码如下所示 -

package com.yiibai.mapping;

import java.util.ArrayList;

import javax.persistence.*;

import com.yiibai.mapping.Student;
import com.yiibai.mapping.Library;

public class OneToManyExample {

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("books_issued");
        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();

        Library lib1 = new Library();
        lib1.setB_id(101);
        lib1.setB_name("Data Structure");

        Library lib2 = new Library();
        lib2.setB_id(102);
        lib2.setB_name("DBMS");

        em.persist(lib1);
        em.persist(lib2);

        ArrayList<Library> list = new ArrayList<Library>();
        list.add(lib1);
        list.add(lib2);

        Student st1 = new Student();
        st1.setS_id(1);
        st1.setS_name("Maxsu");
        st1.setBooks_issued(list);

        em.persist(st1);

        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 |
+------+--------+
|    1 | Maxsu  |
+------+--------+
1 row 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         |
+------+----------------+
|  102 | DBMS           |
|  101 | Data Structure |
+------+----------------+
2 rows in set

Student_library表 - 该表表示学生和图书库表之间的映射。要获取数据，请在MySQL中运行select * from student_library查询。

mysql> select * from student_library;
+--------------+-------------------+
| Student_S_ID | books_issued_B_ID |
+--------------+-------------------+
|            1 |               101 |
|            1 |               102 |
+--------------+-------------------+
2 rows in set