JPA一对多映射

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

@OneToMany 示例

程序输出结果

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