JPA多对多映射

多对多映射表示集合值关联，其中任何数量的实体可以与其他实体的集合关联。在关系数据库中，一个实体的任何行可以被引用到另一个实体的任意数量的行。

完整的项目目录结构如下所示 -

@ManyToMany 示例

在这个例子中，我们将创建学生和图书馆之间的多对多关系，以便可以为任何数量的学生发放任何类型的书籍。

这个例子包含以下步骤 -

第1步: 在com.yiibai.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)，其中包含一个使用@@ManyToMany注解的List类型的Library类对象。

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; @ManyToMany(targetEntity = Library.class) 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(); // TODO Auto-generated constructor stub } 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; } }

第2步: 在包含book id(b_id)，book name(b_name)和@ManyToMany注释的com.yiibai.mapping包下创建另一个实体类Library.java，其中包含List类型的Student类对象。

文件:Library.java 的代码如下所示 -

package com.yiibai.mapping;

import java.util.List; import javax.persistence.*; @Entity public class Library { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int b_id; private String b_name; @ManyToMany(targetEntity = Student.class) 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(); // TODO Auto-generated constructor stub } 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; } }

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

文件:ManyToManyExample.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 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, "Maxsu", null); Student st2 = new Student(2, "Xiaonew", 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, "Data Structure", al1); Library lib2 = new Library(102, "DBMS", al2); em.persist(lib1); em.persist(lib2); 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  |
+------+---------+
|    2 | Xiaonew |
|    1 | Maxsu   |
+------+---------+
2 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         |
+------+----------------+
|  102 | DBMS           |
|  101 | Data Structure |
+------+----------------+
2 rows in set

Library_student表 - 此表包含库的详细信息。要获取数据，请在MySQL中运行select * from library_student查询。

mysql> select * from library_student;
+--------------+-----------+
| Library_B_ID | stud_S_ID |
+--------------+-----------+
|          101 |         1 |
|          101 |         2 |
|          102 |         1 |
|          102 |         2 |
+--------------+-----------+
4 rows in set

@ManyToMany 示例

程序输出结果

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