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
