JPA多對多映射

多對多映射表示集合值關聯，其中任何數量的實體可以與其他實體的集合關聯。 在關係數據庫中，一個實體的任何行可以被引用到另一個實體的任意數量的行。

完整的項目目錄結構如下所示 -

@ManyToMany 示例

在這個例子中，我們將創建學生和圖書館之間的多對多關係，以便可以爲任何數量的學生髮放任何類型的書籍。

這個例子包含以下步驟 -

第1步: 在com.yiibai.mapping包中創建一個實體類Student.java，包含學生ID(s_id)和學生姓名(s_name)，其中包含一個使用@[@ManyToMany](https://github.com/ManyToMany "@ManyToMany")註解的List類型的Library類對象。

文件:Student.java 的代碼如下所示 -

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](https://github.com/ManyToMany "@ManyToMany")註釋的com.yiibai.mapping包下創建另一個實體類Library.java，其中包含List類型的Student類對象。

文件:Library.java 的代碼如下所示 -

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: 將實體類和其他數據庫配置映射到persistence.xml文件中。

文件:persistence.xml 的代碼如下所示 -

<?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包下創建一個持久化類ManyToManyExample，用於將實體對象與數據保持一致。

文件:ManyToManyExample.java 的代碼如下所示 -

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();

    }

}

程序輸出結果

程序執行完成後，下面的表格在MySQL工作臺下生成。

• Student表 - 此表包含學生詳細信息。要獲取數據，請在MySQL中運行select * from student查詢。結果如下所示 -

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

• Library表 - 這個表格代表學生和圖書庫之間的映射。 要獲取數據，請在MySQL中運行select * from library查詢。結果如下所示 -

mysql> select * from library;
+------+----------------+
| B_ID | B_NAME         |
+------+----------------+
|  102 | DBMS           |
|  101 | Data Structure |
+------+----------------+
2 rows in set

• Library_student表 - 此表包含庫的詳細信息。要獲取數據，請在MySQL中運行select * from library_student查詢。

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