JPA Map映射

Map 是一个接口，其中唯一的键与每个值对象相关联。这样，查询、更新、删除等操作都是在key的基础上进行的。

JPA Map映射示例

项目结构如下：

在这个例子中，我们将一个对象嵌入到一个实体类中，并将其定义为集合类型 Map。

private  Map<Integer,Address> map= new  HashMap<Integer,Address>();  

pom依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.yiidian</groupId>
    <artifactId>JPAProject</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.eclipse.persistence</groupId>
            <artifactId>javax.persistence</artifactId>
            <version>2.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.persistence</groupId>
            <artifactId>eclipselink</artifactId>
            <version>2.5.0</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
    </dependencies>
</project>

Address实体类

package com.yiidian.pojo;

import javax.persistence.Embeddable;

@Embeddable
public class Address {
    private int e_pincode;  
    private String e_city;  
    private String e_state;

    public int getE_pincode() {  
        return e_pincode;  
    }  
    public void setE_pincode(int e_pincode) {  
        this.e_pincode = e_pincode;  
    }  
    public String getE_city() {  
        return e_city;  
    }  
    public void setE_city(String e_city) {  
        this.e_city = e_city;  
    }  
    public String getE_state() {  
        return e_state;  
    }  
    public void setE_state(String e_state) {  
        this.e_state = e_state;  
    }
}  

Employee实体类

package com.yiidian.pojo;

import javax.persistence.*;
import java.util.HashMap;
import java.util.Map;

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int e_id;
    private String e_name;

    @ElementCollection
    private Map<Integer,Address> map=new HashMap<Integer,Address>();


    public int getE_id() {
        return e_id;
    }

    public void setE_id(int e_id) {
        this.e_id = e_id;
    }

    public String getE_name() {
        return e_name;
    }

    public void setE_name(String e_name) {
        this.e_name = e_name;
    }

    public Map<Integer, Address> getMap() {
        return map;
    }

    public void setMap(Map<Integer, Address> map) {
        this.map = map;
    }
    
} 

persistence.xml

在resources目录下创建META-INF目录，建立persistence.xml，内容如下：

<?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="Collection_Type">
        <class>com.yiidian.pojo.Address</class>
        <class>com.yiidian.pojo.Employee</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
                      value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                      value="jdbc:mysql://localhost:3306/jpa?serverTimezone=UTC&amp;useSSL=false" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="root" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

MapMapping测试类

package com.yiidian.test;

import com.yiidian.pojo.Address;
import com.yiidian.pojo.Employee;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class MapMapping {
  
    public static void main(String[] args) {
        EntityManagerFactory emf= Persistence.createEntityManagerFactory("Collection_Type");
        EntityManager em=emf.createEntityManager();
          
        em.getTransaction().begin();
        Address a1=new Address();
        a1.setE_pincode(201301);  
        a1.setE_city("广州");
        a1.setE_state("天河区");

        Address a2=new Address();  
        a2.setE_pincode(302001);  
        a2.setE_city("广州");
        a2.setE_state("番禺区");

        Employee e1=new Employee();
        e1.setE_id(1);
        e1.setE_name("Jack");
        e1.getMap().put(1001,a1);

        Employee e2=new Employee();
        e2.setE_id(2);
        e2.setE_name("Rose");
        e1.getMap().put(1002,a2);

        em.persist(e1);
        em.persist(e2);

        em.getTransaction().commit();

        em.close();
        emf.close();
    }  
      
}  

程序执行后，在MySQL下生成下表。

• employee表 ：此表包含员工详细信息。要获取数据，请在 MySQL 中运行 select * from employee 查询。

 

• employee_map表 ： 该表表示员工和地址表之间的映射。要获取数据，请在 MySQL 中运行 seelect * from employee_map 查询。