JPA 映射

2024 年 8 月 29 日 | 阅读 2 分钟

映射是一个接口，其中唯一的键与每个值对象相关联。因此，诸如搜索、更新、删除之类的操作是基于键执行的。

映射示例

在此示例中，我们将对象嵌入到实体类中，并将其定义为集合类型映射。

此示例包含以下步骤： -

在 com.javatpoint.jpa 包下创建一个实体类 Employee.java，其中包含员工 ID、姓名和嵌入对象（员工地址）。注解 @ElementCollection 代表嵌入对象。

Employee.java

package com.javatpoint.jpa;
import java.util.*;

import javax.persistence.*;
@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;
	}

	
	
}

现在，在 com.javatpoint.jpa 包下创建一个嵌入对象 Address.java 类。注解 @Embeddable 代表可嵌入对象。

Address.java

package com.javatpoint.jpa;
import javax.persistence.*;

@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;
	}
	
	
	
}

现在，将实体类和其他数据库配置映射到 Persistence.xml 文件中。

Persistence.xml

<persistence>
<persistence-unit name="Collection_Type">
	
	 <class>com.javatpoint.jpa.Employee</class>
      <class>com.javatpoint.jpa.Address</class>
    

      
     <properties>
      <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name="javax.persistence.jdbc.url" value="jdbc:mysql://:3306/collection_mapping"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value=""/>
         <property name="eclipselink.logging.level" value="SEVERE"/>
         <property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
      </properties>
	
	</persistence-unit>
</persistence>

在 com.javatpoint.collection 包下创建一个持久化类 MapMapping.java 以持久化带有数据的实体对象。

MapMapping.java

package com.javatpoint.collection;
import javax.persistence.*;


import com.javatpoint.jpa.*;
public class ListMapping{

	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("Noida");
		a1.setE_state("Uttar Pradesh");
		
		
		
		
		
		Address a2=new Address();
		a2.setE_pincode(302001);
		a2.setE_city("Jaipur");
		a2.setE_state("Rajasthan");
		
		Address a3=new Address();
		a3.setE_pincode(133301);
		a3.setE_city("Chandigarh");
		a3.setE_state("Punjab");
	
		Address a4=new Address();
		a4.setE_pincode(80001);
		a4.setE_city("Patna");
		a4.setE_state("Bihar");
		
		
	Employee e1=new Employee();
	e1.setE_id(1);
	e1.setE_name("Vijay");
	
	
	Employee e2=new Employee();
	e2.setE_id(2);
	e2.setE_name("Vijay");
	
	Employee e3=new Employee();
	e3.setE_id(3);
	e3.setE_name("William");
	
	Employee e4=new Employee();
	e4.setE_id(4);
	e4.setE_name("Rahul");
	
	e1.getMap().put(1, a1);
	e2.getMap().put(2, a2);
	e3.getMap().put(3, a3);
	e4.getMap().put(4, a4);
	
	em.persist(e1);
	em.persist(e2);
	em.persist(e3);
	em.persist(e4);
	
	em.getTransaction().commit();
	
	em.close();
	emf.close();
		
		
		
	}
	
}

输出

程序执行后，将在 MySQL workbench 下生成以下表。