JPA集映射

Set是包含唯一元素的接口。这些元素不维持任何顺序。当需要以无序方式检索唯一元素时, 可以使用Set。

设置映射示例

在此示例中, 我们将对象嵌入实体类中, 并将其定义为集合类型List。

private Set<Address> address=new HashSet<Address>();

本示例包含以下步骤：-

• 在com.srcmini.jpa包下创建一个实体类Employee.java, 其中包含员工ID, 姓名和嵌入式对象（员工地址）。注释@ElementCollection表示嵌入式对象。

Employee.java

package com.srcmini.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 Set<Address> address=new HashSet<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 Set<Address> getAddress() {
		return address;
	}

	public void setAddress(Set<Address> address) {
		this.address = address;
	}

	
}

• 现在, 在com.srcmini.jpa包下创建一个嵌入式对象Address.java类。批注@Embeddable表示可嵌入对象。

Address.java

package com.srcmini.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, 在Persistence.xml文件中映射实体类和其他数据库配置。

Persistence.xml

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



<properties>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost: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.srcmini.collection包下创建一个持久性类ListMapping.java, 以将实体对象与数据持久化。

SetMapping.java

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


import com.srcmini.jpa.*;
public class SetMapping{

	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.getAddress().add(a1);
	e2.getAddress().add(a2);
	e3.getAddress().add(a3);
	e4.getAddress().add(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工作台下生成以下表格。

• 员工表-此表包含员工详细信息。要获取数据, 请在MySQL中从员工查询中运行select *。

• Employee_address表-此表表示雇员和地址表之间的映射。表中的数据是无序排列的。要获取数据, 请在MySQL中运行employee * address查询中的select *。