JPA 集合（Set）映射
集合(Set)是一個包含唯一元素的接口。 這些元素不保留任何順序。當需要以無序方式檢索唯一元素時，可以使用Set。
完整的項目目錄結構如下所示 -
集合映射實例
在這個例子中，我們將一個對象嵌入到一個實體類中，並將其定義爲一個集合類型 - Set。
private Set<Address> address=new HashSet<Address>();這個例子包含以下步驟 -
第1步: 在com.yiibai.jpa包下創建一個實體類Employee.java，這個類包含員工id，name 和嵌入對象(員工地址)。註解 [@ElementCollection](https://github.com/ElementCollection "@ElementCollection") 表示嵌入對象。
文件:Employee.java 的代碼如下所示 -
package com.yiibai.jpa;
import java.util.*;
import javax.persistence.*;
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;
}
}第2步: 在com.yiibai.jpa包下創建一個嵌入對象Address.java類。 註解[@Embeddable](https://github.com/Embeddable "@Embeddable")表示可嵌入對象。
文件:Address.java 的代碼如下所示 -
package com.yiibai.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;
}
}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="Collection_Type">
<class>com.yiibai.jpa.Employee</class>
<class>com.yiibai.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/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.collection包下創建一個持久化類SetMapping.java，以便將實體對象與數據保持一致。
文件:SetMapping.java 的代碼如下所示 -
package com.yiibai.collection;
import javax.persistence.*;
import com.yiibai.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(501000);
a1.setE_city("Guangzhou");
a1.setE_state("GuangDong");
Address a2 = new Address();
a2.setE_pincode(571100);
a2.setE_city("Haikou");
a2.setE_state("Hainan");
Address a3 = new Address();
a3.setE_pincode(533300);
a3.setE_city("Hangzhou");
a3.setE_state("Zhejiang");
Address a4 = new Address();
a4.setE_pincode(780000);
a4.setE_city("Nanjing");
a4.setE_state("Jiangsu");
Employee e1 = new Employee();
e1.setE_id(1000);
e1.setE_name("Maxsu");
Employee e2 = new Employee();
e2.setE_id(1002);
e2.setE_name("Marry");
Employee e3 = new Employee();
e3.setE_id(1003);
e3.setE_name("William");
Employee e4 = new Employee();
e4.setE_id(1004);
e4.setE_name("Curry");
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();
}
}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。
Employee表 - 此表包含員工詳細信息。要獲取數據，請在MySQL中運行
select * from employee查詢。結果如下所示 -mysql> select * from employee; +------+---------+ | E_ID | E_NAME | +------+---------+ | 1003 | William | | 1004 | Curry | | 1000 | Maxsu | | 1002 | Marry | +------+---------+ 4 rows in setEmployee_address表 - 此表表示員工和地址表之間的映射。 要獲取數據，請在MySQL中運行
select * from employee_address查詢。結果如下所示 -mysql> select * from employee_address; +-----------+-----------+-----------+---------------+ | E_CITY | E_PINCODE | E_STATE | Employee_E_ID | +-----------+-----------+-----------+---------------+ | Hangzhou | 533300 | Zhejiang | 1003 | | Nanjing | 780000 | Jiangsu | 1004 | | Guangzhou | 501000 | GuangDong | 1000 | | Haikou | 571100 | Hainan | 1002 | +-----------+-----------+-----------+---------------+ 4 rows in set