集合(Set)是一个包含唯一元素的接口。 这些元素不保留任何顺序。当需要以无序方式检索唯一元素时，可以使用Set。

完整的项目目录结构如下所示 -

集合映射实例

在这个例子中，我们将一个对象嵌入到一个实体类中，并将其定义为一个集合类型 - Set。

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

这个例子包含以下步骤 -

第1步: 在com.zyiz.jpa包下创建一个实体类Employee.java，这个类包含员工id，name 和嵌入对象(员工地址)。注解 @ElementCollection 表示嵌入对象。

文件:Employee.java 的代码如下所示 -

package com.zyiz.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.zyiz.jpa包下创建一个嵌入对象Address.java类。 注解@Embeddable表示可嵌入对象。

文件:Address.java 的代码如下所示 -

package com.zyiz.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: 将实体类和其他数据库配置映射到persistence.xml文件中。

文件:persistence.xml 的代码如下所示 -

<?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.zyiz.jpa.Employee</class>
        <class>com.zyiz.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.zyiz.collection包下创建一个持久化类SetMapping.java，以便将实体对象与数据保持一致。

文件:SetMapping.java 的代码如下所示 -

package com.zyiz.collection;

import javax.persistence.*;

import com.zyiz.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();

    }

}

程序输出结果

程序执行完成后，下面的表格在MySQL工作台下生成。

• 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。结果如下所示 -

  mysql> select * from employee;
  +------+---------+
  | E_ID | E_NAME  |
  +------+---------+
  | 1003 | William |
  | 1004 | Curry   |
  | 1000 | Maxsu   |
  | 1002 | Marry   |
  +------+---------+
  rows in set
  

• Employee_address表 - 此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select * from employee_address查询。结果如下所示 -

  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 |
  +-----------+-----------+-----------+---------------+
  rows in set