內含 Component 的集合

假設您建立了以下的表格：

create table email (
    id bigint not null,
    address varchar(255)
)

create table user (
    id bigint not null auto_increment,
    name varchar(255),
    primary key (id)
)

alter table email
    add index id (id),
    add constraint id
    foreign key (id)
    references user (id)

一個user可以有多個email，但不可重複，這可以使用 Set 來映射，在對應的物件方法，您可以如下設計物件：

package onlyfun.caterpillar;

import java.util.Set;

public class User {
private Long id;
private Set emails;
....
}

假設您原先預定在Set中儲存的是String型態，而後設計時考慮獨立設計一個MailAddress類別，而Set中將儲存MailAddress的實例，例如：

User.java

package onlyfun.caterpillar;

import java.util.Set;

public class User {
    private Long id;
    private String name;
    private Set emails; 

    // 必須要有一個預設的建構方法
    // 以使得Hibernate可以使用Constructor.newInstance()建立物件
    public User() {}

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set getEmails() {
        return emails;
    }

    public void setEmails(Set emails) {
        this.emails = emails;
    }
 
    public void addEmail(MailAddress mailAddress) {
        this.emails.add(mailAddress);
    }
 
    public void removeEmail(MailAddress mailAddress) {
        this.emails.remove(mailAddress);
    }
}

MailAddress.java

package onlyfun.caterpillar;

public class MailAddress {
    private String address;

    public MailAddress() {}
 
    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public void sendMail() {
        System.out.println("Send mail to " + address);
    }
}

在映射文件方面，可以使用<composite-element>來為MailAddress作映射，如下：

User.hbm.xml

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-mapping 
 PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping> 
    <class name="onlyfun.caterpillar.User" table="user"> 
        <id name="id" column="id"> 
            <generator class="native"/> 
        </id> 

        <property name="name" column="name"/> 
 
        <set name="emails" table="email"> 
            <key column="id" foreign-key="id"/> 
            <composite-element class="onlyfun.caterpillar.MailAddress"> 
                <property name="address" column="address"/> 
            </composite-element> 
        </set> 
    </class> 
</hibernate-mapping>

您可以如下儲存物件：

User user = new User();
user.setName("caterpillar");

user.setEmails(new HashSet());
MailAddress mailAddress = new MailAddress();
mailAddress.setAddress("caterpillar.onlyfun@gmail.com");
user.addEmail(mailAddress);

mailAddress = new MailAddress();
mailAddress.setAddress("caterpillar.onlyfun@yahoo.com");
user.addEmail(mailAddress);

Session session = HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
session.save(user);
tx.commit();
session.close();

則資料庫中會儲存如下的資料：

mysql> select * from user;
+----+-------------+
| id    | name        |
+----+-------------+
| 1    | caterpillar |
+----+-------------+
1 row in set (0.00 sec)

mysql> select * from email;
+----+-------------------------------------------+
| id    | address                      |
+----+-------------------------------------------+
| 1    | caterpillar.onlyfun@yahoo.com |
| 1    | caterpillar.onlyfun@gmail.com    |
+----+-------------------------------------------+
2 rows in set (0.00 sec)

在查詢時，address表格的資料會封裝為MailAddress的實例，一個範例如下：

Session session = HibernateUtil.getSessionFactory().openSession();

User user = (User) session.load(User.class, new Long(1));
Iterator iterator = user.getEmails().iterator();
while(iterator.hasNext()) {
    MailAddress mailAddress = (MailAddress) iterator.next();
    mailAddress.sendMail();
}
session.close();

如果要實現雙向關係的話，也可以在<composite-element>中加上<parent>標籤，作法類似Component 中的作法，例如：
...
        <set name="emails" table="email">
            <key column="id" foreign-key="user_id"/>
            <composite-element class="onlyfun.caterpillar.MailAddress">
<parent name="user"/>
                <property name="address" column="address"/>
            </composite-element>
        </set>
...

From Gossip@caterpillar

Hibernate Gossip: 內含 Component 的集合