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Hibernate Gossip: 内含 Component 的容器

假设您建立了以下的表格：

CREATE TABLE user (
    id INT(11) NOT NULL auto_increment PRIMARY KEY,
    name VARCHAR(100) NOT NULL default ''
);

CREATE TABLE email (
    id INT(11) NOT NULL,
    address VARCHAR(100) NOT NULL
);

一个user可以有多个email，但不可重复，这可以使用 Set 来映像，在对应的对象方法，您可以如下设计对象：

package onlyfun.caterpillar;

import java.util.Set;

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

假设您原先预定在Set中储存的是String型态，而后设计时考虑独立设计一个MailAddress类别，而Set中将储存MailAddress的实例，例如：

User.java

package onlyfun.caterpillar;

import java.util.Set;

public class User {
    private Integer 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 Integer getId() {
        return id;
    }

    public void setId(Integer 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>来为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" type="java.lang.Integer"> 
            <generator class="native"/> 
        </id> 

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

您可以如下储存对象：

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 = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
session.save(user);
tx.commit();
session.close();

则数据库中会储存如下的数据：

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)

在查询时，address表格的数据会封装为MailAddress的实例，一个范例如下：

Session session = sessionFactory.openSession();

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