Map

Hibernate Gossip: Map

關於Map的特性，請參考 HashMap、TreeMap 這兩篇文件。

Map的特性是key/value對，容器中的每一個物件都有一個key與之對應，所以將Map集合的資料儲存至資料庫時，必須一同儲存它的key資訊。

假設您設計了以下的類別：

User.java

package onlyfun.caterpillar;

import java.util.Map;

public class User {
    private Long id;
    private String name;
    private Map items;

    // 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 Map getItems() {
        return items;
    }

    public void setItems(Map items) {
        this.items = items;
    }

    public void addItem(String name, String description) {
        items.put(name, description);
    }
 
    public void removeItem(String name) {
        items.remove(name);
    }
}

您可以建立以下的表格來分別儲存user資訊與當中的items資訊：

create table item (
    id bigint not null,
    description varchar(255),
    name varchar(255) not null,
    primary key (id, name)
)

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

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

在映射文件方面，使用<map>標籤來定義Map的映射，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"/> 
 
        <map name="items" table="item"> 
            <key column="id" foreign-key="id"/> 
            <map-key column="name" type="string"/> 
            <element column="description" type="string"/> 
        </map> 
 
    </class> 

</hibernate-mapping>

假設您使用以下的程式片段來儲存User實例：

User user1 = new User();
user1.setItems(new HashMap());
user1.setName("caterpillar");
user1.addItem("Book", "Java Gossip");
user1.addItem("DC", "Caxxx A80");

User user2 = new User();
user2.setItems(new HashMap());
user2.setName("momor");
user2.addItem("Doll", "Snoppy world");

Session session = HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
session.save(user2);
tx.commit();
session.close();

則資料庫中將儲存以下的資訊：

mysql> select * from user;
+----+--------------+
| id    | name        |
+----+--------------+
| 1    | caterpillar |
| 2 | momor       |
+----+--------------+
2 rows in set (0.00 sec)

mysql> select * from item;
+----+--------+-------------------+
| id    | name | description |
+----+--------+-------------------+
| 1    | DC    | Caxxx A80    |
| 1 | Book | Java Gossip |
| 2 | Doll | Snoppy world |
+----+-------+-------------------+
3 rows in set (0.00 sec)

From Gossip@caterpillar

Hibernate Gossip: Map