hibernate3學習筆記(十七)｜關系映射：多對一

數據表設計如下圖：

DDL文件：

1.CREATE TABLE user (
2.    id INT(11) NOT NULL auto_increment PRIMARY KEY,
3.    name VARCHAR(100) NOT NULL default '',
4.    room_id INT(11)
5.);
6.
7.CREATE TABLE room (
8.    id INT(11) NOT NULL auto_increment PRIMARY KEY,
9.    address VARCHAR(100) NOT NULL default ''
10.);

Room.java

1.package com.hb3.pack_16.model;
2.
3.public class Room {
4.
5.    private Integer id;
6.    private String address;
7.
8.    public Room() {
9.    }
10.
11.    public Integer getId() { 
12.        return id;
13.    } 
14.    public void setId(Integer id) { 
15.        this.id = id;
16.    }
17.    public String getAddress() { 
18.        return address;
19.    }
20.    public void setAddress(String address) { 
21.        this.address = address;
22.    }
23.}

User.java

1.package com.hb3.pack_16.model;
2.
3.public class User {
4.
5.    private Integer id;
6.    private String name;
7.    private Room room;
8.
9.    public User() {
10.    }
11.
12.    public Integer getId() {
13.        return id;
14.    }
15.    public void setId(Integer id) {
16.        this.id = id;
17.    }
18.    public String getName() {
19.        return name;
20.    }
21.    public void setName(String name) {
22.        this.name = name;
23.    }
24.    public Room getRoom() {
25.        return room;
26.    }
27.    public void setRoom(Room room) {
28.        this.room = room;
29.    }
30.}

Room.hbm.xml：

1.<?xml version="1.0" encoding="utf-8"?>
2.<!DOCTYPE hibernate-mapping 
3.    PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
4.    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">5.6.<hibernate-mapping>
7.
8.    <class name="com.hb3.pack_16.model.Room" table="room">
9.
10.        <id name="id" column="id">
11.            <generator class="native"/>
12.        </id>
13.
14.        <property name="address"
15.                  column="address"
16.                  type="java.lang.String"/>
17.    </class>
18.
19.</hibernate-mapping>

User.hbm.xml：

1.<?xml version="1.0" encoding="utf-8"?>
2.<!DOCTYPE hibernate-mapping 
3.    PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
4.    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
5.
6.<hibernate-mapping>
7.
8.    <class name="com.hb3.pack_16.model.User" table="user">
9.
10.        <id name="id" column="id" type="java.lang.Integer">
11.            <generator class="native"/>
12.        </id>
13.
14.        <property name="name" column="name" type="java.lang.String"/>
15.
16.        <many-to-one name="room"
17.                     column="room_id"
18.                     class="com.hb3.pack_16.model.Room"
19.                     cascade="all"
20.                     outer-join="true"/>
21.    </class>
22.
23.</hibernate-mapping>

測試代碼：

1.package com.hb3.pack_16;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.
6.import org.hibernate.Session;
7.import org.hibernate.SessionFactory;
8.import org.hibernate.Transaction;
9.import org.hibernate.cfg.Configuration;
10.
11.import com.hb3.pack_16.model.Room;
12.import com.hb3.pack_16.model.User;
13.
14.
15.public class BusinessService {
16.
17.    public static void main(String[] args) throws IOException, SQLException {
18.
19.        Configuration config = new Configuration().configure();
20.        SessionFactory sessionFactory = config.buildSessionFactory();
21.        Session session = sessionFactory.openSession();
22.
23.        Room room1 = new Room();
24.        room1.setAddress("NTU-M8-419");
25.        Room room2 = new Room();
26.        room2.setAddress("NTU-G3-302");
27.
28.        User user1 = new User();
29.        user1.setName("shenbin");
30.        user1.setRoom(room1);
31.
32.        User user2 = new User();
33.        user2.setName("chenyan");
34.        user2.setRoom(room1);
35.
36.        User user3 = new User();
37.        user3.setName("yangye");
38.        user3.setRoom(room2);
39.
40.        Transaction tx = session.beginTransaction();
41.        session.save(user1);
42.        session.save(user2);
43.        session.save(user3);
44.        tx.commit();
45.
46.        User user = (User) session.load(User.class, new Integer(1));
47.        System.out.println(user.getName());
48.        System.out.println(user.getRoom().getAddress());
49.
50.
51.        session.close();
52.        sessionFactory.close();
53.    }
54.}

在<many-to-one>的設定中：

cascade表示操作時的級聯關系。表示主控方在進行增刪改操作時，被控方是否進行相關操作。

outer-join表示查詢時是否使用外連接查詢方式。

outer-join為true的時候，生成的查詢SQL為：

Hibernate:
select user0_.id as id1_, user0_.name as name0_1_, user0_.room_id as room3_0_1_,
room1_.id as id0_, room1_.address as address1_0_ from user user0_
left outer join room room1_ on user0_.room_id=room1_.id where user0_.id=?
outer-join為false的時候，生成的查詢SQL為：Hibernate:
select user0_.id as id0_, user0_.name as name0_0_, user0_.room_id as room3_0_0_
from user user0_ where user0_.id=?
Hibernate:
select room0_.id as id0_, room0_.address as address1_0_
from room room0_ where room0_.id=?

不過最好使用fetch屬性來表示，及fetch="join"或者fetch="select"。