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hibernate3學習筆記(十八)｜關系映射：一對多

編輯：關於JAVA

接上例：http://blog.csdn.net/kunshan_shenbin/archive/2008/09/03/2873943.aspx

User對Room是多對一的關系，那麼反過來Room對User就是一對多的關系了。

User.java：

1.package com.hb3.pack_17.model;
2.
3.public class User {
4.
5.    private Integer id;
6.    private String name;
7.
8.    public User() {
9.    }
10.
11.    public Integer getId() {
12.        return id;
13.    }
14.    public void setId(Integer id) {
15.        this.id = id;
16.    }
17.    public String getName() {
18.        return name;
19.    }
20.    public void setName(String name) {
21.        this.name = name;
22.    }
23.}

Room.java：

1.package com.hb3.pack_17.model;
2.
3.import java.util.Set;
4.
5.public class Room {
6.
7.    private Integer id;
8.    private String address;
9.    private Set<User> users;
10.
11.    public Room() {
12.    }
13.
14.    public Integer getId() { 
15.        return id;
16.    } 
17.    public void setId(Integer id) { 
18.        this.id = id;
19.    }
20.    public String getAddress() { 
21.        return address;
22.    }
23.    public void setAddress(String address) { 
24.        this.address = address;
25.    }
26.    public Set<User> getUsers() {
27.        return users;
28.    }
29.    public void setUsers(Set<User> users) {
30.        this.users = users;
31.    }
32.    public void addUser(User user) {
33.        users.add(user);
34.    }
35.    public void removeUser(User user) {
36.        users.remove(user);
37.    }
38.}

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_17.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.
18.        <set name="users" table="user" cascade="all">
19.            <key column="room_id"/>
20.            <one-to-many class="com.hb3.pack_17.model.User"/>
21.        </set>
22.
23.    </class>
24.</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_17.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.    </class>
17.
18.</hibernate-mapping>

測試代碼：

1.package com.hb3.pack_17;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.HashSet;
6.
7.import org.hibernate.Session;
8.import org.hibernate.SessionFactory;
9.import org.hibernate.Transaction;
10.import org.hibernate.cfg.Configuration;
11.
12.import com.hb3.pack_17.model.Room;
13.import com.hb3.pack_17.model.User;
14.
15.
16.public class BusinessService {
17.
18.    public static void main(String[] args) throws IOException, SQLException {
19.
20.        Configuration config = new Configuration().configure();
21.        SessionFactory sessionFactory = config.buildSessionFactory();
22.        Session session = sessionFactory.openSession();
23.
24.
25.        User user1 = new User();
26.        user1.setName("yangye");
27.        User user2 = new User();
28.        user2.setName("shenbin");
29.        User user3 = new User();
30.        user3.setName("chenyan");
31.
32.        Room room1 = new Room();
33.        room1.setUsers(new HashSet<User>());
34.        room1.setAddress("NTU-M8-419");
35.        room1.addUser(user1);
36.        room1.addUser(user2);
37.        Room room2 = new Room();
38.        room2.setUsers(new HashSet<User>());
39.        room2.setAddress("NTU-G3-302");
40.        room2.addUser(user3);
41.
42.        Transaction tx = session.beginTransaction();
43.        session.save(room1);
44.        session.save(room2);
45.        tx.commit();
46.
47.
48.        session.close();
49.        sessionFactory.close();
50.    }
51.}

雙向關聯：

以上多對一，一對多的關系都是單向關聯，也就是一方聯系到另一方，而另一方不知道自己被關聯。

如果上方都意識到另一方的存在，則形成雙向關聯。現將上面的代碼改寫如下：

在User類中追加Room類型的字段：

1.package com.hb3.pack_20.model;
2.
3.public class User {
4.
5.    private Integer id;
6.    private String name;
7.    private Room room;
8.
9.    public Room getRoom() {
10.        return room;
11.    }
12.
13.    public void setRoom(Room room) {
14.        this.room = room;
15.    }
16.
17.    public User() {
18.    }
19.
20.    public Integer getId() {
21.        return id;
22.    }
23.    public void setId(Integer id) {
24.        this.id = id;
25.    }
26.    public String getName() {
27.        return name;
28.    }
29.    public void setName(String name) {
30.        this.name = name;
31.    }
32.}

User.hbm.xml中也同樣追加關於Room的信息：

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_20.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_20.model.Room"
19.                     cascade="save-update"
20.                     outer-join="true"/>
21.
22.    </class>
23.
24.</hibernate-mapping>

這裡我們將Room.hbm.xml文件裡set的cascade屬性也設為save-update：

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_20.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.
18.        <set name="users" table="user" cascade="save-update">
19.            <key column="room_id"/>
20.            <one-to-many class="com.hb3.pack_20.model.User"/>
21.        </set>
22.
23.    </class>
24.</hibernate-mapping>

這樣我們可以用多對一的方式來維持管理：

1.package com.hb3.pack_20;
2.
3.import org.hibernate.Session;
4.import org.hibernate.SessionFactory;
5.import org.hibernate.Transaction;
6.import org.hibernate.cfg.Configuration;
7.
8.import com.hb3.pack_20.model.Room;
9.import com.hb3.pack_20.model.User;
10.
11.public class BusinessService {
12.
13.    public static void main(String[] args) {
14.
15.        Configuration config = new Configuration().configure();
16.        SessionFactory sessionFactory = config.buildSessionFactory();
17.        Session session = sessionFactory.openSession();
18.
19.
20.        User user1 = new User();
21.        user1.setName("chenyan");
22.
23.        User user2 = new User();
24.        user2.setName("shenbin");
25.
26.        Room room1 = new Room();
27.        room1.setAddress("NTU-M8-419");
28.
29.        user1.setRoom(room1);
30.        user2.setRoom(room1);
31.
32.        Transaction tx = session.beginTransaction();
33.        session.save(user1);
34.        session.save(user2);
35.        tx.commit();
36.
37.
38.        session.close();
39.        sessionFactory.close();
40.    }
41.}

或者反過來由一對多的方式來維持關系：

1.package com.hb3.pack_20;
2.
3.import java.util.HashSet;
4.
5.import org.hibernate.Session;
6.import org.hibernate.SessionFactory;
7.import org.hibernate.Transaction;
8.import org.hibernate.cfg.Configuration;
9.
10.import com.hb3.pack_20.model.Room;
11.import com.hb3.pack_20.model.User;
12.
13.public class BusinessService {
14.
15.    public static void main(String[] args) {
16.
17.        Configuration config = new Configuration().configure();
18.        SessionFactory sessionFactory = config.buildSessionFactory();
19.        Session session = sessionFactory.openSession();
20.
21.
22.        User user1 = new User();
23.        user1.setName("chenyan");
24.
25.        User user2 = new User();
26.        user2.setName("shenbin");
27.
28.        Room room1 = new Room();
29.        room1.setUsers(new HashSet<User>());
30.        room1.setAddress("NTU-M8-419");
31.        room1.addUser(user1);
32.        room1.addUser(user2);
33.
34.        Transaction tx = session.beginTransaction();
35.        session.save(room1);
36.        tx.commit();
37.
38.
39.        session.close();
40.        sessionFactory.close();
41.    }
42.}

但是一對多的方式來維持的話，先看一下生成的SQL文：

Hibernate: insert into room (address) values (?) Hibernate: insert into user (name, room_id) values (?, ?) Hibernate: insert into user (name, room_id) values (?, ?) Hibernate: update user set room_id=? where id=? Hibernate: update user set room_id=? where id=?

可見，如果把一的一方Room作為主控方，多的一方User因為不知道Room的room_id是多少，所以必須等Room和User存儲之後再更新room_id。所以在多對一，一對多形成雙向關聯的時候，應該把控制權交給多的一方，這樣比較有效率。理由很簡單，就像在公司裡一樣，老板記住所有員工的名字來得快，還是每個員工記住老板的名字來得快。

基於這個理由，我們對Room.hbm.xml再稍作修改：

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_20.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.
18.        <set name="users" table="user" cascade="save-update" inverse="true">
19.            <key column="room_id"/>
20.            <one-to-many class="com.hb3.pack_20.model.User"/>
21.        </set>
22.
23.    </class>
24.</hibernate-mapping>

如此控制權就交給了多的一方。當直接存儲一的一方前，必須讓多的一方意識的一的一方的存在。

1.package com.hb3.pack_20;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.HashSet;
6.
7.import org.hibernate.Session;
8.import org.hibernate.SessionFactory;
9.import org.hibernate.Transaction;
10.import org.hibernate.cfg.Configuration;
11.
12.import com.hb3.pack_20.model.Room;
13.import com.hb3.pack_20.model.User;
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.
24.        User user1 = new User();
25.        user1.setName("bush");
26.
27.        User user2 = new User();
28.        user2.setName("caterpillar");
29.
30.        Room room1 = new Room();
31.        room1.setUsers(new HashSet<User>());
32.        room1.setAddress("NTU-M8-419");
33.        room1.addUser(user1);
34.        room1.addUser(user2);
35.
36.        //多方必須認識到單方的存在
37.        user1.setRoom(room1);
38.        user2.setRoom(room1);
39.
40.        Transaction tx = session.beginTransaction();
41.        session.save(room1);
42.        tx.commit();
43.
44.
45.        session.close();
46.        sessionFactory.close();
47.    }
48.}

此時生成的SQL文為：

Hibernate: insert into room (address) values (?) Hibernate: insert into user (name, room_id) values (?, ?) Hibernate: insert into user (name, room_id) values (?, ?)

從而提高了效率。

如果把代碼中user1.setRoom(room1);和user2.setRoom(room1);這2行移去，你會發現數據庫中room_id的值為null。這個結果就好比在多對一的關系中沒有分配給User一個Room，那麼理所當然room_id的值為null了。