hibernate3學習筆記(二十)｜關系映射：多對多

要實現多對多的對應，一般可以借由一個中間表來完成。也就是借由一對多，多對一來完成。

DDL：

1.CREATE TABLE user (
2.    id INT(11) NOT NULL auto_increment PRIMARY KEY,
3.    name VARCHAR(100) NOT NULL default ''
4.);
5.
6.CREATE TABLE user_server (
7.    user_id INT(11),
8.    server_id INT(11)
9.);
10.
11.CREATE TABLE server (
12.    id INT(11) NOT NULL auto_increment PRIMARY KEY,
13.    address VARCHAR(100) NOT NULL default ''
14.);

User.java

1.package com.hb3.pack_19.model;
2.
3.import java.util.Set;
4.
5.public class User {
6.
7.    private Integer id;
8.    private String name;
9.    private Set<Server> servers;
10.
11.    public User() {
12.    }
13.
14.    public Integer getId() {
15.        return id;
16.    }
17.    public void setId(Integer id) {
18.        this.id = id;
19.    }
20.    public String getName() {
21.        return name;
22.    }
23.    public void setName(String name) {
24.        this.name = name;
25.    }
26.    public Set<Server> getServers() {
27.        return servers;
28.    }
29.    public void setServers(Set<Server> servers) {
30.        this.servers = servers;
31.    }
32.}

Server.java

1.package com.hb3.pack_19.model;
2.
3.import java.util.Set;
4.
5.public class Server {
6.
7.    private Integer id;
8.    private String address;
9.    private Set<User> users;
10.
11.    public String getAddress() {
12.        return address;
13.    }
14.    public void setAddress(String address) {
15.        this.address = address;
16.    }
17.    public Integer getId() {
18.        return id;
19.    }
20.    public void setId(Integer id) {
21.        this.id = id;
22.    }
23.    public Set<User> getUsers() {
24.        return users;
25.    }
26.    public void setUsers(Set<User> users) {
27.        this.users = users;
28.    }
29.}

Server.hbm.xml

1.<?xml version="1.0" encoding="UTF-8"?>
2.
3.<!DOCTYPE hibernate-mapping PUBLIC
4.    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
5.    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
6.
7.<hibernate-mapping>
8.
9.    <class name="com.hb3.pack_19.model.Server" table="server">
10.
11.        <id name="id" column="id">
12.            <generator class="native"/>
13.        </id>
14.
15.        <property name="address" type="java.lang.String"/>
16.
17.        <set name="users"
18.             table="user_server"
19.             inverse="true"
20.             cascade="save-update">
21.
22.             <key column="server_id"/>
23.             <many-to-many class="com.hb3.pack_19.model.User"
24.                           column="user_id"/>
25.        </set>
26.    </class>
27.
28.</hibernate-mapping>

User.hbm.xml

1.<?xml version="1.0" encoding="UTF-8"?>
2.
3.<!DOCTYPE hibernate-mapping PUBLIC
4.    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
5.    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
6.
7.<hibernate-mapping>
8.
9.    <class name="com.hb3.pack_19.model.User" table="user">
10.
11.        <id name="id" column="id" type="java.lang.Integer">
12.            <generator class="native"/>
13.        </id>
14.
15.        <property name="name" column="name" type="java.lang.String"/>
16.
17.        <set name="servers"
18.             table="user_server"
19.             cascade="save-update">
20.
21.            <key column="user_id"/>
22.            <many-to-many class="com.hb3.pack_19.model.Server"
23.                          column="server_id"/>
24.        </set>
25.    </class>
26.
27.</hibernate-mapping>

測試代碼：

1.package com.hb3.pack_19;
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_19.model.Server;
13.import com.hb3.pack_19.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.        Server server1 = new Server();
25.        server1.setAddress("PC-219");
26.        server1.setUsers(new HashSet<User>());
27.
28.        Server server2 = new Server();
29.        server2.setAddress("PC-220");
30.        server2.setUsers(new HashSet<User>());
31.
32.        Server server3 = new Server();
33.        server3.setAddress("PC-221");
34.        server3.setUsers(new HashSet<User>());
35.
36.        User user1 = new User();
37.        user1.setName("shenbin");
38.        user1.setServers(new HashSet<Server>());
39.
40.        User user2 = new User();
41.        user2.setName("chenyan");
42.        user2.setServers(new HashSet<Server>());
43.
44.        user1.getServers().add(server1);
45.        user1.getServers().add(server2);
46.        user1.getServers().add(server3);
47.        server1.getUsers().add(user1);
48.        server2.getUsers().add(user1);
49.        server3.getUsers().add(user1);
50.
51.        user2.getServers().add(server1);
52.        user2.getServers().add(server3);
53.        server1.getUsers().add(user2);
54.        server3.getUsers().add(user2);
55.
56.        Transaction tx= session.beginTransaction();
57.        session.save(user1);
58.        session.save(user2);
59.        tx.commit();
60.
61.
62.        session.close();
63.        sessionFactory.close();
64.    }
65.}

但是，多對多映射由於使用了中介表格，查詢效率不高。這不是一個很好的設計方式，應盡量避免多對多關聯的設計。