[Hibernate] - many to many

Hibernate的多对多实现：

hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://127.0.0.1/testdb</property>
        <property name="connection.username">root</property>
        <property name="connection.password"></property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>

        <mapping resource="com/my/hbm/User.hbm.xml"/>
        <mapping resource="com/my/hbm/Bank.hbm.xml"/>

    </session-factory>

</hibernate-configuration>

mapping配置：

Bank.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.my.bean.Bank" table="bank">
        <id name="bankID" column="bank_id" type="long">
            <generator class="native"></generator>
        </id>
        <set name="users" table="card">
            <key column="bank_id"></key>
            <many-to-many class="com.my.bean.User" column="user_id"></many-to-many>
        </set>
        <property name="bankName" column="bank_name" type="java.lang.String" length="100" not-null="true"></property>
    </class>
</hibernate-mapping>

User.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.my.bean.User" table="user">
        <id name="userID" column="user_id" type="long">
            <generator class="native"></generator>
        </id>
        <set name="banks" table="card">
            <key column="user_id"></key>
            <many-to-many class="com.my.bean.Bank" column="bank_id"></many-to-many>
        </set>
        <property name="userName" column="user_name" type="java.lang.String" length="100" not-null="true"></property>
    </class>
</hibernate-mapping>

POJO

package com.my.bean;

import java.util.Set;

public class Bank {
    private long bankID;
    private String bankName;
    private Set<User> users;

    public long getBankID() {
        return bankID;
    }
    public void setBankID(long bankID) {
        this.bankID = bankID;
    }
    public String getBankName() {
        return bankName;
    }
    public void setBankName(String bankName) {
        this.bankName = bankName;
    }
    public Set<User> getUsers() {
        return users;
    }
    public void setUsers(Set<User> users) {
        this.users = users;
    }
}

package com.my.bean;

import java.util.Set;

public class User {
    private long userID;
    private String userName;
    private Set<Bank> banks;

    public long getUserID() {
        return userID;
    }
    public void setUserID(long userID) {
        this.userID = userID;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public Set<Bank> getBanks() {
        return banks;
    }
    public void setBanks(Set<Bank> banks) {
        this.banks = banks;
    }
}

测试：

package com.my.init;

import java.util.HashSet;

import org.hibernate.Session;
import org.hibernate.Transaction;

import com.my.bean.Bank;
import com.my.bean.User;
import com.my.dao.util.HibernateUtil;

public class Test {

    public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();

        try {
            // create user
            User userRobin = new User();
            userRobin.setUserName("Robin");
            userRobin.setBanks(new HashSet<Bank>());
            User userBen = new User();
            userBen.setUserName("Ben");
            userBen.setBanks(new HashSet<Bank>());

            // create bank
            Bank bankCBC = new Bank();
            bankCBC.setBankName("CBC");
            bankCBC.setUsers(new HashSet<User>());
            Bank bankBBC = new Bank();
            bankBBC.setBankName("BBC");
            bankBBC.setUsers(new HashSet<User>());

            // add relationship
            userRobin.getBanks().add(bankCBC);
            userRobin.getBanks().add(bankBBC);
            userBen.getBanks().add(bankCBC);
            userBen.getBanks().add(bankBBC);

            session.save(userRobin);
            session.save(userBen);
            session.save(bankCBC);
            session.save(bankBBC);

            tx.commit();
        } catch (Exception e) {
            tx.rollback();
            e.printStackTrace();
        }

        session.close();
    }

}

---

上面测试例子会自动生成一张表：card，这张是bank和user表的映射表。里头是bank_id和user_id两个组合字段。

如果想在这张映射表中加入额外的字段，那么hibernate似乎无法做到。因为这需要把多对多写成两个一对多的关系。事实上这样加额外字段到映射表，好像也是违反了DBMS的设计原则。