[Hibernate] - Annotations - Many To Many

Hibernate annotation 多对多：

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下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。

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 class="com.my.bean.User"/>
        <mapping class="com.my.bean.Bank"/>

    </session-factory>

</hibernate-configuration>

HibernateUtil.java

package com.my.dao.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
            Configuration configuration = new Configuration();
            return configuration.configure().buildSessionFactory(
                    new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build());
        } catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

}

Java Beans:

package com.my.bean;

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="user")
public class User {
    @Id @GeneratedValue @Column(name="user_id", nullable=false)
    private long userID;

    @Column(name="user_name", length=100, nullable=false)
    private String userName;

    @Column(name="create_time", nullable=false)
    private Date createTime;

    @ManyToMany(cascade=CascadeType.ALL)
    @JoinTable(name="card", joinColumns={@JoinColumn(name="user_id")}, inverseJoinColumns={@JoinColumn(name="bank_id")})
    private Set<Bank> banks = new HashSet<Bank>();

    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 Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Set<Bank> getBanks() {
        return banks;
    }

    public void setBanks(Set<Bank> banks) {
        this.banks = banks;
    }
}

package com.my.bean;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;

@Entity
@Table(name="bank")
public class Bank {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="bank_id")
    private long bankID;

    @Column(name="bank_name", length=100, nullable=false)
    private String BankName;

    @ManyToMany(mappedBy="banks", cascade={CascadeType.ALL})
    private Set<User> users = new HashSet<User>();

    public long getBankID() {
        return bankID;
    }

    public void setBankID(long bankID) {
        this.bankID = bankID;
    }

    public String getBankName() {
        return BankName;
    }

    public void setBankName(String bankName) {
        BankName = bankName;
    }

    public Set<User> getUsers() {
        return users;
    }

    public void setUsers(Set<User> users) {
        this.users = users;
    }
}

测试：

package com.my.init;
import java.util.Date;

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 {
            //-------------------------------------------------------
            // add user
            //-------------------------------------------------------
            User user = new User();
            user.setUserName("Robin");
            user.setCreateTime(new Date());

            //-------------------------------------------------------
            // add bank
            //-------------------------------------------------------
            Bank bankBCC = new Bank();
            bankBCC.setBankName("BCC");
            Bank bankCBC = new Bank();
            bankCBC.setBankName("CBC");

            //-------------------------------------------------------
            // set relcationship
            //-------------------------------------------------------
            user.getBanks().add(bankBCC);
            user.getBanks().add(bankCBC);

            //-------------------------------------------------------
            // save
            //-------------------------------------------------------
            session.save(user);
            session.save(bankBCC);
            session.save(bankCBC);

            //-------------------------------------------------------
            // remove data
            //-------------------------------------------------------
            session.delete(user);

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

        session.close();
    }

}

参考文档：

http://viralpatel.net/blogs/hibernate-many-to-many-annotation-mapping-tutorial/