[Hibernate] - Annotations - Many To Many

2022-10-13 17:23:25

Hibernate annotation 多对多：

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/

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