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[Hibernate] - Annotations - Many To Many

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/

 

[Hibernate] - Annotations - Many To Many