Hibernate的多对多实现：

 

hibernate.cfg.xml

     
         
              
               
       
        com.mysql.jdbc.Driver
               
       
        jdbc:mysql://127.0.0.1/testdb
               
       
        root
               
               
               
       
        1
               
               
       
        org.hibernate.dialect.MySQL5Dialect
               
               
       
        thread
               
               
       
        org.hibernate.cache.internal.NoCacheProvider
               
               
       
        true
               
               
       
        update
               
               
           
      
     

 

mapping配置：

Bank.hbm.xml

User.hbm.xml

 

POJO

package com.my.bean;import java.util.Set;public class Bank {    private long bankID;    private String bankName;    private Set
     
       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
      
        getUsers() {        return users;    }    public void setUsers(Set
       
         users) {        this.users = users;    }}
       
      
     

package com.my.bean;import java.util.Set;public class User {    private long userID;    private String userName;    private Set
     
       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
      
        getBanks() {        return banks;    }    public void setBanks(Set
       
         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
     
      ());            User userBen = new User();            userBen.setUserName("Ben");            userBen.setBanks(new HashSet
      
       ());                        // create bank            Bank bankCBC = new Bank();            bankCBC.setBankName("CBC");            bankCBC.setUsers(new HashSet
       
        ());            Bank bankBBC = new Bank();            bankBBC.setBankName("BBC");            bankBBC.setUsers(new HashSet
        
         ());                        // 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。