Hibernate集合映射之IDBag - 军军小站|张军博客

假设Team和Student是1对多的关系，而student中只有team_id和name两个属性，我们可以不建立Student实体类，采用element的方式，由于bag可以允许重复，所以，在我们根据team删除其下某一个student的时候，hibernate不知道具体要删除哪一条数据，所以，只有讲team下所有student全部删除，然后再重新插入不应该删除的数据，这样会对效率有很大影响，针对这种情况，可以采用idbag标签，在student中加一个字段cid来唯一标识每一个student

数据库结构：

create table teamIDBag (id varchar ( 32 ),teamname varchar ( 32 ));

create table studentIDBag(team_id varchar ( 32 ),name varchar ( 32 ),cid varchar ( 32 ));

Pojo:

package Collection.IDBag;

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

public class Team {

private String id;

private String teamname;

private List students = new ArrayList();

public String getId() {

return id;

}

public void setId(String id) {

this .id = id;

}

public String getTeamname() {

return teamname;

}

public void setTeamname(String teamname) {

this .teamname = teamname;

}

public List getStudents() {

return students;

}

public void setStudents(List students) {

this .students = students;

}

Team.hbm.xml

<? xml version="1.0" encoding="utf-8" ?>

<! DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >

<!--

Mapping file autogenerated by MyEclipse - Hibernate Tools

-->

< hibernate-mapping >

< class name ="Collection.IDBag.Team" table ="teamIDBag" >

< id name ="id" unsaved-value ="null" >

< generator class ="uuid.hex" ></ generator >

</ id >

< property name ="teamname" type ="string" column ="teamname" ></ property >

< idbag name ="students" table ="studentIDBag" cascade ="all,delete-orphan" >

< collection-id type ="string" column ="cid" >

< generator class ="uuid.hex" ></ generator >

</ collection-id >

< key column ="team_id" ></ key >

< element type ="string" column ="name" ></ element >

</ idbag >

</ class >

</ hibernate-mapping >

Hibernate.cfg.xml

<? xml version='1.0' encoding='UTF-8' ?>

<! DOCTYPE hibernate-configuration PUBLIC

"-//Hibernate/Hibernate Configuration DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd" >

< hibernate-configuration >

< session-factory >

< property name ="connection.username" > root </ property >

< property name ="connection.url" >

jdbc:mysql://localhost:3306/schoolproject?characterEncoding=gb2312 & useUnicode=true

</ property >

< property name ="dialect" >

org.hibernate.dialect.MySQLDialect

</ property >

< property name ="myeclipse.connection.profile" > mysql </ property >

< property name ="connection.password" > 1234 </ property >

< property name ="connection.driver_class" >

com.mysql.jdbc.Driver

</ property >

< property name ="hibernate.dialect" >

org.hibernate.dialect.MySQLDialect

</ property >

< property name ="hibernate.show_sql" > true </ property >

< property name ="current_session_context_class" > thread </ property >

< property name ="jdbc.batch_size" > 15 </ property >

< mapping resource ="Collection/IDBag/Team.hbm.xml" />

</ session-factory >

</ hibernate-configuration >

测试代码：

package Collection.IDBag;

import java.io.File;

import java.util.Map;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

public class Test {

public static void main(String[] args) {

String filePath = System.getProperty( " user.dir " ) + File.separator + " src/Collection/IDBag " + File.separator + " hibernate.cfg.xml " ;

File file = new File(filePath);

System.out.println(filePath);

SessionFactory sessionFactory = new Configuration().configure(file).buildSessionFactory();

Session session = sessionFactory.openSession();

Transaction t = session.beginTransaction();

// Team team1=new Team();

// team1.setTeamname("team1");

// Team team2=new Team();

// team2.setTeamname("team2");

// // 由于Bag映射可以重复，所以这里重复保存student

// team1.getStudents().add("tom1");

// team2.getStudents().add("tom2");

// session.save(team1);

// session.save(team2);

Team t1 = (Team)session.createQuery( " from Team t where t.teamname='team1' " ).uniqueResult();

t1.getStudents().remove( 0 );

t.commit();

}

运行结果：

Hibernate: select team0_.id as id0_, team0_.teamname as teamname0_ from teamIDBag team0_ where team0_.teamname='team1'
Hibernate: select students0_.team_id as team1_0_, students0_.name as name0_, students0_.cid as cid0_ from studentIDBag students0_ where students0_.team_id=?
Hibernate: delete from studentIDBag where cid=?

可以红色部分，删除student时是根据cid删的，如果不使用idbag,则会出类似delete from studentIDBag where team_id=?的操作，然后会接着跟一个insert操作，来恢复本不应该删除的数据

Hibernate集合映射之IDBag

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