hibernate基础-映射关系(多对一I一对一)

多对一关联映射 many-to-one
关联映射本质：
  * 将关联关系映射到数据库，所谓的关联关系是对象模型在内存中的一个或多个引用。
 
<many-to-one/>标签会在多的一端加入一个外键，指定一的一端，这个外键是由<many-to-one>
中的column属性定义的，如果忽略了这个属性那么默认的外键与实体的属性一致

<many-to-one/>标签的定义示例：
  * <many-to-one name="group" column="groupid" cascade="save-update"/>
    其中cascade属性为级联操作，值可以为all、save-update、delete，默认为none
    级联操作：是对象的连锁操作
  * 级联操作的解决方法有两种：1 设cascade属性，2 先进行save操作

<!--
  User:                                      Group:
    private int id;                     private int id;
    private String name;         private String name;
    private Group group;
-->
<class name="com.my.hibernate.Group" table="t_group">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
</class>

<class name="com.my.hibernate.User" table="t_user">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <property name="password"/>
        <property name="createTime"/>
        <property name="expireTime"/>
        <many-to-one name="group" column="groupid" cascade="save-update"/>
</class>

public void testSave1(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            
            session.beginTransaction();
            Group group=new Group();
            group.setName("尚学堂");
            
            User user=new User();
            user.setName("采10");
            user.setPassword("123");
            user.setCreateTime(new Date());
            user.setExpireTime(new Date());
            user.setGroup(group);
            
            User user1=new User();
            group.setName("尚学堂");
            user1.setName("张三");
            user1.setPassword("888");
            user1.setCreateTime(new Date());
            user1.setExpireTime(new Date());
            user1.setGroup(group);
            
            //不能成功保存,抛出TransientObjectException异常
            //因为Group为Transient状态,id没有分配值
            //persistent状态的对象是不能引用trainsient状态的对象的
            session.save(user);
            session.save(user1);
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }
    
    public void testSave2(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            
            session.beginTransaction();
            Group group=new Group();
            group.setName("尚学堂");
            session.save(group);
            
            User user=new User();
            user.setName("采10");
            user.setPassword("123");
            user.setCreateTime(new Date());
            user.setExpireTime(new Date());
            user.setGroup(group);
            
            User user1=new User();
            group.setName("尚学堂");
            user1.setName("张三");
            user1.setPassword("888");
            user1.setCreateTime(new Date());
            user1.setExpireTime(new Date());
            user1.setGroup(group);

            //可以存储，因为对Group进行了save操作
            session.save(user);
            session.save(user1);
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }
    
    public void testSave3(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            
            session.beginTransaction();
            Group group=new Group();
            group.setName("尚学堂");
            User user=new User();
            user.setName("采10");
            user.setPassword("123");
            user.setCreateTime(new Date());
            user.setExpireTime(new Date());
            user.setGroup(group);
            
            User user1=new User();
            group.setName("尚学堂");
            user1.setName("张三");
            user1.setPassword("888");
            user1.setCreateTime(new Date());
            user1.setExpireTime(new Date());
            user1.setGroup(group);
            
            //不会抛出异常,因为采用了cascade属性,所以它会先保存Group
            //采用cascade属性是解决TransientObjectException异常的一种手段
            session.save(user);
            session.save(user1);
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }
    
    public void testload1(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            session.beginTransaction();
            
            User user=(User)session.load(User.class, 3);
            System.out.println(user.getName());
            System.out.println(user.getGroup().getName());
            
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }

一对一关联映射 one-to-one
1 主键关联映射(单向关联Person--->Card)
<one-to-one/>主键关联映射
  * 让两个实体对象的id保持相同，这样可以避免多余的字段被创建
  * 具体映射：
  <id name="id">
   person的主键来源于Card,也就是共享Card的主键
   <generator class="foreign">
    <param name="property">card</param>
   </generator>
  </id>
  <one-to-one/>标签的含义，指示hibernate怎么加载它的关联对象，默认根据主键加载,
  constrained="true",表明当前主键上存在一个约束，person的主键作为外键参照了Card
  <one-to-one name="card" constrained="true"/>

public void testSave1(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            session.beginTransaction();
            
            Card card=new Card();
            card.setCardNo("sadfsaffsf");
            Person person=new Person();
            person.setName("werwer");
            person.setCard(card);
            //不会出现TransientObjectException异常
            //因为一对一主键关联映射中,默认为cascade属性
            session.save(person);
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }
    
    public void testLoad1(){
        Session session=null;
        try{
            session=HibernateUtils.getSession();
            session.beginTransaction();
            
            Person person=(Person)session.load(Person.class, 7);
            System.out.print(person.getName());
            System.out.print(person.getCard().getCardNo());
            
            session.getTransaction().commit();
        }catch(Exception e){
            e.printStackTrace();
            session.getTransaction().rollback();
        }finally{
            HibernateUtils.closeSession(session);
        }
    }

 

一对一关联映射 one-to-one
2 主键关联映射(双向关联Person<--->Card)
需要在Card映射文件中加入<one-to-one/>标签指向person,指示hibernate如何加载person
默认根据主键加载
只能用于查询，load和get

<!--
  Person:                              Card:
    private int id;                  private int id;
    private String name;      private String cardNo;
    private Card card;          private Person person;
-->        
<class name="com.my.hibernate.Card" table="t_card">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="cardNo"/>
        <one-to-one name="person"/>
</class>

<class name="com.my.hibernate.Person" table="t_person">
        <id name="id">
            <generator class="foreign">
                <param name="property">card</param>
            </generator>
        </id>
        <property name="name"/>
        <one-to-one name="card" constrained="true"/>
</class>

<img

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hibernate基础-映射关系(一对多I多对多 ... | hibernate基础-基本映谢标签和属性

• 2009-11-03 15:53
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