Hiberante(四) OneToMany_xml 单向关联

 (1)  oneToMany 单向

   java类：Comment    Message

public class Comment {

	private int id;
	private String content;
      省略get(),set();  
}
public class Message {
  private int id;
  private String title;
  private String content;
  private Set<Comment> comments;
 省略get(),set();
}

Message.hbm.xml文件

<hibernate-mapping package="org.th.model">
    <class name="Message" table="t_message" >
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="title"></property>
        <property name="content"></property>
        
        <!-- 使用了lazy="extra"   当返回结果是数字会使用count，否则使用投影  (智能一点)-->
        <set name="comments"  lazy="extra">
        <!--key用来指定在对方的外键名称  -->
             <key column="mid"></key>
        <!-- class 用来设置列表中的对象类型  -->    
             <one-to-many class="Comment"/>
        </set>
    </class>
</hibernate-mapping>

 Comment.hbm.xml

<hibernate-mapping package="org.th.model">
    <class name="Comment" table="t_comment" >
        <id name="id">
           <generator class="native"></generator>
        </id>
        <property name="content"></property>
    </class>
</hibernate-mapping>

 测试类：OneToMany

public class OneToMany {

	 @Test
	 public void testAdd(){
		 Session session=null;
			try {
				session = HibernateUtil.openSession();
				session.beginTransaction();
				Comment c1=new Comment();
				c1.setContent("黑河");
				Comment c2=new Comment();
				c2.setContent("白河");
				session.save(c1);session.save(c2);
				Message m=new Message();
				m.setTitle("中国好声音");
				m.setContent("123");
				m.addComment(c1);
				m.addComment(c2);
				session.save(m);
				//此时会发出5条sql  2条保存c1，c2。一条保存Message(其中addComment(c1) 由于延迟加载，只是将c1的id存在当中) 
				//当提交事务的 时候session中的对象会与内存中进行比较，不相同即update 
				session.getTransaction().commit();
			} catch (Exception e) {
				e.printStackTrace();
				if(session!=null) session.getTransaction().rollback();
			}finally{
				if(session!=null) HibernateUtil.close(session);
			}
	 }
	 @Test
	 public void testLoad(){
		 Session session=null;
			try {
				session = HibernateUtil.openSession();
				session.beginTransaction();
				Message message=(Message)session.load(Message.class, 1);
                System.out.println(message.getContent());				
			    for(Comment c:message.getComments()){
			    	System.out.println(c.getContent());
			    }
				
				session.getTransaction().commit();
			} catch (Exception e) {
				e.printStackTrace();
				if(session!=null) session.getTransaction().rollback();
			}finally{
				if(session!=null) HibernateUtil.close(session);
			}
	 }
	 //特别注意：OneToMany在添加和维护关系时比较麻烦，所以在开发中不建议使用OneToMany的单向
	 @Test
	 public void testLoad2(){
		 Session session=null;
			try {
				session = HibernateUtil.openSession();
				session.beginTransaction();
				Message message=(Message)session.load(Message.class, 1);
                
				System.out.println(message.getComments().size());				
			    // OneToMany 在不使用特别标签时，
				//Hibernate: select message0_.id as id4_0_, message0_.title as title4_0_, message0_.content as content4_0_ from t_message message0_ where message0_.id=?
				//Hibernate: select comments0_.mid as mid4_1_, comments0_.id as id5_1_, comments0_.id as id5_0_, comments0_.content as content5_0_ from t_comment comments0_ where comments0_.mid=?
					
				//  OneToMany 在使用<set name="comments"  lazy="extra">时    当返回结果是数字会使用count，否则使用投影  (智能一点)
				//Hibernate: select message0_.id as id4_0_, message0_.title as title4_0_, message0_.content as content4_0_ from t_message message0_ where message0_.id=?
			    //Hibernate: select count(id) from t_comment where mid =?
				
				
				session.getTransaction().commit();
			} catch (Exception e) {
				e.printStackTrace();
				if(session!=null) session.getTransaction().rollback();
			}finally{
				if(session!=null) HibernateUtil.close(session);
			}
	 }
}