那天去了达内，试听了2个小时的课，刚好讲的他们的当当网项目，讲hibernate表关系映射这段来了，不过也够呛，投影仪看的眼睛点都不舒服，当时讲的也没认真听，听了个大概，就记住那个老师一句话了，说是有经验的人用的，一般人不告诉，就是说这个join fetch 用法，回来查了一下，用起来比较爽，今晚突然来了兴致，小记一下。

join fetch 其实是相当于 数据库 里面的连接部分。默认是inner join 。也支持left outer join。

join fetch 主要是运用于 HQL 中。

主要体现在一对多、多对多的关系表中

例如:

class Student{

private int id;

private int name;

private Team team;

get{…} ;  set{…};

}

class Team{

private int tid;

private String name;

private Set<Student> students;

get{…}; set{…}';

}

默认查询Team 的时候,fetch=lazy,不会去查询 Team 中所有的Student.

----------------查询---------------------

Team t=(Team)session.load(Team.class, 3);
        Set<Student> sets=t.getStudents();
        for(Student s:sets){
            System.out.println(s.getId()+"   "+s.getName());
        }

----------------查询结果-----------------

Hibernate:
    select
        team0_.cid as cid0_0_,
        team0_.name as name0_0_
    from
        team0_
    where
        team0_.cid=?
Hibernate:
    select
        students0_.cid as cid0_1_,
        students0_.id as id1_,
        students0_.id as id1_0_,
        students0_.cid as cid1_0_,
        students0_.name as name1_0_
    from
        Student students0_
    where
        students0_.cid=?

-----------------------------------------

所以会去查询2次，第一次去查询指定的Team，第二次会查询指定Team下所有的Student.

使用Join fetch 是为了减少查询的次数。

----------------查询---------------------

List<Class> lists = session.createQuery("from Team t  join fetch c.students order by t.cid").list();
        for(Team t : lists){
            System.out.println(t.getCid()+ "   "+t.getName()+"   ");
                for(Student s:t.getStudents()){
                    System.out.println("      "+s.getId()+"   "+ s.getName() +"   "+s.getTeam().getCid());
                }
        }

----------------查询结果-----------------

Hibernate:
    select
        team0_.cid as cid0_0_,
        students1_.id as id1_1_,
        team0_.name as name0_0_,
        students1_.cid as cid1_1_,
        students1_.name as name1_1_,
        students1_.cid as cid0_0__,
        students1_.id as id0__
    from
        team class0_
    inner join
        Student students1_
            on team0_.cid=students1_.cid
    order by
        team0_.cid

-----------------------------------------

结果发现只发错一条SQL语句。经鉴定，Join fetch 是为了用来减少查询次数的。

实质是就是使用了数据库的join.

鉴定完毕。