hibernate join fetch

hibernate join fetch

小洲的博客

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。