hibernate join fetch

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

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