Hiernate 预先抓取策略

预先抓取采用的是fetch="join"的方式

 

预先抓取指的是通过select语句使用outer join 一般是左外连接来获得对象的关联实例或者是关联的集合（集合被初始化了，这是重点）。

 

用上面的例子，加上一个身份证的对象。CARD 他和学生对象是一对一的关系。

 

身份-----立-----学-----立即------班

证-------即------生-----立即-----级

 

注释：max_fetch_depth是在hibernate.hbm.xml中设置。表示可以预先抓取的深度。

如果设置为1，表示只可以外连接一个表，如果这是为0，那么所有的预先抓取的策略将变成立即加载。

三者的对象都采取立即加载的方式。并且设置max_fetch_depth=0

打印：

Hibernate: select team0_.ID as ID0_0_, team0_.Name as Name0_0_ from test.team team0_ where team0_.ID=?

Hibernate: select students0_.teamID as teamID1_, students0_.ID as ID1_, students0_.ID as ID1_0_, students0_.Name as Name1_0_, students0_.TeamID as TeamID1_0_ from test.student students0_ where students0_.teamID=?

Hibernate: select card0_.ID as ID2_0_, card0_.StudentID as StudentID2_0_, card0_.Name as Name2_0_ from test.card card0_ where card0_.ID=?

Hibernate: select card0_.ID as ID2_0_, card0_.StudentID as StudentID2_0_, card0_.Name as Name2_0_ from test.card card0_ where card0_.ID=?

 

可以发现，对应于每个学生都会分别发出一条SQL语句去查询card的信息。

如果有100个学生那么就会发出100条SQL语句去查询card的信息。

这样很明显会降低系统的性能。

 

如果全部设置成预先抓取的话。

Hibernate: select team0_.ID as ID0_2_, team0_.Name as Name0_2_, students1_.teamID as teamID4_, students1_.ID as ID4_, students1_.ID as ID1_0_, students1_.Name as Name1_0_, students1_.TeamID as TeamID1_0_, card2_.ID as ID2_1_, card2_.StudentID as StudentID2_1_, card2_.Name as Name2_1_ from test.team team0_ left outer join test.student students1_ on team0_.ID=students1_.teamID left outer join test.card card2_ on students1_.ID=card2_.ID where team0_.ID=?

就只会发出一条SQL语句。

 

预先抓取和max_fetch_depth有很大的关系。