Hibernate的关联关系中lazy和fetch的设置

Hibernate的关联关系中lazy和fetch的设置会影响到对数据进行查询时候SQL语句的操作,fetch的设置相对于lazy的优先级更高,而且在cl

Hibernate的关联关系中lazy和fetch的设置会影响到对数据进行查询时候SQL语句的操作,fetch的设置相对于lazy的优先级更高,而且在class标签上配置的lazy属性不会影响到关联对象.(本例用的版本是Hibernate3)

本例假设有一个主表为MASTTB,有一个子表为DETAILTB.

主表端的fetch可以取 'join','select'和'subselect'(select为默认值):

join:外连接一次查询.

select:1+n 条select语句,第一条查主表,第n条查第n条主表记录对应的子表记录.

subselect: 以 id in(...)的方式做第二条查询,(如果查询主表的是返回单条记录,subselect和select没有区别,如果查询主表的是返回多条记录的话,对子表查询会以id in 的方式).具体见例4.

lazy可以取'true','extra'以及 'false'(true为默认值):

true:默认取值，它的意思是只有在调用这个集合获取里面的元素对象时,才发出查询语句,加载其集合元素的数据.
false:取消延迟加载特性，即在加载对象的同时,就发出第二条查询语句加载其关联集合的数据.
extra:一种比较聪明的延迟加载策略,即调用集合的size/contains等方法的时候,hibernate并不会去加载整个集合的数据,而是发出一条"聪明"的SQL语句,以便获得需要的值,只有在真正需要用到这些集合元素对象数据的时候,才去发出查询语句加载所有对象的数据.
比如看集合的size:

会发出下面的SQL语句

select
count(DTID)
from
DETAILTB
where
MTID =?

1,对主表进行findById查询的测试,当fetch设为join的时候, 不管lazy是true还是false,都是一次以主表左外连接子表的方式把关联的数据全部查出来.SQL如下:

select
masttb0_.MID as MID1_1_,
masttb0_.MASTINFO as MASTINFO1_1_,
detailtbs1_.MTID as MTID3_,
detailtbs1_.DTID as DTID3_,
detailtbs1_.DTID as DTID2_0_,
detailtbs1_.MTID as MTID2_0_,
detailtbs1_.DETAILINFO as DETAILINFO2_0_
from
MASTTB masttb0_
left outer join
DETAILTB detailtbs1_
on masttb0_.MID=detailtbs1_.MTID
where
masttb0_.MID=?

2,对主表进行findById查询的测试,当fetch设为select的时候,lazy 是true的时候,hibernate先用一条SQL将主表的数据查出来,然后在取子表数据的时候(在访问set的iterator的时候),再以子表的外键作为条件,用SQL语句取子表的数据.
JAVA代码如下:
tx = sessionFactory.getCurrentSession().beginTransaction();
MasttbHome masttbHome = new MasttbHome();

Masttb masttb = masttbHome.findById(new BigDecimal(1));
System.out.println("before getting detai set");

Set set = masttb.getDetailtbs();

System.out.println("after getting detai set");
Iterator itr = set.iterator();
System.out.println("after getting detai set iterator");
while(itr.hasNext()){
Detailtb detailtb = (Detailtb)itr.next();
System.out.println("after getting detai info " + detailtb.getDtid());
}

tx.commit();
运行结果如下:
Hibernate:
select
masttb0_.MID as MID1_0_,
masttb0_.MASTINFO as MASTINFO1_0_
from
MASTTB masttb0_
where
masttb0_.MID=?
before getting detai set
after getting detai set
Hibernate:
select
detailtbs0_.MTID as MTID1_,
detailtbs0_.DTID as DTID1_,
detailtbs0_.DTID as DTID2_0_,
detailtbs0_.MTID as MTID2_0_,
detailtbs0_.DETAILINFO as DETAILINFO2_0_
from
DETAILTB detailtbs0_
where
detailtbs0_.MTID=?
after getting detai set iterator
after getting detai info 2
after getting detai info 1

3,对主表进行findById查询的测试,当fetch设为select的时候,lazy是false的时候,hibernate先用一条SQL将主表的数据查出来,然后马上再以子表的外键作为条件,用SQL语句取子表的数据.
上面例2的代码会打印:
Hibernate:
select
masttb0_.MID as MID1_0_,
masttb0_.MASTINFO as MASTINFO1_0_
from
MASTTB masttb0_
where
masttb0_.MID=?
Hibernate:
select
detailtbs0_.MTID as MTID1_,
detailtbs0_.DTID as DTID1_,
detailtbs0_.DTID as DTID2_0_,
detailtbs0_.MTID as MTID2_0_,
detailtbs0_.DETAILINFO as DETAILINFO2_0_
from
DETAILTB detailtbs0_
where
detailtbs0_.MTID=?
before getting detai set
after getting detai set
after getting detai set iterator
after getting detai info 2
after getting detai info 1


*如果将lazy设为true,fetch设为 select,在session关闭后在去访问set里的值,会出异常.
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.test.hb.Masttb.detailtbs, no session or session was closed

4,对主表进行多条记录查询的测试,当fetch设为subselect的时候,lazy是true的时候,hibernate先用一条SQL将主表的数据查出来,然后用id in 方式的SQL语句取子表的数据.

JAVA代码:

tx = sessionFactory.getCurrentSession().beginTransaction();

List mstlist = sessionFactory.getCurrentSession().createQuery("from com.test.hb.Masttb where id in (1,2)").list();

//下面的hql和上面的hql有同样的效果

//List mstlist = sessionFactory.getCurrentSession().createQuery("from com.test.hb.Masttb where id <3").list();
for (Iterator iter = mstlist.iterator(); iter.hasNext();) {
Masttb masttb = (Masttb) iter.next();
System.out.println("masttb.getMastinfo=" + masttb.getMastinfo());
Set set = masttb.getDetailtbs();
System.out.println(set.size());
if (set != null && !set.isEmpty()) {
for (Iterator itr = set.iterator(); itr.hasNext();) {
Detailtb detailtb = (Detailtb) itr.next();
System.out.println("detailtb.name=" + detailtb.getDetailinfo());
}
}
}

tx.commit();

运行结果:

Hibernate:
select
masttb0_.MID as MID1_,
masttb0_.MASTINFO as MASTINFO1_
from
MASTTB masttb0_
where
masttb0_.MID in (
1 , 2
)
masttb.getMastinfo=mastinfo
Hibernate:
select
detailtbs0_.MTID as MTID1_,
detailtbs0_.DTID as DTID1_,
detailtbs0_.DTID as DTID2_0_,
detailtbs0_.MTID as MTID2_0_,
detailtbs0_.DETAILINFO as DETAILINFO2_0_
from
DETAILTB detailtbs0_
where
detailtbs0_.MTID in (
select
masttb0_.MID
from
MASTTB masttb0_
where
masttb0_.MID in (
1 , 2
)
)
number in detail table: 2
detailtb.name=aaaa
detailtb.name=detailinfo2
masttb.getMastinfo=aaa
number in detail table: 1
detailtb.name=adfasdfa

如果fetch=select,lazy=true的话,运行结果为 1 + 2条SQL语句:

Hibernate:
select
masttb0_.MID as MID1_,
masttb0_.MASTINFO as MASTINFO1_
from
MASTTB masttb0_
where
masttb0_.MID in (
1 , 2
)
masttb.getMastinfo=mastinfo
Hibernate:
select
detailtbs0_.MTID as MTID1_,
detailtbs0_.DTID as DTID1_,
detailtbs0_.DTID as DTID2_0_,
detailtbs0_.MTID as MTID2_0_,
detailtbs0_.DETAILINFO as DETAILINFO2_0_
from
DETAILTB detailtbs0_
where
detailtbs0_.MTID=?
number in detail table: 2
detailtb.name=aaaa
detailtb.name=detailinfo2
masttb.getMastinfo=aaa
Hibernate:
select
detailtbs0_.MTID as MTID1_,
detailtbs0_.DTID as DTID1_,
detailtbs0_.DTID as DTID2_0_,
detailtbs0_.MTID as MTID2_0_,
detailtbs0_.DETAILINFO as DETAILINFO2_0_
from
DETAILTB detailtbs0_
where
detailtbs0_.MTID=?
number in detail table: 1
detailtb.name=adfasdfa