【Hibernate】之查询中get()和load()的区别，list()和iterate()的区别
list()查询
//一次性把数据对象取出来
@Test
public void findTestList(){
Session s=sessionFactory.getCurrentSession();
s.beginTransaction();
List<Person> persons=s.createQuery("from Person").list();
for(Person person:persons){
System.out.println(person.getName()+"----"+person.getId());
}
s.getTransaction().commit();
}
sql语句如下:
16:00:43,118 DEBUG SQL:111 -
select
person0_.id as id1_,
person0_.p_age as p2_1_,
person0_.group_id as group4_1_,
person0_.p_name as p3_1_
from
p_person person0_
张三0----1
张三1----2
张三2----3
张三3----4
张三4----5
张三5----6
张三6----7
张三7----8
张三8----9
张三9----10
iterate()查询
//一次性取出来的是所有对象的主键值，并且会把这些主键值放在Session缓冲中去
//下次需要的话，直接在Session中获取，不需要再次访问数据库
//缺点是将查询出来的主键值，分别再以主键进行取值对象，造成数据库压力巨大
@Test
public void findTestIterate(){
Session s=sessionFactory.getCurrentSession();
s.beginTransaction();
Iterator<Person> persons=s.createQuery("from Person").iterate();
while(persons.hasNext()){
System.out.println(persons.next().getName());
}
s.getTransaction().commit();
}
SQL语句
16:03:55,776 DEBUG SQL:111 -
select
person0_.id as col_0_0_
from
p_person person0_
16:03:55,806 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三0
16:03:55,823 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三1
16:03:55,826 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三2
16:03:55,832 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三3
16:03:55,836 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三4
16:03:55,838 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三5
16:03:55,841 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三6
16:03:55,844 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三7
16:03:55,846 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三8
16:03:55,875 DEBUG SQL:111 -
select
person0_.id as id1_0_,
person0_.p_age as p2_1_0_,
person0_.group_id as group4_1_0_,
person0_.p_name as p3_1_0_
from
p_person person0_
where
person0_.id=?
张三9
get()查询
//立刻发送sql语句，不管我们是否需要，比如person.getName();
@Test
public void findTestget(){
Session s=sessionFactory.getCurrentSession();
s.beginTransaction();
Person person=(Person)s.get(Person.class, 1);
// System.out.println(person.getName());
s.getTransaction().commit();
}
load()查询
//不会立刻发送sql语句，只有当我们需要的时候才会发送sql语句，比如person.getName();
@Test
public void findTestload(){
Session s=sessionFactory.getCurrentSession();
s.beginTransaction();
Person person=(Person)s.load(Person.class, 1);
// System.out.println(person.getName());
s.getTransaction().commit();
}
list()和Iterator()不同之处:
1、list()查询所有
2、Iterator()查主键
3、list()每次查询都访问数据库
4、Iterator()每次查询先访问Session缓冲，存在就直接获取，不存在再查询数据库
