hibernate中Query的list和iterator区别

1、Test_query_list类

public class Test_query_iterator_list {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();        
        String hql="from User where id<=3";
        List<User> users=session.createQuery(hql).list();
        System.out.println("***************************************");
        for (User user: users) {
            System.out.println(user);
        }       
        session.close();       
    }

}

测试结果为：

Hibernate:
select
user0_.id as id0_,
user0_.name as name0_
from
mydb.user user0_
where
user0_.id<=3
***************************************
User [id=1, name=LI]
User [id=2, name=sara2]
User [id=3, name=sara3]

public class Test_query_iterator_list {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();            
        String hql="from User where id<=3";
        Iterator<User> iterator=session.createQuery(hql).iterate();
        System.out.println("***************************************");
        while(iterator.hasNext()) {
            System.out.println(iterator.next());
        }        
        session.close();        
    }
}

输出结果为：

Hibernate:
select
user0_.id as col_0_0_
from
mydb.user user0_
where
user0_.id<=3
***************************************
Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
User [id=1, name=LI]
Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
User [id=2, name=sara2]
Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
User [id=3, name=sara3]

结论：

　　可以看出list每次都是通过一条语句直接操作数据库取出所有的数据返回（并且将对象存入hibernate缓存）；iterator首先通过一条语句取出所有数据的id，然后通过id在hibernate的一级缓存中查找是否存在该对象，如果存在则直接取出，如果没有则再次发出一条sql语句通过id取得对象（并且加入到缓存中），这样如果所有的id在缓存中都没有的话就会出现n+1条sql语句的问题。