Hibernate面试题 --- list和iterator方法的区别

Hibernate面试题  ---  list和iterator方法的区别

1．首先看两个例子来比较一下

（1）在用Query方法查询的时候，通过HQL语句来得到Query对象，并对Query对象进行操作，首先是用list方法获取到Query的List集合并输出:

 1 @Test
 2 public void listQuery() {
 3
 4        Configuration configuration = new Configuration().configure();
 5        SessionFactory factory = configuration.buildSessionFactory();
 6        Session session = factory.openSession();
 7        Query query = session.createQuery("from Customers");
 8        List<Customers> list = query.list();
 9
10        for(Customers entity:list){
11            System.out.println(entity.toString());
12        }
13 }

List的执行sql语句为

1 Hibernate: select customers0_.id as id0_, customers0_.realName as realName0_, customers0_.pass as pass0_, customers0_.sex as sex0_, customers0_.petName as petName0_, customers0_.email as email0_, customers0_.rdate as rdate0_ from customers customers0_cn.csdn.products.domain.Customers@5bf624

（2）通过Query得到Query的对象，并用iterator迭代器输出

 1 （2）通过Query得到Query的对象，并用iterator迭代器输出
 2
 3     public void iterateQuery(){
 4
 5        Configuration configuration = new Configuration().configure();
 6        SessionFactory factory = configuration.buildSessionFactory();
 7        Session session = factory.openSession();
 8        Query query = session.createQuery("from Customers");
 9        Iterator<Customers> it = query.iterate();
10
11        // 遍历出来所有的查询结果
12        while (it.hasNext()) {
13            Customers customer = it.next();
14            System.out.println(customer.toString());
15        }
16     }

Iterator的执行结果：

1 Hibernate: select customers0_.id as col_0_0_ from customers customers0_
2
3 Hibernate: select customers0_.id as id0_0_, customers0_.realName as realName0_0_, customers0_.pass as pass0_0_, customers0_.sex as sex0_0_, customers0_.petName as petName0_0_, customers0_.email as email0_0_, customers0_.rdate as rdate0_0_ from customers customers0_ where customers0_.id=?
4
5 cn.csdn.products.domain.Customers@1d13272

结论：

（1）从上面的执行结果可以看出获取的方式不一样

　　  List的获取方式为：List<Customers> list = query.list();

Iterator的获取方式：Iterator<Customers> it = query.iterate();

（2）从执行结果可以看出list输出一条语句，而iterator输出的是两条sql语句，我们可想一下，为什么会输出这样的效果？

因为他们获取数据的方式不一样，list()会直接查询数据库，iterator()会先到数据库中把id都取出来，然后真正要遍历某个对象的时候先到缓存中找，如果找不到，以id为条件再发一条sql到数据库，这样如果缓存中没有数据，则查询数据库的次数为n+1次

（3）list只查询一级缓存，而iterator会从二级缓存中查

（4）list方法返回的对象都是实体对象，而iterator返回的是代理对象

（5）list, iterator在hibernate中的区别之处, 此处iterator会出现1+N问题而list则不会