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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
Hibernate面试题 --- list和iterator方法的区别
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原文地址：http://www.cnblogs.com/angleBlue/p/4780844.html
