原：http://blog.csdn.net/hanxiaoshuang321123/article/details/7103414
1．首先看兩個例子來比較一下
（1）在用Query方法查詢的時候，通過HQL語句來得到Query對象，並對Query對象進行操作，首先是用list方法獲取到Query的List集合並輸出
public void listQuery() {
Configuration configuration = new Configuration().configure();
SessionFactory factory = configuration.buildSessionFactory();
Session session = factory.openSession();
Query query = session.createQuery("from Customers");
List<Customers> list = query.list();
for(Customers entity:list){
System.out.println(entity.toString());
}
}
輸出的結果為：
List的執行sql語句為：
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）通過Query得到Query的對象，並用iterator迭代器輸出
public void iterateQuery(){
Configuration configuration = new Configuration().configure();
SessionFactory factory = configuration.buildSessionFactory();
Session session = factory.openSession();
Query query = session.createQuery("from Customers");
Iterator<Customers> it = query.iterate();
// 遍歷出來所有的查詢結果
while (it.hasNext()) {
Customers customer = it.next();
System.out.println(customer.toString());
}
}
Iterator的執行結果：
Hibernate: select customers0_.id as col_0_0_ from customers customers0_
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=?
cn.csdn.products.domain.Customers@1d13272
結論：
（1） 從上面的執行結果可以看出獲取的方式不一樣
List的獲取方式為：List<Customers> list = query.list();
Iterator的獲取方式：Iterator<Customers> it = query.iterate();
（2）從執行結果可以看出list輸出一條語句，而iterator輸出的是兩條sql語句，我們可想一下，為什么會輸出這樣的效果？
因為他們獲取數據的方式不一樣，list()會直接查詢數據庫，iterator()會先到數據庫中把id都取出來，然后真正要遍歷某個對象的時候先到緩存中找，如果找不到，以id為條件再發一條sql到數據庫，這樣如果緩存中沒有數據，則查詢數據庫的次數為n+1次
（3）list只查詢一級緩存，而iterator會從二級緩存中查
（4）list方法返回的對象都是實體對象，而iterator返回的是代理對象
（5) session中list第二次發出，仍會到數據庫査詢
（6) iterate 第二次，首先找session 級緩存