hibernate中 list和iterator方法的差別

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，并對Query對象進行操作，首先是用list方法擷取到Query的List集合并輸出

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_

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（2）通過Query得到Query的對象，并用iterator疊代器輸出

    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();

       // 周遊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=?

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結論：

（1）       從上面的執行結果可以看出擷取的方式不一樣

List的擷取方式為：List<Customers> list = query.list();

Iterator的擷取方式：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，為什麼會輸出這樣的效果？

因為他們擷取資料的方式不一樣，list()會直接查詢資料庫，iterator()會先到資料庫中把id都取出來，然後真正要周遊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到資料庫，這樣如果緩存中沒有資料，則查詢資料庫的次數為n+1次

（3）list隻查詢一級緩存，而iterator會從二級緩存中查

（4）list方法傳回的對象都是實體對象，而iterator傳回的是代理對象

（5) session中list第二次發出，仍會到資料庫査詢

（6) iterate 第二次，首先找session 級緩存