List和iterator的區別

（1）在用Query方法查詢的時候，經過HQL語句來獲得Query對象，並對Query對象進行操做，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());

       }

}

輸出的結果爲：

List的執行sql語句爲：

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）經過Query獲得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();

       // 遍歷出來全部的查詢結果

       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）       從上面的執行結果能夠看出獲取的方式不同

List的獲取方式爲：List<Customers> list = query.list();

Iterator的獲取方式：Iterator<Customers> it = query.iterate();

（2）從執行結果能夠看出list輸出一條語句，而iterator輸出的是兩條sql語句，咱們可想一下，爲何會輸出這樣的效果？

由於他們獲取數據的方式不同，list()會直接查詢數據庫，iterator()會先到數據庫中把id都取出來，而後真正要遍歷某個對象的時候先到緩存中找，若是找不到，以id爲條件再發一條sql到數據庫，這樣若是緩存中沒有數據，則查詢數據庫的次數爲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