hibernate iterate list 区别

最新推荐文章于 2018-09-27 23:13:10 发布

原创最新推荐文章于 2018-09-27 23:13:10 发布 · 513 阅读

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本文通过对比iterate与list方法，详细解析了Hibernate中的延迟加载机制。使用iterate时，Hibernate仅先发送获取所有ID的SQL，待实际需要数据时再逐条查询；而list则一次性加载所有数据。文章通过具体代码示例展示了不同加载方式对数据库查询的影响。

iterate:

	        String sql = "from Bigque where id <= 5";
		String sql2 = "from Bigque where id <= 6";
		BigqueDAO dao = new BigqueDAO();
		Session session = dao.getSession();
		Iterator<Bigque> it = session.createQuery(sql).iterate();
		
		while(it.hasNext()){
			System.out.println(it.next().getContent());
		}
		
		System.out.println("===========");
		
                Iterator<Bigque> it2 = session.createQuery(sql2).iterate();
		
		while(it2.hasNext()){
			System.out.println(it2.next().getContent());
		}
		session.close();

结果：

Hibernate: select bigque0_.ID as col_0_0_ from cloudroom.bigque bigque0_ where bigque0_.ID<=5
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
选择题
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
填空题
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
阅读题
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
判断题
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
选择题
===========
Hibernate: select bigque0_.ID as col_0_0_ from cloudroom.bigque bigque0_ where bigque0_.ID<=6
选择题
填空题
阅读题
判断题
选择题
Hibernate: select bigque0_.ID as ID51_0_, bigque0_.Content as Content51_0_, bigque0_.SubjectID as SubjectID51_0_ from cloudroom.bigque bigque0_ where bigque0_.ID=?
填空题

结论：

延迟加载先发送一条sql语句获取所有的id 用到的时候再发送SQL去数据库查找每个对象

在一个session中两次iterate 第二个iterate还是会发送一条SQL获取id 用到的时候再发送SQL去数据库查找每个对象（如果session中有这个对象，就在缓存中查找，不发送SQL）

list:

                String sql = "from Bigque where id <= 5";
	//	String sql2 = "from Bigque where id <= 6";
		BigqueDAO dao = new BigqueDAO();
		Session session = dao.getSession();
		List<Bigque> list = session.createQuery(sql).list();
		for(int i=0; i<list.size(); i++){
			System.out.println(list.get(i).getContent());
		}
		System.out.println("=============");
		
		List<Bigque> list2 = session.createQuery(sql).list();
		for(int i=0; i<list2.size(); i++){
			System.out.println(list2.get(i).getContent());
		}
		session.close();

结果：

Hibernate: select bigque0_.ID as ID51_, bigque0_.Content as Content51_, bigque0_.SubjectID as SubjectID51_ from cloudroom.bigque bigque0_ where bigque0_.ID<=5
选择题
填空题
阅读题
判断题
选择题
=============
Hibernate: select bigque0_.ID as ID51_, bigque0_.Content as Content51_, bigque0_.SubjectID as SubjectID51_ from cloudroom.bigque bigque0_ where bigque0_.ID<=5
选择题
填空题
阅读题
判断题
选择题

结论：list取所有第二次发出仍会到数据库查找