• 缓存的概述

  • 什么是缓存

    • 缓存：是一种优化的方式，将数据存到数据库中，使用的时候直接从缓存中获取，不用通过存储源（数据库、磁盘上的源文件）。

• hibernate的缓存

hibernate缓存中提供了优化手段：缓存和抓取策略。hibernate中提供了二种缓存机制：一级缓存、二级缓存。

• hibernate的一级缓存

  • 概述：称为是Session级别的缓存，一级缓存生命周期与Session一致（一级缓存是由Session中的一系列Java集合构成），并且一级缓存是自带的不可卸载的。hibernate的二级缓存是SessionFactory级别的缓存，需要配置的缓存。

• 证明一级缓存的存在

  	// 证明一级缓存的存在
  	@Test
  	public void demo3() {
  		Session session = HibernateUtils.openSession();
  		Transaction transaction = session.beginTransaction();
  
  		Customer customer1 = session.get(Customer.class, 1l);// 发送SQL语句
  		System.out.println(customer1);
  
  		Customer customer2 = session.get(Customer.class, 1l);// 不发送SQL语句，此时从缓存中获取
  		System.out.println(customer2);
  
  		System.out.println(customer1 == customer2); // 是否为同一个对象
  
  		transaction.commit();
  		session.close();
  	}

  

   

• hibernate的一级缓存的内部结构

  • 一级缓存中的特殊区域：快照区。正是因为有了快照区，持久态对象才会自动更新数据库。

    

     

• 一级缓存的清空

  • clear() : 清空所有。
  • evict() : 清除指定对象。

	@Test
	// 一级缓存的清空
	public void demo5() {
		Session session = HibernateUtils.openSession();
		Transaction transaction = session.beginTransaction();

		Customer customer1 = session.get(Customer.class, 1l);// 发送SQL语句，同时放入到一级缓存中

		session.clear();// 清空所有
		session.evict(customer1);// 清空指定对象

		Customer customer2 = session.get(Customer.class, 1l);// 发送SQL语句
		System.out.println(customer1);
		System.out.println(customer2);
		transaction.commit();
		session.close();
	}

运行结果：
Hibernate: 
    select
        customer0_.cust_id as cust_id1_0_0_,
        customer0_.cust_name as cust_nam2_0_0_,
        customer0_.cust_source as cust_sou3_0_0_,
        customer0_.cust_industry as cust_ind4_0_0_,
        customer0_.cust_level as cust_lev5_0_0_,
        customer0_.cust_phone as cust_pho6_0_0_,
        customer0_.cust_mobile as cust_mob7_0_0_ 
    from
        cst_customer customer0_ 
    where
        customer0_.cust_id=?
Hibernate: 
    select
        customer0_.cust_id as cust_id1_0_0_,
        customer0_.cust_name as cust_nam2_0_0_,
        customer0_.cust_source as cust_sou3_0_0_,
        customer0_.cust_industry as cust_ind4_0_0_,
        customer0_.cust_level as cust_lev5_0_0_,
        customer0_.cust_phone as cust_pho6_0_0_,
        customer0_.cust_mobile as cust_mob7_0_0_ 
    from
        cst_customer customer0_ 
    where
        customer0_.cust_id=?
Customer [cust_id=1, cust_name=James, cust_source=null, cust_industry=null, cust_level=null, cust_phone=null, cust_mobile=null]
Customer [cust_id=1, cust_name=James, cust_source=null, cust_industry=null, cust_level=null, cust_phone=null, cust_mobile=null]