Hibernate学习笔记5（一级缓存）

末蓝、 2021-11-23 00:42 287阅读 0赞

# **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** #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwODE3ODI3_size_16_color_FFFFFF_t_70]  
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]

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