MyBatis源码分析（一）——一级缓存和二级缓存

分手后的思念是犯贱 2023-06-18 03:52 3阅读 0赞

## 前言 ##

看过无数资料，背过无数面试题，我们都知道了mybatis的一级缓存是sqlSession,二级缓存是namespace,但是mybatis对于缓存到底是怎么实现的，怎么处理的呢，我想不少人其实都是一头雾水。今天，就让我们一起来看看，这背后的原理。

## 一、核心类图 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70][]

二、代码实现：

第一步:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 1][]

第二步：看看执行器获取逻辑

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 2][]

第三步：分析执行器simpleExcutor

SimpleExcutor的query方法来自于父类BaseExcutor

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 3][]

那么这里的localCache到底是什么呢？

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 4][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 5][]

那么为啥说一级缓存是sqlSession，而且它和线程绑定？

首先，我们sqlSession持有一个Excutor对象，而excutor对象持有一个PerpetualCache对象，也就是我们的实际存储对象。然后再看看Excutor在提交后会做什么？

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 6][]

对的，清空缓存，那么一旦我们的事务提交，缓存立即清空，也就是说它和当前用户请求线程绑定

第四步：分析执行器cacheExcutor

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 7][]

再看看这个cache哪里来的，我一路追踪，发现其最终实现在MapperBuilderAssistant类的useCacheRef和useNewCache方法

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1ODEzNjUz_size_16_color_FFFFFF_t_70 8][]

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