Mybatis源码学习（一）Mybatis 数据库访问的流程

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直接发送

SQL

一次数据库访问

SqlSessionFactoryBuilder

SqlSessionFactory

SqlSession

db

SqlMapper

1. SqlSessionFactoryBuilder 读取配置信息创建SQLSessionFactory，建造者模式，方法级生命周期，这个东西就是为了创造SqlSessionFactory的，所以一般放在方法内部，用完即销毁

2. SQLSessionFactory 用来创建SqlSession的，工厂单例模式（本身是单例的，用来创建SqlSession的时候体现了工厂模式），存在于程序的整个生命周期

3. SqlSession 代表一次数据库连接，一般通过调用Mapper访问数据库（其实底层也是发送SQL到数据库），也可以直接发送SQL到数据库上，非线程安全，要保证线程独享（方法级）

4. SQLMapper 由一个Java接口和XML文件组成，包含了要执行的SQL语句和结果集的映射规则。方法级生命周期

需要注意的点：

---

#{} 与 ${}

1. #{} 是预解析，SQL中会被解析为?，最后通过传参处理，这种参数不会有有SQL注入的问题

2. ${} 是字符串拼接，一般用于动态SQL，比如动态表名、动态列名、动态排序等，但是这种参数容易出现SQL注入的问题，一定要使用拦截器做好合法性校验

BatchExecutor与foreach+value

1. BatchExecutor是批量执行SQL，但是SQL还是会一条一条的发送到数据库上，最后在数据库上批量执行。（BatchExecutor是通过SqlSession获取来的，应该说所有的Executor都是通过SqlSession获取来的）

2. foreach是动态拼接SQL，最后拼接了一个大SQL，一次性发送给数据库，相比BatchExecutor，foreach因为只有一条SQL，所以节约了网络IO，效率更高。但是也需要注意，不要因为循环过多而导致SQL过大，超过数据库SQL长度限制的SQL执行也是会报错的，建议分批执行，比如500个一批。但是，并不是所有的数据库都支持这种insert方式……

文末

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