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映射器Mapper实现事务的原理：
mapper本质是一个接口代理类，调用接口等某个方法后，获取字符串等于接口全路径名称+方法名去缓存里面找相应的sql语句，然后调用Transaction#getConnection获取数据库连接，后续的提交回滚也是调用这个接口等方法。
sqlsession实现事务的原理：
同上，可以设置自动提交事务
mybatis集成spring实现事务的原理
1、spring实现事务有3种方法，声明式事务包括xml和注解，编程实现事务也就是利用spring事务相关API，xml和注解本质上aop+编程实现事务；
public class UserService { private final PlatformTransactionManager
transactionManager; public UserService(PlatformTransactionManager
transactionManager) { this.transactionManager = transactionManager; } public
void createUser() { 1 TransactionStatus txStatus = transactionManager.
getTransaction(new DefaultTransactionDefinition()); try { 2 userMapper.
insertUser(user); } catch (Exception e) { 3 transactionManager.rollback(txStatus
); throw e; } 4 transactionManager.commit(txStatus); } }
代码1设置自动提交为false，表示开启一个事务；代码2底层会调用Transaction接口的实现类SpringTransaction,把数据库连接放到ThreadLocal，如果后面再有mapper或者sqlsession调用Transaction#getConnection方法，就会获取保存在ThreadLocal里面的数据库连接；代码3和代码4则会获取上面等数据库连接进行提交和回滚。可以知道最开始去获取数据库连接的是代码1，后面代码都是直接从ThreadLocal缓存里面拿。
为什么DataSourceTransactionManager和mybatis需要同一个数据库源？
答：首先DataSourceTransactionManager会去数据库源获取数据库连接，然后放到ThreadLocal，mybatis去SpringTransaction拿数据库连接，得到的是上面ThreadLocal的数据库连接。
还有一个疑问，不在事务里面执行mapper方法是自动提交，在事务里面mapper则是手动提交，这是怎么实现的？
