**关注公众号 [不爱总结的麦穗](https://mp.weixin.qq.com/s?__biz=MzU2ODkyMzA1NQ==\&mid=2247483929\&idx=1\&sn=0b68e59b052042eab55de8dcec6288ab\&chksm=fc87c4d9cbf04dcf7b9eaae3dfa8dd6d94059245ad8eff3a4a09f68eda1b66ec6ea9f7b96742\&token=400735602\&lang=zh_CN#rd) 将不定期推送技术好文** 前面的文章中主要分享了MyBatis是如何初始化和解析配置文件的，这些都是一次性的过程。 **简单回顾了一下MyBatis初始化做了什么？** `MyBatis初始化通过 SqlSessionFactoryBuilder 从 XML 配置文件或一个预先配置的 Configuration 实例来构建SqlSessionFactory 实例。` 具体的大家可以去看看我之前的文章： * [MyBatis初始化基本过程](https://juejin.cn/post/7362547962604306469) * [Mybatis配置解析](https://juejin.cn/post/7404777076089995274) 回到主题，接下通过下面的代码来分享MyBatis 的Sql执行流程 ![image.png](https://p6-xtjj-sign.byteimg.com/tos-cn-i-73owjymdk6/baa35311dfb64f8ca2f812f70870ff53~tplv-73owjymdk6-jj-mark-v1:0:0:0:0:5o6Y6YeR5oqA5pyv56S-5Yy6IEAg5LiN54ix5oC757uT55qE6bqm56mX:q75.awebp?rk3s=f64ab15b&x-expires=1773626543&x-signature=jliIkEPpGlRfa9lgsN65zK790fA%3D) ### 创建 SqlSession ```java sqlSession = sqlSessionFactory.openSession(); ``` 通过`DefaultSqlSessionFactory#openSession()`方法去获取SqlSession对象。 * **DefaultSqlSessionFactory#openSessionFromDataSource** ```java private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { // 通过Confuguration对象去获取相关配置信息, Environment对象包含了数据源和事务的配置 final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 创建执行器 final Executor executor = configuration.newExecutor(tx, execType); // 创建DefaultSqlSession对象 return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } ``` **SqlSession** 是 MyBatis 的核心接口，它负责与数据库进行交互。SqlSession 提供了许多方法，如 selectOne、selectList、insert、update、delete 等，可以执行 SQL 语句并返回结果。 ### 获取MapperProxy ```java ScheduleSettingMapper scheduleSettingMapper = sqlSession.getMapper(ScheduleSettingMapper.class); ``` 在mybatis中，通过MapperProxy动态代理咱们的dao,那怎么获取MapperProxy对象呢？ * **MapperRegistry#getMapper** ```java public T getMapper(Class type, SqlSession sqlSession) { // 从knownMappers获取代理工厂 final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { // 通过mapperProxyFactory返回一个对应映射接口的代理器实例mapperProxy return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } } ``` 代理对象MapperProxy创建过程非常简单，通过[JDK动态代理](https://juejin.cn/post/7320288231194705970)来包装SqlSession ```java // 每一个Mapper接口对应一个MapperProxyFactory代理工厂 Map, MapperProxyFactory> knownMappers = new ConcurrentHashMap<>(); ``` **knownMappers中的映射关系是什么时候产生的呢?** * **XMLConfigBuilder#parseConfiguration** ```java private void mappersElement(XNode context) throws Exception { if (context == null) { return; } for (XNode child : context.getChildren()) { // 1 解析package配置方式 if ("package".equals(child.getName())) { String mapperPackage = child.getStringAttribute("name"); configuration.addMappers(mapperPackage); } else { String resource = child.getStringAttribute("resource"); String url = child.getStringAttribute("url"); String mapperClass = child.getStringAttribute("class"); // 2 解析Resource配置全限定路径xml方式 if (resource != null && url == null && mapperClass == null) { ErrorContext.instance().resource(resource); try (InputStream inputStream = Resources.getResourceAsStream(resource)) { XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); mapperParser.parse(); } } else if (resource == null && url != null && mapperClass == null) { // 3 解析url本地配置或远程配置方式 ErrorContext.instance().resource(url); try (InputStream inputStream = Resources.getUrlAsStream(url)) { XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments()); mapperParser.parse(); } } else if (resource == null && url == null && mapperClass != null) { // 4 解析mapper配置接口路径方式 Class mapperInterface = Resources.classForName(mapperClass); // 向knownMappers存入映射关系 configuration.addMapper(mapperInterface); } else { throw new BuilderException( "A mapper element may only specify a url, resource or class, but not more than one."); } } } } ``` 在解析mybatis-config.xml配置文件时，Mybatis提供了**4种mapper配置方式**，**不论配置哪种方式，最终MyBatis都会将xml映射文件和Mapper接口进行关联** ### 执行Sql ```java ScheduleSettingPO scheduleSettingPO = scheduleSettingMapper.findByJobId("test"); ``` **每个MapperProxy对应一个dao接口，执行Sql，MapperProxy是怎么做的呢？** * **MapperProxy#invoke** ```java @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } return cachedInvoker(method).invoke(proxy, method, args, sqlSession); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } ``` ```java public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable { return mapperMethod.execute(sqlSession, args); } ``` 代理对象调用了MapperMethod#execute方法，该方法主要是判断CRUD类型，然后根据类型去选择到底执行sqlSession中的哪个方法。具体的源码感兴趣的可以去看一下。 * **DefaultSqlSession#selectList** ```java private List selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) { try { MappedStatement ms = configuration.getMappedStatement(statement); dirty |= ms.isDirtySelect(); // CRUD实际上是交给Excetor去处理 return executor.query(ms, wrapCollection(parameter), rowBounds, handler); } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } ``` 通过一层一层的调用，最终会来到doQuery方法。 * **SimpleExecutor#doQuery** ```java public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { Statement stmt = null; try { Configuration configuration = ms.getConfiguration(); // 创建StatementHandler StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); // 进行SQL查询参数的设置 stmt = prepareStatement(handler, ms.getStatementLog()); // 交由StatementHandler处理 return handler.query(stmt, resultHandler); } finally { closeStatement(stmt); } } ``` **StatementHandler是可以被拦截器拦截的，众多物理分页的实现都是在这个地方使用拦截器实现。** StatementHandler参数设置完毕后，执行数据库操作。 * **SimpleStatementHandler#query** ```java public List query(Statement statement, ResultHandler resultHandler) throws SQLException { String sql = boundSql.getSql(); // 调用 JDBC的PreparedStatement执行SQL（数据库操作）了 statement.execute(sql); // 结果给ResultSetHandler 去处理，ResultSetHandler也是可以被拦截的 return resultSetHandler.handleResultSets(statement); } ``` **到这里，整个SQL语句执行流程分析就结束了。** ### 总结 **主要分析了MyBatis的SQL执行流程。在分析流程的过程中，知道了Mapper接口和映射文件是如何进行绑定的，MyBatis中SQL语句的执行流程中Sql参数的设置，SimpleStatementHandler执行、ResultSetHandler结果的处理。**