SQL语句解析执行的过程及原理 SQL语句解析执行的过程及原理

一、sqlSession简单介绍
二、获得sqlSession对象源码分析
三、SQL执行流程，以查询为例

【SQL语句解析执行的过程及原理】
一、sqlSession简单介绍

拿到SqlSessionFactory对象后，会调用SqlSessionFactory的openSesison方法，这个方法会创建一个Sql执行器（Executor），这个Sql执行器会代理你配置的拦截器方法。
获得上面的Sql执行器后，会创建一个SqlSession（默认使用DefaultSqlSession）,这个SqlSession中也包含了Configration对象，所以通过SqlSession也能拿到全局配置；
获得SqlSession对象后就能执行各种CRUD方法了。

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二、获得sqlSession对象源码分析

/** * 通过sqlSessionFactory.openSession进行获取sqlSession对象 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession() */public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); }/** * 通过数据源去获取SqlSession * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean) */private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level,boolean autoCommit) { Transaction tx = null; try {// 获取环境变量final Environment environment = configuration.getEnvironment(); // 获取事务工厂final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); // 获取一个事务tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); // 获取执行器，这边获得的执行器已经代理拦截器的功能final Executor executor = configuration.newExecutor(tx, execType); // 根据获取的执行器创建SqlSessionreturn 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(); }}/** * 获取执行器 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean) */public Executor newExecutor(Transaction transaction, ExecutorType executorType) { // 默认使用SIMPLE的执行器 executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; if (ExecutorType.BATCH == executorType) {// 批量的执行器executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) {// 可重复使用的执行器executor = new ReuseExecutor(this, transaction); } else {// 简单的sql执行器executor = new SimpleExecutor(this, transaction); } // 判断Mybatis的全局配置文件是否开启二级缓存 if (cacheEnabled) {// 开启缓存，吧executor包装为CachingExecutorexecutor = new CachingExecutor(executor); } // 插件的调用：责任链模式 executor = (Executor) interceptorChain.pluginAll(executor); return executor; }

三、SQL执行流程，以查询为例

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/** * 查询的入口方法 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(String, Object) */public T selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many. // 查询数据 List list = this.selectList(statement, parameter); // 长度为1，拿第一个 if (list.size() == 1) {return list.get(0); } else if (list.size() > 1) { // 长度大于一，抛异常throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size()); } else {// 没有拿到返回nullreturn null; }}/** * 查询数据 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(String, Object, RowBounds) */public List selectList(String statement, Object parameter, RowBounds rowBounds) { try {// 通过statement去全局配置文件中获取MappedStatement（得到mapper中增删改查的节点）MappedStatement ms = configuration.getMappedStatement(statement); // 通过执行器去执行SQLreturn executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); } catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database.Cause: " + e, e); } finally {ErrorContext.instance().reset(); }}/** * 执行查询操作的准备工作 * 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler) */public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,ResultHandler resultHandler) throws SQLException { // 通过参数进行sql解析 BoundSql boundSql = ms.getBoundSql(parameterObject); CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }/** * 执行查询操作的准备工作 * 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) */public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { // 判断sql是否开启了缓存 Cache cache = ms.getCache(); // 有缓存 if (cache != null) {// 判断是否需要刷新缓存flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked")// 去二级缓存中获取（装饰者模式）List list = (List) tcm.getObject(cache, key); // 二级缓存没有找到if (list == null) {// 查询数据，并放入缓存list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116}return list; } } // 查询数据 return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }/** * 一级缓存查询的调用 * 源码位置：org.apache.ibatis.executor.BaseExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) */public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,CacheKey key, BoundSql boundSql) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId()); // 已经关闭了，抛异常 if (closed) {throw new ExecutorException("Executor was closed."); } // 清空本地缓存 if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache(); } List list; try {// 从一级缓存中获取数据queryStack++; list = resultHandler == null ? (List) localCache.getObject(key) : null; if (list != null) {// 缓存里面有，进行处理handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else {// 缓存没有，进行查询list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally {queryStack--; } if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load(); }// issue #601deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache(); } } return list; }/** * 在数据库中查询 * 源码位置：org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql) */private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try {// 去数据库查询list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally {localCache.removeObject(key); } // 一级缓存进行缓存 localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter); } return list; }/** * 查询逻辑 * 源码位置：org.apache.ibatis.executor.SimpleExecutor.doQuery(MappedStatement, Object, RowBounds, ResultHandler, BoundSql) */public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,BoundSql boundSql) throws SQLException { Statement stmt = null; try {// 得到整体的配置对象Configuration configuration = ms.getConfiguration(); // 内部封装了ParameterHandler和ResultSetHandlerStatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds,resultHandler, boundSql); stmt = prepareStatement(handler, ms.getStatementLog()); // 执行查询return handler.query(stmt, resultHandler); } finally {closeStatement(stmt); }}/** * 执行查询语句 * 源码位置：org.apache.ibatis.executor.statement.SimpleStatementHandler.query(Statement, ResultHandler) */public List query(Statement statement, ResultHandler resultHandler) throws SQLException { // 得到要执行的sql String sql = boundSql.getSql(); // 执行sql statement.execute(sql); // 处理结果集 return resultSetHandler.handleResultSets(statement); }