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。

二、获得sqlSession对象源码分析

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/**   * 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SqlSession          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();      } }   /**   * 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CachingExecutor          executor = 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> 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<T> list = this.<T>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 null          return null ;      } }   /**   * 查询数据   * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(String, Object, RowBounds)   */ public <E> List<E> 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SQL          return 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 <E> List<E> 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 <E> List<E> 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>      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<E> list = (List<E>) 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.<E>query(ms, parameterObject, rowBounds, resultHandler, key , boundSql);                  tcm.putObject(cache, key , list); // issue #578 and #116              }              return list;          }      }      // 查询数据      return delegate.<E>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 <E> List<E> 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<E> list;      try {          // 从一级缓存中获取数据          queryStack++;          list = resultHandler == null ? (List<E>) 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 #601          deferredLoads.clear();          if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {              // issue #482              clearLocalCache();          }      }      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 <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,          ResultHandler resultHandler, CacheKey key , BoundSql boundSql) throws SQLException {      List<E> 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 <E> List<E> 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ResultSetHandler          StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds,                  resultHandler, boundSql);          stmt = prepareStatement(handler, ms.getStatementLog());            // 执行查询          return handler.<E>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 <E> List<E> 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.<E>handleResultSets(statement); }

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原文链接：https://www.cnblogs.com/zfcq/p/16069357.html