MyBatis源码分析-SQL语句执行的详细流程

MyBatis源码分析-SQL语句执行的完整流程MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以对配置和原生Map使用简单的 XML 或注解，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。如何新建MyBatis源码工程请点击MyBatis源码分析-IDEA新建MyBatis源码工程。MyBatis框架主要完成的是以下2件事情：根据JDBC规范建立与数据库的连接。通过反射打通Java对象与数据库参数交互之间相互转换的关系。MyBatis框架是一种典型的交互式框架，先准备好交互的必要条件，然后构建一个交互的环境，在交互环境中划分会话，在会话中与数据库进行交互数据。1 MyBatis主要的类Configuration MyBatis所有的配置信息都维持在Configuration对象之中。SqlSession 作为MyBatis工作的主要顶层API，表示和数据库交互的会话，完成必要数据库增删改查功能Executor MyBatis执行器，是MyBatis 调度的核心，负责SQL语句的生成和查询缓存的维护StatementHandler 封装了JDBC Statement操作，负责对JDBC statement 的操作，如设置参数、将Statement结果集转换成List集合。ParameterHandler 负责对用户传递的参数转换成JDBC Statement 所需要的参数，ResultSetHandler 负责将JDBC返回的ResultSet结果集对象转换成List类型的集合；TypeHandler 负责java数据类型和jdbc数据类型之间的映射和转换MappedStatement MappedStatement维护了一条节点的封装，SqlSource 负责根据用户传递的parameterObject，动态地生成SQL语句，将信息封装到BoundSql对象中，并返回BoundSql 表示动态生成的SQL语句以及相应的参数信息以上几个类在SQL操作中都会涉及，在SQL操作中重点关注下SQL参数什么时候写入和结果集怎么转换为Java对象，这两个过程正好对应的类是PreparedStatementHandler和ResultSetHandler类。(图片来自深入理解mybatis原理 MyBatis的架构设计以及实例分析)2 SQL执行流程MyBatis主要设计目的还是为了让我们在执行SQL时对输入输出的数据的管理更加方便，所以方便的让我们写出SQL和方便的获取SQL的执行结果是MyBatis的核心竞争力。下面就用一个例子来从源码角度看一下SQL的完整执行流程。新建配置文件conf.xml： conf.xml首先建立数据表，这里就以user表为例 ：复制代码DROP TABLE IF EXISTS user;CREATE TABLE user ( id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, name VARCHAR(32) NOT NULL, password VARCHAR(32) NOT NULL, sex int, email VARCHAR(32), phone VARCHAR(16), admin VARCHAR(16);复制代码然后新建与数据表对应的类User： User再新建usre表的配置文件： userMapper.xml最后新建测试类：复制代码/* * MyBatis测试类 */public class TestMain public static void main(String args) throws IOException String resouce = conf.xml; InputStream is = Resources.getResourceAsStream(resouce); / 构建sqlSession工厂 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is); / 获取sqlSession SqlSession session = sqlSessionFactory.openSession(); User user; try /* * 第一种方式: 直接执行已映射的 SQL 语句 */ String statement = com.luoxn28.dao.UserDao.getById; user = session.selectOne(statement, 1); System.out.println(user); finally session.close(); /* * 第二种方式: 执行更清晰和类型安全的代码 */ UserDao userDao = session.getMapper(UserDao.class);/ user = userDao.getById(1);/ System.out.println(user); 复制代码由于我们分析的是SQL的执行流程，那就重点关注下 user = session.selectOne(statement, 1); 这行代码 注意，传进去的参数是1。session是DefaultSqlSession类型的，因为sqlSessionFactory默认生成的SqlSession是DefaultSqlSession类型。selectOne()会调用selectList()。复制代码/ DefaultSqlSession类public List selectList(String statement, Object parameter, RowBounds rowBounds) try MappedStatement ms = configuration.getMappedStatement(statement); / CURD操作是交给Excetor去处理的 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(); 复制代码在DefaultSqlSession.selectList中的各种CURD操作都是通多Executor进行的，这里executor的类型是CachingExecutor，接着跳转到其中的query方法中。/ CachingExecutor 类public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException BoundSql boundSql = ms.getBoundSql(parameterObject); / 获取绑定的sql命令，比如SELECT * FROM xxx CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);getBoundSql为了获取绑定的sql命令，在创建完cacheKey之后，就进入到CachingExecutor 类中的另一个query方法中。复制代码/ CachingExecutor 类Overridepublic List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException Cache cache = ms.getCache(); if (cache != null) flushCacheIfRequired(ms); if (ms.isUseCache() & resultHandler = null) ensureNoOutParams(ms, parameterObject, boundSql); SuppressWarnings(unchecked) List list = (List) tcm.getObject(cache, key); if (list = null) list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); / issue #578 and #116 return list; return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);复制代码这里真正执行query操作的是SimplyExecutor代理来完成的，接着就进入到了SimplyExecutor的父类BaseExecutor的query方法中。复制代码/ SimplyExecutor的父类BaseExecutor类SuppressWarnings(unchecked)Overridepublic 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(); if (closed) throw new ExecutorException(Executor was closed.); if (queryStack = 0 & ms.isFlushCacheRequired() clearLocalCache(); List list; try queryStack+; /* * localCache是一级缓存，如果找不到就调用queryFromDatabase从数据库中查找 */ list = resultHandler = null ? (List) localCache.getObject(key) : null; if (list != null) handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); else 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;复制代码因为是第一次SQL查询操作，所以会调用queryFromDatabase方法来执行查询。复制代码/ SimplyExecutor的父类BaseExecutor类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 list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); finally localCache.removeObject(key); localCache.putObject(key, list); if (ms.getStatementType() = StatementType.CALLABLE) localOutputParameterCache.putObject(key, parameter); return list;复制代码从数据库中查询数据，进入到SimplyExecutor中进行操作。复制代码/ SimplyExecutor类public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException Statement stmt = null; try Configuration configuration = ms.getConfiguration(); StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); / 子流程1：SQL查询参数的设置 stmt = prepareStatement(handler, ms.getStatementLog(); / StatementHandler封装了Statement / 子流程2：SQL查询操作和结果集的封装 return handler.query(stmt); finally closeStatement(stmt); 复制代码注意，在prepareStatement方法中会进行SQL查询参数的设置，也就是咱们最开始传递进来的参数，其值为1。handler.query(stmt)方法中会进行实际的SQL查询操作和结果集的封装（封装成Java对象）。当流程走到这里时，程序已经压栈有一定深度了，因为接下来程序分析会兵分两路，一方面深入到SQL查询及结果集的设置子流程1中，然后再深入到SQL查询操作和结果集的封装子流程2，因为还会回到这里，所以就来一张调用栈的特写吧：子流程1：SQL查询参数的设置复制代码/ SimplyExecutor类private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException Statement stmt; / 获取一个Connection Connection connection = getConnection(statementLog); stmt = handler.prepare(connection, transaction.getTimeout(); handler.parameterize(stmt); / 设置SQL查询中的参数值 return stmt;复制代码通过getConnection方法来获取一个Connection，调用prepare方法来获取一个Statement（这里的handler类型是RoutingStatementHandler，RoutingStatementHandler的prepare方法调用的是PrepareStatementHandler的prepare方法，因为PrepareStatementHandler并没有覆盖其父类的prepare方法，其实最后调用的是BaseStatementHandler中的prepare方法。是不是绕晕了，那就再看一遍吧 :) ）。调用parameterize方法来设置SQL的参数值（这里最后调用的是PrepareStatementHandler中的parameterize方法，而PrepareStatementHandler.parameterize方法调用的是DefaultParameterHandler中的setParameters方法）。/ PrepareStatementHandler类Overridepublic void parameterize(Statement statement) throws SQLException parameterHandler.setParameters(PreparedStatement) statement);复制代码/ DefaultParameterHandler类Overridepublic void setParameters(PreparedStatement ps) ErrorContext.instance().activity(setting parameters).object(mappedStatement.getParameterMap().getId(); List parameterMappings = boundSql.getParameterMappings(); if (parameterMappings != null) for (int i = 0; i parameterMappings.size(); i+) ParameterMapping parameterMapping = parameterMappings.get(i); if (parameterMapping.getMode() != ParameterMode.OUT) Object value; String propertyName = parameterMapping.getProperty(); if (boundSql.hasAdditionalParameter(propertyName) / issue #448 ask first for additional params value = boundSql.getAdditionalParameter(propertyName); else if (parameterObject = null) value = null; else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass() value = parameterObject; else MetaObject metaObject = configuration.newMetaObject(parameterObject); value = metaObject.getValue(propertyName); TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if (value = null & jdbcType = null) jdbcType = configuration.getJdbcTypeForNull(); try typeHandler.setParameter(ps, i + 1, value, jdbcType); catch (TypeException e) throw new TypeException(Could not set parameters for mapping: + parameterMapping + . Cause: + e, e); catch (SQLException e) throw new TypeException(Could not set parameters for mapping: + parameterMapping + . Cause: + e, e); 复制代码到这里为止，已经给Statement设置了最初传递进去的参数（值为1）了，那么接着分析流程2：流程2：SQL查询及结果集的设置/ RoutingStatementHandler类Overridepublic List query(Statement statement) throws SQLException return delegate.query(statement);复制代码/ RoutingStatementHandler类Overridepublic List query(Statement statement) throws SQLException / 这里就到了熟悉的PreparedStatement了 PreparedStatement ps = (PreparedStatement) statement; / 执行SQL查询操作 ps.execute(); / 结果交给ResultHandler来处理 return resultSetHandler. handleResultSets(ps);复制代码复制代码/ DefaultResultSetHandler类（封装返回值，将查询结果封装成Object对象）Overridepublic List handleResultSets(Statement stmt) throws SQLException ErrorContext.instance().activity(handling results).object(mappedStatement.getId(); final List multipleResults = new ArrayList(); int resultSetCount = 0; ResultSetWrapper rsw = getFirstResultSet(stmt); List resultMaps = mappedStatement.getResultMaps(); int resultMapCount = resultMaps.size(); validateResultMapsCount(rsw, resultMapCount); while (rsw != null & resultMapCount resultSetCount) ResultMap resultMap = resultMaps.get(resultSetCount); handleResultSet(rsw, resultMap, multipleResults, null); rsw = getNextResultSet(stmt); cleanUpAfterHandlingResultSet(); resultSetCount+; String resultSets = mappedStatement.getResultSets(); if (resultSets != null) while (rsw != null & resultSetCount resultSets.length) ResultMapping parentMapping = nextResultMaps.get(resultSetsresultSetCount); if (parentMapping != null) String nestedResultMapId = parentMapping.getNestedResultMapId(); ResultMap resultMap = configuration.getResultMap(nestedResultMapId); handleResultSet(rsw, resultMap, null, parentMapping); rsw = getNextResultSet(stmt); cleanUpAfterHandlingResultSet(); resultSetCount+; return collapseSingleResultList(multipleResults);复制代码ResultSetWrapper是ResultSet的包装类，调用getFirstResultSet方法获取第一个ResultSet，同时获取数据库的MetaData数据，包括数据表列名、列的类型、类序号等，这些信息都存储在ResultSetWrapper类中了。然后调用handleResultSet方法来来进行结果集的封装。复制代码/ DefaultResultSetHandler类private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List multipleResults, ResultMapping parentMapping) throws SQLException try if (parentMapping != null) handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping); else if (resultHandler = null) DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory); handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null); multipleResults.add(defaultResultHandler.getResultList(); else handleRowValues(rsw, resultMap, resultHandler, rowBounds, null); finally / issue #228 (close resultsets) closeResultSet(rsw.getResultSet(); 复制代码这里调用handleRowValues方法来进行值的设置：复制代码/ DefaultResultSetHandler类public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException if (resultMap.hasNestedResultMaps() ensureNoRowBounds(); checkResultHandler(); handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); else / 封装数据 handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); 复制代码复制代码/ DefaultResultSetHandler类/ 封装数据private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException DefaultResultContext resultContext = new DefaultResultContext(); skipRows(rsw.getResultSet(), rowBounds); while (shouldProcessMoreRows(resultContext, rowBounds) & rsw.getResultSet().next() ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null); Object rowValue = getRowValue(rsw, discriminatedResultMap); storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet(); 复制代码复制代码/ DefaultResultSetHandler类private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException final ResultLoaderMap lazyLoader = new ResultLoaderMap(); / createResultObject为新创建的对象，数据表对应的类 Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null); if (resultObject != null & !hasTypeHandlerForResultObject(rsw, resultMap.getType() final MetaObject metaObject = configuration.newMetaObject(resultObject); boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty(); if (shouldApplyAutomaticMpings(resultMap, false) / 这里把数据填充进去，metaObject中包含了resultObject信息 foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) | foundValues; foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) | foundValues; foundValues = lazyLoader.size() 0 | foundValues; resultObject = foundValues ? resultObject : null; return resultObject; return resultObject;复制代码复制代码/ DefaultResultSetHandler类（把ResultSet中查询结果填充到JavaBean中）private boolean apply