Mybatis源码解析6：Mapper执行流程2-三个Handler

- 1.项目结构
- 2. 源码分析
- - 2.1 StatementHandler分析 BaseStatementHandler#prepare
  - 2.2 ParameterHandler分析 DefaultParameterHandler#setParameters
  - 2.3 ResultSetHandler分析

1.项目结构

2. 源码分析

之前已经对 SimpleExecutor#doQuery。StatementHandler是对Statement进行处理，ParameterHandler是对参数的设置进行处理，ResultSetHandler是对Statement执行的结果集进行处理

2.1 StatementHandler分析 BaseStatementHandler#prepare

  @Overridepublic Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {ErrorContext.instance().sql(boundSql.getSql());Statement statement = null;try {statement = instantiateStatement(connection);setStatementTimeout(statement, transactionTimeout);setFetchSize(statement);return statement;} catch (SQLException e) {closeStatement(statement);throw e;} catch (Exception e) {closeStatement(statement);throw new ExecutorException("Error preparing statement.  Cause: " + e, e);}}

获取JDBC层次的Statement对象
设置Statement的超时时间

实例化Statement PreparedStatementHandler#instantiateStatement

  @Overrideprotected Statement instantiateStatement(Connection connection) throws SQLException {String sql = boundSql.getSql();if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {String[] keyColumnNames = mappedStatement.getKeyColumns();if (keyColumnNames == null) {return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);} else {return connection.prepareStatement(sql, keyColumnNames);}} else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {return connection.prepareStatement(sql);} else {return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);}}

获取到sql，此时的sql还没有设置参数。select id, name from blog where id = ?
用过Connection对象获取jdbc层次的Statement

2.2 ParameterHandler分析 DefaultParameterHandler#setParameters

  @Overridepublic void setParameters(PreparedStatement ps) {ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());List<ParameterMapping> 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 paramsvalue = 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 | SQLException e) {throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);}}}}}

parameterMappings是Sql中的参数的信息，而parameterObject是参数所对应的值
通过configuration.newMetaObject(parameterObject)方法，对参数对象进行了封装，里面有反射工厂，会通过反射获取到get、set方法，并且取值和设置值
遍历parameterMappings通过参数的信息，获取到每个参数对应的值，并且获取到值所对应的类型转换器TypeHandler。TypeHandler是在全局配置Configuration中，可通过mybatis-config.xml配置。Configuration中已经默认了常用的类型转换器。
最终通过PreparedStatement设置参数

2.3 ResultSetHandler分析

DefaultResultSetHandler#handleResultSets

  @Overridepublic List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());final List<Object> multipleResults = new ArrayList<>();int resultSetCount = 0;ResultSetWrapper rsw = getFirstResultSet(stmt);List<ResultMap> 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(resultSets[resultSetCount]);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);}

获取JDBC的ResultSet，通过JDK代理ResultSetLogger，ResultSetWrapper封装ResultSetLogger和Configuration
获取resultMapCount条数据，一行一行地去处理结果集

DefaultResultSetHandler#handleResultSet

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> 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());}}

这里就是创建一个结果集处理器，然后解析数据，封装到对象里面

DefaultResultSetHandler#getRowValue


private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)throws SQLException {DefaultResultContext<Object> resultContext = new DefaultResultContext<>();ResultSet resultSet = rsw.getResultSet();skipRows(resultSet, rowBounds);while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);Object rowValue = getRowValue(rsw, discriminatedResultMap, null);storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);}
}private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {final ResultLoaderMap lazyLoader = new ResultLoaderMap();Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {final MetaObject metaObject = configuration.newMetaObject(rowValue);boolean foundValues = this.useConstructorMappings;if (shouldApplyAutomaticMappings(resultMap, false)) {foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;}foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;foundValues = lazyLoader.size() > 0 || foundValues;rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;}return rowValue;}private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);boolean foundValues = false;if (!autoMapping.isEmpty()) {for (UnMappedColumnAutoMapping mapping : autoMapping) {final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);if (value != null) {foundValues = true;}if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {// gcode issue #377, call setter on nulls (value is not 'found')metaObject.setValue(mapping.property, value);}}}return foundValues;}