mybatis中ResultSetHandler的设计与实现

ResultSetHandler主要处理statement,cursor,callstatement三种类型的结果集，其接口定义如下：

public interface ResultSetHandler {<E> List<E> handleResultSets(Statement stmt) throws SQLException;<E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;void handleOutputParameters(CallableStatement cs) throws SQLException;}

在介绍ResultSetHandler之前，先看下与ResultSet相关的一个类，ResultSetWrapper,其包含的成员有ResultSet,列名，及对应的jdbcType和javaType,其根据ResultMap的定义及结果集，得到映射和没有映射的列名集合

public List<String> getMappedColumnNames(ResultMap resultMap, String columnPrefix) throws SQLException {List<String> mappedColumnNames = mappedColumnNamesMap.get(getMapKey(resultMap, columnPrefix));if (mappedColumnNames == null) {loadMappedAndUnmappedColumnNames(resultMap, columnPrefix);mappedColumnNames = mappedColumnNamesMap.get(getMapKey(resultMap, columnPrefix));}return mappedColumnNames;}private void loadMappedAndUnmappedColumnNames(ResultMap resultMap, String columnPrefix) throws SQLException {List<String> mappedColumnNames = new ArrayList<String>();List<String> unmappedColumnNames = new ArrayList<String>();final String upperColumnPrefix = columnPrefix == null ? null : columnPrefix.toUpperCase(Locale.ENGLISH);final Set<String> mappedColumns = prependPrefixes(resultMap.getMappedColumns(), upperColumnPrefix);for (String columnName : columnNames) {final String upperColumnName = columnName.toUpperCase(Locale.ENGLISH);if (mappedColumns.contains(upperColumnName)) {mappedColumnNames.add(upperColumnName);} else {unmappedColumnNames.add(columnName);}}mappedColumnNamesMap.put(getMapKey(resultMap, columnPrefix), mappedColumnNames);unMappedColumnNamesMap.put(getMapKey(resultMap, columnPrefix), unmappedColumnNames);}

1、DefaultRecordSetHandler对RecordSet的处理

其是mybatis中ResultSetHandler的唯一默认实现。

public List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());final List<Object> multipleResults = new ArrayList<Object>();int resultSetCount = 0;//获取第一个结果集，可能有多个（对于存储过程）ResultSetWrapper rsw = getFirstResultSet(stmt);//对于多结果集的，可以配置多个resultMapList<ResultMap> resultMaps = mappedStatement.getResultMaps();int resultMapCount = resultMaps.size();validateResultMapsCount(rsw, resultMapCount);while (rsw != null && resultMapCount > resultSetCount) {ResultMap resultMap = resultMaps.get(resultSetCount);//通过resultMap与recordSet映射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);}

单独处理recordSet是通过handleResultSet来实现的.

当配置的是多个recordsets,而只对应一个resultMap，但是resultMap中的其中一个映射到其中的一个recordSet时，会进入到第一个分支。

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());}}

处理分为简单映射和嵌套映射

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 {handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);}}

简单映射的处理流程为

嵌套映射的处理流程为：前面与简单映射一样，跳到偏移所在的行，在没有到limit时，先根据discriminate找到ResultMap，如果有IdResultMapping就用此来创建CacheKey，否则使用 PropertyResultMapping来创建CacheKey。在嵌套结果对象没有时，创建metaobject,首先根据构造函数设置，根据是否需要自动映射来映射没有指定映射的列，然后映射指定的属性，接着作嵌套映射，在有嵌套结果对象时，直接作嵌套映射，最后调用callResultHandler存储对象

private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {final DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();skipRows(rsw.getResultSet(), rowBounds);Object rowValue = previousRowValue;while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);final CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);Object partialObject = nestedResultObjects.get(rowKey);// issue #577 && #542if (mappedStatement.isResultOrdered()) {if (partialObject == null && rowValue != null) {nestedResultObjects.clear();storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);} else {rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);if (partialObject == null) {storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());}}}if (rowValue != null && mappedStatement.isResultOrdered() && shouldProcessMoreRows(resultContext, rowBounds)) {storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());previousRowValue = null;} else if (rowValue != null) {previousRowValue = rowValue;}}

代理工厂设计的类层次图为

多结果集的处理

private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)throws SQLException {if (propertyMapping.getNestedQueryId() != null) {return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);} else if (propertyMapping.getResultSet() != null) {addPendingChildRelation(rs, metaResultObject, propertyMapping);   // TODO is that OK?return DEFERED;} else {final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);return typeHandler.getResult(rs, column);}}private void addPendingChildRelation(ResultSet rs, MetaObject metaResultObject, ResultMapping parentMapping) throws SQLException {CacheKey cacheKey = createKeyForMultipleResults(rs, parentMapping, parentMapping.getColumn(), parentMapping.getColumn());PendingRelation deferLoad = new PendingRelation();deferLoad.metaObject = metaResultObject;deferLoad.propertyMapping = parentMapping;List<PendingRelation> relations = pendingRelations.get(cacheKey);// issue #255if (relations == null) {relations = new ArrayList<DefaultResultSetHandler.PendingRelation>();pendingRelations.put(cacheKey, relations);}relations.add(deferLoad);ResultMapping previous = nextResultMaps.get(parentMapping.getResultSet());if (previous == null) {nextResultMaps.put(parentMapping.getResultSet(), parentMapping);} else {if (!previous.equals(parentMapping)) {throw new ExecutorException("Two different properties are mapped to the same resultSet");}}}

2、对Cursor的处理

其使用迭代器设计模式，基于DefaultCursor来处理游标的。使用内部类CursorIterator来作迭代，其类关系图为

public <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling cursor results").object(mappedStatement.getId());ResultSetWrapper rsw = getFirstResultSet(stmt);List<ResultMap> resultMaps = mappedStatement.getResultMaps();int resultMapCount = resultMaps.size();validateResultMapsCount(rsw, resultMapCount);if (resultMapCount != 1) {throw new ExecutorException("Cursor results cannot be mapped to multiple resultMaps");}ResultMap resultMap = resultMaps.get(0);return new DefaultCursor<E>(this, resultMap, rsw, rowBounds);}//CursorIterator@Overridepublic boolean hasNext() {if (object == null) {object = fetchNextUsingRowBound();}return object != null;}protected T fetchNextUsingRowBound() {T result = fetchNextObjectFromDatabase();while (result != null && indexWithRowBound < rowBounds.getOffset()) {result = fetchNextObjectFromDatabase();}return result;}protected T fetchNextObjectFromDatabase() {if (isClosed()) {return null;}try {status = CursorStatus.OPEN;resultSetHandler.handleRowValues(rsw, resultMap, objectWrapperResultHandler, RowBounds.DEFAULT, null);} catch (SQLException e) {throw new RuntimeException(e);}T next = objectWrapperResultHandler.result;if (next != null) {indexWithRowBound++;}// No more object or limit reachedif (next == null || (getReadItemsCount() == rowBounds.getOffset() + rowBounds.getLimit())) {close();status = CursorStatus.CONSUMED;}objectWrapperResultHandler.result = null;return next;}public T next() {// Fill next with object fetched from hasNext()T next = object;if (next == null) {next = fetchNextUsingRowBound();}if (next != null) {object = null;iteratorIndex++;return next;}throw new NoSuchElementException();}

3、对存储过程的处理

对绑定的sql的参数映射处理，参数类型为out或者inout类型。

public void handleOutputParameters(CallableStatement cs) throws SQLException {final Object parameterObject = parameterHandler.getParameterObject();final MetaObject metaParam = configuration.newMetaObject(parameterObject);final List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();for (int i = 0; i < parameterMappings.size(); i++) {final ParameterMapping parameterMapping = parameterMappings.get(i);if (parameterMapping.getMode() == ParameterMode.OUT || parameterMapping.getMode() == ParameterMode.INOUT) {if (ResultSet.class.equals(parameterMapping.getJavaType())) {handleRefCursorOutputParameter((ResultSet) cs.getObject(i + 1), parameterMapping, metaParam);} else {final TypeHandler<?> typeHandler = parameterMapping.getTypeHandler();metaParam.setValue(parameterMapping.getProperty(), typeHandler.getResult(cs, i + 1));}}}}private void handleRefCursorOutputParameter(ResultSet rs, ParameterMapping parameterMapping, MetaObject metaParam) throws SQLException {if (rs == null) {return;}try {final String resultMapId = parameterMapping.getResultMapId();final ResultMap resultMap = configuration.getResultMap(resultMapId);final DefaultResultHandler resultHandler = new DefaultResultHandler(objectFactory);final ResultSetWrapper rsw = new ResultSetWrapper(rs, configuration);handleRowValues(rsw, resultMap, resultHandler, new RowBounds(), null);metaParam.setValue(parameterMapping.getProperty(), resultHandler.getResultList());} finally {// issue #228 (close resultsets)closeResultSet(rs);}}

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