入口：DefaultResultSetHandler#handleResultSets

结果集映射

@Override
    public List<Object> handleResultSets(Statement stmt) throws SQLException {
        ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

        // <select>标签的resultMap属性，可以指定多个值，多个值之间用逗号（,）分割
        final List<Object> multipleResults = new ArrayList<>();

        int resultSetCount = 0;
        // 这里是获取第一个结果集，将传统JDBC的ResultSet包装成一个包含结果列元信息的ResultSetWrapper对象
        ResultSetWrapper rsw = getFirstResultSet(stmt);

        // 这里是获取所有要映射的ResultMap（按照逗号分割出来的）
        List<ResultMap> resultMaps = mappedStatement.getResultMaps();
        // 要映射的ResultMap的数量
        int resultMapCount = resultMaps.size();
        validateResultMapsCount(rsw, resultMapCount);
        // 循环处理每个ResultMap，从第一个开始处理
        while (rsw != null && resultMapCount > resultSetCount) {
            // 得到结果映射信息
            ResultMap resultMap = resultMaps.get(resultSetCount);
            // 处理结果集
            // 从rsw结果集参数中获取查询结果，再根据resultMap映射信息，将查询结果映射到multipleResults中
            handleResultSet(rsw, resultMap, multipleResults, null);

            rsw = getNextResultSet(stmt);
            cleanUpAfterHandlingResultSet();
            resultSetCount++;
        }

        // 对应<select>标签的resultSets属性，一般不使用该属性
        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++;
            }
        }

        // 如果只有一个结果集合，则直接从多结果集中取出第一个
        return collapseSingleResultList(multipleResults);
    }

流程图

结果集映射流程.png

流程分析

• DefaultResultSetHandler#handleResultSet

  从rsw结果集参数中获取查询结果，再根据resultMap映射信息，将查询结果映射到multipleResults中

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults,
            ResultMapping parentMapping) throws SQLException {
        try {
            // 处理嵌套结果映射
            if (parentMapping != null) {
                handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
            } else {
                // 第一次一般来说resultHandler为空，则创建DefaultResultHandler来处理结果
                if (resultHandler == null) {
                    DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
                    // 处理行数据，其实就是完成结果映射
                    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());
        }
    }

• DefaultResultSetHandler#handleRowValues

  处理行数据，其实就是完成结果映射

public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler,
            RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
        // 是否有内置嵌套的结果映射
        if (resultMap.hasNestedResultMaps()) {
            ensureNoRowBounds();
            checkResultHandler();
            // 嵌套结果映射
            handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
        } else {
            // 简单结果映射
            handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
        }
    }

• DefaultResultSetHandler#handleRowValuesForSimpleResultMap

  简单结果映射

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap,
            ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
        DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
        // 获取结果集信息
        ResultSet resultSet = rsw.getResultSet();
        // 使用rowBounds的分页信息，进行逻辑分页（也就是在内存中分页）
        skipRows(resultSet, rowBounds);
        while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
            // 通过<resultMap>标签的子标签<discriminator>对结果映射进行鉴别
            ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
            // 将查询结果封装到POJO中
            Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
            // 处理对象嵌套的映射关系
            storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
        }
    }

• 1. DefaultResultSetHandler#getRowValue

  将查询结果封装到POJO中

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
        // 延迟加载的映射信息
        final ResultLoaderMap lazyLoader = new ResultLoaderMap();
        // 创建要映射的PO类对象
        Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
        if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
            final MetaObject metaObject = configuration.newMetaObject(rowValue);
            boolean foundValues = this.useConstructorMappings;
            // 是否应用自动映射，也就是通过resultType进行映射
            if (shouldApplyAutomaticMappings(resultMap, false)) {
                // 根据columnName和type属性名映射赋值
                foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
            }
            // 根据我们配置ResultMap的column和property映射赋值
            // 如果映射存在nestedQueryId，会调用getNestedQueryMappingValue方法获取返回值
            foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
            foundValues = lazyLoader.size() > 0 || foundValues;
            rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
        }
        return rowValue;
    }

• 1.1 DefaultResultSetHandler#createResultObject

  创建映射结果对象

private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader,
            String columnPrefix) throws SQLException {
        this.useConstructorMappings = false; // reset previous mapping result
        final List<Class<?>> constructorArgTypes = new ArrayList<>();
        final List<Object> constructorArgs = new ArrayList<>();
        // 创建结果映射的PO类对象
        Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
        if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
            // 获取要映射的PO类的属性信息
            final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
            for (ResultMapping propertyMapping : propertyMappings) {
                // issue gcode #109 && issue #149
                // 延迟加载处理
                if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
                    // 通过动态代理工厂，创建延迟加载的代理对象
                    resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration,
                            objectFactory, constructorArgTypes, constructorArgs);
                    break;
                }
            }
        }
        this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping
                                                                                                // result
        return resultObject;
    }

private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes,
            List<Object> constructorArgs, String columnPrefix) throws SQLException {
        final Class<?> resultType = resultMap.getType();
        final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
        final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
        if (hasTypeHandlerForResultObject(rsw, resultType)) {
            return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
        } else if (!constructorMappings.isEmpty()) {
            return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes,
                    constructorArgs, columnPrefix);
        } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
            // 对象工厂创建对象
            return objectFactory.create(resultType);
        } else if (shouldApplyAutomaticMappings(resultMap, false)) {
            return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix);
        }
        throw new ExecutorException("Do not know how to create an instance of " + resultType);
    }

• 1.2 DefaultResultSetHandler#applyAutomaticMappings

  根据columnName和type属性名映射赋值

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;
    }

• 1.3 DefaultResultSetHandler#applyPropertyMappings

  根据我们配置ResultMap的column和property映射赋值,如果映射存在nestedQueryId，会调用getNestedQueryMappingValue方法获取返回值

private boolean applyPropertyMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject,
            ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
        final List<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix);
        boolean foundValues = false;
        final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
        for (ResultMapping propertyMapping : propertyMappings) {
            String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
            if (propertyMapping.getNestedResultMapId() != null) {
                // the user added a column attribute to a nested result map, ignore it
                column = null;
            }
            if (propertyMapping.isCompositeResult()
                    || (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH)))
                    || propertyMapping.getResultSet() != null) {
                Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader,
                        columnPrefix);
                // issue #541 make property optional
                final String property = propertyMapping.getProperty();
                if (property == null) {
                    continue;
                } else if (value == DEFERRED) {
                    foundValues = true;
                    continue;
                }
                if (value != null) {
                    foundValues = true;
                }
                if (value != null || (configuration.isCallSettersOnNulls()
                        && !metaObject.getSetterType(property).isPrimitive())) {
                    // gcode issue #377, call setter on nulls (value is not 'found')
                    metaObject.setValue(property, value);
                }
            }
        }
        return foundValues;
    }