MyBatis源码解析（六）：对结果集进行ORM映射

╰+攻爆jí腚メ 2022-02-25 00:12 224阅读 0赞

前言：[MyBatis源码解析（五）：执行Mapper，获取初始结果集][MyBatis_Mapper]中，通过 Mapper 执行 Statement 已经从数据库中获取到数据的基本值，但是并没有和 Java pojo 进行映射，本篇讲继续解析 MyBatis 中的属性映射

一，流程图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTE5NzYzODg_size_16_color_FFFFFF_t_70][]

二，步骤解析

1，获取结果集及结果映射入口

\* PreparedStatementHandler

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        // 转换Statement为JDBC原生的PreparedStatement
        PreparedStatement ps = (PreparedStatement) statement;
        // 通过JDBC底层执行, 并将执行结果封装到PreparedStatement中
        ps.execute();
        // resultSetHandler:结果集处理器, 对查询结果进行ORM映射
        return resultSetHandler.<E> handleResultSets(ps);
      }

2，开始ORM映射接口

\* DefaultResultSetHandler：通过 handleResultSets() 方法开始映射

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;
        // 获取执行结果集, 并封装为ResultSetWrapper
        ResultSetWrapper rsw = getFirstResultSet(stmt);
        // 获取Statement的结果映射, 后台会自动将resultType解析为一个ResultMap
        List<ResultMap> resultMaps = mappedStatement.getResultMaps();
        int resultMapCount = resultMaps.size();
        validateResultMapsCount(rsw, resultMapCount);
        while (rsw != null && resultMapCount > resultSetCount) {
          // 逐个resultMap, 进行结果集解析
          ResultMap resultMap = resultMaps.get(resultSetCount);
          // 处理结果集, 映射数据库查询数据到reslutMap指定VO中
          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++;
          }
        }
        // 返回处理结果集
        return collapseSingleResultList(multipleResults);
      }

\* DefaultResultSetHandler：通过 getFirstResultSet()，获取数据库返回的结果集，并封装为 ResultSetWrapper

private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
        ResultSet rs = stmt.getResultSet();
        while (rs == null) {
          // move forward to get the first resultset in case the driver
          // doesn't return the resultset as the first result (HSQLDB 2.1)
          if (stmt.getMoreResults()) {
            rs = stmt.getResultSet();
          } else {
            if (stmt.getUpdateCount() == -1) {
              // no more results. Must be no resultset
              break;
            }
          }
        }
        return rs != null ? new ResultSetWrapper(rs, configuration) : null;
      }

public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {
        super();
        // 获取类型映射注册信息
        this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
        // 获取结果集
        this.resultSet = rs;
        // 获取数据库元数据列
        final ResultSetMetaData metaData = rs.getMetaData();
        final int columnCount = metaData.getColumnCount();
        // 逐行解析元数据列, 获取元数据列基本信息
        for (int i = 1; i <= columnCount; i++) {
          // 数据库列名, 支持别名
          columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));
          // 数据库列字段类型
          jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));
          // 数据库列字段类型映射的Java数据类型
          classNames.add(metaData.getColumnClassName(i));
        }
      }

2，数据库结果集解析，完成ORM映射

\* DefaultResultSetHandler：调用 handleResultSet() 处理结果集

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
        try {
          // parentMapping 方法传参为null
          if (parentMapping != null) {
            handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
          } else {
            // DefaultSqlSession.selectList方法中, 初始化resultHandler时为null
            if (resultHandler == null) {
              DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
              // 映射查询数据到VO中, 并添加最终VO到结果集中
              handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
              // multipleResults结果集添加ORM映射后的数据
              multipleResults.add(defaultResultHandler.getResultList());
            } else {
              handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
            }
          }
        } finally {
          // issue #228 (close resultsets)
          closeResultSet(rsw.getResultSet());
        }
      }

\* DefaultResultSetHandler：handleRowValues() 处理所有数据行

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 {
          // 不存在嵌套的单条数据查询, 走此方法
          handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
        }
      }

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
          throws SQLException {
        DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
        // 此处基于分页处理, 当前不涉及
        skipRows(rsw.getResultSet(), rowBounds);
        // 遍历每一个数据库查询对象, 进行数据ORM映射
        while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
          // 此处应该意为区分不同的resultMap, 当前只存在一个resultMap, 暂时忽略
          // 此时resultMap只存在一个有效属性, 即type为resultType对应的VO
          ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
          // 映射当前数据库数据到JavaBean, 此行结束后, 返回结果即为JavaBean对象
          Object rowValue = getRowValue(rsw, discriminatedResultMap);
          // 保存结果集到DefaultResultHandler中, 外部通过该对象获取结果集
          storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
        }
      }

\* DefaultResultSetHandler：处理每一行数据映射 getRowValue()

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
        final ResultLoaderMap lazyLoader = new ResultLoaderMap();
        // 初始化JavaBean对象, 初始化为零值, 此时未进行数据赋值
        Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);
        // hasTypeHandlerForResultObject : 判断当前结果集在JDBC->java的默认集合中是否存在
        if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
          final MetaObject metaObject = configuration.newMetaObject(rowValue);
          boolean foundValues = this.useConstructorMappings;
          // 判断是否需要自动映射
          if (shouldApplyAutomaticMappings(resultMap, false)) {
            // 进行自动映射
            foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
          }
          // 涉及ResultMap映射处理
          foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
          foundValues = lazyLoader.size() > 0 || foundValues;
          // 返回结果集
          rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
        }
        return rowValue;
      }

private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
        // 获取还没有建立属性映射的数据库字段
        List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
        boolean foundValues = false;
        if (!autoMapping.isEmpty()) {
          // 未建立映射字段不为空, 为当前属性分别set值
          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;
      }

\* 如果存在自定义 Property 映射，会执行下面方法

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 == DEFERED) {
              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;
      }

\* 最终返回ORM映射后的 Java POJO

3，保存并获取ORM映射后的结果集

\* 保存结果集：映射完成后，会调用 DefaultResultSetHandler.storeObject() 方法，保存结果到 DefaultResultHandler.list属性中

// 保存结果集到DefaultResultHandler中, 外部通过该对象获取结果集
    // org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleRowValuesForSimpleResultMap
          storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());

private void storeObject(ResultHandler<?> resultHandler, DefaultResultContext<Object> resultContext, Object rowValue, ResultMapping parentMapping, ResultSet rs) throws SQLException {
        // parentMapping传递为空
        if (parentMapping != null) {
          linkToParents(rs, parentMapping, rowValue);
        } else {
          callResultHandler(resultHandler, resultContext, rowValue);
        }
      }

private void callResultHandler(ResultHandler<?> resultHandler, DefaultResultContext<Object> resultContext, Object rowValue) {
        resultContext.nextResultObject(rowValue);
        ((ResultHandler<Object>) resultHandler).handleResult(resultContext);
      }

public void handleResult(ResultContext<? extends Object> context) {
        list.add(context.getResultObject());
      }

\* 获取结果集：映射完成后，会从 DefaultResultHandler.list中获取有效结果集

// org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleResultSet
    multipleResults.add(defaultResultHandler.getResultList());

// org.apache.ibatis.executor.result.DefaultResultHandler#getResultList
      public List<Object> getResultList() {
        return list;
      }

4，最终返回结果集到客户端

[MyBatis_Mapper]: https://blog.csdn.net/u011976388/article/details/88919193
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTE5NzYzODg_size_16_color_FFFFFF_t_70]: /images/20220225/777fdc2ec4c7446c931be90b1ef3691f.png