1. ResultSetHandler接口的实现类DefaultResultSetHandler介绍

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1 ） PreparedStatementHandler执行query方法，把执行后的PreparedStatement对象作为入参调用ResultSetHandler类的handleResultSets方法。
2 ) 把Statement对象作为入参调用getFirstResultSet方法返回第一条数据的ResultSetWrapper对象。
3 ）调用mappedStatement.getResultMaps().size()方法获取查询结果记录的条数
4 ) 通过while循环遍历第一条数据的ResultSetWrapper对象或下一条数据的ResultSetWrapper对象，直到第一条或下一条数据为空时，即中止执行。在while循环内部获取每执行一次会调用getNextResultSet方法获取下一条数据的ResultSetWrapper对象。
5 ) 提取最终数据是通过把ResultSetWrapper对象对象作为入参，调用 handleResultSet方法完成的，提取出来的数据会存在multipleResults集合中，最终返回。

2. 结果数据的提取-handleResultSet方法

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DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());

3. handleRowValues方法

这里面会调handleRowValuesForSimpleResultMap方法

    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);
    }

1 ) 通过ResultSetWrapper对象获取到单条数据的ResultSet对象，作为入参调用resolveDiscriminatedResultMap方法，最后返回ResultMap对象。
2 ）把ResultMap对象作为入参调用getRowValue方法，最后返回rowValue.
3 ）获取到rowValue后调用storeObject方法存入resultHandler对象中。

4. getRowValue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 == 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;
  }

获取单个属性源码

  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 DEFERRED;
    } else {
      final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
      final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
      return typeHandler.getResult(rs, column);
    }
  }

获取值BaseTypeHandler

  @Override
  public T getResult(ResultSet rs, String columnName) throws SQLException {
    try {
      return getNullableResult(rs, columnName);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set.  Cause: " + e, e);
    }
  }

调回IntegerTypeHandler

  @Override
  public Integer getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    int result = rs.getInt(columnName);
    return result == 0 && rs.wasNull() ? null : result;
  }