mybatis查询所得list集合，get(0)对象为null问题

问题场景：

代码中获取mybatis查询返回的list集合，获取第一个对象，get方法获取属性值

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问题描述

在上述场景下，获取属性值时报出空指针错误
photoInfo.getValidEndDate()

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原因分析：

源头是查询的sql有问题，原先查询的表中新增了字段，sql查询的数据为本次新增字段，查询结果中所有字段值皆为null。mybatis转换结果集时将null存入List返回，get(0)获取首个存储对象，实际获取为null，导致获取属性值时报出空指针。给新增字段赋值或者查询sql中添加查询一个必有值的字段，解决报错。

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部分mybatis源码：

ResultSetHandler是Mybatis的核心组件，将结果集resultSets转化成结果列表（或cursor）和处理储存过程的输出。
DefaultResultSetHandler是Myabtis为ResultSetHandler提供的唯一一个实现类，查看DefaultResultSetHandler的源码了解结果集resultSet的过程。

类DefaultResultSetHandler >> 方法handleResultSets() >> 方法handleRowValues() >> 方法handleRowValuesForSimpleResultMap() >> 方法getRowValue()
最终getRowValue()返回了null，因此获取mybatis查询的list，get(0)也获取了null导致获取属性报错。

  @Override
  public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    //返回的list集合
    final List<Object> multipleResults = new ArrayList<Object>();

    int resultSetCount = 0;
    //查询结果
    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);
      //处理rsw生成java对象
      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++;
      }
    }
    //返回处理后结果
    return collapseSingleResultList(multipleResults);
  }

  @SuppressWarnings("unchecked")
  private List<Object> collapseSingleResultList(List<Object> multipleResults) {
    return multipleResults.size() == 1 ? (List<Object>) multipleResults.get(0) : multipleResults;
  }

//处理查询的结果集
 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());
    }


  //
  // HANDLE ROWS FOR SIMPLE RESULTMAP
  //
//处理行数据
  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);
    }
  }

  private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
    DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
    skipRows(rsw.getResultSet(), rowBounds);
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      //获取返回对象
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }

 //
  // GET VALUE FROM ROW FOR SIMPLE RESULT MAP
  //

  private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    //生成对象
    Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);
    if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      final MetaObject metaObject = configuration.newMetaObject(rowValue);
      boolean foundValues = this.useConstructorMappings;
      if (shouldApplyAutomaticMappings(resultMap, false)) {
        //自动映射
        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
      }
      //属性映射
      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
      foundValues = lazyLoader.size() > 0 || foundValues;
      //这一步返回了null，returnInstanceForEmptyRow 该配置决定字段全为null是否返回null还是对象，默认false
      rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
    }
    return rowValue;
  }
  

源码分析参考：https://blog.csdn.net/qq_33762302/article/details/115340060