关于MyBatis查询属性封装到对象，对象为null的情况源码分析

源码分析

在DefaultResultSetHandler类中getRowValue方法

1. 创建映射类相应的对象，如果为列匹配到的值标识foundValues是false，表示没有为对象中任何一个字段映射到一个值，则方法返回null，查到的结果也是null

  private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
    if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      final MetaObject metaObject = configuration.newMetaObject(rowValue);
      boolean foundValues = this.useConstructorMappings;
      if (shouldApplyAutomaticMappings(resultMap, false)) {
        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
      }
      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
      foundValues = lazyLoader.size() > 0 || foundValues;
      rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
    }
    return rowValue;
  }

2. 映射查询的列如果不为null，返回foundValues=true,表示有为列匹配到值了

  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;
  }

结论

如果mapper查询结果的对象中存在一个属性映射到了一个不为null（不包括空字符串，空格）的值，则返回封装该属性值相应的该对象

如果mapper查询结果的对象中映射的属性都为null，则返回null