mybatis源码学习之执行过程分析——sql执行后ResultSet的处理及结果返回

今天药忘吃喽~ 2023-02-19 02:29 7阅读 0赞

**mybatis源码学习及分析之执行过程分析——返回结果的处理**  
        上一篇中分析了sql语句的创建和执行过程，使用jdbc时，执行完sql后会返回ResultSet，然后我们会通过getInteger()、getString()等方法拿到数据。而使用Mybatis中我们可以直接将结果转换为POJO对象，下面就来看看mybatis是如何封装ResultSet的。

PreparedStatementHandler.java
    
      public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        PreparedStatement ps = (PreparedStatement) statement;
        ps.execute();
        //在这里开始结果处理
        return resultSetHandler.<E> handleResultSets(ps);
      }

这里用到了ResultSetHandler。

![è¿éåå¾çæè¿°][SouthEast]

DefaultResultSetHandler.java
    
      public List<Object> handleResultSets(Statement stmt) throws SQLException {
        ErrorContext.instance().activity("handling results").object(mapdpedStatement.getId());
    
        final List<Object> multipleResults = new ArrayList<Object>();
    
        int resultSetCount = 0;
        //获取ResultSet并包装为ResultSetWrapper 
        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);
          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++;
          }
        }
    
        return collapseSingleResultList(multipleResults);
      }
    
    
      private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
        //JDBC中的代码，在这里拿到了ResultSet 
        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;
            }
          }
        }
        //将ResultSet包装为ResultSetWrapper
        return rs != null ? new ResultSetWrapper(rs, configuration) : null;
      }

当前调用栈信息：

![è¿éåå¾çæè¿°][SouthEast 1]

当ResultSet不为空时，也就是查到了结果，通过rs和Configuration实例化了ResultSetWrapper。

public class ResultSetWrapper {
    
      private final ResultSet resultSet;
      private final TypeHandlerRegistry typeHandlerRegistry;
      private final List<String> columnNames = new ArrayList<String>();
      private final List<String> classNames = new ArrayList<String>();
      private final List<JdbcType> jdbcTypes = new ArrayList<JdbcType>();
      private final Map<String, Map<Class<?>, TypeHandler<?>>> typeHandlerMap = new HashMap<String, Map<Class<?>, TypeHandler<?>>>();
      private Map<String, List<String>> mappedColumnNamesMap = new HashMap<String, List<String>>();
      private Map<String, List<String>> unMappedColumnNamesMap = new HashMap<String, List<String>>();
    
      public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {
        super();
        //从configuration中拿到了typeHandlerRegistry 
        this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
        this.resultSet = rs;
    
        //拿到了列信息
        final int columnCount = metaData.getColumnCount();
        //将每一列的名称、jdbc类型、java类型分别保存在三个ArrayList中。
        for (int i = 1; i <= columnCount; i++) { 
          columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));
          jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));
          classNames.add(metaData.getColumnClassName(i));
        }
      }
      。。。
     }

当前调用栈如下图：

![è¿éåå¾çæè¿°][SouthEast 2]

返回的ResultSetWrapper信息如图：

![è¿éåå¾çæè¿°][SouthEast 3]

到这里，我们的调用栈就返回到了`DefaultResultHandler#handleResultSets(Statement stmt)`。

接下来通过mappedStatement拿到了xml中配置的ResultMap。

![è¿éåå¾çæè¿°][SouthEast 4]

可以看到ResultMap是我们将jdbc类型数据映射为POJO对象的重要手段。

validateResultMapsCount(rsw, resultMapCount);

对ResultMap进行校验，当查询返回的ResultMap数量小于1时，说明没有找到对应的ResultType或者ResultMap可以将结果映射出来，在这里就会抛出ExecutorException异常。

接下来重要的操作就是handleResultSet(rsw, resultMap, multipleResults, null);这里开始对结果集进行封装。

DefaultResultSetHandler.java
    
      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);
              //处理行信息
              handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
              multipleResults.add(defaultResultHandler.getResultList());
            } else {
                //调用
              handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
            }
          }
        } finally {
          // issue #228 (close resultsets)
          closeResultSet(rsw.getResultSet());
        }
      }
    
    
      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);
          //调用getRowValue
          Object rowValue = getRowValue(rsw, discriminatedResultMap);
          storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
        }
      }
    
      private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
        final ResultLoaderMap lazyLoader = new ResultLoaderMap();
        //创建了User
        Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
        if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
            //拿到列信息
          final MetaObject metaObject = configuration.newMetaObject(resultObject);
          boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
          if (shouldApplyAutomaticMappings(resultMap, false)) {
            foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
          }
          foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
          foundValues = lazyLoader.size() > 0 || foundValues;
          resultObject = foundValues ? resultObject : null;
          return resultObject;
        }
        return resultObject;
      }
    
    //调用User的setter方法为User实例设置信息
      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) {
              //getPropertyMappingValue在这里对了数据转换
            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')
              //在这里使用了setter方法设置值
              metaObject.setValue(property, value);
            }
          }
        }
        return foundValues;
      }

![è¿éåå¾çæè¿°][SouthEast 5]

可以看到调用了User的setName。

![è¿éåå¾çæè¿°][SouthEast 6]

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 DEFERED;
        } else {
           //typeHandler
          final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
          //数据库表的列名
          final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
          //得到数据库中信息
          return typeHandler.getResult(rs, column);
        }
      }

当所有的值都设置完成后，已经得到了我们需要的POJO类信息，调用栈开始出栈操作，方法return。

![è¿éåå¾çæè¿°][SouthEast 7]

至此，结果封装完成，所有调用栈开始出栈操作，最后返回我们需要的User。

[SouthEast]: https://img-blog.csdn.net/20170107163516457?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 1]: https://img-blog.csdn.net/20170108001257787?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 2]: https://img-blog.csdn.net/20170108090650015?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 3]: https://img-blog.csdn.net/20170108090831984?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 4]: https://img-blog.csdn.net/20170108091327190?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 5]: https://img-blog.csdn.net/20170108095840554?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 6]: https://img-blog.csdn.net/20170108102115522?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 7]: https://img-blog.csdn.net/20170108100819574?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvY2NmNTQ3NDYxMjk2/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast