mybatis源碼學習之執行過程分析——sql執行後ResultSet的處理及結果返回

mybatis源碼學習及分析之執行過程分析——返回結果的處理
        上一篇中分析了sql語句的創建和執行過程，使用jdbc時，執行完sql後會返回ResultSet，然後我們會通過getInteger()、getString()等方法拿到數據。而使用Mybatis中我們可以直接將結果轉換爲POJO對象，下面就來看看mybatis是如何封裝ResultSet的。

PreparedStatementHandler.java

  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    //在這裏開始結果處理
    return resultSetHandler.<E> handleResultSets(ps);
  }

這裏用到了ResultSetHandler。

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;
    //獲取ResultSet幷包裝爲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中的代碼，在這裏拿到了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;
        }
      }
    }
    //將ResultSet包裝爲ResultSetWrapper
    return rs != null ? new ResultSetWrapper(rs, configuration) : null;
  }

當前調用棧信息： 

當ResultSet不爲空時，也就是查到了結果，通過rs和Configuration實例化了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();
    //從configuration中拿到了typeHandlerRegistry 
    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.resultSet = rs;

    //拿到了列信息
    final int columnCount = metaData.getColumnCount();
    //將每一列的名稱、jdbc類型、java類型分別保存在三個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));
    }
  }
  。。。
 }

當前調用棧如下圖：

返回的ResultSetWrapper信息如圖： 

到這裏，我們的調用棧就返回到了DefaultResultHandler#handleResultSets(Statement stmt)。

接下來通過mappedStatement拿到了xml中配置的ResultMap。 

可以看到ResultMap是我們將jdbc類型數據映射爲POJO對象的重要手段。

validateResultMapsCount(rsw, resultMapCount);

對ResultMap進行校驗，當查詢返回的ResultMap數量小於1時，說明沒有找到對應的ResultType或者ResultMap可以將結果映射出來，在這裏就會拋出ExecutorException異常。

接下來重要的操作就是handleResultSet(rsw, resultMap, multipleResults, null);這裏開始對結果集進行封裝。
 

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);
          //處理行信息
          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
          multipleResults.add(defaultResultHandler.getResultList());
        } else {
            //調用
          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);
      //調用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();
    //創建了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;
  }

//調用User的setter方法爲User實例設置信息
  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在這裏對了數據轉換
        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')
          //在這裏使用了setter方法設置值
          metaObject.setValue(property, value);
        }
      }
    }
    return foundValues;
  }

可以看到調用了User的setName。

  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();
      //數據庫表的列名
      final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
      //得到數據庫中信息
      return typeHandler.getResult(rs, column);
    }
  }

當所有的值都設置完成後，已經得到了我們需要的POJO類信息，調用棧開始出棧操作，方法return。 

至此，結果封裝完成，所有調用棧開始出棧操作，最後返回我們需要的User。