MyBatis ResultType使用自定义TypeHandler

1、背景

最近在使用MyBatis自定义的TypeHandler在对数据做基础加工，在对ResultMap使用自定义TypeHandler时是没有问题的，可以正常进入TypeHandler中处理数据，但是当结果集被定义为ResultType时总是不进入自定义的TypeHandler，基于这个情况，不得不再次打开MyBatis的源码一探究竟

2、基础代码

/**
 * 自定义TypeHandler
 *
 * @author fulibao
 * @version 1.0
 * @created 2017/7/10 下午4:29
 **/
public class SecurityStringVarcharTypeHandler extends BaseTypeHandler<String> {
    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType) throws SQLException {
        ps.setString(i, parameter);
    }

    @Override
    public String getNullableResult(ResultSet rs, String columnName) throws SQLException {
        //自定义代码

        return rs.getString(columnName);
    }

    @Override
    public String getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        //自定义代码

        return rs.getString(columnIndex);
    }

    @Override
    public String getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        //自定义代码

        return cs.getString(columnIndex);
    }
}

MyBatis XML配置文件:

<select id="getByApplicationIds" parameterType="list" resultType="censorModel">
    <include refid="getAll"/>
    where application_id in
    <foreach collection="list" item="applicationId" index="index"
             open="(" close=")" separator=",">
        #{applicationId}
    </foreach>
</select>

mybatis-config.xml中对于typeHandler的配置

<typeHandlers>
    <typeHandler jdbcType="VARCHAR" javaType="java.lang.String"
                 handler="com.meituan.fd.crm.common.typehandler.SecurityStringVarcharTypeHandler"/>
</typeHandlers>

3、问题排查

3.1、MyBatis执行查询等操作的基础类为：DefaultSqlSession，其代码为：

public <T> T selectOne(String statement) {
  return this.<T>selectOne(statement, null);
}

public <T> T selectOne(String statement, Object parameter) {
  // Popular vote was to return null on 0 results and throw exception on too many.
  List<T> list = this.<T>selectList(statement, parameter);
  if (list.size() == 1) {
    return list.get(0);
  } else if (list.size() > 1) {
    throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
  } else {
    return null;
  }
}

public <K, V> Map<K, V> selectMap(String statement, String mapKey) {
  return this.selectMap(statement, null, mapKey, RowBounds.DEFAULT);
}

public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey) {
  return this.selectMap(statement, parameter, mapKey, RowBounds.DEFAULT);
}

public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) {
  final List<?> list = selectList(statement, parameter, rowBounds);
  final DefaultMapResultHandler<K, V> mapResultHandler = new DefaultMapResultHandler<K, V>(mapKey,
      configuration.getObjectFactory(), configuration.getObjectWrapperFactory());
  final DefaultResultContext context = new DefaultResultContext();
  for (Object o : list) {
    context.nextResultObject(o);
    mapResultHandler.handleResult(context);
  }
  Map<K, V> selectedMap = mapResultHandler.getMappedResults();
  return selectedMap;
}

public <E> List<E> selectList(String statement) {
  return this.selectList(statement, null);
}

public <E> List<E> selectList(String statement, Object parameter) {
  return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    return result;
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

可以看到无论是selectOne、selectMap、selectList最终调用的都是selectList方法，因此我们由selectList入手开始排查问题。

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    //获取本次执行SQL的相关配置信息
    //MappedStatement中包含了一次执行的所以配置信息，包括SQL、配置参数等等
    MappedStatement ms = configuration.getMappedStatement(statement);
    //由executor调度执行查询,executor主要有两类：BaseExecutor(基础)、CachingExecutor(执行二级缓存)
    //现有系统这边没有开启二级缓存，因此我这边进入BaseExecutor
    List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    return result;
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

BaseExecutor.query方法：

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
   //获取执行的BoundSql（详细内容可以参见另外一篇文章：通过BoundSql获取全部执行SQL--MyBatis源码分析）
   BoundSql boundSql = ms.getBoundSql(parameter);
   //创建缓存key
   CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
   //执行真正的查询,见下面方法
   return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
  if (closed) throw new ExecutorException("Executor was closed.”);
  //缓存相关，查看是否需要清理缓存
  if (queryStack == 0 && ms.isFlushCacheRequired()) {
    clearLocalCache();
  }
  List<E> list;
  try {
    queryStack++;
    //resultHandler不为空的情况下由缓存获取，此处resultHandler为空
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      //如果由缓存中取出了数据，那么处理存储过程相关的输出参数
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      //由数据库获取数据及处理数据,方法见下面
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  }
  if (queryStack == 0) {
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    }
    deferredLoads.clear(); // issue #601
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      clearLocalCache(); // issue #482
    }
  }
  return list;
}

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  List<E> list;
  localCache.putObject(key, EXECUTION_PLACEHOLDER);
  try {
    //真正的执行查询的方法,这个方法在BaseExecutor中是一个抽象方法，交由子类来完成，这里我们进入子类：SimpleExecutor中查看
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
  } finally {
    localCache.removeObject(key);
  }
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  }
  return list;
}

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    //获取MyBatis总的配置信息
    Configuration configuration = ms.getConfiguration();
    //创建StatementHandler，它用于执行SQL及处理执行结果
    //它有四个实现类：见下图分析：
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    //将参数等信息注入到执行SQL中
    stmt = prepareStatement(handler, ms.getStatementLog());
    //执行查询,一般的SQL执行都是PreparedStatementHandler，进入它的query中
    return handler.<E>query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  //JDBC级别的SQL执行
  ps.execute();
  //resultSetHandler只有一个实现类：DefaultResultSetHandler,进入看一下：
  return resultSetHandler.<E> handleResultSets(ps);
}

public List<Object> handleResultSets(Statement stmt) throws SQLException {
  ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
  //最终执行完成后获取的结果数据
  final List<Object> multipleResults = new ArrayList<Object>();

  int resultSetCount = 0;
  //ResultSet包装类，内部包含了ResultSet，及其元数据
  ResultSetWrapper rsw = getFirstResultSet(stmt);

  //获取结果集配置
  List<ResultMap> resultMaps = mappedStatement.getResultMaps();
  int resultMapCount = resultMaps.size();
  validateResultMapsCount(rsw, resultMapCount);
  while (rsw != null && resultMapCount > resultSetCount) {
    ResultMap resultMap = resultMaps.get(resultSetCount);
    //处理结果集，MyBatis的查询结果就出自这里,进入看一下。。
    handleResultSet(rsw, resultMap, multipleResults, null);
    rsw = getNextResultSet(stmt);
    cleanUpAfterHandlingResultSet();
    resultSetCount++;
  }

  String[] resultSets = mappedStatement.getResulSets();
  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 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) {
        //默认的结果集处理Handler
        DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
        //处理没一行数据，并将数据存入defaultResultHandler
        handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
        //将数据存入multipleResults
        multipleResults.add(defaultResultHandler.getResultList());
      } else {
        handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
      }
    }
  } finally {
    closeResultSet(rsw.getResultSet()); // issue #228 (close resultsets)
  }
}

private 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 {
    //这里是最终处理数据的位置：
    handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
  }
}

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
    throws SQLException {
  //默认结果集存放处，记录了当前最新的一个结果和记录数
  DefaultResultContext resultContext = new DefaultResultContext();
  //根据RowBounds跳过不需要处理的数据
  skipRows(rsw.getResultSet(), rowBounds);
  //逐行处理数据
  while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
    ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
    //处理一行数据,方法见下面
    Object rowValue = getRowValue(rsw, discriminatedResultMap);
    //存储处理后的一行数据
    storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
  }
}

//ResultType和ResultMap的差异就出现在这里

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
  final ResultLoaderMap lazyLoader = new ResultLoaderMap();
  //获取查询实体的一个实例
  Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
  if (resultObject != null && !typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
    //MyBatis强大的MetaObject
    final MetaObject metaObject = configuration.newMetaObject(resultObject);
    boolean foundValues = resultMap.getConstructorResultMappings().size() > 0;
    if (shouldApplyAutomaticMappings(resultMap, !AutoMappingBehavior.NONE.equals(configuration.getAutoMappingBehavior()))) {  
      //对于未被配置进入的列进行处理（ResultType所对应的列，一律不会被MyBatis配置，都会在这个方法中处理），方法见下面：      
      foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
    }
    //对于配置进入的列进行处理（ResultMap中的列都会被配置，会在此处处理），方法见下面
    foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
    foundValues = lazyLoader.size() > 0 || foundValues;
    resultObject = foundValues ? resultObject : null;
    return resultObject;
  }
  return resultObject;
}

private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
  final List<String> unmappedColumnNames = rsw.getUnmappedColumnNames(resultMap, columnPrefix);
  boolean foundValues = false;
  //对每一列进行处理
  for (String columnName : unmappedColumnNames) {
    //获取列名
    String propertyName = columnName;
    if (columnPrefix != null && columnPrefix.length() > 0) {
      // When columnPrefix is specified,
      // ignore columns without the prefix.
      if (columnName.toUpperCase(Locale.ENGLISH).startsWith(columnPrefix)) {
        propertyName = columnName.substring(columnPrefix.length());
      } else {
        continue;
      }
    }
    //获取对应于实体的属性
    final String property = metaObject.findProperty(propertyName, configuration.isMapUnderscoreToCamelCase());
    if (property != null && metaObject.hasSetter(property)) {
      //获取当前列对应的JavaType
      final Class<?> propertyType = metaObject.getSetterType(property);
      if (typeHandlerRegistry.hasTypeHandler(propertyType)) {
        //获取typeHandler，终于找到了问题的根节点了，在这儿就可以看出，ResultType中的列在MyBatis中也是使用TypeHandler处理的，
        //未进入自定义的TypeHandler的原因只能是MyBatis的查询TypeHandler的方法并没有查询到我们自定义的TypeHandler，我们看一下这个查询方法,见下面
        final TypeHandler<?> typeHandler = rsw.getTypeHandler(propertyType, columnName);
        //由TypeHandler获取当前列执行结果
        final Object value = typeHandler.getResult(rsw.getResultSet(), columnName);
        if (value != null || configuration.isCallSettersOnNulls()) { // issue #377, call setter on nulls
          if (value != null || !propertyType.isPrimitive()) {
            //由metaObject将列值设置进入属性值
            metaObject.setValue(property, value);
          }
          foundValues = true;
        }
      }
    }
  }
  return foundValues;
}

rsw.getTypeHandler(propertyType, columnName); 方法：

public TypeHandler<?> getTypeHandler(Class<?> propertyType, String columnName) {
  TypeHandler<?> handler = null;
  //获取当前列是否已查询过TypeHandler
  Map<Class<?>, TypeHandler<?>> columnHandlers = typeHandlerMap.get(columnName);
  if (columnHandlers == null) {
    //没有则新建一个当前列对应的TypeHandler Map
    columnHandlers = new HashMap<Class<?>, TypeHandler<?>>();
    typeHandlerMap.put(columnName, columnHandlers);
  } else {
    //存在，则直接取出TypeHandler
    handler = columnHandlers.get(propertyType);
  }
  if (handler == null) {
    //由MyBatis配置文件中取出当前JavaType对应的TypeHandler
    //看到问题了吧，查询时，只传入了JavaType，并没有传JdbcType，是否意味着，会查询出JavaType为当前列的JavaType，JdbcType为null的TypeHandler？进入方法看一下就知道了
    handler = typeHandlerRegistry.getTypeHandler(propertyType);
    // Replicate logic of UnknownTypeHandler#resolveTypeHandler
    // See issue #59 comment 10
    if (handler == null || handler instanceof UnknownTypeHandler) {
      final int index = columnNames.indexOf(columnName);
      final JdbcType jdbcType = jdbcTypes.get(index);
      final Class<?> javaType = resolveClass(classNames.get(index));
      if (javaType != null && jdbcType != null) {
        handler = typeHandlerRegistry.getTypeHandler(javaType, jdbcType);
      } else if (javaType != null) {
        handler = typeHandlerRegistry.getTypeHandler(javaType);
      } else if (jdbcType != null) {
        handler = typeHandlerRegistry.getTypeHandler(jdbcType);
      }
    }
    if (handler == null || handler instanceof UnknownTypeHandler) {
      handler = new ObjectTypeHandler();
    }
    columnHandlers.put(propertyType, handler);
  }
  return handler;
}

typeHandlerRegistry.getTypeHandler(propertyType); 方法：

public <T> TypeHandler<T> getTypeHandler(Class<T> type) {
  return getTypeHandler((Type) type, null);
}
继续进入：
private <T> TypeHandler<T> getTypeHandler(Type type, JdbcType jdbcType) {
  //获取当前列对应的JavaType所有已配置的TypeHandler
  Map<JdbcType, TypeHandler<?>> jdbcHandlerMap = TYPE_HANDLER_MAP.get(type);
  TypeHandler<?> handler = null;
  if (jdbcHandlerMap != null) {
    //获取jdbcType对应的TypeHandler
    //当前jdbcType为Null，取出的果然是：JavaType为当前列的JavaType，JdbcType为null的TypeHandler
    //问题揭开了，还是我们配置为有问题，我们自定义的TypeHandler配置的JavaType为：java.lang.String,JdbcType为VARCHAR，所以肯定不会使用我们自定义的TypeHandler
    handler = jdbcHandlerMap.get(jdbcType);
    if (handler == null) {
      handler = jdbcHandlerMap.get(null);
    }
  }
  if (handler == null && type != null && type instanceof Class && Enum.class.isAssignableFrom((Class<?>) type)) {
    handler = new EnumTypeHandler((Class<?>) type);
  }
  @SuppressWarnings("unchecked")
  // type drives generics here
  TypeHandler<T> returned = (TypeHandler<T>) handler;
  return returned;
}

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;
  //获取所有配置的ResultMapping
  final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
  //根据每个ResultMapping来处理每一列的值，ResultMapping中包含了当前列的TypeHandler
  for (ResultMapping propertyMapping : propertyMappings) {
    final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
    if (propertyMapping.isCompositeResult() 
        || (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH))) 
        || propertyMapping.getResultSet() != null) {
      //处理当前列的值,此方法见下面：
      Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);
      final String property = propertyMapping.getProperty(); // issue #541 make property optional
      if (value != NO_VALUE && property != null && (value != null || configuration.isCallSettersOnNulls())) { // issue #377, call setter on nulls
        if (value != null || !metaObject.getSetterType(property).isPrimitive()) {
          metaObject.setValue(property, value);
        }
        foundValues = true;
      }
    }
  }
  return foundValues;
}

getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);方法：

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);
    return NO_VALUE;
  } else if (propertyMapping.getNestedResultMapId() != null) {
    // the user added a column attribute to a nested result map, ignore it
    return NO_VALUE;
  } else {
    //由TypeHandler获取数据
    final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
    final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
    return typeHandler.getResult(rs, column);
  }
}

StatementHandler类：

• 基础实现类：BaseStatementHandler，它有三个实现类，分别对应CallableStatementHandler(存储过程)、PreparedStatementHandler（对应于JDBC中prepareStatement执行的SQL）、SimpleStatementHandler（对应于JDBC中Statement执行的SQL）

• 路由StatementHandler：RoutingStatementHandler，它无业务性，只是提供了一种路由，来产生相应的BaseStatementHandler

4、解决方法

问题排查完毕，是配置自定义TypeHandler时指定了JaveType和JdbcType导致MyBatis找不到自定义的TypeHandler，因此使用如下配置方式即可解决问题：

<typeHandlers>
    <typeHandler javaType="java.lang.String"
                 handler="com.meituan.fd.crm.common.typehandler.SecurityStringVarcharTypeHandler"/>
    <typeHandler jdbcType="VARCHAR" javaType="java.lang.String"
                 handler="com.meituan.fd.crm.common.typehandler.SecurityStringVarcharTypeHandler"/>
</typeHandlers>

此时，javaType为String，JdbcType为VARCHAR和null的都被配置为自定义的TypeHandler