mybatis源码(四)-与JDBC交互

　　之前研究了mybatis大体的执行流程，从上面SqlSession到Executor 内部。这里研究下其内部的处理， 也就是调用JDBC以及对结果出来部分。

　　mybatis有几个重要的对象：ParameterHandler(java参数转为JDBC需要的参数处理器)、ResultSetHandler(负责将JDBC返回的ResultSet 结果集转换成List 类型的集合)、StatementHandler (封装JDBC Statement操作)。其获取都是在Configuration对象，方法如下：

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction) {
    return newExecutor(transaction, defaultExecutorType);
  }

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

1. 前置理解

　　下面是个对象创建都是调用Configuration对象中API进行创建，会走mybatis的插件那一套机制。

1. Executor 创建是在SqlSessionFactory类中openSession 方法创建SqlSession时会创建Executor

2. StatementHandler 是在Executor中执行doUpdate或者doQuery 方法创建

3. ParameterHandler 是在创建StatementHandler 过程中创建，对应方法在父构造org.apache.ibatis.executor.statement.BaseStatementHandler#BaseStatementHandler

4. ResultSetHandler 是在创建StatementHandler 过程中创建，对应方法在父构造org.apache.ibatis.executor.statement.BaseStatementHandler#BaseStatementHandler

2. 分析其过程

　　以org.apache.ibatis.executor.SimpleExecutor#doQuery 执行查询为例子分析其参数设置过程以及结果处理过程。

org.apache.ibatis.executor.SimpleExecutor#doQuery 源码如下：

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }

1. org.apache.ibatis.session.Configuration#newStatementHandler 创建StatementHandler

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

继续调用：org.apache.ibatis.executor.statement.RoutingStatementHandler#RoutingStatementHandler

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

　　这里是根据ms.getStatementType() 来获取，ms.getStatementType() 默认的类型是PREPARED。 我们也可以在xml写sql 的时候进行指定类型。所以会创建PreparedStatementHandler。

最后会调用到：org.apache.ibatis.executor.statement.BaseStatementHandler#BaseStatementHandler (这里创建参数处理器和结果处理器)

  protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }

2. 调用方法prepareStatement 获取Statement 对象

 1》handler.prepare(connection, transaction.getTimeout()); 准备一个Statement 对象， 会调用到：org.apache.ibatis.executor.statement.BaseStatementHandler#prepare

  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      statement = instantiateStatement(connection);
      setStatementTimeout(statement, transactionTimeout);
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

instantiateStatement 调用到：org.apache.ibatis.executor.statement.PreparedStatementHandler#instantiateStatement (这里实际就是创建PrepareStatement 对象)

  @Override
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.prepareStatement(sql);
    }
  }

2》handler.parameterize(stmt); 参数化statement 对象。

会调用到：org.apache.ibatis.executor.statement.PreparedStatementHandler#parameterize

  @Override
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

紧接着调用到： com.baomidou.mybatisplus.MybatisDefaultParameterHandler#setParameters 

    @SuppressWarnings({"rawtypes", "unchecked"})
    @Override
    public void setParameters(PreparedStatement ps) {
        // 反射获取动态参数
        Map<String, Object> additionalParameters = null;
        try {
            additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql);
        } catch (IllegalAccessException e) {
            // ignored, Because it will never happen.
        }
        ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
        if (parameterMappings != null) {
            for (int i = 0; i < parameterMappings.size(); i++) {
                ParameterMapping parameterMapping = parameterMappings.get(i);
                if (parameterMapping.getMode() != ParameterMode.OUT) {
                    Object value;
                    String propertyName = parameterMapping.getProperty();
                    if (boundSql.hasAdditionalParameter(propertyName)) {//issue#448 ask first for additional params
                        value = boundSql.getAdditionalParameter(propertyName);
                    } else if (parameterObject == null) {
                        value = null;
                    } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                        value = parameterObject;
                    } else {
                        MetaObject metaObject = configuration.newMetaObject(parameterObject);
                        value = metaObject.getValue(propertyName);
                        if (value == null && MapUtils.isNotEmpty(additionalParameters)) {
                            // issue #138
                            value = additionalParameters.get(propertyName);
                        }
                    }
                    TypeHandler typeHandler = parameterMapping.getTypeHandler();
                    JdbcType jdbcType = parameterMapping.getJdbcType();
                    if (value == null && jdbcType == null) {
                        jdbcType = configuration.getJdbcTypeForNull();
                    }
                    try {
                        typeHandler.setParameter(ps, i + 1, value, jdbcType);
                    } catch (TypeException | SQLException e) {
                        throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
                    }
                }
            }
        }
    }

 　　这里核心逻辑是for循环遍历所有动态的参数，然后获取到参数的值value、类型处理器 TypeHandler、 JdbcType类型， 然后调用org.apache.ibatis.type.BaseTypeHandler#setParameter：(模板模式的使用)

  public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
    if (parameter == null) {
      if (jdbcType == null) {
        throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
      }
      try {
        ps.setNull(i, jdbcType.TYPE_CODE);
      } catch (SQLException e) {
        throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. " +
                "Cause: " + e, e);
      }
    } else {
      try {
        setNonNullParameter(ps, i, parameter, jdbcType);
      } catch (Exception e) {
        throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different configuration property. " +
                "Cause: " + e, e);
      }
    }
  }

　　这个方法里面核心的设置参数交给子类实现 setNonNullParameter 方法，比如枚举类型参数的设置通过如下设置： org.apache.ibatis.type.EnumTypeHandler#setNonNullParameter(可以看到实际是设置了枚举类型的name() 方法)

  @Override
  public void setNonNullParameter(PreparedStatement ps, int i, E parameter, JdbcType jdbcType) throws SQLException {
    if (jdbcType == null) {
      ps.setString(i, parameter.name());
    } else {
      ps.setObject(i, parameter.name(), jdbcType.TYPE_CODE); // see r3589
    }
  }

3. handler.<E>query(stmt, resultHandler); 进行处理然后处理结果

1. 方法走到org.apache.ibatis.executor.statement.RoutingStatementHandler#query

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    return delegate.<E>query(statement, resultHandler);
  }

2. 然后交给委托类org.apache.ibatis.executor.statement.PreparedStatementHandler#query

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }

这里显示执行，然后交给resultSetHandler.<E> handleResultSets(ps); 处理执行结果。

3.  调用到org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleResultSets处理回传结果：

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    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 List<Object> collapseSingleResultList(List<Object> multipleResults) {
    return multipleResults.size() == 1 ? (List<Object>) multipleResults.get(0) : multipleResults;
  }

1》继续调用org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleResultSet 对结果进行处理：

  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());
    }
  }

　　这里resultHandler 为null， 所以走第8行代码

2》继续调用org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleRowValues

  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);
    }
  }

3》 继续调用到：org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleRowValuesForSimpleResultMap

  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);
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }

这里就是遍历每一行反向映射结果。rowValue 就是获取的结果。然后调用storeObject 将结果收集起来。

3.1》 getRowValue 调用到 org.apache.ibatis.executor.resultset.DefaultResultSetHandler#getRowValue(org.apache.ibatis.executor.resultset.ResultSetWrapper, org.apache.ibatis.mapping.ResultMap)

  private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    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)) {
        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
      }
      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
      foundValues = lazyLoader.size() > 0 || foundValues;
      rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
    }
    return rowValue;
  }

这个方法内部createResultObject 是反射创建对象；

对于rowValue 不为空，且mybatis环境中部包含类型处理器的，也就是类型不是Integer、Long 等类型的，会走 applyAutomaticMappings 是反射调用setter方法设置属性的值。如果是Integer、Long等基本类型的包装类型，!hasTypeHandlerForResultObject(rsw, resultMap.getType()) 返回是false。

(1) createResultObject 反射创建对象逻辑如下：

org.apache.ibatis.executor.resultset.DefaultResultSetHandler#createResultObject

  private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
    this.useConstructorMappings = false; // reset previous mapping result
    final List<Class<?>> constructorArgTypes = new ArrayList<Class<?>>();
    final List<Object> constructorArgs = new ArrayList<Object>();
    Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
    if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
      for (ResultMapping propertyMapping : propertyMappings) {
        // issue gcode #109 && issue #149
        if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
          resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
          break;
        }
      }
    }
    this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
    return resultObject;
  }

createResultObject 创建对象会调用到：

  private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
      throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
    final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
    if (hasTypeHandlerForResultObject(rsw, resultType)) {
      return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
    } else if (!constructorMappings.isEmpty()) {
      return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
    } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
      return objectFactory.create(resultType);
    } else if (shouldApplyAutomaticMappings(resultMap, false)) {
      return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix);
    }
    throw new ExecutorException("Do not know how to create an instance of " + resultType);
  }

　　这里根据不同的方式选择不同的创建对象的方法。

第一种： 对于hasTypeHandlerForResultObject 返回为true的，可以认为是Integer,Long 等基本类型的包装类型的数据，会直接调用createPrimitiveResultObject

  private Object createPrimitiveResultObject(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final String columnName;
    if (!resultMap.getResultMappings().isEmpty()) {
      final List<ResultMapping> resultMappingList = resultMap.getResultMappings();
      final ResultMapping mapping = resultMappingList.get(0);
      columnName = prependPrefix(mapping.getColumn(), columnPrefix);
    } else {
      columnName = rsw.getColumnNames().get(0);
    }
    final TypeHandler<?> typeHandler = rsw.getTypeHandler(resultType, columnName);
    return typeHandler.getResult(rsw.getResultSet(), columnName);
  }

　　可以看到实际就是genjuTypeHandler.getResult 获取返回结果。

第二种： 是接口或者有默认构造方法的调用到：org.apache.ibatis.reflection.factory.DefaultObjectFactory#create(java.lang.Class<T>)

  @Override
  public <T> T create(Class<T> type) {
    return create(type, null, null);
  }

  public <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    Class<?> classToCreate = resolveInterface(type);
    // we know types are assignable
    return (T) instantiateClass(classToCreate, constructorArgTypes, constructorArgs);
  }

  protected Class<?> resolveInterface(Class<?> type) {
    Class<?> classToCreate;
    if (type == List.class || type == Collection.class || type == Iterable.class) {
      classToCreate = ArrayList.class;
    } else if (type == Map.class) {
      classToCreate = HashMap.class;
    } else if (type == SortedSet.class) { // issue #510 Collections Support
      classToCreate = TreeSet.class;
    } else if (type == Set.class) {
      classToCreate = HashSet.class;
    } else {
      classToCreate = type;
    }
    return classToCreate;
  }

  private  <T> T instantiateClass(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    try {
      Constructor<T> constructor;
      if (constructorArgTypes == null || constructorArgs == null) {
        constructor = type.getDeclaredConstructor();
        if (!constructor.isAccessible()) {
          constructor.setAccessible(true);
        }
        return constructor.newInstance();
      }
      constructor = type.getDeclaredConstructor(constructorArgTypes.toArray(new Class[constructorArgTypes.size()]));
      if (!constructor.isAccessible()) {
        constructor.setAccessible(true);
      }
      return constructor.newInstance(constructorArgs.toArray(new Object[constructorArgs.size()]));
    } catch (Exception e) {
      StringBuilder argTypes = new StringBuilder();
      if (constructorArgTypes != null && !constructorArgTypes.isEmpty()) {
        for (Class<?> argType : constructorArgTypes) {
          argTypes.append(argType.getSimpleName());
          argTypes.append(",");
        }
        argTypes.deleteCharAt(argTypes.length() - 1); // remove trailing ,
      }
      StringBuilder argValues = new StringBuilder();
      if (constructorArgs != null && !constructorArgs.isEmpty()) {
        for (Object argValue : constructorArgs) {
          argValues.append(String.valueOf(argValue));
          argValues.append(",");
        }
        argValues.deleteCharAt(argValues.length() - 1); // remove trailing ,
      }
      throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
    }
  }

　　resolveInterface(type); 先对类型进行重置，如果是Map，那么实际类型是HashMap；List或者集合实际类型是ArrayList等。这里需要注意如果返回类型是List<ExtensiveSubject>， 那么mybatis 实际需要的类型是ExtensiveSubject.class，不是List。然后根据不同的类型反射创建不同的对象。

第三种：对于有指定构造方法的调用到org.apache.ibatis.executor.resultset.DefaultResultSetHandler#createByConstructorSignature

  private Object createByConstructorSignature(ResultSetWrapper rsw, Class<?> resultType, List<Class<?>> constructorArgTypes, List<Object> constructorArgs,
                                              String columnPrefix) throws SQLException {
    final Constructor<?>[] constructors = resultType.getDeclaredConstructors();
    final Constructor<?> annotatedConstructor = findAnnotatedConstructor(constructors);
    if (annotatedConstructor != null) {
      return createUsingConstructor(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix, annotatedConstructor);
    } else {
      for (Constructor<?> constructor : constructors) {
        if (allowedConstructor(constructor, rsw.getClassNames())) {
          return createUsingConstructor(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix, constructor);
        }
      }
    }
    throw new ExecutorException("No constructor found in " + resultType.getName() + " matching " + rsw.getClassNames());
  }

  private Constructor<?> findAnnotatedConstructor(final Constructor<?>[] constructors) {
    for (final Constructor<?> constructor : constructors) {
      if (constructor.isAnnotationPresent(AutomapConstructor.class)) {
        return constructor;
      }
    }
    return null;
  }

  private Object createUsingConstructor(ResultSetWrapper rsw, Class<?> resultType, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix, Constructor<?> constructor) throws SQLException {
    boolean foundValues = false;
    for (int i = 0; i < constructor.getParameterTypes().length; i++) {
      Class<?> parameterType = constructor.getParameterTypes()[i];
      String columnName = rsw.getColumnNames().get(i);
      TypeHandler<?> typeHandler = rsw.getTypeHandler(parameterType, columnName);
      Object value = typeHandler.getResult(rsw.getResultSet(), prependPrefix(columnName, columnPrefix));
      constructorArgTypes.add(parameterType);
      constructorArgs.add(value);
      foundValues = value != null || foundValues;
    }
    return foundValues ? objectFactory.create(resultType, constructorArgTypes, constructorArgs) : null;
  }

    。。。

　　可以看到构造方法获取是：先获取AutomapConstructor 注解声明的构造，没有的情况下获取其他有参构造，然后获取到构造参数，遍历参数调用typeHandler.getResult 获取到实际的Java类型之后，再交给objectFactory.create 反射创建对象。机制和上面一样。

(2) applyAutomaticMappings 反射调用setter 源码如下：(遍历映射，调用typeHandler.getResult 获取到值的结果，结果是java类型结果，然后反射修改属性值)

  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;
  }

然后调用org.apache.ibatis.reflection.MetaObject#setValue：

  public void setValue(String name, Object value) {
    PropertyTokenizer prop = new PropertyTokenizer(name);
    if (prop.hasNext()) {
      MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
      if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
        if (value == null && prop.getChildren() != null) {
          // don't instantiate child path if value is null
          return;
        } else {
          metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
        }
      }
      metaValue.setValue(prop.getChildren(), value);
    } else {
      objectWrapper.set(prop, value);
    }
  }

然后走 objectWrapper.set(prop, value); 调用到：org.apache.ibatis.reflection.wrapper.BeanWrapper#set

  public void set(PropertyTokenizer prop, Object value) {
    if (prop.getIndex() != null) {
      Object collection = resolveCollection(prop, object);
      setCollectionValue(prop, collection, value);
    } else {
      setBeanProperty(prop, object, value);
    }
  }

走setBeanProperty， 这里需要注意。对于普通的Bean和Map、List等或有不同的策略。其继承关系图如下：

第一种：org.apache.ibatis.reflection.wrapper.BeanWrapper#setBeanProperty

  private void setBeanProperty(PropertyTokenizer prop, Object object, Object value) {
    try {
      Invoker method = metaClass.getSetInvoker(prop.getName());
      Object[] params = {value};
      try {
        method.invoke(object, params);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    } catch (Throwable t) {
      throw new ReflectionException("Could not set property '" + prop.getName() + "' of '" + object.getClass() + "' with value '" + value + "' Cause: " + t.toString(), t);
    }
  }

　　可以看到这里就是获取到setter，然后反射调用setter 方法修改属性。

第二种：org.apache.ibatis.reflection.wrapper.MapWrapper#set (可以看到如果返回值是Map类型，会走到这里进行put 操作)

  @Override
  public void set(PropertyTokenizer prop, Object value) {
    if (prop.getIndex() != null) {
      Object collection = resolveCollection(prop, map);
      setCollectionValue(prop, collection, value);
    } else {
      map.put(prop.getName(), value);
    }
  }

补充： TypeHandler 是一个类型处理器。在mybatis也有重要的作用。在ParameterHandler 设置过程中，将Java类型转换为JDBC类型；在ResultSetHandler 设置过程中根据JDBC类型转换为Java类型。

源码如下： org.apache.ibatis.type.TypeHandler

public interface TypeHandler<T> {

  void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;

  T getResult(ResultSet rs, String columnName) throws SQLException;

  T getResult(ResultSet rs, int columnIndex) throws SQLException;

  T getResult(CallableStatement cs, int columnIndex) throws SQLException;

}

　　在ParameterHandler 参数设置时会调用setParameter 方法， 在ResultSetHandler 设置查询结果时会调用getResult 方法。可以看到就是根据Java的类型，然后进行处理。这里也是一个模板模式的使用，BaseTypeHandler 是一个抽象类，如下：

public abstract class BaseTypeHandler<T> extends TypeReference<T> implements TypeHandler<T> {

  protected Configuration configuration;

  public void setConfiguration(Configuration c) {
    this.configuration = c;
  }

  @Override
  public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
    if (parameter == null) {
      if (jdbcType == null) {
        throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
      }
      try {
        ps.setNull(i, jdbcType.TYPE_CODE);
      } catch (SQLException e) {
        throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. " +
                "Cause: " + e, e);
      }
    } else {
      try {
        setNonNullParameter(ps, i, parameter, jdbcType);
      } catch (Exception e) {
        throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . " +
                "Try setting a different JdbcType for this parameter or a different configuration property. " +
                "Cause: " + e, e);
      }
    }
  }

  @Override
  public T getResult(ResultSet rs, String columnName) throws SQLException {
    T result;
    try {
      result = getNullableResult(rs, columnName);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set.  Cause: " + e, e);
    }
    if (rs.wasNull()) {
      return null;
    } else {
      return result;
    }
  }

  @Override
  public T getResult(ResultSet rs, int columnIndex) throws SQLException {
    T result;
    try {
      result = getNullableResult(rs, columnIndex);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column #" + columnIndex+ " from result set.  Cause: " + e, e);
    }
    if (rs.wasNull()) {
      return null;
    } else {
      return result;
    }
  }

  @Override
  public T getResult(CallableStatement cs, int columnIndex) throws SQLException {
    T result;
    try {
      result = getNullableResult(cs, columnIndex);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column #" + columnIndex+ " from callable statement.  Cause: " + e, e);
    }
    if (cs.wasNull()) {
      return null;
    } else {
      return result;
    }
  }

  public abstract void setNonNullParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;

  public abstract T getNullableResult(ResultSet rs, String columnName) throws SQLException;

  public abstract T getNullableResult(ResultSet rs, int columnIndex) throws SQLException;

  public abstract T getNullableResult(CallableStatement cs, int columnIndex) throws SQLException;

}

下面具体的Handler 只需要实现其抽象方法，比如对于枚举类型Handler 如下：

package org.apache.ibatis.type;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

/**
 * @author Clinton Begin
 */
public class EnumTypeHandler<E extends Enum<E>> extends BaseTypeHandler<E> {

  private final Class<E> type;

  public EnumTypeHandler(Class<E> type) {
    if (type == null) {
      throw new IllegalArgumentException("Type argument cannot be null");
    }
    this.type = type;
  }

  @Override
  public void setNonNullParameter(PreparedStatement ps, int i, E parameter, JdbcType jdbcType) throws SQLException {
    if (jdbcType == null) {
      ps.setString(i, parameter.name());
    } else {
      ps.setObject(i, parameter.name(), jdbcType.TYPE_CODE); // see r3589
    }
  }

  @Override
  public E getNullableResult(ResultSet rs, String columnName) throws SQLException {
    String s = rs.getString(columnName);
    return s == null ? null : Enum.valueOf(type, s);
  }

  @Override
  public E getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
    String s = rs.getString(columnIndex);
    return s == null ? null : Enum.valueOf(type, s);
  }

  @Override
  public E getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
    String s = cs.getString(columnIndex);
    return s == null ? null : Enum.valueOf(type, s);
  }
}

　　可以看到枚举类型的参数赋值是调用enum.name() 方法； 反向数据库到JavaBean映射值是调用Enum.valueOf(type, s); 方法。