Mybatis源码分析之结果封装ResultSetHandler和DefaultResultSetHandler

ResultSetHandler负责处理两件事：

（1）处理Statement执行后产生的结果集，生成结果列表

（2）处理存储过程执行后的输出参数
ResultSetHandler是一个接口，提供了两个函数分别用来处理普通操作和存储过程的结果，
源码如下：

/**
 * @author Clinton Begin
 */
public interface ResultSetHandler {

   List handleResultSets(Statement stmt) throws SQLException;

  void handleOutputParameters(CallableStatement cs) throws SQLException;

}

ResultSetHandler的具体实现类是DefaultResultSetHandler，其实现的步骤就是将Statement执行后的结果集，按照Mapper文件中配置的ResultType或ResultMap来封装成对应的对象，最后将封装的对象返回即可，
DefaultResultSetHandler源码如下：

//ResultSetHandler具体实现类
public class DefaultResultSetHandler implements ResultSetHandler {

  private static final Object DEFERED = new Object();

  private final Executor executor;
  private final Configuration configuration;
  private final MappedStatement mappedStatement;
  private final RowBounds rowBounds;
  private final ParameterHandler parameterHandler;
  private final ResultHandler resultHandler;
  private final BoundSql boundSql;
  private final TypeHandlerRegistry typeHandlerRegistry;
  private final ObjectFactory objectFactory;
  private final ReflectorFactory reflectorFactory;

  // nested resultmaps
  // 此Map用来保存当前层级内部的结果对象（一对多关系的多方对象），key为combinedKey
  private final Map nestedResultObjects = new HashMap();
  // 此Map用来保存当前层级的根对象（一对多关系中的一方对象），key为absoluteKey
  private final Map ancestorObjects = new HashMap();
  private final Map ancestorColumnPrefix = new HashMap();

  // multiple resultsets
  private final Map nextResultMaps = new HashMap();
  private final Map> pendingRelations = new HashMap>();

  private static class PendingRelation {
    public MetaObject metaObject;
    public ResultMapping propertyMapping;
  }

  public DefaultResultSetHandler(Executor executor, MappedStatement mappedStatement, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql,
      RowBounds rowBounds) {
    this.executor = executor;
    this.configuration = mappedStatement.getConfiguration();
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;
    this.parameterHandler = parameterHandler;
    this.boundSql = boundSql;
    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();
    this.reflectorFactory = configuration.getReflectorFactory();
    this.resultHandler = resultHandler;
  }

  //
  // HANDLE OUTPUT PARAMETER
  //
  //调用存储过程返回结果，将结果值放在参数中
  @Override
  public void handleOutputParameters(CallableStatement cs) throws SQLException {
    final Object parameterObject = parameterHandler.getParameterObject();
    final MetaObject metaParam = configuration.newMetaObject(parameterObject);
    final List parameterMappings = boundSql.getParameterMappings();
	//循环处理每个参数
    for (int i = 0; i < parameterMappings.size(); i++) {
      final ParameterMapping parameterMapping = parameterMappings.get(i);
	  //判断参数的模式
      if (parameterMapping.getMode() == ParameterMode.OUT || parameterMapping.getMode() == ParameterMode.INOUT) {
        if (ResultSet.class.equals(parameterMapping.getJavaType())) {
          handleRefCursorOutputParameter((ResultSet) cs.getObject(i + 1), parameterMapping, metaParam);
        } else {
          final TypeHandler typeHandler = parameterMapping.getTypeHandler();
          metaParam.setValue(parameterMapping.getProperty(), typeHandler.getResult(cs, i + 1));
        }
      }
    }
  }

  private void handleRefCursorOutputParameter(ResultSet rs, ParameterMapping parameterMapping, MetaObject metaParam) throws SQLException {
    try {
      final String resultMapId = parameterMapping.getResultMapId();
      final ResultMap resultMap = configuration.getResultMap(resultMapId);
      final DefaultResultHandler resultHandler = new DefaultResultHandler(objectFactory);
      final ResultSetWrapper rsw = new ResultSetWrapper(rs, configuration);
      handleRowValues(rsw, resultMap, resultHandler, new RowBounds(), null);
      metaParam.setValue(parameterMapping.getProperty(), resultHandler.getResultList());
    } finally {
      // issue #228 (close resultsets)
      closeResultSet(rs);
    }
  }

  //
  // HANDLE RESULT SETS
  //
  //对普通查询到的结果转换
  @Override
  public List