StatementHandler

1. 继承关系
   RoutingStatementHandler会根据MappedStatement中StatementType属性进行创建StatementHandler
   
2. RoutingStatementHandler中StatementHandler的类型
   
3. 真正与数据库交互的StatementHandler
   SimpleStatementHandler 中执行数据库查询后把查询结果交给resultSetHandler进行处理

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