public function getParser()
    {
        if (!isset($this->parser)) {
            // 初始化一个查询解析对象用于辅助生成或链式构造查询语句
            $class = '\\Wslim\\Db\\Parser\\' . $this->options['adapter'] . 'QueryParser';
            if (!class_exists($class)) {
                throw new DbException('QueryParser is not exists:' . $class);
            }
            $this->parser = new $class();
        }
        return $this->parser;
    }