我们直接从gridfs中取文件列表时取出来的文件_id为：

"_id": {
        "timestamp": 1587091947,
        "counter": 9048602,
        "time": 1587091947000,
        "date": "2020-04-17T02:52:27.000+0000",
        "timeSecond": 1587091947,
        "machineIdentifier": 11229770,
        "processIdentifier": 31009
    },

如果要转换成正常的文件id 其实很简单 :

    ObjectId objectId=(ObjectId) map.get("_id");
            map.put("_id",objectId.toString());