所以我用一种简单的方法解决了这个问题。我没有得到gridFSBucket和GridFSFile，并试图将其迁移到新的集合中，这会使所有事情变得复杂，我只是决定直接在mongoCollections bucket1.files和bucket1.chunks上运行find（）方法。
这将返回List documentsList，您可以将其插入到bucket2.files和bucket2.chunks中
我测试了它，我在bucket1中上传的图像可以在遵循此过程后从新的集合bucket2中检索。

MongoCollection<Document> bucket1FileColl = db.getCollection("bucket1.files");
 MongoCollection<Document> bucket1ChunkColl = db.getCollection("bucket1.chunks");
        
        
 MongoCollection<Document> bucket2FileColl = db.getCollection("bucket2.files");
 MongoCollection<Document> bucket2ChunkColl = db.getCollection("bucket2.chunks");

 BasicDBObject searchQuery = new BasicDBObject();
 searchQuery.put("_id", new BasicDBObject("$in", attachmentFileIdsList));
 ArrayList<Document> bucket1Files=
      bucket1FileColl.find(searchQuery).into(new ArrayList<Document>());

  
  if(CollectionUtils.isNotEmpty(bucket1Files)){
     bucket2FileColl.insertMany(bucket1Files);
  }
 

  BasicDBObject searchQueryChunks = new BasicDBObject();
  searchQueryChunks.put("files_id", new BasicDBObject("$in", attachmentFileIdsList));
  
  ArrayList<Document> bucket1Chunks= bucket1ChunkColl.find(searchQueryChunks).into(new ArrayList<Document>());

  
  if(CollectionUtils.isNotEmpty(bucket1Chunks)){
    bucket2ChunkColl.insertMany(bucket1Chunks);
  }