gridFsBucket.downloadStream转为MultipartFilejava代码

下面是将GridFSBucket的下载流转换为MultipartFile的Java代码示例：

import com.mongodb.client.gridfs.GridFSDownloadStream;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.multipart.commons.CommonsMultipartFile;

import java.io.IOException;
import java.io.InputStream;

public class GridFSDownloadStreamToMultipartFileConverter {

    public static MultipartFile convert(GridFSDownloadStream downloadStream, String filename, String contentType) throws IOException {
        InputStream inputStream = downloadStream;
        CommonsMultipartFile multipartFile = new CommonsMultipartFile(null, filename, contentType, inputStream);
        return multipartFile;
    }
}

调用方式如下：

GridFSBucket gridFSBucket = ... // 获取GridFSBucket实例
String fileId = ... // 文件ID
GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(new ObjectId(fileId));
String filename = downloadStream.getGridFSFile().getFilename();
String contentType = downloadStream.getGridFSFile().getMetadata().getString("contentType");
MultipartFile multipartFile = GridFSDownloadStreamToMultipartFileConverter.convert(downloadStream, filename, contentType);

这里使用了Spring的`CommonsMultipartFile`实现，需要添加`spring-web`依赖。如果不想使用Spring的实现，也可以自己实现`MultipartFile`接口。