查询MongoDB GridFS元数据(Java)

import org.bson.Document;
import org.bson.conversions.Bson;

import com.mongodb.MongoClient;
import com.mongodb.MongoClientURI;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.Aggregates;
import com.mongodb.client.model.Filters;

import java.util.Arrays;

import static com.mongodb.client.model.Filters.eq;

// ...
public class GridFSMetadataExample {
    public static void main(String[] args) {
        String connectionString = "mongodb+srv://username:password@cluster0.mongodb.net/test?retryWrites=true&w=majority";
        MongoClient mongoClient = new MongoClient(new MongoClientURI(connectionString));
        MongoDatabase database = mongoClient.getDatabase("test");
        MongoCollection<Document> gridFS = database.getCollection("gridfs");

        // 查询所有文件
        Aggregates.match(new Document("files", new Document("$exists", true)));
        Aggregates.project(new Document("_id", 1)
                .append("filename", 1)
                .append("filesize", 1)
                .append("uploadDate", 1));
        Aggregates.unwind("$files");

        gridFS.aggregate(Arrays.asList(Aggregates.project(new Document("file", "$files.file"))));

        // 查询文件元数据
        Bson match = Aggregates.match(new Document("files", new Document("$exists", true)));
        Bson unwind = Aggregates.unwind("$files");
        Bson project = Aggregates.project(new Document("_id", 1)
                .append("filename", 1)
                .append("filesize", 1)
                .append("uploadDate", 1));

        Document result = gridFS.aggregate(Arrays.asList(match, unwind, project)).get(0);
        System.out.println(result.toJson());

        mongoClient.close();
    }
}