GridFS

GridFS is a specification for storing and retrieving files that exceed the BSON document size limit of 16MB. Instead of storing a file in a single document, GridFS divides a file into parts, or chunks, and stores each of those chunks as a separate document.

When you query a GridFS store for a file, the Java Async driver will reassemble the chunks as needed.

The following code snippets come from the GridFSTour.java example code, which can be found with the driver source on github.

Async Streams

The driver’s GridFS library uses flexible interfaces for asynchronous input and output. The AsyncInputStream interface represents an InputStream and the AsyncOutputStream interface represents an OutputStream. These interfaces should be easy to wrap for any alternative asynchronous I/O implementations such as Netty or Vertx.

In addition to these interfaces, the driver provides the following helpers:

• AsyncStreamHelper, which provides support for:

  • byte[]
  • ByteBuffer
  • InputStream - note: input streams are blocking
  • OutputStream - note: output streams are blocking

• AsynchronousChannelHelper, which provides support for:

  • AsynchronousByteChannel
  • AsynchronousFileChannel

Prerequisites

Include the following import statements:

import com.mongodb.async.client.MongoClient;
import com.mongodb.async.client.MongoClients;
import com.mongodb.async.client.MongoDatabase;

import com.mongodb.async.client.gridfs.GridFSBucket;
import com.mongodb.async.client.gridfs.GridFSBuckets;
import com.mongodb.async.client.gridfs.AsyncInputStream;
import com.mongodb.async.client.gridfs.GridFSDownloadStream;
import com.mongodb.async.client.gridfs.GridFSUploadStream;
import com.mongodb.client.gridfs.model.GridFSFile;
import com.mongodb.client.gridfs.model.GridFSDownloadOptions;
import com.mongodb.client.gridfs.model.GridFSUploadOptions;

import static com.mongodb.async.client.gridfs.helpers.AsyncStreamHelper.toAsyncInputStream;
import static com.mongodb.async.client.gridfs.helpers.AsynchronousChannelHelper.channelToOutputStream;
import com.mongodb.async.SingleResultCallback;

import com.mongodb.Block;
import static com.mongodb.client.model.Filters.eq;
import java.nio.charset.StandardCharsets;
import java.nio.ByteBuffer;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.nio.channels.AsynchronousFileChannel;
import org.bson.Document;
import org.bson.types.ObjectId;
import java.io.FileNotFoundException;
import java.io.IOException;

Consideration

Connect to a MongoDB Deployment

Connect to a MongoDB deployment and declare and define a MongoDatabase instance.

For example, include the following code to connect to a standalone MongoDB deployment running on localhost on port 27017:

MongoClient mongoClient = MongoClients.create();

For additional information on connecting to MongoDB, see Connect to MongoDB.

Create a GridFS Bucket

GridFS stores files in two collections: a chunks collection stores the file chunks, and a files collection stores file metadata. The two collections are in a common bucket and the collection names are prefixed with the bucket name.

The Java Async driver provides the GridFSBuckets.create() method to create the GridFSBucket:

MongoDatabase myDatabase = mongoClient.getDatabase("mydb");

// Create a gridFSBucket using the default bucket name "fs"
GridFSBucket gridFSBucket = GridFSBuckets.create(myDatabase);

You can specify a bucket name to GridFSBuckets.create() method.

// Create a gridFSBucket with a custom bucket name "files"
GridFSBucket gridFSBucketCustom = GridFSBuckets.create(myDatabase, "files");

Upload to GridFS

To upload data into GridFS, you can upload from an InputStream or write data to a GridFSUploadStream.

UploadFromStream

The uploadFromStream method reads the contents of an AsyncInputStream and saves it to the GridFSBucket.

You can use the GridFSUploadOptions to configure the chunk size or include additional metadata.

// Get the input stream
final AsyncInputStream streamToUploadFrom = toAsyncInputStream(
                    "/tmp/mongodb-tutorial.pdf".getBytes(StandardCharsets.UTF_8));

// Create some custom options
GridFSUploadOptions options = new GridFSUploadOptions()
                                    .chunkSizeBytes(1024 * 1024)
                                    .metadata(new Document("type", "presentation"));

gridFSBucket.uploadFromStream("mongodb-tutorial", streamToUploadFrom, options,
    new SingleResultCallback<ObjectId>() {
       @Override
       public void onResult(final ObjectId result, final Throwable t) {
           System.out.println("The fileId of the uploaded file is: " + result.toHexString());
            streamToUploadFrom.close(new SingleResultCallback<Void>() {
                @Override
                public void onResult(final Void result, final Throwable t) {
                    // Stream closed
                }
            });
       }
    }
);

OpenUploadStream

For a finer-grained control of the upload, the driver also provides a GridFSBucket.openUploadStream method, which returns a GridFSUploadStream. You can write data to a GridFSUploadStream which extends AsyncOutputStream.

The GridFSUploadStream buffers data until it reaches the chunkSizeBytes and then inserts the chunk into the chunks collection. When the GridFSUploadStream is closed, the final chunk is written and the file metadata is inserted into the files collection.

Find Files Stored in GridFS

To find the files stored in the GridFSBucket use the find method.

The following example prints out the filename of each file stored:

gridFSBucket.find().forEach(
    new Block<GridFSFile>() {
        @Override
        public void apply(final GridFSFile gridFSFile) {
            System.out.println(gridFSFile.getFilename() + " has file id: " +
                                gridFSFile.getObjectId().toHexString());
        }
    },
    new SingleResultCallback<Void>() {
        @Override
        public void onResult(final Void result, final Throwable t) {
            // Finished
        }
    }
);

You can also provide a custom filter to limit the results returned. The following example prints out the filenames of all files with a “image/png” value set as the contentType in the user defined metadata document:

gridFSBucket.find(eq("metadata.contentType", "image/png")).forEach(
    new Block<GridFSFile>() {
        @Override
        public void apply(final GridFSFile gridFSFile) {
            System.out.println(gridFSFile.getFilename() + " has file id: " +
                               gridFSFile.getObjectId().toHexString());
        }
    },
    new SingleResultCallback<Void>() {
        @Override
        public void onResult(final Void result, final Throwable t) {
            // Finished
        }
    }
);

Download from GridFS

DownloadToStream

The downloadToStream method reads the contents from MongoDB and writes the data directly to the provided AsyncOutputStream.

For the fileId variable, specify an ObjectId value returned from the find operation section in this tutorial.

Path outputPath = Paths.get("/tmp/mongodb-tutorial-out.pdf");
AsynchronousFileChannel streamToDownloadTo = AsynchronousFileChannel.open(outputPath,
        StandardOpenOption.CREATE_NEW,
        StandardOpenOption.WRITE,
        StandardOpenOption.DELETE_ON_CLOSE);

gridFSBucket.downloadToStream(fileId, channelToOutputStream(streamToDownloadTo),
    new SingleResultCallback<Long>() {
      @Override
      public void onResult(final Long result, final Throwable t) {
          System.out.println("downloaded file sized: " + result);
      }
});
// Note: AsynchronousFileChannel was opened with option delete on close
streamToDownloadTo.close();

If the _id of the file is unknown but you know the filename, then you can pass the filename to the downloadToStream method. By default, it will download the latest version of the file. Use the GridFSDownloadOptions to configure which version to download.

Path outputPath = Paths.get("/tmp/mongodb-tutorial-out.pdf");
AsynchronousFileChannel streamToDownloadTo = AsynchronousFileChannel.open(outputPath,
        StandardOpenOption.CREATE_NEW,
        StandardOpenOption.WRITE,
        StandardOpenOption.DELETE_ON_CLOSE);

GridFSDownloadOptions downloadOptions = new GridFSDownloadOptions().revision(0);

gridFSBucket.downloadToStream("mongodb-tutorial", channelToOutputStream(streamToDownloadTo), downloadOptions,
    new SingleResultCallback<Long>() {
      @Override
      public void onResult(final Long result, final Throwable t) {
          System.out.println("downloaded file sized: " + result);
      }
});
// Note: AsynchronousFileChannel was opened with option delete on close
streamToDownloadTo.close();

OpenDownloadStream

For a finer-grained control of the upload, the driver also provides a openDownloadStream method, which returns a GridFSDownloadStream.

Rename Files

If you should need to rename a file, then use the rename method.

The following example renames a file to “mongodbTutorial”:

ObjectId fileId; // The id of a file uploaded to GridFS, initialize to valid file id 
...
gridFSBucket.rename(fileId, "mongodbTutorial", new SingleResultCallback<Void>() {
    @Override
    public void onResult(final Void result, final Throwable t) {
        System.out.println("Renamed file");
    }
});

Delete Files

To delete a file from the GridFSBucket use the delete method.

The following example deletes a file from the GridFSBucket:

ObjectId fileId; // The id of a file uploaded to GridFS, initialize to valid file id 
...
gridFSBucket.delete(fileId, new SingleResultCallback<Void>() {
    @Override
    public void onResult(final Void result, final Throwable t) {
        System.out.println("Deleted file");
    }
});