Incomplete file returned by GridFS

I'm working on a Java project to store and retrieve files from MongoDB using GridFS specification. I'm using the code snippets provided in MongoDB Java driver documentation from <https://mongodb.github.io/mongo-java-driver/4.1/driver/tutorials/gridfs/>.

While using OpenDownloadStream to retrieve the file, I noticed that if the file is divided into more than one chunks, it returns only the first chunk, and not the full file.

ObjectId fileId;

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteTo = new byte[fileLength];
downloadStream.read(bytesToWriteTo);    /*read file contents */
downloadStream.close();

System.out.println(new String(bytesToWriteTo, StandardCharsets.UTF_8));

Any solutions to this?

Looking at the class GridFSDownloadStreamImpl which implements GridFSDownloadStream, it looks like the method read(byte[]) reads chunk by chunk:

@Override
public int read(final byte[] b) {
    return read(b, 0, b.length);
}

@Override
public int read(final byte[] b, final int off, final int len) {
    checkClosed();

    if (currentPosition == length) {
        return -1;
    } else if (buffer == null) {
        buffer = getBuffer(chunkIndex);
    } else if (bufferOffset == buffer.length) {
        chunkIndex += 1;
        buffer = getBuffer(chunkIndex);
        bufferOffset = 0;
    }

    int r = Math.min(len, buffer.length - bufferOffset);
    System.arraycopy(buffer, bufferOffset, b, off, r);
    bufferOffset += r;
    currentPosition += r;
    return r;
}

Therefore, you have to loop until all expected bytes are actually read:

byte[] bytesToWriteTo = new byte[fileLength];
int bytesRead = 0;
while(bytesRead < fileLength) {
    int newBytesRead = downloadStream.read(bytesToWriteTo);
    if(newBytesRead == -1) {
        throw new Exception();
    }
    bytesRead += newBytesRead;
}
downloadStream.close();

Note that I was not able to test above code so please use with caution.

I ended up using readAllBytes() method and it returns the whole file.

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteTo = new byte[fileLength];
bytesToWriteTo = downloadStream.readAllBytes();
downloadStream.close();