driver-async/src/main/com/mongodb/async/client/gridfs/helpers/AsynchronousChannelHelper.java
public static AsyncOutputStream channelToOutputStream(final AsynchronousFileChannel asynchronousFileChannel) {
        notNull("asynchronousByteChannel", asynchronousFileChannel);
        return new AsyncOutputStream() {
            private int position = 0;
            @Override
            public void write(final ByteBuffer src, final SingleResultCallback<Integer> callback) {
                asynchronousFileChannel.write(src, position, null, new CompletionHandler<Integer, Object>() {
                    @Override
                    public void completed(final Integer result, final Object attachment) {
                        position += result;
                        callback.onResult(result, null);
                    }

                    @Override
                    public void failed(final Throwable exc, final Object attachment) {
                        callback.onResult(null, exc);
                    }
                });
            }

            @Override
            public void close(final SingleResultCallback<Void> callback) {
                try {
                    asynchronousFileChannel.close();
                    callback.onResult(null, null);
                } catch (IOException e) {
                    callback.onResult(null, e);
                }
            }
        };
    }
public static AsyncOutputStream channelToOutputStream(final AsynchronousFileChannel asynchronousFileChannel) {
        notNull("asynchronousByteChannel", asynchronousFileChannel);
        return new AsyncOutputStream() {
            private int position = 0;
            @Override
            public void write(final ByteBuffer src, final SingleResultCallback<Integer> callback) {
                asynchronousFileChannel.write(src, position, null, new CompletionHandler<Integer, Object>() {
                    @Override
                    public void completed(final Integer result, final Object attachment) {
                        position += result;
                        callback.onResult(result, null);
                    }

                    @Override
                    public void failed(final Throwable exc, final Object attachment) {
                        callback.onResult(null, exc);
                    }
                });
            }

            @Override
            public void close(final SingleResultCallback<Void> callback) {
                try {
                    asynchronousFileChannel.close();
                    callback.onResult(null, null);
                } catch (IOException e) {
                    callback.onResult(null, e);
                }
            }
        };
    }
Converts a {@link AsynchronousFileChannel} into a {@link AsyncOutputStream}

@param asynchronousFileChannel the AsynchronousFileChannel
@return the AsyncOutputStream