driver-async/src/main/com/mongodb/async/client/gridfs/helpers/AsyncStreamHelper.java
public static AsyncOutputStream toAsyncOutputStream(final OutputStream outputStream) {
        notNull("outputStream", outputStream);
        return new AsyncOutputStream() {
            @Override
            public void write(final ByteBuffer srcByteBuffer, final SingleResultCallback<Integer> callback) {
                notNull("src", srcByteBuffer);
                notNull("callback", callback);
                if (!srcByteBuffer.hasRemaining()) {
                    callback.onResult(-1, null);
                    return;
                }

                int amount = srcByteBuffer.remaining();
                byte[] bytes = new byte[amount];
                try {
                    srcByteBuffer.get(bytes);
                    outputStream.write(bytes);
                } catch (Throwable t) {
                    callback.onResult(null, new MongoGridFSException("Error reading from input stream", t));
                    return;
                }
                callback.onResult(amount, null);
            }

            @Override
            public void close(final SingleResultCallback<Void> callback) {
                try {
                    outputStream.close();
                } catch (Throwable t) {
                    callback.onResult(null, new MongoGridFSException("Error closing from output stream", t));
                    return;
                }
                callback.onResult(null, null);
            }
        };
    }
public static AsyncOutputStream toAsyncOutputStream(final OutputStream outputStream) {
        notNull("outputStream", outputStream);
        return new AsyncOutputStream() {
            @Override
            public void write(final ByteBuffer srcByteBuffer, final SingleResultCallback<Integer> callback) {
                notNull("src", srcByteBuffer);
                notNull("callback", callback);
                if (!srcByteBuffer.hasRemaining()) {
                    callback.onResult(-1, null);
                    return;
                }

                int amount = srcByteBuffer.remaining();
                byte[] bytes = new byte[amount];
                try {
                    srcByteBuffer.get(bytes);
                    outputStream.write(bytes);
                } catch (Throwable t) {
                    callback.onResult(null, new MongoGridFSException("Error reading from input stream", t));
                    return;
                }
                callback.onResult(amount, null);
            }

            @Override
            public void close(final SingleResultCallback<Void> callback) {
                try {
                    outputStream.close();
                } catch (Throwable t) {
                    callback.onResult(null, new MongoGridFSException("Error closing from output stream", t));
                    return;
                }
                callback.onResult(null, null);
            }
        };
    }
Converts a {@link OutputStream} into a {@link AsyncOutputStream}

@param outputStream the OutputStream
@return the AsyncOutputStream