VaadinDownloadFile.java

public class LettingUserDownladFile extends UI {

    @Override
    protected void init(VaadinRequest request) {
        Button downloadButton = new Button("Download image");

        StreamResource myResource = createResource();
        FileDownloader fileDownloader = new FileDownloader(myResource);
        fileDownloader.extend(downloadButton);

        setContent(downloadButton);
    }

    private StreamResource createResource() {
        return new StreamResource(new StreamSource() {
            @Override
            public InputStream getStream() {
                String text = "My image";

                BufferedImage bi = new BufferedImage(100, 30, BufferedImage.TYPE_3BYTE_BGR);
                bi.getGraphics().drawChars(text.toCharArray(), 0, text.length(), 10, 20);

                try {
                    ByteArrayOutputStream bos = new ByteArrayOutputStream();
                    ImageIO.write(bi, "png", bos);
                    return new ByteArrayInputStream(bos.toByteArray());
                } catch (IOException e) {
                    e.printStackTrace();
                    return null;
                }

            }
        }, "myImage.png");
    }

}