Creating QR Code as Base64 String using Spring Boot

I am creating a microservice using spring boot where in i have a file handling.
My task is to write the QR code to image file and base64 encode to transfer over network.
Because I need to write to an image file and then read and then base64 encode it , I need to first create the image file and then write to it.
Creating a temperory folder and keep the file ?, create a folder in root directory and keep the file? or use the java.io.tmpdir ....
Note: I have no use of this file once I encode it. Also there are lot of user to whom we will be exposing this service as rest api.
I want to know what is the best way to do this.

If you are using the https://github.com/zxing/zxing library i.e.

    <dependency>
      <groupId>com.google.zxing</groupId>
      <artifactId>javase</artifactId>
      <version>3.2.1</version>
    </dependency>

Then something like the following will work.

@RestController
public class TestController {

    @GetMapping(value = "/test")
    public QrInfo getQrInfo() throws Exception {

        String url = "https://news.bbc.co.uk";

        int imageSize = 200;
        BitMatrix matrix = new MultiFormatWriter().encode(url, BarcodeFormat.QR_CODE,
            imageSize, imageSize);
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        MatrixToImageWriter.writeToStream(matrix, "png", bos);
        String image = Base64.getEncoder().encodeToString(bos.toByteArray()); // base64 encode

        // return QrInfo
        QrInfo qrInfo = new QrInfo();
        qrInfo.setUrl(url);
        qrInfo.setImage(image);
        return qrInfo;
    }

}


@Data // lombok for brevity
class QrInfo {

    private String url;
    private String image;

}

> NOTE: This approach doesn't write any files but does it all in-memory using a ByteArrayOutputStream .

If you hit this endpoint you'll see the following: -

{
	"url": "https://news.bbc.co.uk",
	"image": "iVBORw0KGgoAAAANSUhEUgAAAMgAAADIAQAAAACFI5MzAAABGUlEQVR42u2YSw7DIAxEzYpjcFM+N+UYrErtMUkjpd2WWQQlyudtLI89JpH5a8lDHvJnUkVXmkMPKcMeAg1peo70inrpRbm/ISFDwkhNX4NUSWxEo26WVFKisgc2ArWncSO3OthJvEs0nTju/bOT+NJKzJK++c5OovJWRIob2AwNsf6YXWJ3eFGbgXS4skgEGafaDGSifVONS/ZCQ/Q2YI5l8BdSS0ImwtTezehjiM9C3FG8fbVdykft/URTeEY918hlIZZFC9Yq0Rw6ns63nyxXtkTCYK6VuJv4NKvmMdgFMBHfBbRjb8JFxgoWW04RPmKfEaY2pgcZcT/OsL3GQ5baFrUN23iZZrvJ6pKjDJFXFvL8P3jIfvIGvNX7jsCaJvEAAAAASUVORK5CYII="
}

If you paste the Base64 into e.g https://codebeautify.org/base64-to-image-converter and point camera from your phone you will see the URL.

If you rendering this out it is easy i.e. in React (any JavaScript approach will be similar).

<img src={`data:image/png;base64,${image}`} />

I like this approach as if you are e.g. generate 2FA codes you can pass down both the random secret and the QR code - useful to have both as a backup if the user doesn't have access to a mobile device.

You can do this in a designated folder in your file-system easily and once you are done with the transfer, you can remove the file. But since, this operation is exposed over a rest api, concurrency and file collision can be an issue. To avoid that you can give unique to every file-name, that way same file-name collision can be avoided.

One approach could be to UUID for writing the file-name and storing the file with this name in the file-system. It will ensure collision doesn't occur.

String fileName = UUID.randomUUID().toString();

</details>