This assumes a few things, that you know what the output file name will be and that your data comes as a string. I'm sure you can modify the following to meet your needs:

// Needed Imports
import java.io.ByteArrayInputStream;
import sun.misc.BASE64Decoder;


def sourceData = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAPAAAADwCAYAAAA+VemSAAAgAEl...==';

// tokenize the data
def parts = sourceData.tokenize(",");
def imageString = parts[1];

// create a buffered image
BufferedImage image = null;
byte[] imageByte;

BASE64Decoder decoder = new BASE64Decoder();
imageByte = decoder.decodeBuffer(imageString);
ByteArrayInputStream bis = new ByteArrayInputStream(imageByte);
image = ImageIO.read(bis);
bis.close();

// write the image to a file
File outputfile = new File("image.png");
ImageIO.write(image, "png", outputfile);

Please note, this is just an example of what parts are involved. I haven't optimized this code at all and it's written off the top of my head.

Had the same problem when I recieved the base64 String throught a @RequestParam, it recieved "+" as " ".

Base64.getDecoder().decode() gave me the same exception (java.lang.IllegalArgumentException: Illegal base64 character 20) and Base64.getMimeDecoder() just ignored the spaces and saved the image with errors.

fix:

public byte[] convertImage(String base64) { String base64Image = base64.replaceAll(" ", "+"); return byte[] imageBytes = java.util.Base64.getDecoder().decode(base64Image); }

And it worked for me.

I think you might want:

String encodedFile = Base64.getEncoder().encodeToString(bytes);