DrawIoConverter.java

import com.mxgraph.canvas.mxICanvas;
import com.mxgraph.canvas.mxSvgCanvas;
import com.mxgraph.io.mxCodec;
import com.mxgraph.util.mxCellRenderer;
import com.mxgraph.util.mxDomUtils;
import com.mxgraph.util.mxUtils;
import com.mxgraph.util.mxXmlUtils;
import com.mxgraph.view.mxGraph;

public class DrawIoConverter{
    //https://github.com/jgraph/mxgraph/tree/master/java
    //http://stackoverflow.com/questions/35274068/rendering-xml-from-draw-io-as-an-image-using-mxcellrenderer/35322632
    private void convert(Path file, Path outputPath) {
        getLog().info("Converting: " + file.toString() + " to " + outputPath.toString());
        try {
            Files.createDirectories(outputPath.getParent());
            mxGraph graph = new mxGraph();

            Element node =  DocumentBuilderFactory
                    .newInstance()
                    .newDocumentBuilder()
                    .parse(new ByteArrayInputStream(Files.readAllBytes(file)))
                    .getDocumentElement();
            String diagramEncoded = node.getElementsByTagName("diagram").item(0).getTextContent();

            byte[] bytes = DatatypeConverter.parseBase64Binary(diagramEncoded);

            Inflater decompresser = new Inflater(true);
            decompresser.setInput(bytes);
            ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            while (!decompresser.finished()) {
                int count = decompresser.inflate(buffer);
                outputStream.write(buffer, 0, count);
            }
            outputStream.close();
            decompresser.end();

            String decoded = URLDecoder.decode(outputStream.toString(), "UTF-8");

            // Parses XML into graph
            Document doc = mxXmlUtils.parseXml(decoded);
            mxCodec codec = new mxCodec(doc);
            codec.decode(doc.getDocumentElement(), graph.getModel());

            // Renders graph to SVG
            mxSvgCanvas canvas = (mxSvgCanvas) mxCellRenderer.drawCells(graph,
                    null, 1, null, new mxCellRenderer.CanvasFactory() {
                        public mxICanvas createCanvas(int width, int height) {
                            return new mxSvgCanvas(mxDomUtils
                                    .createSvgDocument(width, height));
                        }
                    });

            mxUtils.writeFile(mxXmlUtils.getXml(canvas.getDocument()), outputPath.toString());
        } catch (IOException | DataFormatException | SAXException | ParserConfigurationException e) {
            getLog().error("Error converting xml to svg; file: " + file.toString() + " ; error: " + e.getMessage());
        }
    }
    
}