@Slf4j
public class Service {
    @Data
    public static final class Request {
        String originalXmlRequest;
        Node root;
    }

    private final ExecutorService executor = Executors.newFixedThreadPool(1);

    public void enqueue(Request req) {
        LOGGER.info("Got XML: {}", req.originalXmlRequest);
        executor.execute(() -> process(req.root));
    }

    private synchronized void process(Node root) {
        // do stuff on parsed xml DOM tree starting on 'root'
        Thread.sleep(5000);
    }
}