- servlet, that returns an image:
    response.setContentType("image/jpeg");
    BufferedImage result = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    Graphics2D gr = result.createGraphics();
    gr.setBackground(Color.white);
    gr.fillRect(0, 0, width, height);
    gr.setColor(Color.BLACK);
    gr.drawLine(x1, y1, x2, y2);
    OutputStream out = response.getOutputStream();
    ImageIO.write(result, "jpg", out);
    
- asynchronous servlet:
    - read about asynchronous servlets at the end of:
       https://dai.fmph.uniba.sk/courses/java2/jsp/jsp/kap3.pdf
    - also check asyncronous section and nonblocking I/O subchapter here:
       http://dai.fmph.uniba.sk/courses/java2/glassfish/j7.pdf
    - create simple async servlet for assigning "first" "second"
       to two users on the same URL - see video
        link: https://dai.fmph.uniba.sk/courses/java2/first_and_second.zip  
    - download servlet from LIST homework, and study that,
       version for IDEA and Jakarta9 available here:
        https://dai.fmph.uniba.sk/courses/java2/AsyncChat.zip  
    - implement new async servlet for single-line chat for two
      users as required in LIST hw