How to successfully draw background JPanel once an

Question

I have a custom JLayeredPane, and I am repainting it in my game loop. There are two custom JPanels added into the JLayeredPane. These are foreground and background JPanels. How do I successfully only draw my background JPanel once, (And repaint when window is re-sized or any other reason) to reduce impact on system resources, while continuing to update my foreground JPanel constantly.

To re-iterate, I dont want to constantly repaint the background JPanel in a loop. I want to repaint it only when it is nessessary, as the background does not change. and is large.

In my attempt to do this, I have only drawn the background once. However. the background JPanel is simply not visible. while the foreground JPanel updates as normal. It is almost as if the foreground JPanel paints ontop of the background JPanel, even though I have both of the JPanels set to setOpaque(false)

I have made a mvce which shows my attempt at only drawing the background JPanel once, while updating the foreground JPanel constantly.

The problem with my code is that the background JPanel does not show.

Now. I know that if I were to draw it constantly it would show. But that defeats the purpose of what i'm trying to do. I am trying to only draw it once, and have be seen at the same time

My code successfully only draws the background JPanel once. The problem is that the background JPanel does not show. How do I fix THIS problem

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;

public class Main extends JLayeredPane {
    static JFrame frame;
    static Main main;
    static Dimension screenSize;
    public Main() {     
        JPanel backPanel = new BackPanel();
        JPanel frontPanel = new FrontPanel();
        add(backPanel, new Integer(7));
        add(frontPanel, new Integer(8));

        new Thread(() -> {
            while (true){
                repaint();
            }
        }).start();

    }

    public static void main(String[] args) {
        screenSize = Toolkit.getDefaultToolkit().getScreenSize();

        frame = new JFrame("Game"); // Just use the constructor

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        main = new Main();
        frame.add(main, BorderLayout.CENTER);

        frame.pack();
        frame.setSize(screenSize);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);

    }
    public class BackPanel extends JPanel{
        public boolean drawn = false;
        public BackPanel(){
            setVisible(true);
            setOpaque(false);
            setSize(screenSize);
            JLabel test1 = new JLabel("Test1");
            JLabel test2 = new JLabel("Test2");
            add(test1);
            add(test2);
        }
        @Override
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            drawOnce(g);
        }
        public void drawOnce(Graphics g){
            if (!drawn){
                g.setColor(Color.red);
                g.fillRect(0, 0, screenSize.width, 200);
                drawn=true;
            }

        }
    }
    public class FrontPanel extends JPanel{

        public FrontPanel(){
            setVisible(true);
            setOpaque(false);
            setSize(screenSize);
            JLabel test = new JLabel("Test");
            add(test);
        }
        @Override
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            g.setColor(Color.blue);
            g.fillRect(0+screenSize.width/2, 0, screenSize.width/4, 300);
        }
    }
}

Answer 1

Try RepaintManager.currentManager(component).markCompletelyClean(component). It will prevent the component from repainting. You might need to do this after each time you add new components.

http://docs.oracle.com/javase/6/docs/api/javax/swing/RepaintManager.html#markCompletelyClean%28javax.swing.JComponent%29

Answer 2

I don't know if this two lines of code

super.paintComponent(g);
drawOnce(g);

are the root of problem, I sincerly don't remember how paintComponent works (a test could help) but try to swap them :

drawOnce(g);
super.paintComponent(g);

maybe, on your original version, you tells JVM to paint the whole component and, only after the AWTEvent has been added to the queue, to draw what you need. I guess that the awt's documentation will explain it.