Paint over all other components (Swing, Java)

Question

In my application I need to draw grid lines just like those like Photoshop has - e.g, an user can drag lines over the document to help him align layers. Now, the problem is that I am able to draw such lines (it's just plain simple Java2D painting using Line2D), but I am not being able to keep such lines on top of everything else, because when children components draw themselves, my grid line is erased.

The program structure is something like this: JFrame -> JPanel -> JScrollPane -> JPanel -> [many others JPanels, which are like layers]

As a test, I added the draw code to JFrame, which correctly shows my Line2D instance on top of everything else. However, when I do anything in an child component that requires that child to repaint itself, the line that was drawn in the JFrame is erased.

I understand that this is the expected Swing behavior - that is, it will only repaint those areas that have changed. However, I am looking for some approach that continuously draws line grid lines on top of everything else.

The only way I was able to get it working was to use a Swing Timer that calls repaint() on my root component every 10ms, but it consumes a lot of CPU.

UPDATE
Working code of an example is below. Please mind that in my real application I have dozens of different components that could trigger a repaint(), and none of them have a reference to the component that does the grid line drawing (of course I can pass it to everyone, but that appears to be the latest option)

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Line2D;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class GridTest extends JFrame {
    public static void main(String[] args) {
        new GridTest().run();
    }

    private void run() {
        setLayout(null);
        setPreferredSize(new Dimension(200, 200));
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JPanel p = new JPanel();
        p.setBounds(20, 20, 100, 100);
        p.setBackground(Color.white);
        add(p);

        JButton b = new JButton("Refresh");
        b.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                // When I call repaint() here, the paint() method of
                // JFrame it's not called, thus resulting in part of the
                // red line to be erased / overridden.

                // In my real application application, I don't have
                // easy access to the component that draws the lines
                p.repaint();
            }
        });
        b.setBounds(0, 150, 100, 30);
        add(b);

        pack();
        setVisible(true);
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);

        Graphics2D gg = (Graphics2D)g.create();
        Line2D line = new Line2D.Double(0, 50, getWidth(), 50);
        gg.setStroke(new BasicStroke(3));
        gg.setColor(Color.red);
        gg.draw(line);
        gg.dispose();
    }
}

Answer 1

if you want to paint over JComponents placed to the JScrollPane then you can paint to the JViewPort, example here

EDIT:

1) beacuse your code painted to the wrong Container, to the JFrame, sure is possible to paint to the JFrame, but you have to extract RootPane or GlassPane

2) you have to learn how to LayoutManagers works, I let your code with original Sizing, not nice and very bad

3) paint to the GlassPane or JViewPort

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Line2D;
import javax.swing.*;

public class GridTest extends JFrame {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        new GridTest().run();
    }

    private void run() {
        setLayout(null);
        setPreferredSize(new Dimension(200, 200));
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JPanel p = new JPanel() {
            private static final long serialVersionUID = 1L;

            @Override
            public void paint(Graphics g) {

                super.paint(g);
                Graphics2D gg = (Graphics2D) g.create();
                Line2D line = new Line2D.Double(0, 50, getWidth(), 50);
                gg.setStroke(new BasicStroke(3));
                gg.setColor(Color.red);
                gg.draw(line);
                //gg.dispose();

            }
        };
        p.setBounds(20, 20, 100, 100);
        p.setBackground(Color.white);
        add(p);

        JButton b = new JButton("Refresh");
        b.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                p.repaint();
            }
        });
        b.setBounds(0, 150, 100, 30);
        add(b);

        pack();
        setVisible(true);
    }
}

EDIT: 2, if you expecting single line, on the fixed Bounds

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Line2D;
import javax.swing.*;
import javax.swing.border.LineBorder;

public class GridTest extends JFrame {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        new GridTest().run();
    }

    private void run() {
        setPreferredSize(new Dimension(200, 200));
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JPanel p = new JPanel() {
            private static final long serialVersionUID = 1L;

            @Override
            public void paint(Graphics g) {
                super.paint(g);
                Graphics2D gg = (Graphics2D) g.create();
                Line2D line = new Line2D.Double(0, 50, getWidth(), 50);
                gg.setStroke(new BasicStroke(3));
                gg.setColor(Color.red);
                gg.draw(line);
                gg.dispose();
            }
        };
        JPanel p1 = new JPanel();
        p1.setBorder(new LineBorder(Color.black,1));
        JPanel p2 = new JPanel();
        p2.setBorder(new LineBorder(Color.black,1));
        JPanel p3 = new JPanel();
        p3.setBorder(new LineBorder(Color.black,1));
        p.setLayout(new GridLayout(3,0));
        p.add(p1);
        p.add(p2);
        p.add(p3);
        p.setBounds(20, 20, 100, 100);
        p.setBackground(Color.white);
        add(p, BorderLayout.CENTER);

        JButton b = new JButton("Refresh");
        b.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                p.repaint();
            }
        });
        add(b, BorderLayout.SOUTH);

        pack();
        setVisible(true);
    }
}

Answer 2

One possible solution is to override the JPanel's repaint method so that it calls the contentPane's repaint method instead. Another point is that you probably shouldn't draw grid lines directly in the JFrame but rather in its contentPane. Counter to what I usually recommend, I think you're better off overriding the either the contentPane's paint method (or that of some other containing JPanel), not its paintComponent method so that it can call after the children have been painted. For example:

import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Stroke;
import java.awt.event.ActionEvent;

import javax.swing.*;

@SuppressWarnings("serial")
public class GridTest2 extends JPanel {
   private static final Stroke LINE_STROKE = new BasicStroke(3f);
   private boolean drawInPaintComponent = false;

   public GridTest2() {
      final JPanel panel = new JPanel() {
         @Override
         public void repaint() {
            JRootPane rootPane = SwingUtilities.getRootPane(this);
            if (rootPane != null) {
               JPanel contentPane = (JPanel) rootPane.getContentPane();
               contentPane.repaint();
            }
         }
      };
      panel.setBackground(Color.white);
      panel.setPreferredSize(new Dimension(100, 100));

      JPanel biggerPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
      biggerPanel.setBorder(BorderFactory.createEmptyBorder(20, 20, 0, 0));
      biggerPanel.setOpaque(false);
      biggerPanel.add(panel);

      JButton resetButton = new JButton(new AbstractAction("Reset") {
         public void actionPerformed(ActionEvent arg0) {
            panel.repaint();
         }
      });
      JPanel btnPanel = new JPanel();
      btnPanel.add(resetButton);

      setLayout(new BorderLayout());
      add(biggerPanel, BorderLayout.CENTER);
      add(btnPanel, BorderLayout.SOUTH);
   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(300, 300);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (drawInPaintComponent ) {
         drawRedLine(g);
      }
   }

   @Override
   public void paint(Graphics g) {
      super.paint(g);
      if (!drawInPaintComponent ) {
         drawRedLine(g);
      }
   }

   private void drawRedLine(Graphics g) {
      Graphics2D g2 = (Graphics2D) g;
      g2.setStroke(LINE_STROKE);
      g2.setColor(Color.red);
      g2.drawLine(0, 50, getWidth(), 50);
   }

   private static void createAndShowGui() {
      GridTest2 mainPanel = new GridTest2();

      JFrame frame = new JFrame("GridTest2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}