Re: Copying the View of a JPanel

Knute Johnson <>
Wed, 02 Jul 2008 16:19:22 -0700
X wrote:

I have a JPanel that a user can edit and add drawings and Swing
components to, and I want to create a second JPanel that shows the
same thing as what's on the first JPanel. Does anyone know how to best
accomplish this?

My current implementation consists of calling the first JPanel's
paint() method with the second JPanel's Graphics context. This works
fine until the first Swing component is added to the screen. The
second JPanel shows everything but then "freezes" after a Swing
component is added to the first JPanel. Does anyone know what might be
wrong, or does anyone have an alternate suggestion as to how I can
have this second JPanel copy the view from the first JPanel?

I tried to write a program to emulate yours but I couldn't do it. I
would be really curious to see your code if you have an SSCCE.

In any case I took a different tact, I copy the data from the first
component with a Robot and then draw it on the second. This works
pretty good but if you put something over the first panel such as
another window, that's what shows up in the second panel. Also I just
refresh the second panel every 100ms. You could capture events on the
first window and trigger a repaint then as well.

This was an interesting question but I am curious as to what the purpose
of it is. Are you trying to make some sort of whiteboard connected
across the net?

Anyway, here is my implementation.

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.swing.*;

public class test extends JPanel implements Runnable {
     JComponent c;
     Robot robot;

     public test(JComponent c) {
         this.c = c;
         try {
             robot = new Robot();
         } catch (AWTException awte) {

     public void run() {
         while (true) {
             try {
             } catch (InterruptedException ie) { }

     public void paint(Graphics g) {
         Point p = c.getLocationOnScreen();
         Rectangle r = new Rectangle(p,c.getSize());
         BufferedImage bi = robot.createScreenCapture(r);

     public static void main(String[] args) {
         SwingUtilities.invokeLater(new Runnable() {
             public void run() {
                 final JFrame f = new JFrame();
                 f.setLayout(new FlowLayout());

                 final JPanel p = new JPanel();
                 p.setPreferredSize(new Dimension(400,300));
                 p.setBackground(new Color(0,0,255,40));
                 JButton b = new JButton("Add Component");
                 b.addActionListener(new ActionListener() {
                     public void actionPerformed(ActionEvent ae) {
                         JLabel l = new JLabel("A JLabel");

                 test t = new test(p);
                 t.setPreferredSize(new Dimension(400,300));


                 new Thread(t).start();


Knute Johnson
email s/nospam/knute2008/

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