Why does my custom Swing label take so long to initialize?
-
27-10-2019 - |
Question
I've made a custom subclass of JLabel
. I have a single instance of this, inside a single JPanel
, inside a single JFrame
. I do not override the paintComponent()
method; all the class does is change the background color when the cursor hovers over it.
The JFrame
loads immediately, but for several seconds the JPanel
is left undrawn. I verified that this is because of my custom class by overriding paintComponent()
and adding some debug println()
statements.
public void paintComponent(Graphics context)
{
System.out.println("Painting...");
super.paintComponent(context);
System.out.println("Painted.");
}
The strange thing is, it's drawn instantly when I use Panel
instead of JPanel
or Label
instead of JLabel
.
Where is this lag coming from?
EDIT: Some example code. Nothing is actually drawn; look at the console message delay.
import javax.swing.JFrame;
import javax.swing.JLabel;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.awt.GridLayout;
import javax.swing.JPanel;
public class Example extends JLabel implements MouseListener
{
private static final long serialVersionUID = 0;
public Example()
{
super();
System.out.println("Constructed.");
}
public void paintComponent(java.awt.Graphics g)
{
System.out.println("Painting component...");
super.paintComponent(g);
System.out.println("Painted.");
}
public void mouseEntered(MouseEvent event) { }
public void mouseExited(MouseEvent event) { }
public void mouseReleased(MouseEvent event) { }
public void mousePressed(MouseEvent event) { }
public void mouseClicked(MouseEvent event) { }
public static void main(final String[] arguments)
{
JPanel panel = new JPanel();
panel.setLayout(new GridLayout(1, 1));
panel.add(new Example());
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(panel);
frame.setVisible(true);
System.out.println("Set visible.");
}
}
Solution
My code doesn't lag:
My SSCCE:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.*;
@SuppressWarnings("serial")
public class LabelTest extends JPanel {
public LabelTest() {
add(new MyLabel("Fubar!"));
}
private static void createAndShowGui() {
LabelTest mainPanel = new LabelTest();
JFrame frame = new JFrame("LabelTest");
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();
}
});
}
}
@SuppressWarnings("serial")
class MyLabel extends JLabel {
private static final Color BACKGROUND_DEFAULT = new Color(200, 200, 255);
private static final Color BACKGROUND_MOUSEOVER = new Color(255, 200, 200);
private static final int PREF_W = 200;
private static final int PREF_H = 100;
public MyLabel(String text) {
super(text, SwingConstants.CENTER);
setOpaque(true);
setBackground(BACKGROUND_DEFAULT);
addMouseListener(new MouseAdapter() {
@Override
public void mouseEntered(MouseEvent arg0) {
setBackground(BACKGROUND_MOUSEOVER);
}
@Override
public void mouseExited(MouseEvent e) {
setBackground(BACKGROUND_DEFAULT);
}
});
}
@Override
public Dimension getPreferredSize() {
int width = Math.max(super.getPreferredSize().width, PREF_W);
int height = Math.max(super.getPreferredSize().height, PREF_H);
return new Dimension(width, height);
}
}
This suggests to me that the problem isn't in the concept of a JLabel whose background changes via a MouseListener, but rather you've got a bug somewhere in your code. Where? Who knows until you post compilable runnable code, an SSCCE, like the one I've posted above.