Why does calling dispose() on Graphics object cause JPanel to not render any components
Solution 1
The thing is that the Graphics
context you are using in paintComponent
is created and provided by the caller (the framework), which is also responsible for disposing of it.
You only need to dispose of Graphics
when you actually create it yourself (for example by calling Component.getGraphics()
). In your case, you're not creating it, you're just casting it, so do not call dispose in this case.
Solution 2
Because .dispose()
releases your resources. Yes, everything.
David Kroukamp
Note worthy Badges: 10th user to be awarded Gold Badge for swing (1st in South Africa) 140th user to be awarded Gold Badge for java (2nd in South Africa) 7th user to be awarded Silver Badge for jpanel 7th user to be awarded Bronze Badge for jframe 5th user to be awarded Bronze Badge for paintcomponent Featured Poster @ Daniweb View David Kroukamp's profile on LinkedIn My dream is to write kernel drivers, create my own Anti-Virus... Oh and my own Operating System. I am also interested in creating games, hacking, virus writing and malware analysis and reverse engineering - for educational purposes only though.
Updated on July 22, 2022Comments
-
David Kroukamp almost 2 years
After learning that
dispose()
should be called onGraphics
/Graphics2D
object after use, I went about changing my game to incorporate this.When I added
g2d.dispose()
in overriddenpaintComponent(Graphics g)
ofJPanel
, my components which I added (extensions ofJLabel
class) where not rendered I was able to still click on them etc but they would not be painted.I tested with a normal
JLabel
andJButton
with same effect (thoughJButton
is rendered when mouse is over it).So my question is why does this happen?
Here is an SSCCE to demonstrate:
after uncommenting call to
dispose()
inpaintComponent
ofMainMenuPanel
class:import java.awt.Color; import java.awt.Dimension; import java.awt.Font; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.GridBagConstraints; import java.awt.GridBagLayout; import java.awt.Image; import java.awt.RenderingHints; import java.awt.event.FocusAdapter; import java.awt.event.FocusEvent; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import java.awt.image.BufferedImage; import java.net.URL; import java.util.logging.Level; import java.util.logging.Logger; import javax.imageio.ImageIO; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JOptionPane; import javax.swing.JPanel; import javax.swing.SwingUtilities; public class Test { public Test() { try { initComponents(); } catch (Exception ex) { Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex); } } public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { @Override public void run() { new Test(); } }); } private void initComponents() throws Exception { JFrame frame = new JFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setResizable(false); MainMenuPanel mmp = new MainMenuPanel(); frame.add(mmp); frame.pack(); frame.setVisible(true); } } class MainMenuPanel extends JPanel { //create labels for Main Menu private PopUpJLabel versusesModeLabel; private PopUpJLabel singlePlayerModeLabel; private PopUpJLabel optionsLabel; private PopUpJLabel helpLabel; private PopUpJLabel aboutLabel; //create variable to hold background private Image background; private Dimension preferredDimensions; public static String gameType; public static final String SINGLE_PLAYER = "Single Player", VERSUS_MODE = "VS Mode"; /** * Default constructor to initialize double buffered JPanel with * GridBagLayout */ public MainMenuPanel() { super(new GridBagLayout(), true); try { initComponents(); } catch (Exception ex) { JOptionPane.showMessageDialog(null, "Could not load main menu background!", "Main Menu Error: 0x004", JOptionPane.ERROR_MESSAGE); System.exit(4); } } /* * Create JPanel and its components */ private void initComponents() throws Exception { //set prefered size of JPanel preferredDimensions = new Dimension(800, 600); background = scaleImage(800, 600, ImageIO.read(new URL("http://photos.appleinsider.com/12.08.30-Java.jpg"))); //create label instances singlePlayerModeLabel = new PopUpJLabel("Single Player Mode"); singlePlayerModeLabel.setEnabled(false); versusesModeLabel = new PopUpJLabel("Versus Mode"); optionsLabel = new PopUpJLabel("Options"); helpLabel = new PopUpJLabel("Help"); aboutLabel = new PopUpJLabel("About"); //create new constraints for gridbag GridBagConstraints gc = new GridBagConstraints(); gc.fill = GridBagConstraints.HORIZONTAL; gc.ipady = 50;//vertical spacing //add newGameLabel to panel with constraints gc.gridx = 0; gc.gridy = 0; add(singlePlayerModeLabel, gc); gc.gridy = 1; add(versusesModeLabel, gc); //add optionsLabel to panel with constraints (x is the same) gc.gridy = 2; add(optionsLabel, gc); //add helpLabel to panel with constraints (x is the same) gc.gridy = 3; add(helpLabel, gc); //add aboutLabel to panel with constraints (x is the same) gc.gridy = 4; add(aboutLabel, gc); } public static BufferedImage scaleImage(int w, int h, BufferedImage img) throws Exception { BufferedImage bi; //bi = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB); bi = new BufferedImage(w, h, img.getType()); Graphics2D g2d = (Graphics2D) bi.createGraphics(); g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON); g2d.addRenderingHints(new RenderingHints(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY)); g2d.drawImage(img, 0, 0, w, h, null); g2d.dispose(); return bi; } /* * Will return the preffered size of JPanel */ @Override public Dimension getPreferredSize() { return preferredDimensions; } /* * Will draw the background to JPanel with anti-aliasing on and quality rendering */ @Override protected void paintComponent(Graphics grphcs) { super.paintComponent(grphcs); //convert graphics object to graphics2d object Graphics2D g2d = (Graphics2D) grphcs; //set anti-aliasing on and rendering etc //GamePanel.applyRenderHints(g2d); //draw the image as the background g2d.drawImage(background, 0, 0, null); //g2d.dispose();//if I uncomment this no LAbels will be shown } } class PopUpJLabel extends JLabel { public final static Font defaultFont = new Font("Arial", Font.PLAIN, 50); public final static Font hoverFont = new Font("Arial", Font.BOLD, 70); PopUpJLabel(String text) { super(text); setHorizontalAlignment(JLabel.CENTER); setForeground(Color.ORANGE); setFont(defaultFont); //allow component to be focusable setFocusable(true); //add focus adapter to change fints when focus is gained or lost (used for transversing labels with keys) addFocusListener(new FocusAdapter() { @Override public void focusGained(FocusEvent fe) { super.focusGained(fe); if (isEnabled()) { setFont(getHoverFont()); } } @Override public void focusLost(FocusEvent fe) { super.focusLost(fe); setFont(getDefaultFont()); } }); addMouseListener(new MouseAdapter() { @Override public void mouseEntered(MouseEvent me) { super.mouseEntered(me); if (isEnabled()) { setFont(getHoverFont()); } //call for focus mouse is over this component requestFocusInWindow(); } }); } Font getDefaultFont() { return defaultFont; } Font getHoverFont() { return hoverFont; } }
-
David Kroukamp over 11 years+1 thank you i see my mistake I though it should be called every time we use the object like in paintComponent etc. will accept when I can
-
Guillaume Polet over 11 yearsI don't understand why you state You only need to dispose of Graphics when you actually create it yourself (for example by calling Component.getGraphics()). When you call
Component.getGraphics()
do you create a newGraphics
? My understanding was thatGraphics.createGraphics()
does, but not a simplegetGraphics()
. And anyway, I believe that callinggetGraphics()
is usually a bad idea and should be avoided. -
Thorsten Dittmar over 11 yearsYou are right that
getGraphics
should be avoided, but it is explicitly stated in the docs as a way to obtain aGraphics
context. Also,getGraphics
documentation say: Creates a Graphics context..., so I assume that a new one is created every time. Of course,BufferedImage.createGraphics
is also a way to create aGraphics
context.