Java：透明的JDialog被重新粉刷成白色

Question

I am experiencing a very strange problem with a quite large Java program on Windows. I have written a small test program to reproduce the issue.

A custom, transparent JDialog gets repainted completely white after a UAC prompt overlay has been opened by Windows.

Given the following simple test class:

import java.awt.Color;
import java.awt.EventQueue;

import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;

public class DialogTests extends JDialog {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }
        EventQueue.invokeLater(() -> {
            new DialogTests().setVisible(true);
        });
    }

    public DialogTests() {
        this.setAutoRequestFocus(false);
        this.setUndecorated(true);
        this.setAlwaysOnTop(true);
        this.setFocusableWindowState(true);
        this.setBackground(new Color(0,255,255,0));

        JPanel contentPane = new JPanel();
        contentPane.setBackground(new Color(0,0,0,200));
        setContentPane(contentPane);
        setBounds(200, 200, 500, 500);

        JLabel label = new JLabel("this is just to see something!");
        label.setForeground(new Color(255,0,0,255));
        contentPane.add(label);

        JButton button1 = new JButton("test button 1");
        button1.setBackground(new Color(0,0,0,0));
        contentPane.add(button1);

        JButton button2 = new JButton("test button 2");
        button2.setBackground(new Color(0,0,0,0));
        contentPane.add(button2);
    }

}

the following sequence of actions is able to reproduce the issue for me:

1. Launch the program. Don't click any buttons or move your mouse over the dialog!
2. Force an UAC prompt to appear. For example, disable or enable a network adapter when you have UAC set to the highest security level. Confirm the prompt.
3. Click on "test button 2". The dialog becomes repainted white, only the two buttons remain visible (because they are redrawn for the System Look&Feel effects)

If you do not want to or cannot reproduce the issue, here are two screenshots:

Before:

After:

I would like to know an explanation for this bug, or a possible workaround. Preferably both :)

Some details about the system I am working with:

• Windows 7 x64
• Java 8 u60
• eclipse Mars 4.5.0 (used for launching & debugging)

Thank you very much!

Answer 1

Don't use opacity in a Color if you don't have to. This breaks the painting contract regarding opaqueness and can cause painting artifacts. See Background With Transparency for the problems that can result.

When using 255, the component is opaque. When using 0, the component is non-opaque. So just use:

component.setOpaque(true or false);

When using transparency on a frame or dialog you can set the opactity directly. Try using:

this.setBackground(new Color(0,255,255)); // play with this color
this.setOpacity(0.75f);  // play with this opacity.
...
//contentPane.setBackground(new Color(0,0,0,200));
contentPane.setOpaque(false);

Answer 2

I have found a workaround myself, which involves using the JNA library . It appears that by using WindowUtils.setWindowTransparent() the render mode gets changed in a way which does not lead to the bug anymore.

The following augmented code should be working fine:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;

import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;

import com.sun.jna.platform.WindowUtils;

public class DialogTests extends JDialog {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        try {
            System.setProperty("sun.java2d.noddraw", "true");
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }
        EventQueue.invokeLater(() -> {
            new DialogTests().setVisible(true);
        });
    }

    public DialogTests() {
        this.setAutoRequestFocus(false);
        this.setUndecorated(true);
        this.setAlwaysOnTop(true);
        this.setFocusableWindowState(true);
        this.setBackground(new Color(0,255,255,0));
        WindowUtils.setWindowTransparent(this, true);

        JPanel contentPane = new JPanel();
        contentPane.setBackground(new Color(0,0,0,200));
        contentPane.setPreferredSize(new Dimension(200,200));
        setContentPane(contentPane);
        setBounds(200, 200, 500, 500);

        JLabel label = new JLabel("this is just to see something!");
        label.setForeground(new Color(255,0,0,255));
        contentPane.add(label);

        JButton button1 = new JButton("test button 1");
        button1.setBackground(new Color(0,0,0,0));
        contentPane.add(button1);

        JButton button2 = new JButton("test button 2");
        button2.setBackground(new Color(0,0,0,0));
        contentPane.add(button2);
    }

}

The only things to consider:

• System.setProperty("sun.java2d.noddraw", "true"); has to be set before creating the window
• window properties, such as undecorated and background color, have to be set before using WindowUtils.setWindowTransparent(this, true); and cannot be changed afterwards
• as a (for me desirable) side effect, all not-drawn transparent pixels are now click-through (means mouse events are propagated to other windows behind this window)