How to pass events through a JFrame?

Question

I have an undecorated JFrame with some contents on it (Labels, Images, etc.). I need the JFrame to pass all the events through it. For example: when a click is made on that JFrame, I want it to pass that click through, to the window/anything that is underneath the frame.

Problem Example:

public static void main(String[] args) {
    JFrame.setDefaultLookAndFeelDecorated(true);
    JFrame f = new JFrame("Test");
    f.setAlwaysOnTop(true);
    Component c = new JPanel() {
        @Override
        public void paintComponent(Graphics g) {
            Graphics2D g2 = (Graphics2D)g.create();
            g2.setColor(Color.gray);
            int w = getWidth();
            int h = getHeight();
            g2.fillRect(0, 0, w,h);
            g2.setComposite(AlphaComposite.Clear);
            g2.fillRect(w/4, h/4, w-2*(w/4), h-2*(h/4));
        }
    };
    c.setPreferredSize(new Dimension(300, 300));
    f.getContentPane().add(c);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.pack();
    f.setVisible(true);
    com.sun.awt.AWTUtilities.setWindowOpaque(f,false);
}

• In this case, the JFrame has a border with close/minimize/fullscreen controls, and graphics on the JFrame still catch the events while just the transparent parts pass them through. I need both (transparent parts, and with graphics) to pass through the events.

Video Example of my goal: https://www.youtube.com/watch?v=irUQGDDSk_g

Similar question:

Pass mouse events to applications behind from a Java UI

• This question tries to achieve a similar goal, but the JFrame is decorated (has close/minimize controls with an outer frame), and graphics still catch the user control events.

Question: How can I make a JFrame with graphics, that will not catch the events from the user controlling, but to pass in through?

Answer 1

This is not an answer, but a correction to the code example

The example you provide is actually doing some very dangerous things, first it's painting a translucent color onto an opaque component, this means that Swing doesn't know that it should actually be painting anything under the component and could also result in a number of very nasty paint artifacts, as Swing only knows about opaque and transparent component, it doesn't know about semi-transparent components, so you need to trick the API.

When performing custom painting, you should always call super.paintComponent to ensure that the Graphics context is setup correctly before painting. In your case, you should also make the component transparent using setOpaque and passing it false, for example...

import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestFrame {

    public static void main(String[] args) {
        new TestFrame();
    }

    public TestFrame() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setUndecorated(true);
                frame.setAlwaysOnTop(true);
                frame.setBackground(new Color(0, 0, 0, 0));
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            setOpaque(false);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setColor(Color.BLUE);
            g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
            g2d.fill(new Rectangle(0, 0, getWidth(), getHeight()));
            g2d.dispose();
        }

    }

}