如果 `set setOpaque(false);`，JWindow 上的 JPanel 不透明 - 为什么？

Question

在这个例子中如何让 JPanel 透明？渐变背景不可见：

package test;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.BoxLayout;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JWindow;

public class PaintJPanelOnJWindow extends JWindow {

    public PaintJPanelOnJWindow() {

        JPanel panel = new JPanel();

        panel.setPreferredSize(new Dimension(350, 120));
        panel.setMinimumSize(new Dimension(350, 120));
        panel.setMaximumSize(new Dimension(350, 120));

        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
        panel.setOpaque(false);

        JLabel someText = new JLabel("I'm not transparent and my JPanel too :(");
        someText.setOpaque(false);
        panel.add(someText);
        add(panel);

        pack();
        setLocationRelativeTo(null);
        setVisible(true);
    }

    @Override
    public void paint(Graphics g) {

        Graphics2D g2d = (Graphics2D) g.create();
        try {
            int w = getWidth(), h = getHeight();
            g2d.setPaint(new GradientPaint(0, 0, Color.RED, 0, h, Color.WHITE));
            g2d.fillRect(0, 0, w, h);
        } finally {
            g2d.dispose();
        }
        super.paint(g);
    }
}

Answer 1

当前的问题是

super.paint(g);

在paint 方法中的自定义绘画代码之后被调用，这将导致任何先前的绘画丢失。调用panel.setOpaque(false) 对paint 方法中的操作没有影响。无需为问题中的任何组件调用setOpaque - 默认情况下，当正确实现自定义绘画时会显示背景。

这应该通过覆盖paintComponent 方法来完成。这意味着创建一个新的JPanel 并将自定义绘画功能放置在那里，而不是放在顶级容器中，例如JWindow。

例子：

public class PaintJPanelApp {


    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame frame = new JFrame("Gradient App");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setUndecorated(true);
                frame.setLocationRelativeTo(null);

                JLabel someText = new JLabel("I AM transparent and my JPanel too :)");
                GradientPanel gradientPanel = new GradientPanel();
                gradientPanel.add(someText);
                frame.add(gradientPanel);

                frame.pack();
                frame.setVisible(true);
            }
        });
    }

    static class GradientPanel extends JPanel {


        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            int w = getWidth(); 
            int h = getHeight();
            Graphics2D g2d = (Graphics2D) g;
            g2d.setPaint(new GradientPaint(0, 0, Color.RED, 0, h, Color.WHITE));
            g2d.fillRect(0, 0, w, h);
        }
    }
}

Answer 2

来自JavaDocs：

create()：创建一个新的 Graphics 对象，它是此 Graphics 对象的副本。

这意味着Graphics对象g2d不是指JWindowGraphics对象，它指的是JWindowGraphics对象的副本。

你需要改变

Graphics2D g2d = (Graphics2D) g.create(); //creates a copy, wrong object

到

Graphics2D g2d = (Graphics2D) g;          //refers to the right Graphics object

---

更新

但是，这不是正确的方法。您应该覆盖JPanel 的paintComponent 方法，而不是破坏窗口的绘制过程。首先，从您的类中删除 paint() 方法覆盖。然后，初始化JPanel如下：

JPanel panel = new JPanel() {

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        int w = getWidth(), h = getHeight();
        g2d.setPaint(new GradientPaint(0, 0, Color.RED, 0, h, Color.WHITE));
        g2d.fillRect(0, 0, w, h);
    }
};