绘制带有 JLabel 的 JPanel 渐变背景时出现问题

Question

我有以下代码来绘制 JPanel 的背景：

@Override
 public void paintComponent(Graphics g) {
    UIDefaults uid = UIManager.getDefaults();
    Graphics2D g2d = (Graphics2D)g;
    Dimension d = this.getSize();

    g2d.setPaint(new GradientPaint(0, 0, uid.getColor("ToolBar.light"), 0, d.height, uid.getColor("ToolBar.shadow"), true));
    g2d.fillRect(0, 0, d.width , d.height);

    g2d.setColor(uid.getColor("ToolBar.light"));
    g2d.drawLine(0, d.height-2, d.width, d.height-2);

    g2d.setColor(uid.getColor("ToolBar.highlight"));
    g2d.drawLine(0, d.height-1, d.width, d.height-1);
 }

我在这个 JPanel 中有一个 JLabel （不透明 = false），但 JLabel 的背景仍然是某种灰色。如何在 JPanel 背景上绘制渐变，并为 JLabel 提供透明背景？

完整的工作示例如下：

DemoFrame.java:

import javax.swing.UIManager;

public class DemoFrame extends javax.swing.JFrame {

    public DemoFrame() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        panelHeader = new Extras.JPanelHeader();
        labelTitle = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        panelHeader.setOpaque(false);
        panelHeader.setPreferredSize(new java.awt.Dimension(172, 29));
        panelHeader.setLayout(new java.awt.FlowLayout(java.awt.FlowLayout.LEFT));

        labelTitle.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        labelTitle.setText("Criar Novo Utilizador");
        labelTitle.setOpaque(true);
        panelHeader.add(labelTitle);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(panelHeader, javax.swing.GroupLayout.DEFAULT_SIZE, 200, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(panelHeader, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(83, Short.MAX_VALUE))
        );

        pack();
    }// </editor-fold>

    public static void main(String args[]) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception ex) {
            ex.printStackTrace();
        }

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new DemoFrame().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    private javax.swing.JLabel labelTitle;
    private Extras.JPanelHeader panelHeader;
    // End of variables declaration

}

JPanelHeader.java:

import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;
import javax.swing.UIDefaults;
import javax.swing.UIManager;

public class JPanelHeader extends JPanel {

    @Override
     public void paintComponent(Graphics g) {
        UIDefaults uid = UIManager.getDefaults();
        Graphics2D g2d = (Graphics2D)g;
        Dimension d = this.getSize();

        g2d.setPaint(new GradientPaint(0, 0, uid.getColor("ToolBar.shadow"), 0, d.height, uid.getColor("ToolBar.background"), false));
        g2d.fillRect(0, 0, d.width , d.height);

        g2d.setColor(uid.getColor("ToolBar.light"));
        g2d.drawLine(0, d.height-2, d.width, d.height-2);

        g2d.setColor(uid.getColor("ToolBar.highlight"));
        g2d.drawLine(0, d.height-1, d.width, d.height-1);
     }

}

Answer 1

你一定使用了某种奇怪的 LAF。如果是这种情况，那么应该将其发布在您的问题中。事实上，SSCCE 应该与每个问题一起发布，以便我们可以准确地看到您在做什么。

对背景使用 alpha 值可能会导致问题，如 背景透明度。

这是一个简单的 SSCCE，它表明您不需要执行任何特殊操作即可正确绘制标签：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class FaceComponent2 extends JPanel
{
    protected void paintComponent(Graphics g)
    {
        UIDefaults uid = UIManager.getDefaults();
        Graphics2D g2d = (Graphics2D)g;
        Dimension d = this.getSize();

        g2d.setPaint(new GradientPaint(0, 0, uid.getColor("ToolBar.light"),
            0, d.height, uid.getColor("ToolBar.shadow"), true));
        g2d.fillRect(0, 0, d.width , d.height);

        g2d.setColor(uid.getColor("ToolBar.light"));
        g2d.drawLine(0, d.height-2, d.width, d.height-2);

        g2d.setColor(uid.getColor("ToolBar.highlight"));
        g2d.drawLine(0, d.height-1, d.width, d.height-1);
    }


    public static void main(String args[])
        throws Exception
    {
        JComponent face = new FaceComponent2();
        face.setLayout( new GridLayout(0, 1) );
        face.setPreferredSize( new Dimension(250, 250) );
        face.add( new JLabel("Line1") );
        face.add( new JLabel("Line2") );
        face.add( new JLabel("Line3") );

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add( face );
        frame.pack();
        frame.setLocationRelativeTo( null );
        frame.setVisible( true );

    }
}