Java Swing 圆形JButton鼠标悬停后出现无法消失的方形背景问题

我在使用Java Swing开发时，只要将鼠标悬停在任意圆形按钮上，按钮后方就会出现一块方形背景且再也不会消失。按钮本身仍可正常点击，但该方形背景无交互响应。我没有实现任何鼠标事件，不清楚这块背景出现的原因。

左：点击后状态，中：鼠标悬停后状态，右：初始状态

完整代码示例 #

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

public class GUI implements ActionListener {
    private JLabel textLabel;
    private JButton buttonL, buttonConfirm, buttonR;
    
    public GUI() {
        JPanel mainDisplay = new JPanel();
        mainDisplay.setBackground(new Color(172, 181, 176));
        mainDisplay.setPreferredSize(new Dimension(150, 160));
        mainDisplay.setLayout(new BorderLayout());
        textLabel = new JLabel("1");
        textLabel.setHorizontalAlignment(SwingConstants.CENTER);
        mainDisplay.add(textLabel, BorderLayout.CENTER);
        
        JPanel container = new JPanel();
        container.setLayout(new GridBagLayout());
        GridBagConstraints gc = new GridBagConstraints();
        gc.gridwidth = GridBagConstraints.REMAINDER;
        gc.insets = new Insets(75, 0, 0, 0);
        container.add(mainDisplay, gc);
        
        Dimension buttonDimension = new Dimension(18, 18);
        Color buttonColor = new Color(242, 240, 241);
        buttonL = new RoundButton();
        buttonL.setBackground(buttonColor);
        buttonL.setPreferredSize(buttonDimension);
        buttonL.addActionListener(this);
        gc.gridwidth = GridBagConstraints.RELATIVE;
        gc.insets = new Insets(10, 20, 0, 0);
        container.add(buttonL, gc);
        
        buttonConfirm = new RoundButton();
        buttonConfirm.setBackground(buttonColor);
        buttonConfirm.setPreferredSize(buttonDimension);
        buttonConfirm.addActionListener(this);
        gc.insets = new Insets(40, -35, 0, 0);
        container.add(buttonConfirm, gc);
        
        buttonR = new RoundButton();
        buttonR.setBackground(buttonColor);
        buttonR.setPreferredSize(buttonDimension);
        buttonR.addActionListener(this);
        gc.insets = new Insets(10, -50, 0, 0);
        container.add(buttonR, gc);
        
        JFrame frame = new JFrame();
        frame.add(container);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        if(e.getSource() == buttonL) {
            textLabel.setText("L");
        } else if(e.getSource() == buttonConfirm) {
            textLabel.setText("");
        } else if(e.getSource() == buttonR) {
            textLabel.setText("R");
        }
    }
    
    public static void main(String[] args) {        
            new GUI();
    }
}

原圆形JButton实现代码 #

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Shape;
import java.awt.geom.Ellipse2D;
import javax.swing.JButton;

public class RoundButton extends JButton {
    Shape shape;
    
    @Override
    protected void paintComponent(Graphics g) {
        g.setColor(getBackground());
        g.fillOval(0, 0, getSize().width - 1, getSize().height - 1);
    }

    @Override
    protected void paintBorder(Graphics g) {
        g.setColor(Color.darkGray);
        g.drawOval(0, 0, getSize().width - 1, getSize().height - 1);
    }

    @Override
    public boolean contains(int x, int y) {
        if (shape == null || !shape.getBounds().equals(getBounds())) {
            shape = new Ellipse2D.Float(0, 0, getWidth(), getHeight());
        }
        return shape.contains(x, y);
    }
}

问题根因 #

Swing默认JButton为不透明组件，交互（悬停、获焦）时UI代理会自动绘制默认的方形背景，你自定义重写paintComponent时没有清除原有区域内容，也没有关闭默认的背景绘制逻辑，导致方形背景绘制后残留无法消失。

修复后的RoundButton代码 #

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Shape;
import java.awt.geom.Ellipse2D;
import javax.swing.JButton;

public class RoundButton extends JButton {
    Shape shape;

    public RoundButton() {
        // 关闭默认不透明填充，禁止UI自动绘制方形背景
        setOpaque(false);
        // 关闭默认焦点框绘制，避免点击后出现方形焦点边框
        setFocusPainted(false);
        // 关闭内容区域自动填充，适配不同观感实现，避免多余绘制
        setContentAreaFilled(false);
    }
    
    @Override
    protected void paintComponent(Graphics g) {
        g.setColor(getBackground());
        g.fillOval(0, 0, getSize().width - 1, getSize().height - 1);
        // 后续如果需要给按钮添加文字、图标，保留下方super调用即可，无内容可删除
        super.paintComponent(g);
    }

    @Override
    protected void paintBorder(Graphics g) {
        g.setColor(Color.darkGray);
        g.drawOval(0, 0, getSize().width - 1, getSize().height - 1);
    }

    @Override
    public boolean contains(int x, int y) {
        if (shape == null || !shape.getBounds().equals(getBounds())) {
            shape = new Ellipse2D.Float(0, 0, getWidth(), getHeight());
        }
        return shape.contains(x, y);
    }
}

修改后重新运行即可消除悬停残留的方形背景。

内容的提问来源于stack exchange，提问作者Egg