尝试image在ImageIcon更新后包装内部。同样，Graphics#dispose当您完成对图形上下文的渲染时，还应该调用。

    public void jButton1_ActionPerformed(ActionEvent evt) {
        BufferedImage image=new BufferedImage(jPanel1.getWidth(), jPanel1.getHeight(), BufferedImage.TYPE_3BYTE_BGR);
        Graphics2D g = image.createGraphics();
        g.fillRect(0, 0, image.getWidth(), image.getHeight());
        g.setColor(Color.BLUE);
        g.drawLine(0, 0, 300, 400);
        g.dispose();
        JLabel l=new JLabel(new ImageIcon(image));
        jPanel1.add(l);
    }

您还应该依靠布局管理器，而不是自己动手做，这只会使您的生活更轻松。

就个人而言，我认为将其直接绘制到自定义组件会更容易JPanel。查看执行自定义绘画以获取更多详细信息

用示例更新

基本上，我将您的示例更改为

1. 使用布局管理器
2. 在EDT上下文中加载UI
3. revalidate 的 jPanel1

    public class BadLabel extends JFrame {
        // start attributes

        private JPanel jPanel1 = new JPanel(new BorderLayout(), true);
        private JButton jButton1 = new JButton();
        private JButton jButton2 = new JButton();
        // end attributes

        public BadLabel(String title) {
            // Frame-Init
            super(title);
            setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
            int frameWidth = 300;
            int frameHeight = 300;
            setSize(frameWidth, frameHeight);
            Dimension d = Toolkit.getDefaultToolkit().getScreenSize();
            int x = (d.width - getSize().width) / 2;
            int y = (d.height - getSize().height) / 2;
            setLocation(x, y);
    //        setResizable(false);
            Container cp = getContentPane();
    //        cp.setLayout(null);
            // start components

    //        jPanel1.setBounds(48, 24, 209, 145);
            jPanel1.setOpaque(true);
            jPanel1.setBackground(Color.RED);
            cp.add(jPanel1);

            JPanel buttons = new JPanel();
    //        jButton1.setBounds(88, 208, 75, 25);
            jButton1.setText("jButton1");
            jButton1.setMargin(new Insets(2, 2, 2, 2));
            jButton1.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent evt) {
                    jButton1_ActionPerformed(evt);
                }
            });
            buttons.add(jButton1);
    //        jButton2.setBounds(184, 208, 75, 25);
            jButton2.setText("jButton2");
            jButton2.setMargin(new Insets(2, 2, 2, 2));
            jButton2.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent evt) {
                    jButton2_ActionPerformed(evt);
                }
            });
            buttons.add(jButton2);
            // end components

            cp.add(buttons, BorderLayout.SOUTH);

            setVisible(true);
        } // end of public BadLabel

        // start methods
        public void jButton1_ActionPerformed(ActionEvent evt) {
            BufferedImage image = new BufferedImage(jPanel1.getWidth(), jPanel1.getHeight(), BufferedImage.TYPE_3BYTE_BGR);
            Graphics2D g = image.createGraphics();
            g.fillRect(0, 0, image.getWidth(), image.getHeight());
            g.setColor(Color.BLUE);
            g.drawLine(0, 0, 300, 400);
            g.dispose();
            JLabel l = new JLabel(new ImageIcon(image));
            l.setBorder(new LineBorder(Color.BLUE));
            jPanel1.add(l);
            jPanel1.revalidate();
        } // end of jButton1_ActionPerformed

        public void jButton2_ActionPerformed(ActionEvent evt) {
            // TODO add your code here
        } // end of jButton2_ActionPerformed

        // end methods
        public static void main(String[] args) {
            EventQueue.invokeLater(new Runnable() {
                @Override
                public void run() {
                    try {
                        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                    } catch (Exception exp) {
                    }
                    new BadLabel("BadLabel");
                }
            });
        } // end of main
    } // end of class BadLabel}