面板需要根据需要进行布局它的要求。如果面板尚未在屏幕上实现，它可能无法按预期的方式呈现

The panel needs to be laid out based on it's requirements. If the panel hasn't being realized on the screen yet, it may not render the way you expect it do

以下示例假定面板没有显示在屏幕上...

The following example assumes that the panel has not being displayed on the screen...

setSize(getPreferredSize());
BufferedImage image = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);
Graphics2D g = image.createGraphics();
printAll(g);
g.dispose();
try { 
    ImageIO.write(image, "png", new File([location goes here]); 
} catch (IOException e) {
    e.printStackTrace();
}

你应该避免调用 paint 你自己，如果组件没有在屏幕上实现，它可以抛出异常，而是使用 printAll

You should avoid calling paint yourself, it can throw an exception if the component has not being realized on the screen, instead, use printAll

另外，如果你创建一个资源，你应该处理它;）

Also, if your create a resource, you should dispose of it ;)

更新

我做了这个简单的例子。屏幕截图位于顶部，jpeg位于左侧，png位于右侧。

I did this quick example. Screen shoot on top, jpeg on left, png on right.

jpeg为30kb，png为320kb

jpeg is 30kb and png is 320kb

我用它来创建它...

I used this to create it...

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class PaintComponent {

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

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

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private JPanel paintPane;

        public TestPane() {

            paintPane = new JPanel(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;

            paintPane.add(new JLabel("I'm a label"), gbc);
            paintPane.add(new JTextField("I'm a text field", 20), gbc);
            paintPane.add(new JLabel(new ImageIcon("some\pretty\picture")), gbc);

            setLayout(new BorderLayout());
            add(paintPane);

            JButton paint = new JButton("Capture");
            paint.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    BufferedImage image = new BufferedImage(paintPane.getWidth(), paintPane.getHeight(), BufferedImage.TYPE_INT_RGB);
                    Graphics2D g = image.createGraphics();
                    paintPane.printAll(g);
                    g.dispose();
                    try {
                        ImageIO.write(image, "jpg", new File("Paint.jpg"));
                        ImageIO.write(image, "png", new File("Paint.png"));
                    } catch (IOException exp) {
                        exp.printStackTrace();
                    }
                }
            });
            add(paint, BorderLayout.SOUTH);

        }
    }
}

我会确保你实际上正在查看正确的文件;）

I would make sure that you are actually looking at the correct files ;)