将 JPanel 保存为图像

我正在开发一个应用程序，允许用户在图层中加载图像。用户可以在另一层上绘制一些图片并仅保存用户绘制的图像。这是我的代码：

import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.ImageIcon;
import javax.swing.JApplet;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JPanel;

public class LayerDemo extends JApplet {

    private JLayeredPane mainLayer;

    private JPanel layer1;

    private JPanel layer2;

    private JLabel label;

    private ImageIcon imgIcon;


    /**
     * Create the applet.
     */
    public LayerDemo() {    
    }

    public void init() {
        Dimension mainDemension = new Dimension(1024,768);
        setSize(mainDemension);

        mainLayer = new JLayeredPane();   
        layer1 = new JPanel();
        layer1.setOpaque(false);
        layer1.setBounds(0, 0, this.getWidth(), this.getHeight());
        imgIcon = new ImageIcon("bear.jpg");
        label = new JLabel(imgIcon);
        label.setBounds(0, 0, imgIcon.getIconWidth(), imgIcon.getIconHeight());
        layer1.add(label);

        layer2 = new PaintDemo(true);
        layer2.setOpaque(false);
        layer2.setBounds(0, 0, this.getWidth(), this.getHeight());

        mainLayer.add(layer1, 1);
        mainLayer.add(layer2, 2);
        this.setContentPane(mainLayer);
    }

    public void paint(Graphics g) {    
    }

}

这是用户绘制的类：

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import javax.imageio.ImageIO;
import javax.swing.JPanel;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.event.MouseWheelListener;
import java.awt.event.MouseWheelEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class PaintDemo extends JPanel {
    /**
     * field explanation
     */
    private Point startPoint = new Point();

    private Point endPoint = new Point();

    private Graphics2D g2;

    private int minX;

    private int minY;

    private int maxX;

    private int maxY;

    private int height;

    private int width;


    /**
     * Create the panel.
     */

    public PaintDemo(boolean isDoubleBuffer) {

        addMouseWheelListener(new MouseWheelListener() {
            public void mouseWheelMoved(MouseWheelEvent e) {
            }
        });
        this.setDoubleBuffered(isDoubleBuffer);
        addMouseMotionListener(new MouseMotionAdapter() {
            @Override
            public void mouseDragged(MouseEvent e) {
                endPoint = e.getPoint();
                Graphics g = PaintDemo.this.getGraphics();
                paintComponent(g);
                minX = minX < endPoint.x ? minX : endPoint.x;
                minY = minY < endPoint.y ? minY : endPoint.y;
                maxX = maxX > endPoint.x ? maxX : endPoint.x;
                maxY = maxY > endPoint.y ? maxY : endPoint.y;
                startPoint = endPoint;
            }
        });
        addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                startPoint = e.getPoint();
                minX = startPoint.x;
                minY = startPoint.y;
                maxX = startPoint.x;
                maxY = startPoint.y;
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                endPoint = e.getPoint();
                Graphics g = PaintDemo.this.getGraphics();
                paintComponent(g);
                minX = minX < endPoint.x ? minX : endPoint.x;
                minY = minY < endPoint.y ? minY : endPoint.y;
                maxX = maxX > endPoint.x ? maxX : endPoint.x;
                maxY = maxY > endPoint.y ? maxY : endPoint.y;
                minX = minX > 0 ? minX : 0;
                minY = minY > 0 ? minY : 0;
                maxX = maxX < 1024 ? maxX : 1024;
                maxY = maxY < 768 ? maxY : 768;
                width = maxX - minX;
                height = maxY - minY;
                saveImage();     
                startPoint = new Point();
                endPoint = new Point();

            }
        });
    }

    /**
     * Paint method
     * 
     * {@inheritDoc}
     */
    @Override
    public void paintComponent(Graphics g) {
        g2 = (Graphics2D)g;
        g2.setStroke(new BasicStroke(2, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
        g2.setFont(new Font("Serif", Font.BOLD, 18));
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setColor(Color.red);
        g2.drawLine(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
    }

    public void saveImage() {
        BufferedImage bi = new BufferedImage(PaintDemo.this.getWidth(), PaintDemo.this.getHeight(), BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = bi.createGraphics();
        paintComponent(g2);
        g2.dispose();
        try
        {
            ImageIO.write(bi, "jpg", new File("clip.jpg"));
        }
        catch(IOException ioe)
        {
            System.out.println("Clip write help: " + ioe.getMessage());
        }
    }
}

保存图像时，它只是空白图像。请帮助我。非常感谢。 P/S：我已经按照你的想法编辑了我的代码，但它不起作用。结果是没有图像背景，仍然保存空白图像。 :(

自定义绘制是通过重写面板的paintComponent()方法来完成的。然后使用 Graphics 对象进行绘画。

你不应该有一个空的paint()方法。

drawline() 方法不应使用 getGraphics() 方法。相反，应该将该代码移至 PaintComponent() 方法，然后使用传递给该方法的 Graphics 对象。

另外，您不应该重写小程序的 Paint() 方法。由于您的代码按实际大小绘制图像，因此您应该仅使用 JLabel 通过创建 ImageIcon 来显示图像。然后将标签添加到分层窗格以用作背景图像。

Edit:

为什么仍然有空的paint()方法？摆脱它，无需重写 Paint() 方法。

当我运行代码时，我收到一个安全异常，因为小程序无法写入文件，所以我无法测试这部分代码。但如果你有兴趣我用屏幕图像 http://tips4java.wordpress.com/2008/10/13/screen-image/创建组件的图像。

然而，你的主要问题是绘画代码是错误的。是的，您会看到绘制的线条，但它们不是永久性的。当您想要进行永久绘画时，切勿使用组件的 getGraphics() 方法。尝试画一些线，然后最小化小程序，然后恢复小程序，您就会明白我的意思。

解决方案是在 BufferedImage 上进行绘图。请参阅DrawOnImage示例来自定制绘画方法 http://tips4java.wordpress.com/2009/05/08/custom-painting-approaches/.