java - 我可以从 JPanel 创建 BufferedImage 而不在 JFrame 中渲染吗？

Question

是否可以从 JPanel 创建 BufferedImage 而无需先在 JFrame 中渲染它？我搜索了所有我能想到的地方，但找不到答案。谁能帮忙？

这是一些示例代码。如果我不取消注释 JFrame 代码，我的 BufferedImage 就是空白的。

    test(){
//      JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        Dimension dim = new Dimension(50,50);
        panel.setMinimumSize(dim);
        panel.setMaximumSize(dim);
        panel.setPreferredSize(dim);
        JLabel label = new JLabel("hello");
        panel.add(label);
//      frame.add(panel);
//      frame.pack();
        BufferedImage bi = getScreenShot(panel);

        //...code that saves bi to a jpg
    }

    private BufferedImage getScreenShot(JPanel panel){
        BufferedImage bi = new BufferedImage(panel.getWidth(), panel.getHeight(), BufferedImage.TYPE_INT_ARGB);
        panel.paint(bi.getGraphics());
        return bi;
    }

Answer 1

参见 this answer至 Swing: Obtain Image of JFrame以及Why does the JTable header not appear in the image?有关绘制尚未渲染的组件的提示。我希望您的问题的解决方案显示在 LabelRenderTest.java 的标签中。

JLabel textLabel = new JLabel(title);
textLabel.setSize(textLabel.getPreferredSize());

更新

Dimension dim = new Dimension(50,50);
panel.setSize(dim);  // very important!
panel.setMinimumSize(dim);
panel.setMaximumSize(dim);
panel.setPreferredSize(dim);
// ...

或者这里是完整的源代码。标签的大小也需要设置。

import java.awt.Color;
import java.awt.Dimension;
import java.awt.image.BufferedImage;

import javax.swing.*;

public class RenderTest {

    RenderTest() {
        JPanel panel = new JPanel();
        panel.setBackground(Color.RED);
        Dimension dim = new Dimension(50,50);
        panel.setSize(dim);
        panel.setMinimumSize(dim);
        panel.setMaximumSize(dim);
        panel.setPreferredSize(dim);
        JLabel label = new JLabel("hello");
        label.setSize(label.getPreferredSize());
        panel.add(label);

        BufferedImage bi = getScreenShot(panel);
        JOptionPane.showMessageDialog(null, new JLabel(new ImageIcon(bi)));
    }

    private BufferedImage getScreenShot(JPanel panel){
        BufferedImage bi = new BufferedImage(
            panel.getWidth(), panel.getHeight(), BufferedImage.TYPE_INT_ARGB);
        panel.paint(bi.getGraphics());
        return bi;
    }

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