尝试将缓冲图像绘制到文件不起作用

Question

我正在尝试将jpanel转换为bufferedimage，然后将该图像保存到文件中。

DrawdPanel是扩展Jpanel的类。

public class J extends JFrame {
    public J(){
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(500,500);
        DrawingPanel panel = new DrawingPanel();
        add(panel);
        BufferedImage bi = (BufferedImage) panel.createImage(500,500);
        File file = new File("file.png");
        setVisible(true);
        try {
            ImageIO.write(bi, "PNG", file);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

我有一个错误： IllegalArgumentException: image == null!此错误发生在此行中：

ImageIO.write(bi, "PNG", file);

有什么问题？谢谢

编辑：Drawspanel类：

public class DrawingPanel extends JPanel {
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        setBackground(Color.BLACK);
    }
}

Answer 1

很难猜测您的问题，因为您的绘图板不可用，但这可能是由于您在GUI启用之前尝试获取图像的原因，实际上是在此之前完全渲染了任何东西，仅在jframe上调用setVisible(true)之后才发生。

，例如，

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.RenderingHints;
import java.awt.image.*;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.*;
public class ImageFoo extends JFrame {
   public ImageFoo() {
      setDefaultCloseOperation(EXIT_ON_CLOSE);
      // setSize(500, 500);
      DrawingPanel panel = new DrawingPanel();
      add(panel);
      // BufferedImage bi = (BufferedImage) panel.createImage(500, 500);
      // File file = new File("file.png");
      pack();
      setLocationRelativeTo(null);
      setVisible(true);
      BufferedImage bi = (BufferedImage) panel.createImage(500, 500);
      File file = new File("file.png");
      try {
         ImageIO.write(bi, "PNG", file);
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
   private static void createAndShowGui() {
      JFrame frame = new ImageFoo();
   }
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}
class DrawingPanel extends JPanel {
   private static final int PREF_W = 500;
   private static final int PREF_H = PREF_W;
   public DrawingPanel() {
      setBackground(Color.black);
   }
   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      Paint paint = new GradientPaint(0, 0, Color.red, 20, 20, Color.blue, true);
      g2.setPaint(paint);
      g2.fillOval(0, 0, PREF_W, PREF_H);
      // setBackground(Color.BLACK); // never do in paintComponent
   }
   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }
}

---

编辑：我喜欢这种尝试更好

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.RenderingHints;
import java.awt.image.*;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.*;
public class ImageFoo extends JFrame {
   public ImageFoo() {
      setDefaultCloseOperation(EXIT_ON_CLOSE);
      // setSize(500, 500);
      final DrawingPanel panel = new DrawingPanel();
      add(panel);
      // BufferedImage bi = (BufferedImage) panel.createImage(500, 500);
      // File file = new File("file.png");
      pack();
      setLocationRelativeTo(null);
      setVisible(true);
      new Thread(new Runnable() {
         public void run() {
            // try {
            //    Thread.sleep(300);
            // } catch (InterruptedException e1) {
            // }
            // BufferedImage bi = (BufferedImage) panel.createImage(500, 500);
            BufferedImage bi = new BufferedImage(DrawingPanel.PREF_W,
                  DrawingPanel.PREF_H, BufferedImage.TYPE_INT_ARGB);
            Graphics2D g2 = bi.createGraphics();
            panel.paintAll(g2);
            g2.dispose();
            File file = new File("file.png");
            try {
               ImageIO.write(bi, "PNG", file);
            } catch (IOException e) {
               e.printStackTrace();
            }
         }
      }).start();
   }
   private static void createAndShowGui() {
      JFrame frame = new ImageFoo();
   }
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}
class DrawingPanel extends JPanel {
   public static final int PREF_W = 500;
   public static final int PREF_H = PREF_W;
   public DrawingPanel() {
      setBackground(Color.black);
   }
   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      Paint paint = new GradientPaint(0, 0, Color.red, 20, 20, Color.blue, true);
      g2.setPaint(paint);
      g2.fillOval(0, 0, PREF_W, PREF_H);
      // setBackground(Color.BLACK); // never do in paintComponent
   }
   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }
}