设置 JFrame 背景，为什么这不起作用? [英] setting a JFrame background, why this doesn't work?

问题描述

我想为我的 jFrame 设置背景，我正在使用以下代码:

import java.awt.Graphics;导入 java.awt.Image;导入 java.awt.MediaTracker;导入 java.awt.Toolkit;导入 javax.swing.JFrame;导入 javax.swing.JPanel;公共类 DemoBackgroundSwing 扩展 JPanel {私人图像 img;公共 DemoBackgroundSwing() {System.out.println("完成");img = Toolkit.getDefaultToolkit().createImage("red.png");System.out.println("完成");加载图像(img)；System.out.println("完成");}私有无效加载图像(图像 img){尝试 {MediaTracker track = new MediaTracker(this);track.addImage(img, 0);track.waitForID(0);} catch (InterruptedException e) {e.printStackTrace();}}受保护的无效paintComponent(图形g){设置不透明(假)；System.out.println("完成");g.drawImage(img, 0, 0, null);super.paintComponent(g);}公共静态无效主(字符串... argv){JFrame frame = new JFrame("演示背景图片");DemoBackgroundSwing back = new DemoBackgroundSwing();System.out.println("完成");frame.getContentPane().add(back);frame.setSize(400, 287);frame.setVisible(true);}}

当我执行它时，在系统输出中我有 5 个完成".所以这意味着所有任务都被执行.我不明白错误在哪里.请帮帮我！

解决方案

打印已加载图片的宽度，如果为 -1，则图片未正确加载.

img = Toolkit.getDefaultToolkit().createImage("red.png");System.out.println(img.getWidth(null));//检查打印的内容

值得阅读上的Java教程使用 getResource 加载图像

<小时>

您可以根据图像位置尝试任何一种.

//从同一个包中读取ImageIO.read(getClass().getResourceAsStream("c.png"));//从项目中与 src 平行的图像文件夹中读取ImageIO.read(new File("images/c.jpg"));//从 src/images 文件夹中读取ImageIO.read(getClass().getResource("/images/c.png"))//从 src/images 文件夹中读取ImageIO.read(getClass().getResourceAsStream("/images/c.png"))

阅读更多...><小时>

几点:

• 在重写的 paintComponent() 方法的第一行调用 super.paintComponent(g);.

• 使用 ImageIO 而不是 Toolkit 来加载图像.

• 使用 frame.pack() 而不是 frame.setSize() 以适应组件的首选大小.

• 覆盖 getPreferredSize() 以在自定义绘画的情况下设置 JPanel 的首选大小.

• 使用 SwingUtilities.invokeLater() 或 EventQueue.invokeLater() 以确保 EDT 已正确初始化.

  阅读更多

  • 为什么在 main 方法中使用 SwingUtilities.invokeLater?

  • SwingUtilities.invokeLater

  • 我们应该将 EventQueue.invokeLater 用于 Java 桌面应用程序中的任何 GUI 更新吗?

I want to set a background to my jFrame, and I'm using this code:

import java.awt.Graphics;
import java.awt.Image;
import java.awt.MediaTracker;
import java.awt.Toolkit;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class DemoBackgroundSwing extends JPanel {

  private Image img;

  public DemoBackgroundSwing() {
      System.out.println("done");
    img = Toolkit.getDefaultToolkit().createImage("red.png");
    System.out.println("done");
    loadImage(img);
    System.out.println("done");
  }

  private void loadImage(Image img) {
    try {
      MediaTracker track = new MediaTracker(this);
      track.addImage(img, 0);
      track.waitForID(0);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }

  protected void paintComponent(Graphics g) {
    setOpaque(false);
    System.out.println("done");
    g.drawImage(img, 0, 0, null);
    super.paintComponent(g);
  }

  public static void main(String... argv) {
    JFrame frame = new JFrame("Demo Background Image");
    DemoBackgroundSwing back = new DemoBackgroundSwing();
    System.out.println("done");
    frame.getContentPane().add(back);
    frame.setSize(400, 287);
    frame.setVisible(true);
  }
}

when I execute it, in system out I have 5 "done". so this means that all tasks are executed. I don't understand where's the error. Please Help me!

解决方案

Print the width of loaded image if it's -1 then image is not properly loaded.

img = Toolkit.getDefaultToolkit().createImage("red.png");
System.out.println(img.getWidth(null)); // check what it prints

It's worth reading Java Tutorial on Loading Images Using getResource

---

You can try any one based on image location.

// Read from same package 
ImageIO.read(getClass().getResourceAsStream("c.png"));

// Read from images folder parallel to src in your project
ImageIO.read(new File("images/c.jpg"));

// Read from src/images folder
ImageIO.read(getClass().getResource("/images/c.png"))

// Read from src/images folder
ImageIO.read(getClass().getResourceAsStream("/images/c.png"))

Read more...

---

Some Points:

• call super.paintComponent(g); at first line in the overridden paintComponent() method.

• Use ImageIO instead of Toolkit to load the image.

• Use frame.pack() instead of frame.setSize() that fits the components as per component's preferred size.

• Override getPreferredSize() to set the preferred size of the JPanel in case of custom painting.

• Use SwingUtilities.invokeLater() or EventQueue.invokeLater() to make sure that EDT is initialized properly.

  Read more

  • Why to use SwingUtilities.invokeLater in main method?

  • SwingUtilities.invokeLater

  • Should we use EventQueue.invokeLater for any GUI update in a Java desktop application?

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