为什么我的自定义Swing标签需要这么长时间来初始化

Question

我已经创建了JLabel的自定义子类。我有一个单独的例子，在一个单独的JPanel中，在一个单独的JFrame中。我做不覆盖paintComponent()方法;这个类所做的就是当光标悬停在它上面时改变它的背景颜色。

JFrame立即加载，但JPanel在几秒钟内未被绘制。我通过重写paintComponent()和添加一些调试println()语句来验证这是因为我的自定义类。

public void paintComponent(Graphics context)
{
    System.out.println("Painting...");
    super.paintComponent(context);
    System.out.println("Painted.");
}

奇怪的是，当我使用Panel而不是JPanel或Label而不是JLabel时，它会立即绘制。

这个延迟是从哪里来的?

编辑:一些示例代码。实际上没有画出任何东西;查看控制台消息延迟

import javax.swing.JFrame;
import javax.swing.JLabel;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.awt.GridLayout;
import javax.swing.JPanel;
public class Example extends JLabel implements MouseListener
{
    private static final long serialVersionUID = 0;
    public Example()
    {
        super();
        System.out.println("Constructed.");
    }
    public void paintComponent(java.awt.Graphics g)
    {
        System.out.println("Painting component...");
        super.paintComponent(g);
        System.out.println("Painted.");
    }
    public void mouseEntered(MouseEvent event) { }
    public void mouseExited(MouseEvent event) { }
    public void mouseReleased(MouseEvent event) { }
    public void mousePressed(MouseEvent event) { }
    public void mouseClicked(MouseEvent event) { }
    public static void main(final String[] arguments)
    {
        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(1, 1));
        panel.add(new Example());
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.setVisible(true);
        System.out.println("Set visible.");
    }
}

Answer 1

我的代码没有延迟:

我SSCCE

:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.*;
@SuppressWarnings("serial")
public class LabelTest extends JPanel {
   public LabelTest() {
      add(new MyLabel("Fubar!"));
   }
   private static void createAndShowGui() {
      LabelTest mainPanel = new LabelTest();
      JFrame frame = new JFrame("LabelTest");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}
@SuppressWarnings("serial")
class MyLabel extends JLabel {
   private static final Color BACKGROUND_DEFAULT = new Color(200, 200, 255);
   private static final Color BACKGROUND_MOUSEOVER = new Color(255, 200, 200);
   private static final int PREF_W = 200;
   private static final int PREF_H = 100;
   public MyLabel(String text) {
      super(text, SwingConstants.CENTER);
      setOpaque(true);
      setBackground(BACKGROUND_DEFAULT);
      addMouseListener(new MouseAdapter() {
         @Override
         public void mouseEntered(MouseEvent arg0) {
            setBackground(BACKGROUND_MOUSEOVER);
         }
         @Override
         public void mouseExited(MouseEvent e) {
            setBackground(BACKGROUND_DEFAULT);
         }
      });
   }
   @Override
   public Dimension getPreferredSize() {
      int width = Math.max(super.getPreferredSize().width, PREF_W);
      int height = Math.max(super.getPreferredSize().height, PREF_H);
      return new Dimension(width, height);
   }
}

这对我来说，问题不在于通过MouseListener更改背景的JLabel概念，而是您的代码中的某个地方存在错误。在哪里?谁知道，直到你发布可编译的可运行代码，一个SSCCE，就像我上面发布的。