在JPanel上画图时为何paint()方法会被多次调用？

[tripofdream]

代码如下：

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.swing.*;

public class InfoPanel extends JPanel
{
private Image img = Toolkit.getDefaultToolkit().getImage(
InfoPanel.class.getResource("about.jpg"));
private int count = 0;

public InfoPanel()
{

}

public void paint(Graphics g){
super.paint(g);
g.drawImage(img,0,0,this.getWidth(),this.getHeight(),this);
count ++;
System.out.println(count);
}

public static void main(String[] args){
JFrame f = new JFrame();
f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
f.setSize(400,200);
InfoPanel info = new InfoPanel();
f.getContentPane().add(info);
f.setVisible(true);
}
}

运行结果：
1
2
3
4
......
301
302
说明paint方法被多次调用，为什么？这样图片需要很长时间才能显示，怎样让图片立即显示？

[jncz]

你的竟然可以运行

在我这里死活都无法运行

uncatch error

[liad]

http://java.sun.com/docs/books/tutorial/uiswing/misc/icon.html

[cbhyk]

通过实现PanelUI在JPanel上绘图：

import java.awt.Image;
import java.awt.MediaTracker;
import java.awt.Toolkit;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class ImgPanel extends JPanel
{
private Image img;

public ImgPanel()
{
}

public void updateUI()
{
setUI(ImgPanelUI.getInstance());
}

public Image getImg()
{
return img;
}

public void setImg(Image image)
{
img = image;
}

public static void main(String[] args)
{
try
{
JFrame frame = new JFrame("test");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Image img = Toolkit.getDefaultToolkit().getImage(ImgPanel.class.getResource("test.jpg"));
ImgPanel panel = new ImgPanel();

MediaTracker tracker = new MediaTracker(panel);
tracker.addImage(img, 1);
tracker.waitForID(1);
System.out.println("load image ok");

panel.setImg(img);

panel.add(new JButton("test1"));
panel.add(new JButton("test2"));
frame.getContentPane().add(panel);
frame.setSize(640, 480);
frame.setLocation(80, 60);
frame.setVisible(true);
}
catch(Throwable e)
{
e.printStackTrace();
System.exit(1);
}
}
}

import java.awt.Graphics;

import javax.swing.JComponent;
import javax.swing.plaf.ComponentUI;
import javax.swing.plaf.basic.BasicPanelUI;


public class ImgPanelUI extends BasicPanelUI
{
private static ImgPanelUI instance = new ImgPanelUI();

public static ComponentUI getInstance()
{
return instance;
}

public void paint(Graphics g, JComponent c)
{
System.out.println("draw ImgPanel");
ImgPanel panel = (ImgPanel) c;
g.drawImage(panel.getImg(), 0, 0, panel.getWidth(), panel.getHeight(), panel);
super.paint(g, c);
}
}

这里的paint方法只会被调用一次

[wangyanqiu]

up

[tripofdream]

Applet的例子就请不要拿过了，我要的是Frame下的。运行环境是win2000+jdk1.3.1

[whyxx]

我在我的机子上测试过了,只打到15就可以了,但估计不停的paint像是一个缓冲一样,在图片很大,没法一次画出来的时候,隔几秒就自动执行一下paint,直到图片出来为止

[tripofdream]

没有人能解释清楚吗？

[tripofdream]

T

[veijerd]

Awt被设计为假定图象可能是从缓慢的网络连接上得来的。java创建了一个新的线程来加载该文件，然后在文件实际加载完之前便返回。
在你的paint()中，因为图片尚未加载，所以paint一次次地尝试调用绘制图片，count的增加值由加载的速度决定。

可以采用MediaTracker来完成，这是它的典型应用
你的改正代码如下,我调试过了，你看看 :)

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.MediaTracker;
import javax.swing.*;

public class InfoPanel extends JPanel
{
private Image img = Toolkit.getDefaultToolkit().getImage(
InfoPanel.class.getResource("DDG51.gif"));


private int count = 0;

public InfoPanel()
{
MediaTracker tracker=new MediaTracker(this);
tracker.addImage(img,0);
try
{
tracker.waitForID(0);
}
catch(InterruptedException e)
{
e.printStackTrace();
}
}

public void paint(Graphics g){
super.paint(g);
g.drawImage(img,0,0,this.getWidth(),this.getHeight(),this);
count ++;
System.out.println(count);
}

public static void main(String[] args){
JFrame f = new JFrame();
f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
f.setSize(400,200);
InfoPanel info = new InfoPanel();
f.getContentPane().add(info);
f.setVisible(true);
}
}

[tripofdream]

图片为400*300*24

[LingFengNB]

up

[tripofdream]

再加100，重赏之下必有勇夫？!

[LingFengNB]

good