java swing 实现随机背景验证码的效果

]/**
* 使用图形输出验证码效果（随机数字）
* Cavas Random Graphics
**/

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class Exseven extends JFrame implements ActionListener {
JButton button;
ImgCanvas canvas;
public Exseven() {
super("验证码-CSDN.huadis");
setBackground(Color.white);
setLayout(new BorderLayout());
setSize(300, 150);
setLocation(200, 200);

button = new JButton("验证码");
button.addActionListener(this);

canvas = new ImgCanvas();
canvas.setSize(180, 70);
canvas.setOpaque(true);
canvas.setVisible(true);

add(canvas,BorderLayout.CENTER);
add(button,BorderLayout.SOUTH);

setVisible(true);
}

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

public void actionPerformed(ActionEvent e) {
if(e.getSource()==button){
canvas.repaint();
}
}

private class ImgCanvas extends JComponent{
public ImgCanvas(){
super();
}
public void paintComponent(Graphics g){
Random ra=new Random();
char st[]="1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();

g.setColor(new Color(ra.nextInt(256),ra.nextInt(256),ra.nextInt(256)));
g.fillRect(0, 0, getWidth(), getHeight());

Font f=new Font("Fixedsys",Font.PLAIN,50);
g.setColor(new Color(ra.nextInt(100),ra.nextInt(100),ra.nextInt(100)));//设置验证码随机色彩
g.setFont(f);
//随机生成四个数字与字母的组合,+""必须有,意思是把四个字符连结起来变成字符串String
String ss=new String(st[ra.nextInt(36)]+""+st[ra.nextInt(36)]+st[ra.nextInt(36)]+st[ra.nextInt(36)]);
g.drawString(ss, 20, 50);

g.drawLine(10, 10, 170, 60);
g.drawLine(10, 40, 170, 20);
}
}
}