<servlet>
	<servlet-name>validateImg</servlet-name>
	<servlet-class>com.cytest.ValidateImg</servlet-class>
</servlet>
<servlet-mapping>
	<servlet-name>validateImg</servlet-name>
	<url-pattern>/validateImage.jpg</url-pattern>
</servlet-mapping>

package com.cytest;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.awt.*;
import java.awt.image.*;
import java.util.*;
import javax.imageio.*; 

public class ValidateImg extends HttpServlet {
	//定义图形验证码中绘制字符的字体
	private final Font mFont = new Font("Arial Black", Font.PLAIN, 16);
	//定义图形验证码的大小
	private final int IMG_WIDTH = 70;
	private final int IMG_HEIGTH = 20;
	
	
	/**
	 * 定义一个获取随机颜色的方法
	 */
	private Color getRandColor(int fc,int bc) {
		Random random = new Random();
		if(fc > 255) fc = 255;
		if(bc > 255) bc=255;
		int r = fc + random.nextInt(bc - fc);
		int g = fc + random.nextInt(bc - fc);
		int b = fc + random.nextInt(bc - fc);
		//得到随机颜色
		return new Color(r , g , b);
	}
	
	
	/**
	 * 重写service方法，生成对客户端的响应
	 */
	public void service(HttpServletRequest request, HttpServletResponse response) 
		throws ServletException, IOException {
		//设置禁止缓存
		response.setHeader("Pragma","No-cache");
		response.setHeader("Cache-Control","no-cache");
		response.setDateHeader("Expires", 0);
		response.setContentType("image/jpeg");
		
		BufferedImage image = new BufferedImage(IMG_WIDTH , IMG_HEIGTH , BufferedImage.TYPE_INT_RGB);
		Graphics g = image.getGraphics();
		Random random = new Random();
		g.setColor(getRandColor(200 , 250));
		//填充背景色
		g.fillRect(1, 1, IMG_WIDTH - 1, IMG_HEIGTH - 1); 
		//为图形验证码绘制边框
		g.setColor(new Color(102 , 102 , 102));
		g.drawRect(0, 0, IMG_WIDTH - 1, IMG_HEIGTH - 1);
		g.setColor(getRandColor(160,200));
		//生成随机干扰线
		for (int i = 0 ; i < 80 ; i++) {
			int x = random.nextInt(IMG_WIDTH - 1);
			int y = random.nextInt(IMG_HEIGTH - 1);
			int xl = random.nextInt(6) + 1;
			int yl = random.nextInt(12) + 1;
			g.drawLine(x , y , x + xl , y + yl);
		}
		g.setColor(getRandColor(160,200));
		//生成随机干扰线
		for (int i = 0 ; i < 80 ; i++) {
			int x = random.nextInt(IMG_WIDTH - 1);
			int y = random.nextInt(IMG_HEIGTH - 1);
			int xl = random.nextInt(12) + 1;
			int yl = random.nextInt(6) + 1;
			g.drawLine(x , y , x - xl , y - yl);
		}
		//设置绘制字符的字体
		g.setFont(mFont);
		//用于保存系统生成的随机字符串
		String sRand = "";
		for (int i = 0 ; i < 4 ; i++) {
			String tmp = getRandomChar();
			sRand += tmp;
			//获取随机颜色
			g.setColor(new Color(20 + random.nextInt(110)
				,20 + random.nextInt(110)
				,20 + random.nextInt(110)));
			//在图片上绘制系统生成的随机字符
			g.drawString(tmp , 15 * i + 10,15);
		}
		//获取HttpSesssion对象
		HttpSession session = request.getSession(true);
		//将随机字符串放入HttpSesssion对象中 
		session.setAttribute("randNum" , sRand);
		g.dispose();
		//向输出流中输出图片
		ImageIO.write(image, "JPEG", response.getOutputStream());
	}
		
	/**
	 * 获取随机字符串
	 * @return
	 */
	private String getRandomChar() {
		//生成一个0、1、2的随机数字
		int rand = (int)Math.round(Math.random() * 2);
		long itmp = 0;
		char ctmp = '\u0000';
		switch (rand) {
			//生成大写字母
			case 1:
				itmp = Math.round(Math.random() * 25 + 65);
				ctmp = (char)itmp;
				return String.valueOf(ctmp);
			//生成小写字母
			case 2:
				itmp = Math.round(Math.random() * 25 + 97);
				ctmp = (char)itmp;
				return String.valueOf(ctmp);
			//生成数字
			default :
				itmp = Math.round(Math.random() * 9);
				return  itmp + "";
		}
	}
}