此servlet生成一个验证码图片可以通过请求参数设置此图片
/*
 * 创建日期 2006-7-23
 *
 * TODO 要更改此生成的文件的模板，请转至
 * 窗口 － 首选项 － Java － 代码样式 － 代码模板
 * 参数：
 * count:验证码的字符个数
 * width:验证码图片宽度
 * height:验证码图片高度
 * type:类型 0-纯数字 1-纯大写字母 2-数字和大学字母混合
 * 功能：生成验证码图片，将验证码保存进session里
 */
package hrl.util;

import java.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.awt.*;
import java.awt.image.*;
import java.util.*;
import javax.imageio.*;
/**
 * @author Administrator
 *
 * TODO 要更改此生成的类型注释的模板，请转至
 * 窗口 － 首选项 － Java － 代码样式 － 代码模板
 */
public class ValidateCodeServlet extends HttpServlet {
    
    private final int TYPE_NUMBER = 0;
    private final int TYPE_LETTER = 1;
    private final int TYPE_MULTIPLE = 2;
    private int width;
    private int height;
    private int count;
    private int type;
    private String  validate_code;
    private Random random;
    private Font font;
    private int line;

    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        //TODO Method stub generated by Lomboz
        width = 150;
        height = 50;
        count = 4;
        type = TYPE_NUMBER;
        random = new Random();
        line = 200;
    }

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        //TODO Method stub generated by Lomboz

        response.setHeader("Pragma","No-cache");
        response.setHeader("Cache-Control","no-cache");
        response.setDateHeader("Expires", 0);
        response.setContentType("image.jpeg");

        String reqCount = request.getParameter("count");
        String reqWidth = request.getParameter("width");
        String reqHeight = request.getParameter("height");
        String reqType = request.getParameter("type");

        if(reqCount!=null && reqCount!="")this.count = Integer.parseInt(reqCount);
        if(reqWidth!=null && reqWidth!="")this.width = Integer.parseInt(reqWidth);
        if(reqHeight!=null && reqHeight!="")this.height = Integer.parseInt(reqHeight);
        if(reqType!=null && reqType!="")this.type = Integer.parseInt(reqType);

        font = new Font("Courier New",Font.BOLD,width/count);

        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        Graphics g = image.getGraphics();

        g.setColor(getRandColor(200,250));
        g.fillRect(0, 0, width, height);

        g.setColor(getRandColor(160,200));
        for (int i=0;i<line;i++){
            int x = random.nextInt(width);
            int y = random.nextInt(height);
            int xl = random.nextInt(12);
            int yl = random.nextInt(12);
            g.drawLine(x,y,x+xl,y+yl);
        }

        g.setFont(font);
        validate_code = getValidateCode(count,type);
        request.getSession().setAttribute("validate_code",validate_code);
        for (int i=0;i<count;i++){
            g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
            int x = (int)(width/count)*i;
            int y = (int)((height+font.getSize())/2)-5;
            g.drawString(String.valueOf(validate_code.charAt(i)),x,y);
        }

        g.dispose();
        ImageIO.write(image, "JPEG", response.getOutputStream());
    }

    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        //TODO Method stub generated by Lomboz
        doGet(request, response);
    }
    
    private Color getRandColor(int from,int to){
        Random random = new Random();
        if(to>255) from=255;
        if(to>255) to=255;
        int rang = Math.abs(to - from);
        int r=from+random.nextInt(rang);
        int g=from+random.nextInt(rang);
        int b=from+random.nextInt(rang);
        return new Color(r,g,b);
    }
    
    private String getValidateCode(int size,int type){
        StringBuffer validate_code = new StringBuffer();
        for(int i = 0; i < size; i++){
            validate_code.append(getOneChar(type));
        }
        return validate_code.toString();
    }
    private String getOneChar(int type){
        String result = null;
        switch(type){
            case TYPE_NUMBER:
                result = String.valueOf(random.nextInt(10));
                break;
                
            case TYPE_LETTER:
                result = String.valueOf((char)(random.nextInt(26)+65));
                break;
            
            case TYPE_MULTIPLE:
                if(random.nextBoolean()){
                    result = String.valueOf(random.nextInt(10));
                }else{
                    result = String.valueOf((char)(random.nextInt(26)+65));
                }
                break;
            default:
                result=null;
                break;
        }
        if(result==null)throw new NullPointerException("获取验证码出错");
        return result;
    }
}
web.xml配置：
<servlet><servlet-name>yan</servlet-name><display-name>yan</display-name><description>yan</description><servlet-class>hrl.util.ValidateCodeServlet</servlet-class></servlet><servlet>
<servlet-mapping><servlet-name>yan</servlet-name><url-pattern>/ValidateCodeServlet</url-pattern></servlet-mapping><servlet-mapping>

实例：
show.jsp:
<%@ page language="java" pageEncoding="GB2312" %>
<!DOCTYPE HTML PUBLIC "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<title>Lomboz JSP</title>
</head>
<body bgcolor="#FFFFFF">
<form action="heck.jsp" method=post>
<table align=center>
<tr><td>
请输入验证码：<input type='text' name='validate_code' size=20>
</td><td>
<img src="ValidateCodeServlet" width=150 height=50>
</td></tr>
<tr><td>
<input type="submit" value="提交">
</td><td>
<input type="reset" value="重填">
</td></tr>
</table>
</body>
</html>

check.jsp:
<%@ page language="java" pageEncoding="GB2312" %>
<!DOCTYPE HTML PUBLIC "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<title>Lomboz JSP</title>
</head>
<body bgcolor="#FFFFFF">

<%
String session_validate_code = (session.getAttribute("validate_code")).toString();
if(session_validate_code==null)System.out.println("validate code has not set");
else{
 String request_code = request.getParameter("validate_code");
 if(request_code==null)out.println("未输入");
 else{
  if(request_code.equalsIgnoreCase(session_validate_code))out.println("匹配");
  else out.println("不匹配");
 }
}
%>
</body>
</html>

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