我用例子解释吧：

先来一个在Servlet中编写验证码的例子：

package com.servlet.checkImage;import java.awt.Color;import java.awt.Font;import java.awt.Graphics2D;import java.awt.image.BufferedImage;import java.io.IOException;import java.io.PrintWriter;import java.util.Random;import javax.servlet.ServletException;import javax.servlet.ServletOutputStream;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import com.sun.image.codec.jpeg.JPEGCodec;import com.sun.image.codec.jpeg.JPEGImageEncoder;public class checkImageDemo extends HttpServlet {    public static final char[] CHARS={'2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','J','K','L','M','N','P','Q','R','S','T','U','V','W','X','Y','Z'};    public static Random random=new Random();        public static String getRandomString(){        StringBuffer buffer=new StringBuffer();        for(int i=0;i<6;i++){            buffer.append(CHARS[random.nextInt(CHARS.length)]);        }        return buffer.toString();    }    public static Color getRandomColor(){        return new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));    }        public static Color getReverseColor(Color c){
    //返回某颜色的反色        return new Color(255-c.getRed(),255-c.getGreen(),255-c.getBlue());    }        public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        request.setCharacterEncoding("UTF-8");        response.setCharacterEncoding("UTF-8");        response.setContentType("image/jpeg");//设置输出类型，必须的                String randomString=getRandomString();        request.getSession(true).setAttribute("randomString", randomString);                int width=100;        int height=30;                Color color=getRandomColor();//用于背景颜色        Color reverse=getReverseColor(color);//用于前景颜色                BufferedImage bi=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);//创建一个彩色图片        Graphics2D g=bi.createGraphics();//获取绘图对象        g.setFont(new Font(Font.SANS_SERIF,Font.BOLD,16));//设置字体        g.setColor(color);        g.fillRect(0, 0, width, height);//绘制背景        g.setColor(reverse);        g.drawString(randomString, 18, 20);        for(int i=0,n=random.nextInt(100);i

再来一个在类中编写验证码的例子：

package com.servlet.checkImage;import java.awt.Color;import java.awt.Graphics;import java.awt.image.BufferedImage;import java.io.IOException;import java.io.OutputStream;import java.util.Random;import javax.imageio.ImageIO;public class Image1 {    public Random random=new Random();        public String[] getRandomString(){        String validateCode[]=new String[4];        for(int i=0;i<4;i++){            validateCode[i]=random.nextInt(10)+"";        }                return validateCode;    }        public Color getRandomColor(){        Color color=new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));        return color;    }        public Color getResverseColor(Color color){        Color reverse=new Color(255-color.getRed(),255-color.getGreen(),255-color.getBlue());//获取该颜色的反色，如果需要噪点或者切割线可以使用这个        return reverse;    }        public String getImage(OutputStream os){        int width=80;        int height=25;                BufferedImage bi=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);        Graphics g=bi.getGraphics();//获得一个画笔        //g.setFont(new Font(Font.SANS_SERIF,Font.BOLD,16));//设置字体                //边框        g.setColor(Color.black);        g.drawRect(0, 0, width-1, height-1);                Color back=getRandomColor();        g.setColor(back);        g.fillRect(0, 0, width, height);//填充背景                            String[] code=getRandomString();        for(int i=0;i

在类中编写的验证码调用需要一个jsp作为中间介质：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><%@page contentType="image/jpeg" %>                      
          
          
              
          
          
                 
           <%        String str=image.getImage(response.getOutputStream());        session.setAttribute("checkCode",str);                out.clear();        out=pageContext.pushBody();     %>      

在Servlet中的验证码可以直接调用：

两个的调用示例在这个例子里面：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%>                      
          
          
              
          
          
                            
          

至于一些配置的就不啰嗦了。认真看看代码就基本验证码该会制作了。