Java验证码功能的实现方法

package com.bjpowernode.drp.util.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
/**
 * 验证码功能生成
 * @author Ares
 *
 */
public class AuthImage extends HttpServlet{
     private static final String CONTENT_TYPE = "text/html; charset=gb2312"; 
      //设置字母的大小,大小 
      private Font mFont = new Font("Times New Roman", Font.PLAIN, 17); 
      public void init() throws ServletException 
      { 
        super.init(); 
      } 
      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); 
        return new Color(r,g,b); 
      } 

      public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
      { 
        response.setHeader("Pragma","No-cache"); 
        response.setHeader("Cache-Control","no-cache"); 
        response.setDateHeader("Expires", 0); 
        //表明生成的响应是图片 
        response.setContentType("image/jpeg"); 

        int width=100, height=18; 
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); 

        Graphics g = image.getGraphics(); 
        Random random = new Random(); 
        g.setColor(getRandColor(200,250)); 
        g.fillRect(1, 1, width-1, height-1); 
        g.setColor(new Color(102,102,102)); 
        g.drawRect(0, 0, width-1, height-1); 
        g.setFont(mFont); 

        g.setColor(getRandColor(160,200)); 

        //画随机线 
        for (int i=0;i<155;i++) 
        { 
          int x = random.nextInt(width - 1); 
          int y = random.nextInt(height - 1); 
          int xl = random.nextInt(6) + 1; 
          int yl = random.nextInt(12) + 1; 
          g.drawLine(x,y,x + xl,y + yl); 
        } 

        //从另一方向画随机线 
        for (int i = 0;i < 70;i++) 
        { 
          int x = random.nextInt(width - 1); 
          int y = random.nextInt(height - 1); 
          int xl = random.nextInt(12) + 1; 
          int yl = random.nextInt(6) + 1; 
          g.drawLine(x,y,x - xl,y - yl); 
        } 

        //生成随机数,并将随机数字转换为字母 
        String sRand=""; 
        for (int i=0;i<6;i++) 
        { 
          int itmp = random.nextInt(26) + 65; 
          char ctmp = (char)itmp; 
          sRand += String.valueOf(ctmp); 
          g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110))); 
          g.drawString(String.valueOf(ctmp),15*i+10,16); 
        } 

        HttpSession session = request.getSession(true); 
        session.setAttribute("rand",sRand); 
        g.dispose(); 
        ImageIO.write(image, "JPEG", response.getOutputStream()); 
      } 
      public void destroy() 
      { 
      } 
}

  //servlet名字和位置
  <servlet>
    <servlet-name>AuthImage</servlet-name>
    <servlet-class>com.bjpowernode.drp.util.servlet.AuthImage</servlet-class>
  </servlet>
  //servlet 对那些映射起作用
  <servlet-mapping>
    <servlet-name>AuthImage</servlet-name>
    <url-pattern>/servlet/login/AuthImage</url-pattern>
  </servlet-mapping>

<img id="img" style="cursor:pointer;" alt="验证码" src="${pageContext.request.contextPath}/servlet/login/AuthImage" onclick="changeImg()">

//点击验证码变换图片
function changeImg(){
  var img =document.getElementById("img");
  img.src="${pageContext.request.contextPath}/servlet/login/AuthImage?d="+Math.random();
  return false;
}