參考了幾篇文章，寫了個用servlet生產唯一驗證碼(改造了原作者代碼，在此致謝，后附鏈接)。附件是可直接使用的工程。明天試用一個開源項目，可以比較方便的生成各種風格的驗證碼，甚至還有聲音，比較有意思。第一次記錄自己的學習，以后爭取每天寫一點。
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package com.stone.study.validate;

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

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class ImgValidateServlet2 extends HttpServlet {
	private static int WIDTH = 90;
    private static int HEIGHT = 20;
    private static int LENGTH = 6;

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		 HttpSession session = request.getSession();

         response.setContentType("image/jpeg");

         ServletOutputStream sos = response.getOutputStream();

         //設置瀏覽器不要緩存此圖片
         response.setHeader("Pragma","No-cache");
         response.setHeader("Cache-Control","no-cache");
         response.setDateHeader("Expires", 0);

         //創建內存圖象并獲得其圖形上下文
         BufferedImage image = 
                new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB); 

         Graphics g = image.getGraphics();

         //產生隨機的認證碼
         char [] rands = generateCheckCode();

         //產生圖像
         drawBackground(g);

         drawRands(g,rands);

         //結束圖像的繪制過程，完成圖像
         g.dispose();

         //將圖像輸出到客戶端
         ByteArrayOutputStream bos = new ByteArrayOutputStream();

         ImageIO.write(image, "JPEG", bos);

         byte [] buf = bos.toByteArray();

         response.setContentLength(buf.length);

         //下面的語句也可寫成：bos.writeTo(sos);
         sos.write(buf);
         bos.close();
         sos.close();

         //將當前驗證碼存入到Session或者數據庫中,下一個表單處理中驗證客戶端提交的驗證碼是否正確
         session.setAttribute("check_code",new String(rands));
	}

	private static char [] generateCheckCode()
    {
       //定義驗證碼的字符表
       String chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

       char [] rands = new char[LENGTH];
       for(int i=0; i<LENGTH; i++)

       {
          int rand = (int)(Math.random() * 36);
          rands[i] = chars.charAt(rand);
       }

       return rands;
    }
	
	   private void drawRands(Graphics g , char [] rands) 
       {
          g.setColor(Color.BLACK);
          g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));

          //在不同的高度上輸出驗證碼的每個字符         
          g.drawString("" + rands[0],1,17);
          g.drawString("" + rands[1],16,15);
          g.drawString("" + rands[2],31,18);
          g.drawString("" + rands[3],46,16);
          g.drawString("" + rands[4],61,14);
          g.drawString("" + rands[5],76,19);

          System.out.println(rands);
       }

       private void drawBackground(Graphics g)

       {
           //畫背景
          g.setColor(new Color(0xDCDCDC));
          g.fillRect(0, 0, WIDTH, HEIGHT);

          //隨機產生120個干擾點
          for(int i=0; i<120; i++)

          {
             int x = (int)(Math.random() * WIDTH);

             int y = (int)(Math.random() * HEIGHT);

             int red = (int)(Math.random() * 255);

             int green = (int)(Math.random() * 255);

             int blue = (int)(Math.random() * 255);

             g.setColor(new Color(red,green,blue));        

             g.drawOval(x,y,1,0);

          }
          
          //加兩條干擾線
          g.drawLine(0, 5, 90, 5);
          g.drawLine(0, 15, 90, 15);

       }

       public static void main(String args[]) {
    	   char[] chars = generateCheckCode();

    	   System.out.println(chars);
       }
}


  

代碼出處:《深入體驗Java Web開發內幕——核心基礎》

servlet生產唯一圖片驗證碼