第一步：建一个动态工程check,一个check.java(servlet类)，一个login.jsp文件
第二步：编写check.java文件，内容为：
01.package check;
02.import javax.servlet.ServletException;
03.import javax.servlet.http.*;
04.import java.io.*;
05.import java.awt.*;
06.import java.awt.image.*;
07.import java.util.*;
08.import javax.imageio.*;
09.
10./**
11. * @author yeeku.H.lee kongyeeku@163.com
12. * @version 1.0
13. * <br>Copyright (C), 2005-2008, yeeku.H.Lee
14. * <br>This program is protected by copyright laws.
15. * <br>Program Name:
16. * <br>Date:
17. */
18.public class check extends HttpServlet
19.{
20. private Font mFont = new Font("Arial Black", Font.PLAIN, 16);
21. public void init() throws ServletException
22. {
23. super.init();
24. }
25. Color getRandColor(int fc,int bc)
26. {
27. Random random = new Random();
28. if(fc>255) fc=255;
29. if(bc>255) bc=255;
30. int r=fc+random.nextInt(bc-fc);
31. int g=fc+random.nextInt(bc-fc);
32. int b=fc+random.nextInt(bc-fc);
33. return new Color(r,g,b);
34. }
35.
36. public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
37. {
38. response.setHeader("Pragma","No-cache");
39. response.setHeader("Cache-Control","no-cache");
40. response.setDateHeader("Expires", 0);
41. response.setContentType("image/jpeg");
42.
43. int width=100, height=18;
44. BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
45.
46. Graphics g = image.getGraphics();
47. Random random = new Random();
48. g.setColor(getRandColor(200,250));
49. g.fillRect(1, 1, width-1, height-1);
50. g.setColor(new Color(102,102,102));
51. g.drawRect(0, 0, width-1, height-1);
52. g.setFont(mFont);
53.
54. g.setColor(getRandColor(160,200));
55. for (int i=0;i<155;i++)
56. {
57. int x = random.nextInt(width - 1);
58. int y = random.nextInt(height - 1);
59. int xl = random.nextInt(6) + 1;
60. int yl = random.nextInt(12) + 1;
61. g.drawLine(x,y,x + xl,y + yl);
62. }
63. for (int i = 0;i < 70;i++)
64. {
65. int x = random.nextInt(width - 1);
66. int y = random.nextInt(height - 1);
67. int xl = random.nextInt(12) + 1;
68. int yl = random.nextInt(6) + 1;
69. g.drawLine(x,y,x - xl,y - yl);
70. }
71.
72. String sRand="";
73. for (int i=0;i<6;i++)
74. {
75. String tmp = getRandomChar();
76. sRand += tmp;
77. g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));
78. g.drawString(tmp,15*i+10,15);
79. }
80.
81. HttpSession session = request.getSession(true);
82. session.setAttribute("rand",sRand);
83. g.dispose();
84. ImageIO.write(image, "JPEG", response.getOutputStream());
85. }
86. private String getRandomChar()
87. {
88. int rand = (int)Math.round(Math.random() * 2);
89. long itmp = 0;
90. char ctmp = '\u0000';
91. switch (rand)
92. {
93. case 1:
94. itmp = Math.round(Math.random() * 25 + 65);
95. ctmp = (char)itmp;
96. return String.valueOf(ctmp);
97. case 2:
98. itmp = Math.round(Math.random() * 25 + 97);
99. ctmp = (char)itmp;
100. return String.valueOf(ctmp);
101. default :
102. itmp = Math.round(Math.random() * 9);
103. return String.valueOf(itmp);
104. }
105. }
106.}
第三步：编写login.jsp文件，内容为：
01.<%@ page language="java" import="java.util.*" pageEncoding="GB18030"%>
02.<%
03.String path = request.getContextPath();
04.String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
05.%>
06.
07.<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
08.<html>
09. <head>
10. <base href="<%=basePath%>">
11.
12. <title>My JSP 'index.jsp' starting page</title>
13. <meta http-equiv="pragma" content="no-cache">
14. <meta http-equiv="cache-control" content="no-cache">
15. <meta http-equiv="expires" content="0">
16. <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
17. <meta http-equiv="description" content="This is my page">
18. <!--
19. <link rel="stylesheet" type="text/css" href="styles.css">
20. -->
21. <script>
22. function refresh()
23. {
24. document.getElementById("authImg").src='check?now=' + new Date();
25. }
26. </script>
27. </head>
28.
29. <body>
30. <%
31.
32.
33. %>
34. 验证码如图：<img src="check" id="authImg"/>看不清？<a href="#" onClick="refresh()">单击此处刷新</a>
35. </body>
36.</html>
第四步：在web.xml文件中配置代码，内容为：
01.<servlet>
02. <servlet-name>img</servlet-name>
03. <servlet-class>check.check</servlet-class>
04. </servlet>
05.
06. <servlet-mapping>
07. <servlet-name>img</servlet-name>
08. <url-pattern>/check</url-pattern>
09. </servlet-mapping>
第五步：在tomcat服务器上发布项目check,运行login.jsp,运行结果为：
