package com;

 import java.awt.Color;

 import java.awt.Font;

 import java.awt.Graphics2D;

 import java.awt.image.BufferedImage;

 import java.io.IOException;

 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 IdentityServlet extends HttpServlet {

     public static final char[] CHARS = { '2', '3', '4', '5', '6', '7', '8',

             '9', '0', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K',

             'L', 'M', 'N', 'O', 'P', 'K', 'R', 'S', 'T', 'U', 'V', 'W', 'X',

             'Y', 'Z' };

     public static Random random = new Random();

     public static String getRandomString() {// 随机获取6位

         StringBuffer buffer = new StringBuffer();

         for (int i = 0; i < 6; i++) {

             buffer.append(CHARS[random.nextInt(CHARS.length)]);// 随机获取一个字符

         }

         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) {// 取反色

         return new Color(255 - c.getRed(), 255 - c.getGreen(),

                 255 - c.getBlue());

     }

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("image/jpeg");

         String randomString = getRandomString();

         request.getSession(true).setAttribute("randomString", randomString);

         int width = 100;

         int height = 30;

         Color color = getRandomColor();

         Color reverseColor = getReverseColor(color);

         BufferedImage bImage = new BufferedImage(width, height,

                 BufferedImage.TYPE_INT_RGB);

         Graphics2D graphics2d = bImage.createGraphics();

         graphics2d.setFont(new Font(Font.SANS_SERIF, Font.BOLD, 16));

         graphics2d.setColor(color);

         graphics2d.fillRect(0, 0, width, height);

         graphics2d.setColor(reverseColor);

         graphics2d.drawString(randomString, 18, 20);

         for (int i = 0, n = random.nextInt(100); i < n; i++) {

             graphics2d.drawRect(random.nextInt(width), random.nextInt(height),

                     1, 1);

         }

         ServletOutputStream outputStream = response.getOutputStream();

         JPEGImageEncoder coder = JPEGCodec.createJPEGEncoder(outputStream);

         coder.encode(bImage);

         outputStream.flush();

     }

 }

需要注意的是在myEclipse中处理图片，需要引入两个包：
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;
报错:
Access restriction: The type JPEGImageEncoder is not accessible due to
restriction on required library C:\Java\jre1.6.0_07\lib\rt.jar

此时解决办法：
MyEclipse默认把这些受访问限制的API设成了ERROR。只要把Windows-Preferences-Java-Complicer-Errors/Warnings里面的Deprecated
 and restricted API中的Forbidden references(access rules)选为Warning就可以编译通过。

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<%

String path = request.getContextPath();

String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

  <head>

    <base href="<%=basePath%>">

    <title>My JSP 'index.jsp' starting page</title>

    <meta http-equiv="pragma" content="no-cache">

    <meta http-equiv="cache-control" content="no-cache">

    <meta http-equiv="expires" content="0">

    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">

    <meta http-equiv="description" content="This is my page">

    <!--

    <link rel="stylesheet" type="text/css" href="styles.css">

    -->

  </head>

  <body>

    <img src = "servlet/IdentityServlet" id = "identity" onload = "btn.disabled=false;"/>

    <input type = button value="next" onclick="reloadImage()" id = "btn"/>

  </body>

    <script>

        function reloadImage(){

            document.getElementById('btn').disabled = true;

            document.getElementById('identity').src = 'servlet/IdentityServlet?ts='+new Date().getTime();

        }

    </script>

</html>
<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">

  <display-name>servlet</display-name>

  <servlet>

    <description>This is the description of my J2EE component</description>

    <display-name>This is the display name of my J2EE component</display-name>

    <servlet-name>IdentityServlet</servlet-name>

    <servlet-class>com.IdentityServlet</servlet-class>

  </servlet>

  <servlet-mapping>

    <servlet-name>IdentityServlet</servlet-name>

    <url-pattern>/servlet/IdentityServlet</url-pattern>

  </servlet-mapping>

  <welcome-file-list>

    <welcome-file>index.html</welcome-file>

    <welcome-file>index.htm</welcome-file>

    <welcome-file>index.jsp</welcome-file>

    <welcome-file>default.html</welcome-file>

    <welcome-file>default.htm</welcome-file>

    <welcome-file>default.jsp</welcome-file>

  </welcome-file-list>

</web-app>

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