response生成图片验证码

新建一个java web工程

src 目录下xieyuan包MyServlet.java文件（Servlet文件）

package xieyuan;  

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.net.URLEncoder;
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.corba.se.impl.javax.rmi.CORBA.Util;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;  

public class MyServlet extends HttpServlet {  

    /**
     * Constructor of the object.
     */
    public MyServlet() {
        super();
    }  

    /**
     * Destruction of the servlet. <br>
     */
    public void destroy() {
        super.destroy(); // Just puts "destroy" string in log
        // Put your code here
    }  

    /**
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     *
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        execute(request, response);
    }  

    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     *
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        execute(request, response);
    }  

    private 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();
    //生成随机数字，len为需要随机数字的个数
    public static String getRandomString(int len)
    {
        StringBuilder builder=new StringBuilder();
        for(int i=0;i<len;i++)
        {
            builder.append(CHARS[random.nextInt(CHARS.length)]) ;
        }
        return builder.toString();
    }
    //随机生成颜色，座位背景色
    public static Color getColor()
    {
        return new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));
    }
    //取颜色的反色
    public static Color getReverseColor(Color color)
    {
        return new Color(255-color.getRed(),255-color.getGreen(),255-color.getBlue());
    }  

    private void execute(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
    {
        response.setCharacterEncoding("utf-8");
        //设置返回的文件编码
        response.setContentType("image/jpeg");  

        //获取随机码
        String getRandomCode=getRandomString(5);
        //将随机码放到Session中
        request.getSession().setAttribute("randomcode", getRandomCode);
        int width=100;
        int height=30;
        Color color=getColor();
        Color reverseColor=getReverseColor(color);
        //创建一个彩色图片
        BufferedImage bi=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
        Graphics2D g=bi.createGraphics();
        g.setFont(new Font(null,Font.BOLD,16));
        g.setColor(color);
        g.fillRect(0,0,width,height);
        g.setColor(reverseColor);
        g.drawString(getRandomCode, 18,20);
        //绘制噪点,最多100个
        for(int i=0,n=random.nextInt(100);i<n;i++)
        {
            g.drawRect(random.nextInt(width), random.nextInt(height), 1,1);
        }
        ServletOutputStream out=response.getOutputStream();
        JPEGImageEncoder encoder=JPEGCodec.createJPEGEncoder(out);
        encoder.encode(bi);
        out.flush();
    }  

    /**
     * Initialization of the servlet. <br>
     *
     * @throws ServletException if an error occurs
     */
    public void init() throws ServletException {
    }  

}  

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

<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>MyServlet</servlet-name>
    <servlet-class>xieyuan.MyServlet</servlet-class>
  </servlet>  

  <servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/servlet/MyServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

index.jsp

<%@ 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">
    -->
    <script language="JavaScript" >
        function reloadImage()
        {
           <!--将按钮状态设置为不可用，当图片加载完成触发onload后，按钮状态就为可用了。这样可用避免重复获取-->
           document.getElementById("btn").disabled=true;
           <!--第一次连接的时候不会有问题，第二次连接时，假如你后面没有new Date().getTime(),加参数就会连接的时候拿缓存，没有连到服务器。加上时间函数就能保证你每次得到的不是浏览器的缓存。-->
           document.getElementById("img").src="servlet/MyServlet?timestamp="+new Date().getTime();
         }
    </script>
  </head>  

  <body>
     <img src="servlet/MyServlet" id="img" onload="btn.disabled=false;" /><br/><br/>
     <input type="button" value="换一张图片" onClick="reloadImage()" id="btn" /><br/>
     <script>document.write("页面最后更新："+document.lastModified)</script>
  </body>
</html>

　　将java web放在服务器上启动，最后访问http://localhost:8088/firstWeb/，展示效果如下：

注：参考http://www.2cto.com/kf/201309/241744.html