为啥验证码不出来，求指教

//ImageServlet
package com.muke;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ImageServlet  extends HttpServlet{

	public void doGet(HttpServletRequest request,HttpServletResponse response) throws IOException{
		
		BufferedImage bi= new BufferedImage(68,22,BufferedImage.TYPE_INT_RGB);
	    Graphics g=bi.getGraphics();
	    Color c = new Color(200,150,255);
	    g.setColor(c);
	    g.fillRect(0, 0, 68, 22);
	    
	    //验证码里面内容字母数字组合
	    char[] ch ="ABCDEFGHIGKLMNOPQRSTUVWXYZ0123456789".toCharArray();
		Random r =new Random();
		int len=ch.length,index;
		StringBuffer sb=new StringBuffer();
		for(int i=0;i<4;i++){
          index=r.nextInt(len);
        g.setColor(new Color(r.nextInt(88),r.nextInt(188),r.nextInt(255)));  //随机产生颜色
		g.drawString(ch[index]+"", (i*15)+3, 18);
		sb.append(ch[index]);
		}
		request.getSession().setAttribute("piccde", sb.toString());
		ImageIO.write(bi,"JPG",response.getOutputStream());
	}
}

//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>
	      <servlet-name>ImageServlet</servlet-name>
	      <servlet-class>com.muke.ImageServlet</servlet-class>
	</servlet>
	<servlet-mapping>
	        <servlet-name>ImageServlet</servlet-name>
	        <url-pattern>/servlet/ImageServlet</url-pattern>
	</servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>