用通过severlet配置的验证图显示不出来，而用JSP做的图没问题，估计是WEB.XML配置有问题，求教高手，
是不是severlet请求被其他Filter拦截了？
生成验证图片的JAVA类
Java code
package com.dingding.web;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.awt.*;
import java.awt.image.*;
import java.util.*;
import javax.imageio.*;
/**
* Description:
* <br/>Copyright (C), 2001-2010, Yeeku.H.Lee
* <br/>This program is protected by copyright laws.
* <br/>Program Name:
* <br/>Date:
* @author Yeeku.H.Lee kongyeeku@163.com
* @version 1.0
*/
public class AuthImg extends HttpServlet
{
//定义图形验证码中绘制字符的字体
private final Font mFont =
new Font("Arial Black", Font.PLAIN, 16);
//定义图形验证码的大小
private final int IMG_WIDTH = 100;
private final int IMG_HEIGTH = 18;
//定义一个获取随机颜色的方法
private Color getRandColor(int fc,int bc)
{
Random random = new Random();
if(fc > 255) fc = 255;
if(bc > 255) bc=255;
