用kaptcha生成的图片显示的验证码和实际获取到的验证码不一致

如图所示，图片里显示的是gaey。

可是我的程序输出的却不是这样的，我的程序输出如下：

上图中前者是我input输入框的值，而后者是通过下面这句程序获得的值：

String kaptchaExpected = (String)request.getSession().getAttribute(com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY);

我搞不清这是怎么一回事，希望有老师能帮忙分析分析。

下面上相关的代码：

web.xml文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns="http://xmlns.jcp.org/xml/ns/javaee"
	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
	id="WebApp_ID" version="4.0">

	<!-- 初始化InitServlet -->
	<servlet>
		<servlet-name>InitServlet</servlet-name>
		<servlet-class>com.imooc.servlet.InitServlet</servlet-class>
		<load-on-startup>2</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>InitServlet</servlet-name>
		<url-pattern>/InitServlet</url-pattern>
	</servlet-mapping>

	<!-- 字符编码过滤器 -->
	<filter>
		<filter-name>EncodeFilter</filter-name>
		<filter-class>com.imooc.filter.EncodeFilter</filter-class>
		<init-param>
			<param-name>charset</param-name>
			<param-value>UTF-8</param-value>
		</init-param>
	</filter>

	<filter-mapping>
		<filter-name>EncodeFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>

	<!-- 配置kaptcha -->
	<servlet>
		<servlet-name>Kaptcha</servlet-name>
		<servlet-class>com.google.code.kaptcha.servlet.KaptchaServlet</servlet-class>
		<init-param>
			<param-name>kaptcha.image.width</param-name>
			<param-value>120</param-value>
		</init-param>
		<init-param>
			<param-name>kaptcha.image.height</param-name>
			<param-value>50</param-value>
		</init-param>
		<init-param>
			<param-name>kaptcha.textproducer.font.size</param-name>
			<param-value>40</param-value>
		</init-param>
		<init-param>
			<param-name>kaptcha.textproducer.char.length</param-name>
			<param-value>4</param-value>
		</init-param>
	</servlet>
	
	<servlet-mapping>
		<servlet-name>Kaptcha</servlet-name>
		<url-pattern>/kaptcha.jpg</url-pattern>
	</servlet-mapping>
	
	<!-- 配置LoginServlet -->
	<servlet>
		<servlet-name>LoginServlet</servlet-name>
		<servlet-class>com.imooc.servlet.LoginServlet</servlet-class>
	</servlet>
	
	<servlet-mapping>
		<servlet-name>LoginServlet</servlet-name>
		<url-pattern>/LoginServlet</url-pattern>
	</servlet-mapping>

</web-app>

2.LoginServlet.java

package com.imooc.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.imooc.service.CourseServiceImpl;

/*
 * 控制登录的Servlet
 */
@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//校验验证码是否正确
		//从session中取出servlet生成的验证码text值
		String kaptchaExpected = (String)request.getSession().getAttribute(com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY);
		//获取用户页面输入的验证码
		String kaptchaReceived = request.getParameter("checkCode");
		System.out.println(kaptchaReceived + "===" + kaptchaExpected);
		if (kaptchaReceived == null || !kaptchaReceived.equalsIgnoreCase(kaptchaExpected)){
			//如果验证失败，弹出弹窗提示：验证码错误
			response.setContentType("text/html;charset=gb2312");
			PrintWriter out = response.getWriter();
			out.print("<script language='javascript'>alert('验证码错误');window.location.href='index.jsp';</script>");
			return;
		}
		//获取到登录页面提交的用户名和密码信息
		String username = request.getParameter("username");
		String password = request.getParameter("password");
		//判断数据库中是否存在用户信息
		CourseServiceImpl courseServiceImpl = (CourseServiceImpl) this.getServletContext().getAttribute("courseServiceImpl");
		if(courseServiceImpl.login(username, password)) {
			//如果登录成功则在session中设置loginUser属性，并重定向到server.jsp页面
			HttpSession session = request.getSession();
			session.setAttribute("loginUser", username);
			response.sendRedirect(request.getContextPath() + "/server.jsp");
		}else {
			//如果登录失败，弹出弹窗提示：用户名或密码错误
			response.setContentType("text/html;charset=gb2312");
			PrintWriter out = response.getWriter();
			out.print("<script language='javascript' charset='utf-8'>alert('用户名或密码错误');window.location.href='index.jsp';</script>");
		}
	}

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}

}

3.index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
	pageEncoding="UTF-8"%>
<%
	String basePath = request.getScheme() + ":" + "//" + request.getServerName() + ":" + request.getServerPort()
			+ request.getServletContext().getContextPath();
%>
<!DOCTYPE html>
<html>
<head>
<base href="<%=basePath%>">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>登录页面</title>
<style type="text/css">
.code {
	cursor: pointer;
}

a {
	text-decoration: none;
	font-size: 12px;
	color: #288bc4;
}

a:hover {
	text-decoration: underline;
}
</style>
<script>
	//刷新验证码
	function changeCode() {
		var inputCode = document.getElementById("inputCode").value;
		document.getElementById("code").src = "http://localhost:8080/class_management/kaptcha.jpg?d=" + new Date().getTime();
	}
</script>
</head>
<body>
	<center>
		<h1>用户登录</h1>
		<form action="<%=basePath%>/LoginServlet" method="post" onsubmit="return validateCode()">
			<table width="400px" height="200px" cellspacing="0px" cellpadding="0px" border="1px">
				<tr>
					<td>用户名</td>
					<td colspan="2">
						<input type="text" name="username" placeholder="用户名为3-12位字母数字或下划线组合"
								pattern="[a-zA-Z_0-9]{3,12}" required="required" >
					</td>
				</tr>
				<tr>
					<td>密&nbsp;码</td>
					<td colspan="2">
						<input type="password" name="password" placeholder="长度为5-12位字母数字或下划线组合" 
								pattern="[a-zA-Z_0-9]{5,12}" required="required">
					</td>
				</tr>
				<tr>
					<td>验证码</td>
					<td style="border-right-style: none;">
						<input type="text" name="checkCode" placeholder="请输入验证码" id="inputCode">
					</td>
					<td style="border-left-style: none;">
						<div class="code" id="checkCode">
							<img src="http://localhost:8080/class_management/kaptcha.jpg" id="code" onclick="changeCode()" />
						</div>
					</td>
				</tr>
				<tr>
					<td colspan="3" style="text-align: center">
					<input type="submit" value="登录"> <input type="reset" value="取消">
					</td>
				</tr>
			</table>
		</form>
	</center>
</body>
</html>

慕数据4154996 2018-09-03

源自：项目：商品展示及购物车 5-3 作业题