验证码一致提示出错

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<style>#code{height:30px;}</style>
</head>
<body>
<form action="submit.action">
    <p>验证码：<input type="text" name="kaptcha" id="code" maxlength="4" placeholder="请输入验证码" />
     <img src="http://localhost:8080/code/kaptcha.jpg" id="checkcode" />
    </p>
    <p><input type="button" value="登陆" id="login" ></p>
    <div id="result"></div>
</form>
<script src="http://localhost:8080/code/js/jquery-1.12.4.min.js"></script>
<script>
$(function(){
    $("#changecode").on("click",function(){
        $(this).attr("src","http://localhost:8080/code/kaptcha.jpg?d="+new Date().getTime());
    });
     
    //登陆按钮绑定点击事件
    $("#login").on("click",function(){
        var code=$("#code").val();
        alert(code);
        var params={"code":code};
        $.post("http://localhost:8080/code/loginServlet",params,function(data){
            if(data=="success"){
                $("#result").html("验证码输入正确");
            }else{
                $("#result").html("验证码输入不正确");
                $("#code").val("").focus();
                 
            }
        })
    })
});
</script>
</body>
</html>

package com.imooc;
 
import java.io.IOException;
import java.io.PrintWriter;
 
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
import org.eclipse.jdt.internal.compiler.impl.Constant;
 
import com.google.code.kaptcha.Constants;
 
/**
 * Servlet implementation class loginServlet
 */
@WebServlet("/loginServlet")
public class loginServlet extends HttpServlet {
     
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //设置编码
        request.setCharacterEncoding("UTF-8");
        response.setCharacterEncoding("UTF-8");
        //获取浏览器输出流对象
        PrintWriter out=response.getWriter();
        //获取用户传递过来的验证码,图片是后台产生输送给客户端的
        String code=request.getParameter("code");
        //获取验证码框产生的验证码
        String sessionCode=(String)request.getSession().getAttribute(Constants.KAPTCHA_SESSION_KEY);
         
        out.println("sessionCode："+sessionCode);
         
        if(code!=null&&sessionCode!=null) {
             
            if(code.equalsIgnoreCase(sessionCode)) {
                out.print("success");
            }else {
                out.println("fail");
            }
        }else {
            out.println("fail");
        }
         
        out.flush();
        out.close();
    }
 
 
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }
 
}

<?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" 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>code</display-name>
 <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>200</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.char.length</param-name>
        <param-value>4</param-value>
       
    </init-param>
    <init-param>
        <param-name>kaptcha.noise.impl</param-name>
        <param-value>com.google.code.kaptcha.impl.NoNoise</param-value>
       
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>Kaptcha</servlet-name>
    <url-pattern>/kaptcha.jpg</url-pattern>
</servlet-mapping>
</web-app>