在填写好数据后点击注册按钮没有反应，一直在转圈，主要在学后端，对前端的也不太了解，想问一下一般这是什么原因？
代码：
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page isELIgnored="false" %>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>欢迎注册</title>
<link href="css/register.css" rel="stylesheet">
</head>
<body>
<div class="form-div">
<div class="reg-content">
<h1>欢迎注册</h1>
<span>已有帐号？</span> <a href="login.jsp">登录</a>
</div>
<form id="reg-form" action="/cookie_demo/registerService" method="post">
<table>
<tr>
<td>用户名</td>
<td class="inputs">
<input name="username" type="text" id="username">
<br>
<span id="username_err" class="err_msg">${fail}</span>
</td>
</tr>
<tr>
<td>密码</td>
<td class="inputs">
<input name="password" type="password" id="password">
<br>
<span id="password_err" class="err_msg" style="display: none">密码格式有误</span>
</td>
</tr>
<tr>
<td>验证码</td>
<td class="inputs">
<input name="checkCode" type="text" id="checkCode">
<img id="checkCodeImag" src="/cookie_demo/checkCodeServlet">
<a href="#" id="changeImg">看不清？</a>
</td>
</tr>
</table>
<div class="buttons">
<input value="注 册" type="submit" id="reg_btn">
</div>
<br class="clear">
</form>
</div>
<script>
document.getElementById("changeImg").onclick=function () {
document.getElementById("checkCodeImag").src="/cookie_demo/checkCodeServlet?"+new Date().getMilliseconds();
}
</script>
</body>
</html>
结构：
