新手求教，在线等啊

<%@ page language="java" import="java.util.*" pageEncoding="gbk"%>
<%@ taglib prefix="s" uri="/struts-tags" %>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'register2.jsp' starting page</title>
    
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">    
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->


    <script type="text/javascript">
    
 function validate(}
 {
  var usernameValue = document.getElementById("usernameId").value;
  var passwordValue = document.getElementById("passwordId").value;
  var repasswordValue = document.getElementById("repasswordId").value;  
  if(usernameValue.length == 0)
  {
     alert("用户名不能为空");
     return false;
  }
   if(passwordValue.length == 0)
  {
     alert("密码不能为空");
     return false;
  }
   if(repasswordValue.length == 0)
  {
     alert("再次输入密码不能为空");
     return false;
  }
   if（passwordValue != repasswordValue）
  {
     alert("两次输入不一致");
     return false;
  }  return true;
 
 }    </script>
  </head>
  
  <body>
    <table align="center" width="20%" >
  <tr>
  <td >
  <s:fielderror cssStyle="color:red"/>
  </td>
  </tr>
  </table>
  
  
    <s:form action="register" theme="simple" onsubmit="return validate();">
    <table align="center" width="20%" border="1">
    <tr>
    <td>
    username:
    </td>
    <td>
    <s:textfield name="username" label="username" id="usernameId"></s:textfield>
    </td>
    </tr>
    <tr>
    <td>
    password:
    </td>
    <td>
    <s:password name="password" label="password" id="passwordId"></s:password>
    </td>
    </tr>
    <tr>
    <td>
    repassword:
    </td>
    <td>
    <s:password name="repassword" label="repassword" id="repasswordId"></s:password>
    </td>
    </tr>
    <tr>
    <td>
    age:
    </td>
    <td>
    <s:textfield name="age" label="age" id="ageId"></s:textfield>
    </td>
    </tr>
    <tr>
    <td>
    birthday:
    </td>
    <td>
    <s:textfield name="birthday" label="birthday" id="birthdayId"></s:textfield>
    </td>
    </tr>
    <tr>
    <td>
    graduation:
    </td>
    <td>
    <s:textfield name="graduation" label="graduation" id="graduationId"></s:textfield>
    </td>
    </tr>
    <tr>
    <td>
    <s:submit label="submit"></s:submit>
    </td>
    <td>
    <s:reset label="reset"></s:reset>
    </td>
    </tr>
    </table>
    </s:form>
  </body>
</html>
我这样写，为什么运行后页面不弹出错误信息，比如用户名不填，也不弹出用户名不能为空