JSP与JS交互问题 值传递

<script language="javascript">
<!--
var str="123";
<%
String aa;
%>
-->
</script>

<script language="javascript">
function returnValue(){
var myform=document.form1;
var str=myform.text1.value;
<%
String aa;
//怎么让这个aa 等于js中的str???
%>
}
</script>
<form name="form1">
<input type="text" name="text1" onchange="returnValue();">
</form>

<form name="form1">
<input type="hidden" name="hide">
</form>
script中定义var str=“123”;
document.form1.hide.value=str;