变量值从javascript传递把JSP

我有以下情况

<select name="dropdown_Source" onchange="call()">
    <% for (int i=0 ; i < a1.length; i  ) { if (a1[i]==n ull) break; %>
        <option value="<%=a1[i]%>">
            <%=a1[i]%>
        </option>
    <% } %>
</select>

<script>
    function call() {
        var source = document.forms[0].dropdown_source.value;
        // Now this 'source' value i have to pass to jsp function 
        <%
            Admin a = new Admin(); //this is class defined
            a.getResult(source); //but source is not resolved here
        %>
    }
</script>

我应该如何传入此来源值JSP功能？
a.getResult（） - 函数将返回下拉元素列表，并列出
我必须填入另一个 < option> tag。

How this source value should I pass in JSP function? a.getResult() - function will return me list of dropdown elements and that list I have to populate in another <option> tag.