JSP Ajax 级联问题，请求帮助

报错是：
严重: Servlet.service() for servlet [jsp] in context with path [/menu] threw exception [java.lang.IllegalStateException: Exception occurred when flushing data] with root cause
java.io.IOException: Stream closed
at org.apache.jasper.runtime.JspWriterImpl.ensureOpen(JspWriterImpl.java:210)
at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:115)
at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:190)
at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:123)
at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:80)
at org.apache.jsp.menu2_jsp._jspService(menu2_jsp.java:103)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:563)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:399)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:317)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:204)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:311)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
我是用menu1.jsp里调用menu2.jsp
menu1.jsp中javascript代码时如下：
<script language="javascript">
    var XMLHttpReq;
    var currentSort;
     //创建XMLHttpRequest对象      
    function createXMLHttpRequest() {
        if(window.XMLHttpRequest) { //Mozilla 浏览器
            XMLHttpReq = new XMLHttpRequest();
        }
        else if (window.ActiveXObject) { // IE浏览器
            try {
                XMLHttpReq = new ActiveXObject("Msxml2.XMLHTTP");
            } catch (e) {
                try {
                    XMLHttpReq = new ActiveXObject("Microsoft.XMLHTTP");
                } catch (e) {}
            }
        }
    }
    //发送请求函数
    function sendRequest(url) {
        createXMLHttpRequest();
        XMLHttpReq.open("GET", url, true);
        XMLHttpReq.onreadystatechange = processResponse;//指定响应函数
        XMLHttpReq.send(null);  // 发送请求
    }
    // 处理返回信息函数
    function processResponse() {
        if (XMLHttpReq.readyState == 4) { // 判断对象状态
            if (XMLHttpReq.status == 200) { // 信息已经成功返回，开始处理信息
                updateMenu();
            } else { //页面不正常
                  alert("您所请求的页面有异常。");
            }
        }
    }
    //更新菜单函数
    function updateMenu() {
        //alert(1);
        var res=XMLHttpReq.responseXML.getElementsByTagName_r("res");
       alert("启动了更行操作");
        //var subMenu = "";
        //for(var i = 0; i < res.length; i++) {
        //    subMenu = subMenu + "  " + res[i].firstChild.data + "";
        //}
        //currentSort.innerHTML = subMenu;
        //var list = document.all.list;
        //List list = new ArrayList();
        var list = document.getElementsByName("list").list;
        list.options.length=0;
        //alert(1);
        list.options[list.options.length] = new Option("---请选择---","");
        //list.add(new Option("---请选择---",""));
        for(var i=0;i<res.length;i++){
            list.options[list.options.length]=new Option(res[i].firstChild.data,res[i].firstChild.data);
            alert(res[i].firstChild.data);
        }
    }
    // 创建级联菜单函数
    function showSubMenu(obj) {
        //alert(obj);
        //currentSort =document.getElementByIdx(obj);
        //currentSort.parentNode.style.display = "";
        sendRequest("menu2.jsp?sort=" + obj);
       
        //document.all.mli.options[0].selected=true;
    }</script>menu2.jsp的代码如下：<%@ page contentType="text/html; charset=utf-8" import="java.util.*,java.sql.*,dao.DB_conn" %>
<%
    String sort=request.getParameter("sort");
    System.out.println("已经进入menu2："+sort);
    Connection conn = null;
    Statement st = null;
    ResultSet rs = null;
    List cities = new ArrayList();    try {
        conn = DB_conn.getConnection();
        st=conn.createStatement();
        rs=st.executeQuery("SELECT * FROM city where proid='"+sort+"'");
        System.out.println("已经执行SQL语句：");
        while (rs.next())
        {
            cities.add(rs.getString(2));
            System.out.println("添加了一条语句："+rs.getString(2));
        }
        rs.close();
        st.close();
        conn.close();
    }  catch (Exception e)  {  
        e.printStackTrace();
    }
    response.setContentType("text/xml; charset=UTF-8");
    response.setHeader("Cache-Control", "no-cache");
    out.println("<response>");
    for(int i=0;i<cities.size();i++)
    {
        out.println("<res>" + cities.get(i).toString() + "</res>");
        System.out.println(cities.get(i).toString());
    }
    out.println("</response>");
    out.close();
%>这个该如何处理，请求大神帮助。