用ajax 做个二级联动的菜单，改变第一个下拉框中的值，第二个下拉框跟着改变，前三次请求能正确完成，三次以上则不行，在网上搜的答案都试过了，附jsp和servlet在下面。
package com.pynon.crawler;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.pynon.base.util.MysqlConnections;
public class subtype extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setCharacterEncoding("UTF-8");
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
PrintWriter out = response.getWriter();
//String supertype = new String(request.getParameter("supertype").getBytes("ISO-8859-1"),"gbk");
String supertype=request.getParameter("supertype");
// System.out.println(supertype);
List list = new ArrayList();
Connection conn = MysqlConnections.getConnection();
if (conn != null) {
Statement stmt = null;
ResultSet rs = null;
try {
stmt = conn.createStatement();
rs = stmt
.executeQuery("select id,classtype from dt_talknet where typename='"
+ supertype + "';");
while (rs.next()) {
Map map = new HashMap();
map.put("id", rs.getInt(1));
map.put("classtype", rs.getString(2));
list.add(map);
// System.out.println(rs.getInt(1) + " " +
// rs.getString(2));
}
} catch (SQLException e) {
MysqlConnections.freeConnection(conn);
e.printStackTrace();
} finally {
try {
if (rs != null) {
rs.close();
rs = null;
}
if (stmt != null) {
stmt.close();
stmt = null;
}
} catch (SQLException e1) {
e1.getMessage();
}
}
}
if (list.size() > 0) {
out.print("<response>");
for (int i = 0; i < list.size(); i++) {
Map map = (Map) list.get(i);
out.print("<subtypeids>" + map.get("id") + "</subtypeids>");
out.print("<subtypenames>" + map.get("classtype")
+ "</subtypenames>");
}
out.print("</response>");
}
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
<body>
<fieldset><script type="text/javascript">
var XMLHttpReq;
//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) {
}
}
}
}
function go4()
{
createXMLHttpRequest();
var supertype = document.getElementById("supertype").value;//获取选中的大类别.
var url = "/pynon/subtype?supertype="+supertype+"¶m="+ new Date().getTime();
XMLHttpReq.onreadystatechange = processResponse;//指定响应函数
XMLHttpReq.open("POST",url,true);
XMLHttpReq.setRequestHeader("If-Modified-Since","0");
XMLHttpReq.setRequestHeader("Cache-Control","no-cache");
XMLHttpReq.send(null);
}
function processResponse() {
if (XMLHttpReq.readyState == 4) { // 判断对象状态
if (XMLHttpReq.status == 200 || XMLHttpReq.status==0) { // 信息已经成功返回，开始处理信息
updateList();
} else { //页面不正常
window.alert("您所请求的页面有异常。");
}
}
}
function updateList()
{
clearList();
var subtype = document.getElementById("subtype");//小类别的控件对象
var subtypeids = XMLHttpReq.responseXML.getElementsByTagName("subtypeids");
var subtypenames = XMLHttpReq.responseXML.getElementsByTagName("subtypenames");
var option=null;
for(var i=0;i<subtypeids.length;i++)
{
option = document.createElement("option");
option.value = subtypeids[i].firstChild.nodeValue;
option.appendChild(document.createTextNode(subtypenames[i].firstChild.nodeValue));
subtype.appendChild(option);
}
}
function clearList() {
var subtype = document.getElementById("subtype");
while(subtype.childNodes.length > 0) {
subtype.removeChild(subtype.childNodes[0]);
}
}
</script>
<div>分析模板：<select id="supertype" onChange="go4()"
style="width: 100px">
<option>请选择类别</option>
<%
Connection cont = MysqlConnections.getConnection();
if (cont != null) {
Statement stmt = null;
ResultSet rs = null;
try {
stmt = cont.createStatement();
rs = stmt
.executeQuery("select distinct typename from dt_talknet");
while (rs.next()) {
%>
<option value="<%=rs.getString("typename")%>"><%=rs.getString("typename")%></option>
<%
}
%>
</select> <%
if (rs != null) {
rs.close();
rs = null;
}
stmt.close();
stmt = null;
MysqlConnections.freeConnection(cont);
} catch (SQLException e) {
MysqlConnections.freeConnection(cont);
e.printStackTrace();
} finally {
try {
if (rs != null) {
rs.close();
rs = null;
}
if (stmt != null) {
stmt.close();
stmt = null;
}
} catch (SQLException e1) {
e1.getMessage();
}
}
}
%>
<select style="width: 200px" id="subtype" name="subtype">
<option value="0">==请选择==</option>
</select>
</div>
<br>
<br>
<br>
</fieldset>
</body>
问题补充：
lovewhzlq 那个我现在加了，这个问题到底是为什么呀，帮我看看呀！谢谢啦
问题补充：
三次都正常执行了，按第四次的时候二级类别的就不动了，还是显示的第三次出来的子类别，如果把浏览器关了，重新来，又是只能执行三次
