Jquery通过ajax实现三级联动

jap部分

    <select id="sheng">
<option value="--">请选择</option>
<option value="hn">河南</option>
<option value="bj">北京</option>
</select>
<select id="city">
<option value="--">请选择</option> 
</select>
<select id="xian">
<option value="--">请选择</option> 
</select>

js部分

<script type="text/javascript">

function pp() {

var s = document.getElementById("city");
var index = document.getElementById("city").selectedIndex;

return s[index].value;
 }


 $("#sheng").change(
function() {
var val = $("#sheng").val();
var json = {
'sheng' : val
 };
var url = "${pageContext.request.contextPath}/CheckOut?id="
 + new Date().getTime();
 $.post(url, json,
function(response, status, xhr) {
var jsonObject = eval("(" + response + ")");
 document.getElementById("city").length = 1;
for ( var i = 0; i < jsonObject.length; i++) {
var option = document
 .createElement("option");
 option.value = jsonObject[i].key;
 option.innerHTML = jsonObject[i].value;

 document.getElementById("city")
 .appendChild(option);
 }

if (pp() == "--") {
 document.getElementById("xian").length = 1;
 }

 })
 })

//-----------------------------------------------------------------------

 $("#city").change(
function() {
var val = $("#city").val();
var json = {
'city' : val
 };
var url = "${pageContext.request.contextPath}/SanJi?id="
 + new Date().getTime();

 $.post(url, json,
function(response, status, xhr) {
var jsonObject = eval("(" + response + ")");

 document.getElementById("xian").length = 1;
for ( var i = 0; i < jsonObject.length; i++) {
var option = document
 .createElement("option");
 option.value = jsonObject[i].key;
 option.innerHTML = jsonObject[i].value;

 document.getElementById("xian")
 .appendChild(option);

 }

 })

 })
</script>

action部分

public class CheckOut extends ActionSupport {

private String sheng;
private List<City> list=new ArrayList<City>();
public List<City> getList() {
return list;
    }
public void setList(List<City> list) {
this.list = list;
    }
public String getSheng() {
return sheng;
    }
public void setSheng(String sheng) {
this.sheng = sheng;
    }

@Override
public String execute() throws Exception {
if("hn".equals(sheng)){
                    City c1=new City("zz","郑州");
                    City c2=new City("xx","新乡");
                    City c3=new City("kf","开封");
                    list.add(c1);
                    list.add(c2);
                    list.add(c3);
                }
if("bj".equals(sheng)){
                    City c1=new City("zy","朝阳");
                    City c2=new City("hd","海淀");
                    City c3=new City("cp","昌平");
                    list.add(c1);
                    list.add(c2);
                    list.add(c3);
                }

                JSONArray fromObject = JSONArray.fromObject(list);
                String string = fromObject.toString();
                HttpServletResponse response = ServletActionContext.getResponse();
                response.setContentType("text/html;charset=UTF-8");
                PrintWriter writer = response.getWriter();
                writer.write(string);
                writer.close();
return NONE;
            }
}

//-------------------------------------------


public class SanJi extends ActionSupport {
private String city;
private List<Xian> list=new ArrayList<Xian>();
public List<Xian> getList() {
return list;
    }
public void setList(List<Xian> list) {
this.list = list;
    }
public String getCity() {
return city;
    }
public void setCity(String city) {
this.city = city;
    }
@Override
public String execute() throws Exception {
if("zz".equals(city)){
                    Xian c1=new Xian("zy","中原区");
                    Xian c2=new Xian("eq","二七区");
                    Xian c3=new Xian("gx","高新区");
                    list.add(c1);
                    list.add(c2);
                    list.add(c3);
                }
if("xx".equals(city)){
                    Xian c1=new Xian("yy","原阳");
                    Xian c2=new Xian("sz","私宅");
                    Xian c3=new Xian("db","大宾");
                    list.add(c1);
                    list.add(c2);
                    list.add(c3);
                }
                System.out.println(list);
                JSONArray fromObject = JSONArray.fromObject(list);
                System.out.println(fromObject);
                String string = fromObject.toString();
                System.out.println(string);
                HttpServletResponse response = ServletActionContext.getResponse();
                response.setContentType("text/html;charset=UTF-8");
                PrintWriter writer = response.getWriter();
                writer.write(string);
                writer.close();
return NONE;


            }
}