如下：
request.jsp
代码如下：

%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%
!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"
html
head
meta http-equiv="Content-Type" content="text/html; charset=UTF-8"
titleInsert title here/title
script language="javascript"!--
function GetXmlHttpObject(){
    var xmlHttp = null;
    try{
        xmlHttp = new XMLHttpRequest();
    }catch(e){
        try{
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
        }catch(e){
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
    }
    return xmlHttp;

}
function showMsg(str){
    xmlHttp = GetXmlHttpObject();
    if(xmlHttp == null){
        alert ("you browser don't support the ajax");
         return;

    }
    var url = "response.jsp";
    url = url + "?q="+ str;
    url = url + "&sid ="+ Math.random();
    xmlHttp.onreadystatechange = stateChanged;
    xmlHttp.open("GET", url, true);
    xmlHttp.send(null);
}
function stateChanged()
{
    if(xmlHttp.readyState==4)
    {
        document.getElementById("city").value = xmlHttp.responseText;
    }
}
// --/script
/head
body
form name="form1" action="" method="post"
    label City Code:/label
    input type="text" name="code" onblur = "showMsg(this.value)" /
    br/br
    labelCity Name:/label
    input type="text" name="city" id="city" /input
/form
/body
/html

response.jsp
代码如下：

%@ page language="java" contentType="text/plain; charset=UTF-8"
pageEncoding="UTF-8"%
%@ page import="com.lwf.eus.util.*,java.util.*,com.lwf.eus.entity.*,com.lwf.eus.bean.*" %
%
String code = request.getParameter("q");
System.out.println(code);
if(code.equals("140"))
    out.print("上海");
else if(code.equals("150"))
    out.print("北京");
else if(code.equals("160"))
    out.print("天津");
else
    out.print("未知地");
%

这里要注意的是由于返回的结果要在文本框中显示，因此在response.jsp中没有html等标签，因为测试发现如果有这些标签的话，在cityname文本框中这些标签也会显示。