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[1]写index.jsp文件
<%@ page contentType="text/html;charset=gb2312"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="description" content="This is my page">
<link rel="stylesheet" type="text/css" href="styles.css">
</head>
<body>
<script type="text/javascript">
var req;
function validate() {
var idField = document.getElementById("userid");
var url = "servlet/ValidateServlet?id=" + escape(idField.value);
if (window.XMLHttpRequest) {
alert("0");
req = new XMLHttpRequest();
}else if (window.ActiveXObject) {
alert("1");
req = new ActiveXObject("Microsoft.XMLHTTP");
}
if(req){
req.open("GET", url, true);
req.onreadystatechange = callback;
req.send(null);
}
}
function callback() {
if (req.readyState == 4) {
if (req.status == 200) {
parseMessage();
// update the HTML DOM based on whether or not message is valid
}else{
alert ("Not able to retrieve description" + req.statusText);
}
}
}
function parseMessage() {
var message = req.responseXML.getElementsByTagName("message")[0];
var name = req.responseXML.getElementsByTagName("name")[0];
setMessage(message.firstChild.data,name.firstChild.data);
}
function setMessage(message,name) {
var userMessageElement = document.getElementById("userIdMessage");
userMessageElement.innerHTML = "<font color="red">" + message + " you "+name+"</font>";
}
</script>
<div id="userIdMessage"></div>
<input type="text"
size="20"
id="userid"
name="id"
onkeyup="validate();">
</body>
</html>
[2] 写servlet/ValidateServlet.java类
/*
* 创建日期 2005-8-3
*
* TODO 要更改此生成的文件的模板，请转至
* 窗口 － 首选项 － Java － 代码样式 － 代码模板
*/
package com;//com包需要自己创建.
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* @author Administrator
*
* TODO 要更改此生成的类型注释的模板，请转至
* 窗口 － 首选项 － Java － 代码样式 － 代码模板
*/
public class ValidateServlet extends HttpServlet {
/**
* Constructor of the object.
*/
private ServletContext context;
private HashMap users = new HashMap();
public ValidateServlet() {
super();
}
/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}
/**
* The doGet method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to get.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
String targetId = request.getParameter("id");
System.out.println(targetId.trim());
if ((targetId != null) && users.containsKey(targetId.trim())) {
response.getWriter().write("<info><message>welcome</message><name>sdl</name></info>");
} else {
response.getWriter().write("<info><message>kill</message><name>bush</name></info>");
System.out.print("invalid");
}
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occure
*/
public void init(ServletConfig config) throws ServletException {
this.context = config.getServletContext();
users.put("greg","account data");
users.put("duke","account data");
}
}
[3]写web.xml文件<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>ValidateServlet</servlet-name>
<servlet-class>com.ValidateServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ValidateServlet</servlet-name>
<url-pattern>/servlet/ValidateServlet</url-pattern>
</servlet-mapping>
</web-app>
[4]说明：
你可以在IE或FireFox里测试，在文本输入框里输入，当按键抬起，会在层中显示”kill you bush”。其中index.htm中的styles.css只是美化页面，没有列出来源代码。如果在servlet向客户端输出中文，需要编码转换。
[多此一举]
你需要了解tomcat工程。熟悉xmlHttpRequest,java servlet,html等知识。 | 
