AJAX用于判定用户是否注册

html>
head>
title>/title>
script type="text/javascript">
var xmlHttp;
var flag;
function createXMLHttp(){
if(window.XMLHttpRequest){
xmlHttp=new XMLHttpRequest();
}else{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
}

function checkUserid(userid){
createXMLHttp();
xmlHttp.open("POST","CheckServlet?userid="+userid);
xmlHttp.onreadystatechange=checkUseridCallback;
xmlHttp.send();
document.getElementById("msg").innerHTML="正在验证。。。";
}

function checkUseridCallback(){
if(xmlHttp.readyState==400){
if(xmlHttp.status==200){
var text=xmlHttp.responseText;
if(text=="true"){
flag=false;
document.getElementById("msg").innerHTML="用户ID重复，无法使用";

}else{
flag=true;
document.getElementById("msg").innerHTML="此用户ID可以注册";
}
}
}
}

function checkForm(){
return flag;
}

/script>
/head>
body>

form action="tt.jsp" method="post" onsubmit="return checkForm()">
用户ID input type="text" name="userid" onblur="checkUserid(this.value)">span id="msg">/span>br>
姓名：input type="text" name="name">br>
密码：input type="password" name="password">br>
input type="button" value="注册">
input type="reset" value="重置">
/form>
/body>
/html>

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class CheckServlet extends HttpServlet{
public static final String DBDRIVER = "oracle.jdbc.OracleDriver";
public static final String DBURL = "jdbc:oracle:thin:@59.173.240.149:1521:heer";
public static final String DBUSER = "hnsyu_dev";
public static final String DBPASS = "hnsyuok";

public void doGet(HttpServletRequest request,HttpServletResponse response)
throws ServletException,IOException{
this.doPost(request, response);
}

public void doPost(HttpServletRequest request,HttpServletResponse response)
throws ServletException,IOException{
request.setCharacterEncoding("gbk");
response.setContentType("text/html");
Connection connection = null;
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
PrintWriter out = response.getWriter();
String userid = request.getParameter("userid");
try {
Class.forName(DBDRIVER);
connection = DriverManager.getConnection(DBURL, DBUSER, DBPASS);
String sql = "select count(userid) from userdemo where userid=?";
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setString(1,userid);
resultSet = preparedStatement.executeQuery();
if (resultSet.next()) {
if(resultSet.getInt(1)>0){
out.print("false");
System.out.println("true");
}else {
out.print("false");
}
}
out.close();
} catch (Exception e) {
e.printStackTrace();
}finally{
try {
connection.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}

?xml version="1.0" encoding="UTF-8"?>
web-app version="3.0" 
xmlns="http://java.sun.com/xml/ns/javaee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
 display-name>/display-name> 
 welcome-file-list>
  welcome-file>index.jsp/welcome-file>
 /welcome-file-list>
  servlet>
 servlet-name>CheckServlet/servlet-name>
 servlet-class>CheckServlet/servlet-class>
 /servlet>
 servlet-mapping>
 servlet-name>CheckServlet/servlet-name>
 url-pattern>/CheckServlet/url-pattern>
 /servlet-mapping> 
/web-app>