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ajax技術(shù)現(xiàn)在越來越受到大家的歡迎，因為它能很好的解決一些以前不好解決的問題，動態(tài)檢查文本框中的數(shù)據(jù)就是體現(xiàn)之一。現(xiàn)在已經(jīng)加上了數(shù)據(jù)庫mysql，可以說得上是一個比較完整的例子了。

下面就是怎樣實現(xiàn)動態(tài)檢查文本框中的數(shù)據(jù)一個例子：
工程目錄如下：

CheckServlet.java:

/**
?*?
?*/
package?com;

import?java.io.IOException;

import?javax.servlet.RequestDispatcher;
import?javax.servlet.ServletConfig;
import?javax.servlet.ServletException;
import?javax.servlet.http.HttpServlet;
import?javax.servlet.http.HttpServletRequest;
import?javax.servlet.http.HttpServletResponse;

/**
?*?@author?Administrator
?*
?*/
public?class?CheckServlet?extends?HttpServlet?{

????public?String[]?usernameList;
????
????public?CheckServlet()?{
????????super();
????????//?TODO?Auto-generated?constructor?stub
????}
????
????private?boolean?IsContain(String?param){
????????for(int?i=0;i<usernameList.length;i++){
????????????if(usernameList[i].equals(param)){
????????????????return?true;
????????????}else{
????????????????continue;
????????????}
????????}
????????return?false;
????}
????
????public?void?doGet(HttpServletRequest?request,HttpServletResponse?response)throws?IOException,ServletException{
????????String?username=(String)request.getParameter("username");
????????String?password=(String)request.getParameter("password");
????????String?repassword=(String)request.getParameter("repassword");
????????String?email=(String)request.getParameter("email");
????????
????????if(username.equals("")||username==null){
????????????request.setAttribute("error.message","用戶名不能為空!");
????????????RequestDispatcher?requsetDispatcher=request.getRequestDispatcher("error.jsp");
????????????requsetDispatcher.forward(request,response);
????????}else?if(password.equals("")||password==null){
????????????request.setAttribute("error.message","密碼不能為空!");
????????????RequestDispatcher?requsetDispatcher=request.getRequestDispatcher("error.jsp");
????????????requsetDispatcher.forward(request,response);
????????}else?if(!password.equals(repassword)){
????????????request.setAttribute("error.message","兩次輸入的密碼不一致!");
????????????RequestDispatcher?requsetDispatcher=request.getRequestDispatcher("error.jsp");
????????????requsetDispatcher.forward(request,response);
????????}else?if(this.IsContain(username)){
????????????request.setAttribute("error.message","您輸入的用戶名已經(jīng)存在!");
????????????RequestDispatcher?requsetDispatcher=request.getRequestDispatcher("error.jsp");
????????????requsetDispatcher.forward(request,response);
????????}else{
????????????RequestDispatcher?requsetDispatcher=request.getRequestDispatcher("success.jsp");
????????????requsetDispatcher.forward(request,response);
????????}
????}
????
????public?void?doPost(HttpServletRequest?request,HttpServletResponse?response)throws?IOException,ServletException{
????????doGet(request,response);
????}
????
????public?void?init(ServletConfig?config)?throws?ServletException{
????????usernameList=new?String[]{"Tom","Jerry","Brain"};
????}
????
}

PreServlet.java:

package?com;

import?java.io.IOException;
import?java.io.PrintWriter;
import?java.util.Iterator;
import?java.util.List;

import?javax.servlet.ServletException;
import?javax.servlet.http.HttpServlet;
import?javax.servlet.http.HttpServletRequest;
import?javax.servlet.http.HttpServletResponse;

public?class?PreServlet?extends?HttpServlet?{

????public?List?usernameList;?
????/**
?????*?Constructor?of?the?object.
?????*/
????public?PreServlet()?{
????????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?username=(String)request.getParameter("user_name");
????????System.out.println(username);
????????String?xml="<?xml?version=\"1.0\"?encoding=\"UTF-8\"?>";
????????if(username.equals("")||username==null){
????????????xml+="<message><info>Username?is?required!</info></message>";
????????}
????????else?if(this.IsContain(username)){
????????????xml+="<message><info>The?username?has?existes,Please?choose?other?username!</info></message>";
????????}
????else{
????????????xml+="<message><info>Username?is?approved!</info></message>";
????????}
????????response.getWriter().write(xml);
????}

????/**
?????*?The?doPost?method?of?the?servlet.?<br>
?????*
?????*?This?method?is?called?when?a?form?has?its?tag?value?method?equals?to?post.
?????*?
?????*?@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?doPost(HttpServletRequest?request,?HttpServletResponse?response)
????????????throws?ServletException,?IOException?{

????????doGet(request,response);
????}

????/**
?????*?Initialization?of?the?servlet.?<br>
?????*
?????*?@throws?ServletException?if?an?error?occure
?????*/
????public?void?init()?throws?ServletException?{
????????//?Put?your?code?here
????????DBOperator?dBOperator=new?DBOperator();
????????usernameList=dBOperator.getUsernameList();
????}
????
????private?boolean?IsContain(String?param){
??? ?? //這里修改了以前的代碼，現(xiàn)在是讀取數(shù)據(jù)庫的數(shù)據(jù)。
????????Iterator?it=usernameList.iterator();
????????while(it.hasNext()){
????????????if(it.next().toString().equals(param)){
????????????????return?true;
????????????}else{
????????????????continue;
????????????}
????????}
????????return?false;
????}
}

DBOperator.java:

package?com;

import?java.sql.Connection;
import?java.sql.ResultSet;
import?java.sql.Statement;
import?java.util.ArrayList;
import?java.util.List;

public?class?DBOperator?{
????public?DBOperator(){}
????
????public?Connection?Connect(){
????????Connection?con?=?null;
????????try?{
????????????Class.forName("com.mysql.jdbc.Driver").newInstance();
????????????con?=?java.sql.DriverManager.getConnection("jdbc:mysql://localhost/test?useUnicode=true&characterEncoding=GBK","root","");
????????}?catch?(Exception?e)?{
????????????//?TODO?Auto-generated?catch?block
????????????e.printStackTrace();
????????}
????????return?con;
????}
????
????public?List?getUsernameList(){
????????Connection?con=Connect();
????????List?usernameList=new?ArrayList();
????????try?{
????????????Statement?stmt=con.createStatement();
????????????ResultSet?rst=stmt.executeQuery("select?*?from?m_stuinfo");
????????????while(rst.next()){
????????????????usernameList.add(rst.getString("m_stuinfo_name"));
????????????}
????????}?catch?(Exception?e)?{
????????????//?TODO?Auto-generated?catch?block
????????????e.printStackTrace();
????????}
????????return?usernameList;
????}
}

index.jsp:

<%@?page?language="java"?contentType="text/html;?charset=utf-8"%>
<html>
????<head>
????????????<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8">
????????????<title>注冊</title>
????????????<SCRIPT?type="text/javascript">
????????????????var?req;
????????????????function?UsrNameCheck()
????????????????{
????????????????????var?username=document.getElementById('username').value;
????????????????????var?url="pre?user_name="+username;
????????????????????
????????????????????if(window.XMLHttpRequest)
????????????????????{
????????????????????????req=new?XMLHttpRequest();
????????????????????}else?if(window.ActiveXObject)
????????????????????{
????????????????????????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();
????????????????????????????}else{
????????????????????????????????alert("Not?able?to?retrieve?description"+req.statusText);
????????????????????????????}
????????????????????????}else{
????????????????????????????document.getElementById('check_username').innerHTML="正在驗證用戶名

";
????????????????????????}
????????????????????}
????????????????????
????????????????????function?parseMessage()
????????????????????{
????????????????????????var?xmlDoc=req.responseXML.documentElement;
????????????????????????alert(xmlDoc);
????????????????????????var?node=xmlDoc.getElementsByTagName('info');
????????????????????????document.getElementById('check_username').innerHTML=node[0].firstChild.nodeValue;
????????????????????}
????????????????????
????????????????????function?Form_Submit()
????????????????????{
????????????????????????if(regForm.username.value=="")
????????????????????????{
????????????????????????????alert("用戶名不能為空");
????????????????????????????return?false;
????????????????????????}else?if(regForm.password.value=="")
????????????????????????{
????????????????????????????alert("密碼不能為空");
????????????????????????????return?false;
????????????????????????}else?if(regForm.password.value!=regForm.repassword.value)
????????????????????????{
????????????????????????????alert("兩次輸入的密碼不一致");
????????????????????????????return?false;
????????????????????????}
????????????????????????regForm.submit();
????????????????????}
????????????????}
????????????</SCRIPT>
????</head>
????<body>
????????<div?align="center">
????????<form?name="regForm"?method="post"?action="/reg/regservlet">
????????????<table?width="70%"?border="1">
????????????????<tr?align="center">
????????????????????<td?colspan="2">用戶注冊</td>
????????????????</tr>
????????????????<tr>
????????????????????<td?width="24%"?align="center">用戶名:?</td>
????????????????????<td?width="76%"?><input?name="username"?type="text"?id="username"?onBlur="UsrNameCheck()">
????????????????????????<SPAN?id="check_username"></SPAN>
????????????????????</td>
????????????????</tr>
????????????????<tr>
????????????????????<td?align="center">密碼:?</td>
????????????????????<td?><input?name="password"?type="password"?id="password"></td>
????????????????</tr>
????????????????<tr>
????????????????????<td?align="center">重復(fù)密碼:?</td>
????????????????????<td?><input?name="repassword"?type="password"?id="repassword"></td>
????????????????</tr>
????????????????<tr>
????????????????????<td?align="center">email:?</td>
????????????????????<td?><input?name="email"?type="text"?id="email"></td>
????????????????</tr>
????????????????<tr?align="center">
????????????????????<td?colspan="2"><input?type="button"?name="Submit"?value="提交"?onClick="Form_Submit()"></td>
????????????????</tr>
????????????</table>
????????</form>
????????</div>
????</body>
</html>

success.jsp:

<%@?page?language="java"?contentType="text/html;?charset=GB2312"%>
<!DOCTYPE?HTML?PUBLIC?"-//W3C//DTD?HTML?4.01?Transitional//EN">
<html>
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=GB2312">
<title>注冊成功</title>
</head>
<body>
????恭喜你，注冊成功！<br>
</body>
</html>

error.jsp:

<%@?page?language="java"?contentType="text/html;?charset=GB2312"%>
<!DOCTYPE?HTML?PUBLIC?"-//W3C//DTD?HTML?4.01?Transitional//EN">
<html>
????<head>
????????<meta?http-equiv="Content-Type"?content="text/html;?charset=GB2312">
????????<title>失敗</title>
????</head>
????<body>
????????<div?align="center">
????????????<table?width="70%"?border="1">
????????????????<tr>
????????????????????<td>注冊失敗</td>
????????????????</tr>
????????????????<tr>
????????????????????<td> <%=(String)request.getAttribute("error.message")?%></td>
????????????????</tr>
????????????????<tr>
????????????????????<td><a?href="index.jsp">返回注冊頁</a></td>
????????????????</tr>
????????????</table>
????????</div>
????</body>
</html>

web.xml:

<?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">
????
????<display-name>reg</display-name>
????
????
????<servlet>
????????<servlet-name>regservlet</servlet-name>
????????<servlet-class>com.CheckServlet</servlet-class>
????</servlet>
????<servlet>
????????<servlet-name>PreServlet</servlet-name>
????????<servlet-class>com.PreServlet</servlet-class>
????</servlet>

????
????
??????<servlet-mapping>
????????<servlet-name>regservlet</servlet-name>
????????<url-pattern>/regservlet</url-pattern>
??????</servlet-mapping>
????<servlet-mapping>
????????<servlet-name>PreServlet</servlet-name>
????????<url-pattern>/pre</url-pattern>
????</servlet-mapping>
??????
????<welcome-file-list>
????????<welcome-file>index.jsp</welcome-file>
????</welcome-file-list>
</web-app>

運行結(jié)果是: