在很多注冊的頁面上，我們可能會遇到以下情況，當我們注冊用戶名的時候可能會提示該用戶名已注冊，其實現是應用AJAX技術。
首先寫一個登陸頁面
<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="正在驗證。。。";}function checkUseridCallback(){if(xmlHttp.readyState==400){if(xmlHttp.status==200){var text=xmlHttp.responseText;if(text=="true"){flag=false;document.getElementById("msg").innerHTML="用戶ID重復，無法使用";}else{flag=true;document.getElementById("msg").innerHTML="此用戶ID可以注冊";}}}}function checkForm(){return flag;}</script></head><body><form action="tt.jsp" method="post" onsubmit="return checkForm()">用戶ID <input type="text" name="userid" onblur="checkUserid(this.value)"><span id="msg"></span><br>姓名：<input type="text" name="name"><br>密碼：<input type="password" name="password"><br><input type="button" value="注冊"><input type="reset" value="重置"></form></body></html>隨后寫一個servlet的Java代碼
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();}}}}
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