在任何網站註冊用戶的時候，都會檢查用戶是否已經存在。很久以前的處理方式是將所有數據提交到服務器端進行驗證，很顯然這種方式的用戶體驗很不好；後來有了Ajax，有了異步交互，當用戶輸完用戶名繼續填寫其他信息的時候，Ajax就將信息發到了服務器去檢查該用戶名是否已經被註冊了，這樣如果用戶名已經存在，不用等用戶將所有數據都提交就可以給出提示。採用這種方式大大改善了用戶體驗。
regist.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!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">
<title>Insert title here</title>
<script type="text/javascript">
var xmlHttp;
//創建Ajax核心對象XMLHttpRequest
function createXMLHttp(){
if(window.XMLHttpRequest){
xmlHttp = new XMLHttpRequest();
}else{
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
}
function checkUsername(username){
createXMLHttp();
//設置請求方式爲GET，設置請求的URL，設置爲異步提交
xmlHttp.open("GET","CheckServlet?username="+username,true);
//將方法地址複製給onreadystatechange屬性
//類似於電話號碼
xmlHttp.onreadystatechange = checkUsernameCallback();
//將設置信息發送到Ajax引擎
xmlHttp.send(null);
}
function checkUsernameCallback(){
//Ajax引擎狀態爲成功
if(xmlHttp.readyState == 4){
//HTTP協議狀態爲成功
if(xmlHttp.status == 200){
var text = xmlHttp.responseText;
if(text == "true"){
document.getElementById("msg").innerHTML = "此用戶名已存在，無法使用！";
}else{
document.getElementById("msg").innerHTML = "此用戶名可以使用";
}
}
}
}
</script>
</head>
<body>
<form action="regist.jsp" method="post">
用戶名:<input type="text" name="username" onblur="checkUsername(this.value)"><span id="msg"></span><br/>
密 碼:<input type="password" name="password"><br/>
<input type="submit" value="註冊">
<input type="reset" value="重置">
</form>
</body>
</html>
public class CheckServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public static final String DBDRIVER = "com.microsoft.sqlserver.jdbc.SQLServerDriver";
public static final String DBURL = "jdbc:sqlserver://localhost:1433;DatabaseName=bbs";
public static final String DBUSER = "sa";
public static final String DBPASS = "pass";
public CheckServlet() {
super();
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doPost(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html");
Connection conn = null;
PreparedStatement pst = null;
ResultSet rs = null;
PrintWriter out = response.getWriter();
String username = request.getParameter("usernaem");
try{
Class.forName(DBDRIVER);
conn = DriverManager.getConnection(DBURL,DBUSER,DBPASS);
String sql = "select count(username) from user where username=?";
pst = conn.prepareStatement(sql);
pst.setString(1,username);
rs = pst.executeQuery();
if(rs.next()){
if(rs.getInt(1)>0){//用戶名已經存在了
out.print("true");
}else{
out.print("false");
}
}
}catch(Exception e){
e.printStackTrace();
}finally{
try{
conn.close();
}catch(Exception e){
e.printStackTrace();
}
}
}
}
CheckServlet.java