<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<form method="post" action="user.do">
用戶名:<input type="text" onkeyup="check()" id="username" name="username"><span id="span"></span><br>
密碼:<input type="password">
</form>
<script>
function check(){
var request = new XMLHttpRequest();//第一條線打通(創建ajax對象)
var username = document.getElementById("username").value;//創建一個名叫username的變量使其值等於用戶輸入的用戶名
//第二條線打通(打開此ajax引擎,並向服務器發送http請求)
request.open("get", "user.do?username="+username, true);//指明向哪裏發出http請求以及請求的方式。此處true定義了此請求爲異步
request.send(null);
//第三條線打通(服務器返回的http響應)
request.onreadystatechange = function (){
if(request.readyState == 4 && request.status == 200){//條件成立則說明得到服務器相應成功
var text = request.responseText;
if(text == "true"){
document.getElementById("span").innerHTML= "yes";
document.getElementById("span").style.color="green";
}else{
document.getElementById("span").innerHTML= "no";
document.getElementById("span").style.color="red";
}
}
}
}
</script>
serlvet代碼如下:
package ajax12demo;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class UserServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String userName = request.getParameter("username");
if(userName.equals("ZwZ")){
response.getWriter().print(false);
}else{
response.getWriter().print(true);
}
System.out.println(userName);
}
}
以上代碼可以實現對用戶名驗證的效果。
當請求爲同步,即request.open( )中async參數值爲false時,其中js代碼的順序應如下:
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<form method="post" action="user.do">
用戶名:<input type="text" onkeyup="check()" id="username" name="username"><span id="span"></span><br>
密碼:<input type="password">
</form>
<script>
function check(){
var request = new XMLHttpRequest();//第一條線打通(創建ajax對象)
var username = document.getElementById("username").value;
//第三條線打通(服務器返回的http響應)
request.onreadystatechange = function (){
if(request.readyState == 4 && request.status == 200){//條件成立則說明得到服務器相應成功
var text = request.responseText;
if(text == "true"){
document.getElementById("span").innerHTML= "yes";
document.getElementById("span").style.color="green";
}else{
document.getElementById("span").innerHTML= "no";
document.getElementById("span").style.color="red";
}
}
}
//第二條線打通(向服務器發送http請求)
request.open("get", "user.do?username="+username, false);
request.send(null);
}
</script>
原因:與ajax同步與異步機制和js的解釋運行原理有關。