Ajax案例3-->判断用户名是否被占用

jsp页面--thirdajax.jsp

 1 <%@ page language="java" contentType="text/html; charset=UTF-8"
 2     pageEncoding="UTF-8"%>
 3 <%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
 4 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
 5 <html>
 6 <head>
 7 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
 8 <title>Insert title here</title>
 9 </head>
10 <script type="text/javascript">
11     function createXMLHttpRequest() {
12         try {
13             return new XMLHttpRequest();
14         } catch (e) {
15             try {
16                 return new ActiveXObject("Msxml2.XMLHTTP");
17             } catch (e) {
18                 try {
19                     return new ActiveXObject("Microsoft.XMLHTTP");
20                 } catch (e) {
21                     alert("浏览器不兼容");
22                 }
23             }
24         }
25     }
26 
27     window.onload = function() {
28         var usernameEle = document.getElementById("username");
29         usernameEle.onblur = function() {//给文本框的失去焦点事件添加监听器
30             //alert("hello");
31 
32             var xmlHttp = createXMLHttpRequest();
33             xmlHttp.open("POST", "<c:url value ='/ThirdServlet'/>", true);
34             xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
35             xmlHttp.send("username=" + usernameEle.value);
36 
37             xmlHttp.onreadystatechange = function() {
38                 if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
39 
40                     var text = xmlHttp.responseText;//得到的是String
41                     var flag = eval("(" + text + ")");//eval()执行字符串
42                     var error = document.getElementById("errorMsg");
43                     if (flag) {
44                         error.innerHTML = "用户名已被注册";
45                     }else{
46                         error.innerHTML="";
47                     }
48                 }
49             };
50         };
51     };
52 </script>
53 <body>
54 
55     <h1>注册表单</h1>
56     <form action="" method="POST">
57         姓名<input type="text" id="username" name="username" /><span id="errorMsg"></span><br />             
58         密码<input type="password" /><br />
59          <input    type="submit" value="提交" />
60     </form>
61 </body>
62 </html>

Servlet-->ThirdServlet.java

 1 package ajax;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 
11 public class ThirdServlet extends HttpServlet {
12 
13     private static final long serialVersionUID = 8739665601659312476L;
14 
15     public void doPost(HttpServletRequest request, HttpServletResponse response)
16             throws ServletException, IOException {
17 
18         request.setCharacterEncoding("utf-8");
19         response.setContentType("text/html;charset=utf-8");
20 
21         String username = request.getParameter("username");
22 
23         /*
24          * 实际开发中需要查询数据库
25          * 这里进行了简化
26          */
27         if ("lisa".equalsIgnoreCase(username)) {
28             response.getWriter().print(true);
29         } else {
30             response.getWriter().print(false);
31         }
32 
33     }
34 
35 }

图片展示:

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原文地址：https://www.cnblogs.com/vmkash/p/5522016.html