2.ajax+servlet实现注册时用户名验证

效果：

精灵图（来源：从百度注册中download下来的）：

userVerify.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>
<style type="text/css">
    *{
        margin:0;
        padding:0;
    }
    input{
        height:24px;
    }
    span{
        display:none;
        float: right;
        position: relative;
        width: 250px;
        top: 5px;
        left: 5px;
        height: 16px;
        line-height: 14px;
        padding-left: 20px;
        background:url(img/reg_icons.png) -80px 0 no-repeat;
    }
</style>

</head>
<body>
<table>
    <tr>
        <td>用户名：</td>
        <td><input type="text" name="uname" id="uname" onblur="checkUserName()"><span id="tip"></span></td>
    </tr>
    <tr>
        <td colspan="2"><input type="button" value="注册"></td>
    </tr>
</table>    
<script type="text/javascript">
    function checkUserName(){
        var uname = document.getElementById("uname").value;
        var xmlHttp;
        if(window.XMLHttpRequest){
            xmlHttp = new XMLHttpRequest();
        }else{
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlHttp.onreadystatechange = function(){
            //判断返回状态
            if(xmlHttp.readyState == 4 && xmlHttp.status == 200){
                document.getElementById("tip").style.display = "block";
                var dataObj = eval("("+xmlHttp.responseText+")");
                if(dataObj.exist == "success"){
                    document.getElementById("tip").innerText = "该用户可用！";
                    document.getElementById("tip").style.color = "#5BC92E";
                    document.getElementById("tip").style.backgroundPosition = "-80px 0";
                }else if(dataObj.exist == "error"){
                    document.getElementById("tip").innerText = "请输入用户名！";
                    document.getElementById("tip").style.color = "#fc4343";
                    document.getElementById("tip").style.backgroundPosition = "-80px -24px";
                }else{
                    document.getElementById("tip").innerText = "该用户已存在！";
                    document.getElementById("tip").style.color = "#fc4343";
                    document.getElementById("tip").style.backgroundPosition = "-80px -24px";
                    
                }
                
            }
            
        }
        
        xmlHttp.open("get","checkUserName?action=checkUserName&uname="+uname,true);
        xmlHttp.send();
    }
</script>
</body>
</html>

CheckUserNameAjaxServlet.java

package com.xxc.ajax;

import java.io.IOException;
import java.io.PrintWriter;

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

import net.sf.json.JSONObject;


public class CheckUserNameAjaxServlet extends HttpServlet{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        String action = req.getParameter("action");
        if("checkUserName".equals(action)) {
            this.checkUserName(req, resp);
        }
    }
    
    public void checkUserName(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        
        PrintWriter out = resp.getWriter();
        String uname = req.getParameter("uname");
        JSONObject resultJson = new JSONObject();//注意：使用此对象需要导入包，下面会介绍
        if(uname.trim()==null || "".equals(uname.trim())){
            resultJson.put("exist", "error");//用户名为空
        }else if("xxc".equals(uname)) {
            resultJson.put("exist", true);//用户名已存在
        }else {
            resultJson.put("exist", "success");//用户名不存在
        }
        out.println(resultJson);
        out.flush();
        out.close();
    }

}

web.xml

<servlet>
      <servlet-name>CheckUserNameAjaxServlet</servlet-name>
      <servlet-class>com.xxc.ajax.CheckUserNameAjaxServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>CheckUserNameAjaxServlet</servlet-name>
      <url-pattern>/checkUserName</url-pattern>
  </servlet-mapping>

实现ajax 的所需的json包（版本不一定是我所列的）：

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原文地址：https://www.cnblogs.com/alex-xxc/p/10011941.html