用Ajax请求后台数据

我们先不讲ajax的原理，还是先以实战为主，看一下这个东西到底怎么用的？

form表单：

<%@ 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>
</head>
<body>
    <div
        style="width: 300px; padding: 20px; margin: 20px; border: 4px dashed #ccc;">
        <form id="myform" name="myform" method="post" action="#">
            <table>
                <tr>
                    <td>用户名：</td>
                    <td><input type="text" id="username" name="username" placeholder="请输入用户名"
                        onfocus="this.placeholder=''" onblur="this.placeholder='请输入用户名'" /></td>
                </tr>
                <tr>
                    <td>密码：</td>
                    <td><input type="password" id="password" name="password" placeholder="请输入密码"
                        onfocus="this.placeholder=''" onblur="this.placeholder='请输入密码'" /></td>
                </tr>
                <tr>
                    <td colspan="2"><input type="button" value="提交"
                        onclick="doLogin('weixin');" /></td>
                </tr>
            </table>
            <input type="hidden" name="loginType" id="loginType" value="weixin" />
        </form>
         
    </div>
    <div id="errMsg" style="color: red">${errMsg}</div>
    <script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
    <script type="text/javascript">
        function doLogin(type) {
            $.ajax({
                type : "GET",
                url : "login.do",
                data : {
                    username : $("#username").val(),
                    password : $("#password").val(),
                    type : type
                },
                dataType : "json", //预期服务器返回的数据
                success : function(data) {
                    if (data.errCode < 0) {
                        alert("登录失败！")
                        $("#errMsg").show().html(data.errMsg).stop(true, true)
                                .fadeOut(3000);
                    } else {
                        //登录成功，做其他处理
                        alert("恭喜你，登录成功！");
                    }
                }
            });
        }
    </script>
</body>
</html>

LoginServlet

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("用户登录...");
        System.out.println("开始存入map...");
        Map<String,Object> map=StringUtils.getParameters(req);
        System.out.println("存入map成功！");
        System.out.println(map);
         
        if(StringUtils.isEmpty(map.get("username"))){
            StringUtils.writeObject(resp, "{\"errCode\":-1,\"errMsg\":\"用户名不能为空！\"}");
            System.out.println("用户名不能为空！");
            return;
        }
        if(StringUtils.isEmpty(map.get("password"))){
            StringUtils.writeObject(resp,"{\"errCode\":-2,\"errMsg\":\"密码不能为空！\"}");
            System.out.println("密码不能为空！");
            return;
        }
        System.out.println("登陆成功！");
        StringUtils.writeObject(resp,"{\"errCode\":0,\"errMsg\":\"登录成功!\"}");
    }