用Ajax请求后台数据

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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!\"}");
}