案例：

* 校验用户名是否存在
	1. 服务器响应的数据，在客户端使用时，要想当做json数据格式使用。有两种解决方案：
		1. $.get(type):将最后一个参数type指定为"json"
		2. 在服务器端设置MIME类型
			response.setContentType("application/json;charset=utf-8");

前端代码

	<!DOCTYPE html>
<html lang="en">
<head>
    <Meta charset="UTF-8">
    <title>注册</title>
    <script src="js/jquery-3.3.1.min.js"></script>
    <script>
        $(function () {
            //绑定blur事件
            $("#username").blur(function () {
                //获取文本
                var username = $(this).val();
                //发送请求
                $.get("findUserServlet",{username:username},function (data) {
                    alert(data)
                    var span = $("#s_username");
                    if (data.userExit){
                        span.css("color","red");
                        span.html(data.msg);
                    }else{
                        span.css("color","green");
                        span.html(data.msg);
                    }
                },"json")
            });
        });
    </script>
</head>
<body>
<form action="">
     <input type="text" name="username" id="username" placeholder="请输入用户名">
    <span id="s_username"></span>
    <br>
    <input type="password" name="password" placeholder="请输入密码"><br>
     <input type="submit" value="提交">
</form>
</body>
</html>

servlet代码

package cn.pluto.web.servlet;

import com.fasterxml.jackson.databind.ObjectMapper;

import javax.servlet.servletexception;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;


@WebServlet("/findUserServlet")
public class FindUserServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws servletexception, IOException {
        response.setContentType("text/html;charset=utf-8");
        //获取值
        String username = request.getParameter("username");
        //调用service
        Map<String,Object> map = new HashMap<String, Object>();
        if ("tom".equals(username)){
            //存在
            map.put("userExit",true);
            map.put("msg","此用户忙太受欢迎，换一个");
        }else{
            //不存在
            map.put("userExit",false);
            map.put("msg","欢迎可用");
        }
        //将map转为json 传递给客户端
        ObjectMapper mapper = new ObjectMapper();
        mapper.writeValue(response.getWriter(),map);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws servletexception, IOException {
        this.doPost(request, response);
    }
}

ajax案例

案例：

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