3、Ajax

AJAX（ASynchronous JavaScript And XML）异步的JavaScript和XML, 是一种无需重新加载整个网页的情况下，能够更新部分网页的技术。

实现方式

原生JS实现方式（了解）
JQuery实现方式

　　　　$.ajax()

　　　　$.get()

　　　　$.post()

JQery - $.ajax()

后端代码如下

@WebServlet("/AjaxServlet")
public class AjaxServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String age = request.getParameter("age");
        System.out.println("username:" + username + "--" + "age:" + age);

        // 休眠5秒
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        response.getWriter().write("you are ok");
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

前端代码发送ajax请求

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link href="css/bootstrap.min.css" rel="stylesheet">
    <script src="js/jquery-3.3.1.min.js"></script>
    <script src="js/bootstrap.min.js"></script>

    <script>
        $(function () {
            $("#myBtn").click(function () {
                var $btn = $(this).button('loading');
                $.ajax({
                    url: "AjaxServlet", // 请求路径
                    type: "post",  //请求方式
                    data: {"username": "heboan", "age": 18},  //请求参数
                    success: function (data) {
                        $btn.button('reset');
                        console.log(data);
                    }, //响应成功后的回调函数
                    error: function () {
                        $btn.button('reset');
                        console.log("出错了...");
                    }, //表示如果请求响应出现错误，会执行的回调函数
                    dataType:"text"//设置接受到的响应数据的格式
                });
            })
        });
    </script>
</head>
<body>

    <button type="button" id="myBtn" data-loading-text="加载中..." class="btn btn-primary" autocomplete="off">
        发送异步请求
    </button>

</body>
</html>

JQuery - $.get()

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/jquery-3.3.1.min.js"></script>
    <script>
        
        //定义方法
        function  fun() {

            $.get("ajaxServlet",{username:"rose", "age": 18},function (data) {
                alert(data);
            },"text");

        }
        
    </script>
    
    
</head>
<body>

    <input type="button" value="发送异步请求" onclick="fun();">

    <input>
</body>
</html>

JQuery - $.post()

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/jquery-3.3.1.min.js"></script>
    <script>
        
        //定义方法
        function  fun() {

            $.post("ajaxServlet",{username:"rose", "age": 18},function (data) {
                alert(data);
            },"text");

        }
        
    </script>
    
    
</head>
<body>

    <input type="button" value="发送异步请求" onclick="fun();">

    <input>
</body>
</html>

案例 - 用户名检测

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <script src="js/jquery-3.3.1.min.js"></script>


    <script>
        $(function () {
            // 给username绑定blur(失去焦点)事件
            $("#username").blur(function () {
                // 获取表单值
                var username = $(this).val();

                /*
                发送ajax请求
                期望服务器响应回的json数据格式
                    {"userExsit":true,"msg":"此用户名太受欢迎,请更换一个"}
                    {"userExsit":false,"msg":"用户名可用"}
                */

                var span = $("#check_username_msg");

                $.post("findUserServlet",{username:username},function (data) {
                    if(data.userExsit) {
                        // 用户名存在
                        span.css("color", "red");
                        span.html(data.msg);
                    } else {
                        span.css("color", "green");
                        span.html(data.msg);
                    }
                },"json");
            });
        });
    </script>
</head>
<body>


    <form>
        <input type="text" placeholder="请输入用户名" id="username" />
        <span id="check_username_msg"></span>
        <br />
        <input type="password" placeholder="请输入密码" id="password">
        <br />
        <input type="button" value="注册">
    </form>


</body>
</html>

前端-register.html

package com.heboan.www;

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;

@WebServlet("/findUserServlet")
public class findUserServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // 获取用户名
        String username = request.getParameter("username");

        /*
        期望服务器响应回的数据格式:
             {"userExsit":true,"msg":"此用户名太受欢迎,请更换一个"}
             {"userExsit":false,"msg":"用户名可用"}
        */
        HashMap<String, Object> map = new HashMap<>();


        // 设置响应的数据格式为json
        response.setContentType("application/json;charset=utf-8");

        if ("heboan".equals(username)) {
            map.put("userExsit", true);
            map.put("msg", "此用户名太受欢迎,请更换一个");
        } else {
            map.put("userExsit", false);
            map.put("msg", "用户名可用");
        }

        // 将map转为json，并且传递给客户端
        ObjectMapper mapper = new ObjectMapper();
        mapper.writeValue(response.getWriter(),map);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

后端-findUserServlet.java