AJAX请求详解同步异步 GET和POST

　　上一篇博文(http://www.cnblogs.com/mengdd/p/4191941.html)介绍了AJAX的概念和基本使用,附有一个小例子,下面基于这个例子做一些探讨.

同步和异步

在准备请求的时候,我们给open方法里传入了几个参数,其中第三个参数为true时,表示是异步请求:

//1. prepare request

xmlHttpRequest.open("GET", "AjaxServlet", true);

// XMLHttpRequest.prototype.open = function(method,url,async,user,password) {};

　　为了模拟服务器的响应,并且不使用缓存内容,我们把服务器代码改成如下,加了5秒延时:

public class HelloAjaxServlet extends HttpServlet {

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

        System.out.println("doGet invoked!");

        //mock the processing time of server

        try {

            Thread.sleep(5000L);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        //no cache

        response.setHeader("pragma","no-cache");

        response.setHeader("cache-control","no-cache");

        PrintWriter out = response.getWriter();

        out.println("Hello World");

        out.flush();

    }

}

　　下面就可以比较出同步和异步的差别了:

　　xmlHttpRequest.open()方法,第三个参数设置为async=true时,异步:

　　点击按钮后过了5秒出现Hello World字样,但是这5秒过程中页面中的其他地方都是不受影响的,可以操作.

　　xmlHttpRequest.open()方法,第三个参数设置为async=false时,同步:

　　同样是点击按钮5秒后出现Hello World字样,但是这5秒中,按钮是不可点击状态,页面不可做其他操作.

　　当使用同步设置时,其实不需要写回调函数,直接把响应的操作放在后面的语句即可.

　　注:不推荐使用async=false.

GET和POST

让我们把原来的程序改得复杂一点,计算两个输入框的值.

加入两个输入框,然后在发送请求之前获取它们的值:

<body>

    <input type="button" value="get content from server" onclick="ajaxSubmit();"><br>

    <input type="text" value="value1" id="value1Id">

    <input type="text" value="value2" id="value2Id">

    <div id="div1"></div>

</body>

服务器端获取参数值并返回计算结果:

public class HelloAjaxServlet extends HttpServlet {

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

        System.out.println("doGet invoked!");

        process(request, response);

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        System.out.println("doPost invoked!");

        process(req, resp);

    }

    private void process(HttpServletRequest request, HttpServletResponse response) throws IOException {

        System.out.println("process invoked!");

        String v1 = request.getParameter("v1");

        String v2 = request.getParameter("v2");

        String result = String.valueOf(Integer.valueOf(v1) + Integer.valueOf(v2));

        //mock the processing time of server

//        try {

//            Thread.sleep(5000L);

//        } catch (InterruptedException e) {

//            e.printStackTrace();

//        }

        //no cache

        response.setHeader("pragma", "no-cache");

        response.setHeader("cache-control", "no-cache");

        PrintWriter out = response.getWriter();

        out.println("Hello World: " + result);

        out.flush();

    }

}

首先用GET方法:

GET方法的参数拼接在url的后面:

xmlHttpRequest.open("GET", "AjaxServlet?v1=" + value1 + "&v2=" + value2, true);//GET

xmlHttpRequest.send(null);//GET

POST方法:

xmlHttpRequest.open("POST", "AjaxServlet", true);//POST

xmlHttpRequest.send("v1=" + value1 + "&v2=" + value2);//POST requset needs params here, for GET, just leave params empty or set it to null.

　　注意,使用POST方法提交,在请求发送之前,必须要加上如下一行:

xmlHttpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

　　完整index.jsp代码:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<html>

<head>

    <title>Hello Ajax</title>

    <script type="text/javascript">

        var xmlHttpRequest;

        function ajaxSubmit() {

            if (window.XMLHttpRequest) {//in JavaScript, if it exists(not null and undifine), it is true.

                // code for IE7+, Firefox, Chrome, Opera, Safari

                xmlHttpRequest = new XMLHttpRequest();

            } else if (window.ActiveXObject) {

                // code for IE6, IE5

                xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");

            } else {

                //very rare browsers, can be ignored.

            }

            //also, we can handle IE5,6 first using:

            /*

             if (window.ActiveXObject) {

             //code for IE6, IE5

             }

             else {

             //code for others

             }

             */

            //send request

            if (null != xmlHttpRequest) {

                //get parameters from DOM

                var value1 = document.getElementById("value1Id").value;

                var value2 = document.getElementById("value2Id").value;

                //1. prepare request

//                xmlHttpRequest.open("GET", "AjaxServlet?v1=" + value1 + "&v2=" + value2, true);//GET

                xmlHttpRequest.open("POST", "AjaxServlet", true);//POST

                // XMLHttpRequest.prototype.open = function(method,url,async,user,password) {};

                //2. set callback function to handle events

                xmlHttpRequest.onreadystatechange = ajaxCallback;

                //3. do sending request action

//                xmlHttpRequest.send(null);//GET

                //POST

                xmlHttpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

                xmlHttpRequest.send("v1=" + value1 + "&v2=" + value2);//POST requset needs params here, for GET, just leave params empty or set it to null.

            }

        }

        function ajaxCallback() {

            //alert("test");//this alert will show several times when click the button.

            if (4 == xmlHttpRequest.readyState && 200 == xmlHttpRequest.status) {

                var responseText = xmlHttpRequest.responseText;

                document.getElementById("div1").innerHTML = responseText;

            }

        }

    </script>

</head>

<body>

<input type="button" value="get content from server" onclick="ajaxSubmit();"><br>

<input type="text" value="" id="value1Id">

<input type="text" value="" id="value2Id">

<div id="div1"></div>

</body>

</html>

index.jsp

参考资料:

　　圣思园张龙老师JavaWeb视频教程64 POST与GET方式提交Ajax请求的区别，深度解读HTTP协议.