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.