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.