var http_request=null;

function create_ajax(){
    http_request = false;
    //开始初始化XMLHttpRequest对象
    if(window.XMLHttpRequest) { //Mozilla 浏览器
        http_request = new XMLHttpRequest();
        if (http_request.overrideMimeType) {//设置MiME类别
            http_request.overrideMimeType('text/xml');
        }
    }
    else if (window.ActiveXObject) { // IE浏览器
        try {
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                http_request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {}
        }
    }
    if (!http_request) { // 异常，创建对象实例失败
        window.alert("不能创建XMLHttpRequest对象实例.");
        return false;
    }
}

var obj1;

function get_ajax(url,obj) {//初始化、指定处理函数、发送请求的函数
obj1=obj;
create_ajax();
http_request.onreadystatechange =processobj; // 确定发送请求的方式和URL以及是否同步执行下段代码
http_request.open("POST", url, true);
http_request.send(null);
}

function processobj(){
document.getElementById(obj1).innerHTML="Loading...";
if (http_request.readyState == 4) { // 判断对象状态
if (http_request.status == 200) { // 信息已经成功返回，开始处理信息
document.getElementById(obj1).innerHTML=http_request.responseText;
} else { //页面不正常
document.getElementById(obj1).innerHTML="loading error.";
}
}
}

<script type="text/javascript" src="ajax.js" src="ajax.js" ></script>
Welcome.
<div id="d1" onclick="get_ajax('ajax/get1.jsp','d1');">d1</div>
<div id="d2" onclick="get_ajax('ajax/get2.jsp','d2');">d2</div>
<div id="a">d2</div>