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   <script type="text/javascript">
        function createXMLHttpRequest() {
            try {
                return new XMLHttpRequest();
            } catch (e) {
                try {
                    return ActiveXObject("Msxml2.XMLHTTP");//IE6
                } catch (e) {
                    try {
                        return ActiveXObject("Microsoft.XMLHTTP");//IE5.5及以前版本
                    } catch (e) {
                        alert("不支持的浏览器");
                        throw e;
                    }
                }
            }
        }

        window.onload = function () {
            var btn = document.getElementById("btn");
            btn.onclick = function () {
                var xmlHttp = createXMLHttpRequest();
                xmlHttp.open("GET", "<c:url value='/AjaxServlet'/>", true);
                xmlHttp.send(null);
                xmlHttp.onreadystatechange = function () {
                    if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                        var text = xmlHttp.responseText;
                        var h1 = document.getElementById("h1");
                        h1.innerHTML = text;
                    }
                }
            }
        }
    </script>