Javascript Ajax如何检查用户名是否已经存在？

问题是什么？

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我认为代码是正确的。只删除返回空；在servlet中。

欢迎使用SO。请访问，了解如何提问。提示：问一个实际的问题

<script type="text/javascript">
function createXMLHttpRequest() {
    try {
        return new XMLHttpRequest();
    } catch (e) {
        try {
            return ActiveXObject("Msxm12.XMLHTTP");
        } catch (e) {
            try {
                return ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                alert("dude,What kind of browser you have?");
                throw e;
            }
        }
    }
}

window.onload = function () {
    var username = document.getElementById("loginname");
    username.onblur = function () {
        var xmlHttp = createXMLHttpRequest();

        xmlHttp.open("POST", "<c:url value='/ajaxValidateLoginname'/>", true);

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

        xmlhttp.send("loginname=" + username.value);

        xmlhttp.onreadystatechange = function () {
            if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
                var text = xmlhttp.responseText;
                var label = document.getElementById("loginnameError");
                if (text === false) {
                    label.innerHTML = "The user name has been registered!";
                }else{
                    label.innerHTML = "";
                }
            }
        };
    };
};

public void doPost ajaxValidateLoginname(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String loginname = request.getParameter("loginname");
        boolean flag = us.ajaxValidateLoginname(loginname);
        response.getWriter().print(flag);
        return null;
    }