form action="servlet/LoginServlet" method="post">
    table>
        tr>
            td>用戶賬號：/td>
            td>input type="text" name="username" onblur="checkUser(this)"/>/td>
        /tr>
        tr>
            td>用戶密碼：/td>
            td>input type="password" name="password"/>/td>
        /tr>
        tr>
            td>input type="submit" value="注冊"/>/td>
            td>input type="reset" value="重置">/td>
        /tr>
    /table>
/form>

servlet>
servlet-name>LoginServlet/servlet-name>
servlet-class>login.LoginServlet/servlet-class>
/servlet>

servlet-mapping>
servlet-name>LoginServlet/servlet-name>
url-pattern>/servlet/LoginServlet/url-pattern>
/servlet-mapping>

public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

        response.setContentType("text/html;charaet=UTF-8");
        PrintWriter out = response.getWriter();
        String name=request.getParameter("username");
        System.out.println(name);
        if(name.equals("admin"))
            out.print(false);
        else
            out.print(true);
        out.flush();
        out.close();
}

script type="text/javascript">
    //創(chuàng)建XMLHttpRequest
    function createXmlHttpRequest(){
        if(window.XMLHttpRequest){
            return new XMLHttpRequest();
        }else{
            return new ActiveXObject("Microsoft.XMLHTTP");
        }
    }
    //當用戶賬號輸入框失去焦點時調用該方法
    function checkUser(obj){
        //獲取輸入框輸入的值
        var user = obj.value;
        //如果輸入框中的值為空，那么彈窗提示，并且讓該輸入框獲得焦點
        if(!user){
            alert("用戶名不能為空！");
            obj.focus();
            return;
        }
        //不為空時，使用Ajax請求向后臺發(fā)送信息，驗證該用戶名是否可用
        //get請求字符串
        var url="servlet/LoginServlet?username="+user;
        //調用方法創(chuàng)建XMLHttpRequest對象
        XmlHttpRequest = createXmlHttpRequest();
        //設置回調函數
        XmlHttpRequest.onreadystatechange=finish;
        //初始化xmlhttprequest
        XmlHttpRequest.open("GET",url,true);
        //發(fā)送請求
        XmlHttpRequest.send(null);
    }
    //回調函數
    function finish(){
        if(XmlHttpRequest.readyState == 4 XmlHttpRequest.status == 200){
            var result = XmlHttpRequest.responseText;
            if(result =="true"){
                alert("用戶名可用！");
            }else{
                alert("用戶名不可用！");
            }
        }
    }
/script>