jsp+ajax发送GET请求的方法

<form action="servlet/LoginServlet" method="post">
    <table>
        <tr>
            <td>用户账号</td>
            <td><input type="text" name="username" onblur="checkUser(this)"/></td>
        </tr>
        <tr>
            <td>用户密码</td>
            <td><input type="password" name="password"/></td>
        </tr>
        <tr>
            <td><input type="submit" value="注册"/></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">
    //创建XMLHttpRequest
    function createXmlHttpRequest(){
        if(window.XMLHttpRequest){
            return new XMLHttpRequest();
        }else{
            return new ActiveXObject("Microsoft.XMLHTTP");
        }
    }
    //当用户账号输入框失去焦点时调用该方法
    function checkUser(obj){
        //获取输入框输入的值
        var user = obj.value;
        //如果输入框中的值为空，那么弹窗提示，并且让该输入框获得焦点
        if(!user){
            alert("用户名不能为空！");
            obj.focus();
            return;
        }
        //不为空时，使用Ajax请求向后台发送信息，验证该用户名是否可用
        //get请求字符串
        var url="servlet/LoginServlet?username="+user;
        //调用方法创建XMLHttpRequest对象
        XmlHttpRequest = createXmlHttpRequest();
        //设置回调函数
        XmlHttpRequest.onreadystatechange=finish;
        //初始化xmlhttprequest
        XmlHttpRequest.open("GET",url,true);
        //发送请求
        XmlHttpRequest.send(null);
    }
    //回调函数
    function finish(){
        if(XmlHttpRequest.readyState == 4&& XmlHttpRequest.status == 200){
            var result = XmlHttpRequest.responseText;
            if(result =="true"){
                alert("用户名可用！");
            }else{
                alert("用户名不可用！");
            }
        }
    }
</script>