下拉框没有反应

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<header class="navbar navbar-fixed-top navbar-shadow">
        <div class="navbar-branding">
            <a class="navbar-brand" href="dashboard.html">
                <b>油画商城</b>
            </a>
            <span id="toggle_sidemenu_l" class="ad ad-lines"></span>
        </div>
        <ul class="nav navbar-nav navbar-right">
            <li class="dropdown menu-merge">
                <a href="#" class="dropdown-toggle fw600 p15" data-toggle="dropdown">
                    <img src="${pageContext.request.contextPath }/assets/img/avatars/5.jpg" alt="avatar" class="mw30 br64">
                    <span class="hidden-xs pl15"> ${ existUser.username } </span>
                    <span class="caret caret-tp hidden-xs"></span>
                </a>
                <ul class="dropdown-menu list-group dropdown-persist w250" role="menu">
                    <li class="list-group-item">
                        <a href="/self" class="animated animated-short fadeInUp">
                            <span class="fa fa-user"></span> 个人信息
                            <span class="label label-warning"></span>
                        </a>
                    </li>
                    <li class="list-group-item">
                        <a href="/to_change_password" class="animated animated-short fadeInUp">
                            <span class="fa fa-gear"></span> 设置密码 </a>
                    </li>
                    <li class="dropdown-footer">
                        <a href="${ pageContext.request.contextPath }/UserServlet?method=logout" class="">
                            <span class="fa fa-power-off pr5"></span> 退出 </a>
                    </li>
                </ul>
            </li>
        </ul>
    </header>