在jsp中用jstl取不到值而用java能取到

<%System.out.println(request.getSession().getAttribute("user")); %>可以得到而用下面的得不到<c:choose>
<c:when test="${sessionScope.user==null}">
<li id="_topLogin"><a href="Tologin">登录</a>|<a href="Tozhuce">注册</a>|</li>
</c:when>
<c:otherwise>
<li id="_topLogin">欢迎您，${sessionScope.user}!|<a href="SessionDisppear">退出</a>|</li>
</c:otherwise>
</c:choose>