session.getAttribute give null when retreiving session Data

Question

I have no idea why my session.getAttribute("xxx") return null when servlet to jsp to servlet.

My code is the following.

AccountListServlet

ArrayList<UserInfo> userInfoList = new ArrayList<UserInfo>();
UserInfo userInfo = new UserInfo();
userInfo.setID(rs.getInt("id"));
userInfoList.add(userInfo);

HttpSession session = request.getSession(true);
session.setAttribute("accountInfoList", userInfoList);

AccountList.jsp

<%@ page session="true" %>
<%
    ArrayList<UserInfo> UserInfoList = (ArrayList<UserInfo>)session.getAttribute("accountInfoList");
%>

in the html body

<div class="text">
    <form method="post" id="form" name="form" action="./accountcontrol">
        <input type="submit" value="Edit" id="accountcontrol" />
    </form>
</div>

AccountControlServlet

ArrayList<UserInfo> UserInfoList = (ArrayList<UserInfo>)session.getAttribute("accountInfoList");

At AccountList.jsp, I retrieve session data.And then when button clicked at AccountList.jsp and go to the AccountControlServlet, the session data is null.

How can I work around for this?