为什么Java会话属性返回null

Question

im试图将logggedin字符串对象用作标志。当我在日志中打印出来时，为什么它会返回空值？

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    String loggedIn = "false"; //create a marker to tell if the user is logged in
    String url = "/index.jsp";
    HttpSession session = request.getSession();
    // get a user object from the session. 
    User sessionUser =(User) session.getAttribute("user");
    // create empty message strings
    String messageSuccess = "";
    String passwordMismatch = "";

    session.setAttribute("loggedIn", loggedIn);
    System.out.println("The loggedIn attribute of the session object is: " + session.getAttribute(loggedIn));
    String action = request.getParameter("action");
    if (action == null) {
        action = "join";
    }
    if (action.equals("join")) {
        url = "/new_customer.jsp";
    } else if (action.equals("signIn")) {
//      User sessionUser = (User) session.getAttribute("sessionUser");
        if (sessionUser == null) {
            url = "/login_failure.jsp";
        } else {
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            if (username.equals(sessionUser.getUserName()) && password.equals(sessionUser.getPassword())) {
                url = "/account_activity.jsp";
                loggedIn = "true";
                session.setAttribute("loggedIn", true);
            } else {
                url = "/login_failure.jsp";
            }
        }
    }

Answer 1

因为

session.getAttribute(loggedIn)

与

结合

String loggedIn = "false";

将评估

session.getAttribute("false")

，该属性很可能在您的会话中未设置。

您可能正在寻找

session.getAttribute("loggedIn")