通过Java类，我假设你的意思是一个Servlet class作为任意Java类中的设置会话属性没有意义。你可以在你的servlet的doGet / doPost方法中做这样的事情

By Java class, I am assuming you mean a Servlet class as setting session attribute in arbitrary Java class does not make sense.You can do something like this in your servlet's doGet/doPost methods

public void doGet(HttpServletRequest request, HttpServletResponse response) {

    HttpSession session = request.getSession();
    String username = (String)request.getAttribute("un");
    session.setAttribute("UserName", username);
}