String username = (String) request.getParameter("username").trim();
            if (username.equals("")) {
                return;
            }
            String password = (String) request.getParameter("password").trim();
            if (password.equals("")) {
                return;
            }
            String password2 = (String) request.getParameter("password2").trim();
            if (password2.equals("") || !password2.endsWith(password)) {
                return;
            }
            String email = (String) request.getParameter("email").trim();
            if (email.equals("")) {
                return;
            }
            User user = new User(username, password, email);
//            persist(user);后面的不知道怎么写了,高手提点一下啊。