If the object itself is empty, an error will be reported when the attribute of the object is retrieved from the session

Solution:
original code:

< span th:text=”${session.user.username}”>& lt;/ span>

After change:

< span th:text=”${session.user?.username}”>& lt;/ span>

Why add one?You can solve
because this is a method for thymeleaf to judge whether the object is empty. Thymeleaf shows that an object uses ${username}, but if the username is null, thymeleaf will report an error