EL (expression language)
1. Grammatical structure: $ ()
(1) $ (username): Find the name of a range of variables for the username
(2) If the scope is not specified, the default start page will be the scope to find, if not found, then turn in reqeust, session, application scope of search
Way to find username, on the direct return, or keep looking down, if all areas are not found then return null.
(3) to specify the scope
$ (RequestScope.username) is equivalent to JSP syntax <% = request.getAttribute ("username")%>
$ (SessionScope.username) is equivalent to JSP syntax <% = session.getAttribute ("username")%>