try this -> {sessionScope.username}
By default page, request, session and application objects are available to JSP pages. So you can access then using EL syntax.
And following table shows IMPLICIT objects available to EL.
Implicit object Description
1. pageScope Scoped variables from page scope
2. requestScope Scoped variables from request scope
3. sessionScope Scoped variables from session scope
4. applicationScope Scoped variables from application scope
5. param Request parameters as strings
6. paramValues Request parameters as collections of strings
7. header HTTP request headers as strings
8. headerValues HTTP request headers as collections of strings
9. initParam Context-initialization parameters
10. cookie Cookie values
11. pageContext The JSP PageContext object for the current page
Reference: Are session and sessionScope the same in JSP EL?