This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
i hope you mean <% request.setAttribute("username", "user123"); %>
You can do this using JSTL EL,
or if userName is a request parameter.
but as Christophe said, try to avoid setting variables in request, session, application scopes inside JSP (instead use controller). If you want to create temporary variables inside jsp, create those in "page" scope (in c:set, simply dont mention "scope" attribute).