Check if parameter exists in Expression Language [duplicate]

Answer 1

If I may add a comment...

To test if the request parameter "username" does not exist in the JSP page "a-jsp.jsp", we can write an "if" clause in the page "a-jsp.jsp":

http://server/webapp/a-jsp.jsp

or 

http://server/webapp/a-jsp.jsp?username=

We won't if the requested URL is:

http://server/webapp/a-jsp.jsp?username=foo