jsp - Check if parameter exists in Expression Language -

this question has answer here:

• evaluate empty or null jstl c tags 8 answers

<c:if test="${param.username}" > </c:if> 

how check if param.username exists??

use not empty check.

<c:if test="${not empty param.username}" > </c:if> 

edit: if have parameter of form ?username (no value), safer use ${param.username ne null}