问题
<c:if test="${param.username}" >
</c:if>
How do I check if param.username exists??
回答1:
Use the not empty check.
<c:if test="${not empty param.username}" >
</c:if>
Edit: If you have a parameter of the form ?username (no value), it is safer to use ${param.username ne null}
回答2:
If you want to check if parameter exists, just test if it is not null, in your case:
<c:if test="${param.username != null}"></c:if>
Broader explanation:
If you want to check:
if yourParam exists/ is not null:
<c:if test="${param.yourParam != null}"></c:if>if yourParam does not exist/ is null
<c:if test="${param.yourParam == null}"></c:if>if yourParam is not empty (not empty string and not null)
<c:if test="${!empty param.yourParam}"></c:if>if yourParam is empty (empty string or null)
<c:if test="${empty param.yourParam}"></c:if>if yourParam evaluates to 'true'
<c:if test="${yourParam}"></c:if>if yourParam evaluates to 'false' (string other than 'true')
<c:if test="${!yourParam}"></c:if>
回答3:
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":
<c:if test="${empty param['username']>
...
</c:if>
We'll go through that "if" clause if the requested URL is:
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
来源：https://stackoverflow.com/questions/10932677/check-if-parameter-exists-in-expression-language