c:set var question

[Yogesh Hingmire]

If i use a <c:set var = "Fido" value = null/> and i have two attributes one in page and other in request scope.
Would the execution of this tag remove the Fido attribute from page scope as well as the request scope?

Thank You

[Marc Peabody]

I don't think you can say value=null. And if you say value="null", well you'll get a String with the characters n, u, l, and l.

value="${null}" might work.

It will only change one scope's value - the first one it finds. Since you did not specify a scope, it will first look in the default page scope (which it finds) and that should be the only one effected.

[Charles Lyons]

I don't think you can say value=null

No, indeed not! If you want to remove something from a scope, use <c:remove />:

<c:remove var="Fido" />

In that case it'll remove the attribute called "Fido" from all scopes (by the semantics of PageContext's removeAttribute method).

Back to your original question, if you use:

<c:set var="Fido" value="SomethingElse" />

The value of the "Fido" page-scoped attribute will be set to "SomethingElse", overwriting any previous value stored in that attribute. It will only affect the page scope here as you haven't supplied a scope explicitly. <c:set /> operates according to the semantics of PageContext's setAttribute method when no scope is supplied.

[Yogesh Hingmire]

Thanks Marc/Charles.
I understand, that <c:remove> is the way to go for attribute removal, but my question is HFSJ mentions that <c:set has the effect of removing an attribute from a scope, when you set it to a ${null} value.
From Marc's answer it seems it will only remove the attribute from the first scope it finds it in.
so say for eg : if i have this :

<c:set var = "Fido" value = "${null}"/>.

I have Fido in request and session scope.

per Marc, it will only remove the attribute in request scope and not the session scope.

Please let me know if my understanding is correct.

Thanks

[Narendra Dhande]

HI,

<c:set var = "Fido" value = "${null}"/>.

This tag removes all the attribute Fido from all scopes.

If the attribute scope is specified, the scope variable is removed according to the semantics of PageContext.removeAttribute(varName, Scope). That is the attribute is removed from particular scope.

If the attribute scope is NOT specified, the scope variable is removed according to the semantics of PageContext.removeAttribute(varName). That is the attribute is removed from ALL scope.

Alternately, As per Charles said, you can use

<c:remove var="Fido" />

for the same effect.


Thanks
[ July 28, 2006: Message edited by: Narendra Dhande ]

[Marc Peabody]

That didn't sound right to me, so I had to test it.

The reason it didn't sound right is that when you use c:set to replace a value without specifying scope it will only replace the first one it finds. Why should a value that evaluates to null act differently? But it does!

 <%@ taglib uri="/WEB-INF/c.tld" prefix="c" %>
 
<c:set var="myTest" value="aaa" scope="page"/>
<c:set var="myTest" value="bbb" scope="request"/>
<c:set var="myTest" value="ccc" scope="session"/>
<c:set var="myTest" value="ddd" scope="application"/>
 
<c:out value="${pageScope.myTest}" />
<c:out value="${requestScope.myTest}" />
<c:out value="${sessionScope.myTest}" />
<c:out value="${applicationScope.myTest}" />
---
<c:set var="myTest" value="zzz"/>
<c:out value="${pageScope.myTest}"/>
<c:out value="${requestScope.myTest}" />
<c:out value="${sessionScope.myTest}" />
<c:out value="${applicationScope.myTest}" />
---
<c:set var="myTest" value="${null}"/>
<c:out value="${pageScope.myTest}"/>
<c:out value="${requestScope.myTest}" />
<c:out value="${sessionScope.myTest}" />
<c:out value="${applicationScope.myTest}" />
---   

That code gave this output:
aaa bbb ccc ddd --- zzz bbb ccc ddd --- ---

Totally weird!!!
[ July 28, 2006: Message edited by: Marc Peabody ]

[Yogesh Hingmire]

Thanks Narendra/Marc,

Can we conclude that <c:set> behaves differently when you set by specicfying some value as opposed to when you set by specifying an expression that evaluates to null ?

Yogesh

[Marc Peabody]

Yes. I believe my test proved that.

[Narendra Dhande]

Thanks Marc for your testing result,

I had not tested the <c:set> behaviour for null values. But I read it from the JSTL 1.1 spec. There is one seperate para on this behaviour.

Thanks