double EL evaluating

[Craig Longman]

i've searched the archives, but have been unable to find an answer to this, other than a question shawn made about wanting a real-world case for it. i think i might have one, and i'm not sure how to proceed, so i'll see what y'all think.



the situation is that i have an 'imported' page that displays a menu, but the options in the menu change depending on what page imports it. i achieve this by sending different parameters to the imported page, and then the imported page goes from there. one of the parameters passed in are parent_objectName, which, for example, can contain 'Product' or 'Service'. what i want to do is this:
c:if test${not empty param.typeID}
 c:set var=${param.parent_objectName}_typeID value=${param.typeID}/
/c:if
then later:
c:out value=${${parent_objectName}_typeID}/
this would store an attribute named either Product_typeID or Service_typeID that could then be used elsewhere in the page.



unfortunately, the problems are plenty. it seems c:set.var isn't EL enabled (there is nothing set by the name of Service_typeID for example) and one can't 'double evaluate' in the c:out.value attribute.



the only way i can see of doing this now, is to create a custom tag that performs the magic, which might make sense for me in this case anyway, but i'd thought i'd throw this into the pit to be picked apart.



cheers,





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Re: double EL evaluating

[Shawn Bayern]

On 24 Sep 2002, Craig Longman wrote:

 what i want to do is this:
 
 c:if test${not empty param.typeID}
  c:set var=${param.parent_objectName}_typeID value=${param.typeID}/
 /c:if
 
 then later:
 
 c:out value=${${parent_objectName}_typeID}/
 
 this would store an attribute named either Product_typeID or
 Service_typeID that could then be used elsewhere in the page.
 
 unfortunately, the problems are plenty.  it seems c:set.var isn't EL
 enabled (there is nothing set by the name of Service_typeID for
 example) and one can't 'double evaluate' in the c:out.value attribute.
 
 the only way i can see of doing this now, is to create a custom tag
 that performs the magic, which might make sense for me in this case
 anyway, but i'd thought i'd throw this into the pit to be picked
 apart.

You don't have to go that far.  Indeed, you can't use dynamic values with
'var', and you can't begin a subordinate expression within a top-level
expression.  However, using collections, it should be straightforward to
do what you want:

 %-- Set up a Map.  Of course, a back-end servlet could do this too. --%
 jsp:useBean id=types class=java.util.Map /

 c:if test=${not empty param.typeID}
  c:set target=${types}
 property=${param.parent_objectName}_typeID
 value=${param.typeID} /
 /c:if
 
This saves information, which you can later retrieve with

 c:set var=propertyName value=${param.parent_objectName}_typeID /
 c:out value=${types[propertyName} /

Note the c:set, which lets us build up a complex temporary key into
'types'; you can't concatenate strings in the JSTL EL, so the
concatenation must occur outside an expression -- in this case, in the
attribute value of c:set.

Hope that helps,

-- 
Shawn Bayern
JSTL in Action   http://www.jstlbook.com


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