Evaluation of macro arguments
-----------------------------
Key: VELOCITY-752
URL: https://issues.apache.org/jira/browse/VELOCITY-752
Project: Velocity
Issue Type: Bug
Components: Engine
Affects Versions: 1.6.2
Environment: Windows XP SP2, Java 1.5
Reporter: Steve O'Hara
Priority: Minor
I've just come across a little gotcha that has taken a few hours to track down
which might help other people.
An example;
#macro(myMacro $FieldName)
#foreach ($Name in $FieldList)
#if ($FieldName=="empty")
#set ($tmp=$FieldsFound.put($FieldName,$velocityCount))
#end
#end
#end
#foreach ($X in $MyFields)
#myMacro("Field$velocityCount")
#end
All looks OK doesn't it, but........
The problem is one of when arguments are evaluated or in fact to be more
precise, are not evaluated at all.
Once you bear that in mind you can see the logical problem above - the use of
$velocityCount (or indeed any variable) in the argument list of myMacro i.e.
#myMacro("Field$velocityCount")
This sort of usage is dangerous because in effect, what is happening is that
the macro argument literal is being placed within the macro body without being
evaluated first.
So, the body of the macro becomes this;
#foreach ($Name in $FieldList)
#if ("Field$velocityCount"=="empty")
#set ($tmp=$FieldsFound.put("Field$velocityCount",$velocityCount)
#end
#end
Not what you intended at all and depending on the names of your variables,
could cause all kinds of mayhem with values coming into scope that you hadn't
intended.
The workaround is to set a variable before the call to the macro and send that
instead e.g.
#foreach ($X in $MyFields)
#set ($tmp="Field$velocityCount")
#myMacro($tmp)
#end
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