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From: Erik Hatcher <erik@ehatchersolutions.com>
Subject: Re: Embedded Properties
Date: Tue, 13 Apr 2004 16:09:32 -0400
To: "Ant Users List" <user@ant.apache.org>
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On Apr 13, 2004, at 3:26 PM, Ivan Ivanov wrote:
> Hi Nick, Stefan and all we can do this also:
> <property name=3D"Lib.Name.Static" value=3D"foo"
> id=3D"Lib.Name.Static"/>
> <property name=3D"Lib.Name.Dynamic" value=3D"bar"
> id=3D"Lib.Name.Dynamic" />
> <property name=3D"Lib.Name" refid=3D"Lib.Name.${Type}" />
>
> BUT, the Holy Book[1] says it is bad :))

It does?  I had to go search for refid in the PDF to find what I said=20
about it.  The full section is pasted below.  Yeah, I suppose I did say=20=

it is bad, but only because of what I thought of a better alternative=20
to property swapping.  I would argue that for most cases, what folks=20
really should use is the .properties file trick shown below, but having=20=

dynamically dereferenced properties is handy in some cases too.  I do,=20=

however, use the <propertycopy> task from ant-contrib, and now with=20
Stefan's macrodef example, I'll likely switch to that.

	Erik


Dereferencing properties
Makefile experts, and others desiring tricky variable dereferencing may=20=

be disappointed
to find that Ant does not have advanced evaluation of properties. They=20=

are
simply string substitutions and nesting properties does not accomplish=20=

what some
may expect. For example

   <property name=3D"X" value=3D"Y"/>
   <property name=3D"Y" value=3D"Z"/>
   <property name=3D"A" value=3D"${${X}}"/>
   <property name=3D"B" value=3D"$${${X}}"/> The =93$$=94 is replaced by =
=93$=94
   <echo message=3D"A =3D ${A}"/>
   <echo message=3D"B =3D ${B}"/>

The output of the above is

   [echo] A =3D ${${X}}
   [echo] B =3D ${Y}

It is possible, however, to accomplish this, though rarely, if ever,=20
would this particular
technique be needed in a build file. Make has a feature called=20
=93computed variable
names,=94 which is similar to our first attempts at dereferencing, yet=20=

with different
results. (In other words, A would have equaled Z.) Using an additional=20=

property is
required as a selector:

   <property name=3D"X" value=3D"Y" id=3D"X.prop"/>
   <property name=3D"Y" value=3D"Z" id=3D"Y.prop"/>
   <property name=3D"selector" value=3D"${X}"/>
   <property name=3D"A" refid=3D"${selector}.prop"/>
   <echo message=3D"A =3D ${A}"/>

While this appears fairly straightforward, it is actually taking=20
advantage of some fairly
complex capability of Ant, that of assigning an id to a task (in this=20
case <property>).
The value of selector becomes Y, and the assignment of A uses the value
of the referenced =93object=94 (in this case a task) by the name of =
Y.prop.=20
Avoid this
kind of wackiness at almost all costs because there are much more=20
standard and
clearer ways to choose a different set of properties, such as

   <property name=3D"props" value=3D"default"/>
   <property file=3D"${props}.properties"/>

In this case, we load default.properties unless the property props has=20=

been
overridden previously, perhaps with

   ant -Dprops=3Dmy

This would load my.properties instead, thanks to property immutability=20=

and -D
setting props first.

NOTE There is a third-party task <propertycopy> provided at the=20
Sourceforge
ant-contrib project that more cleanly accomplishes property=20
dereferencing.
We recommend using this task instead of the craziness shown
here. See section 10.6 for details on <propertycopy>.


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