Centralising enviroment variables in resource files

It's always a pain to manage different resource files, which most of times, share non-environmental properties with environmental dependent properties. Keeping one copy of each file per environment is not always a good option, for the following reasons:

• To create a new environment, it's needed to go through all files looking for environment dependent ones.
• Changes to environment dependent files need to be in sync with all different environment
• Duplication of the same properties in different resource files
  

One solution of this problems is to parametrise the environment dependent values into a central and single file, which is then processed using Ant (or other tool if you prefer).

In this article, Pete Eakle, shows how to use Ant to replace tokens in css files and resource files.

I think it was missing what Ant replacement can do best, to separate the environmental dependencies in one central file per environment (and possibly also per application).

The following figure shows how the approach would work:



The env files (eg: dev.properties, qa.properties) would contain only environmental name=values entries. Those names would be tokens in the central resource files.

For example:

dev.properties:
...
#HOST FOR THE DB
DB_HOST=xdbserver
...

and inside my central resource file:

myspring.properties :

db_host=${DB_HOST}

In this way, i can change the properties on the fly during build, and before deploying. Using the replacement token ${xxx} is not mandatory and can be changed in the ant script.

The example should be used with the following structure:

root
|-resources-|-myspring.properties
|           |-x.properties
|-build.xml
|-dev.properties
|-live.properties
|-target-
|-resources

The build file can be something like the following:

<?xml version="1.0"?>
<project name="resources_merging" basedir="." default="filter_resources">
<target name="filter_resources" depends="describe" if="env_type">
<echo message="Replacing tokens for resource files using ${env_type}.properties">
<copy todir="./target/resources/" overwrite="true">
<filterset begintoken="$${" endtoken="}">
<filtersfile file="./${env_type}.properties" />
</filterset>
<fileset dir="./resources" includes="**/*.*" />
</copy>
</target>
<target name="describe" unless="env_type">
<echo message="Deployment type required, eg.: ant -Denv_type=local filter_resources" />
</target>

</project>

To use this scrip, you need to provide an environment parameter, like:

ant -Denv_type=dev filter_resources

This will replace the values provided in dev.properties in all the resource files, and copy all of them to the target/resources folder (that needs to be created beforehand).

It uses the filterset option of the copy ant task, which will search for all the parameters in all the files (in this case in a recursive way).

If you have any doubt about this approach or would like to discuss it, please let me know!!

Emerson Cargnin