Resource Filtering with Gradle

My team has recently started a new Java web application project and we picked gradle as our build tool. Most of us were extremely familiar with maven, but decided to give gradle a try.
Today I had to figure out how to do resource filtering in gradle. And to be honest it wasn't as easy as I thought it should be; as least coming from a maven background. I eventually figured it out, but wanted to post my solution to make it easier for others.

What is Resource Filtering?
First, for those that may not know, what is resource filtering? It's basically a way to avoid hard coding values in files and make them more dynamic. For example, I may want to display the version of my application in my application. The version is usually defined in your build file and this value can be injected or replaced in your configuration file during assembly. So I could have a file called config.properties under src/main/resources with the following content:
application.version=${application.version}. With resource filtering the ${application.version} value gets replaced with 1.0.0 during assembly, then my application can load config.properties and display the application version.

It's an extremely valuable and powerful feature in build tools like maven and one that I took advantage of often.

Resource Filtering in Gradle
With this being my first gradle project, I needed to find the recommended way to enable resource filtering in gradle. My first problem I had to figure out was where to define the property. In maven this would typically be defined in the project's pom.xml file as a maven property:

<properties>
    <application.version>1.0.0</application>
</properties>

For gradle the appropriate place seemed to be the project's gradle.properties file. So you would add the following to your project's gradle.properties file (Note, I'm not suggesting you would hardcode the modules version in the gradle.properties file. Obviously the value would be derived from the version property in your project. I'm just using this for a simple example):

application.version=1.0.0

The next, and most difficult, problem I had to track down was how to actually enable resource filtering. I was hoping to just set some enableFiltering option and define the includes/excludes list, but that doesn't seem to be the case (extra tip: don't do filtering on binary files like images). I did find some resources online, but this one seemed to be the best approach. So you will need to add the following to your build.gradle file:

import org.apache.tools.ant.filters.*

processResources {
    filter ReplaceTokens, tokens: [
        "application.version": project.property("application.version")
    ]
}

Next you need to update your resource file. So put a config.properties file under src/main/resources and add this:

application.version=@application.version@

Note, the use of @ instead of ${}. This is because gradle is based on ant, and ant by default uses the @ character as the token identifier whereas maven uses ${}.

Finally, if you build your project you can look under build/resources/main and you should see a config.properties file with a value of 1.0.0. You can also open up your artifact and see the same result.

Dot notation
One thing to note is I typically use a period or dot to separate words for properties: application.version instead of applicationVersion. So you will notice the surrounding quotes around "application.version" in the build.gradle file. This is required as failing to surround the key by quotes will fail the build. Probably because groovy's dynamic nature thinks you are traversing an object.

Overriding
I also investigated the best approach to overriding properties in gradle, as this appeared to be slightly different then how it's done in maven. In maven, properties can be overridden by properties defined in the user's setting.xml file or on the command line with the -D option. To override application.version in gradle on the command line I had to run the following:

gradle assemble -Papplication.version=2.0.0

If you want to override it for all projects you can add the property in your gradle.properties file under /user_home/.gradle.

Also, if you are overriding the value via the command line and your property value contains special characters like a single quote, you can wrap the value with double quotes like the following to get it to work:

gradle assemble -Papplication.version="2.0.0'6589"

Summary
Well I hope this helps and if anyone from the gradle community sees a better way to perform resource filtering I'd love to hear about it. I'd also like to see something as important as resource filtering becoming easier to perform in gradle. I think it's crazy having to add an import statement to perform something so simple.