Writing simple Gradle plugin

- 4 mins

Recently, in my current project we have stumbled upon an issue with Gradle. To give some context, we use gradle as our build tool. We use it also to execute Cucumber and Selenium based end2end tests. As the test suite grew we started to add more and more properties to its configuration. We started also to execute those tests not only on CI environment but also on other machines (development environments for example). Soon we wanted to be able to overwrite the test properties while scheduling the build. Having experience from Maven, we simply thought that adding -D parameter to the build with properties specified, would overwrite those which we have defined. Unfortunately after some digging it turned out that gradle will not by default pass to test execution any properties specified with -P nor -D. More on that can be read in many stackoverflow topics:

I have quickly tackled the problem with a little bit of test task configuration:

test {
    gradle.startParameter.projectProperties.each { key, value -> systemProperty key, value }
}

I used project properties (-P) to keep compatibility with our previous approach. But nothing actually stands in the way to make use of system arguments(-D). To use those, the configuration needs only a small change:

test {
    gradle.startParameter.systemPropertiesArgs.each { key, value -> systemProperty key, value }
}

Plugin

That is mostly it as it comes to giving some context. After the fix I thought that maybe it could be nice to write a plugin which gives exactly the same functionality. The solution for the problem isn’t any rocket science, that it would require a plugin, but it seemed like a good idea to try something different. So I started to read about how to start. It actually turned out to be quite easy. You can define a plugin for gradle in the build.gradle file itself.

apply plugin: GradlePlugin  

class GradlePlugin implements Plugin<Project> {
    @Override
    void apply(Project project) {
    }
}

And that’s it. Simple plugin created and ready to be used. And after couple of changes and tests, my plugin ended up on my github and the end result looked like that:

package com.gmaslowski.gradle.plugin.property

import org.gradle.api.Plugin
import org.gradle.api.Project

class PropertyGradlePlugin implements Plugin<Project> {

    @Override
    void apply(Project project) {
        project.gradle.taskGraph.whenReady { graph ->
            if (graph.hasTask(':test')) {
                project.tasks.getByName('test').configure {
                    project.gradle.startParameter.systemPropertiesArgs.each { key, value -> systemProperty key, value }
                }
            }
        }
    }
}

There’s nothing really fancy going on there. The plugin waits until the task graph is being created, and than if a test task exists, it gets configured to set system properties for test execution from the command line arguments. So invoking the tests like that gradle test -Dproperty=value would set property property with value value for tests execution. The source code can be found at https://github.com/gmaslowski/property-gradle-plugin.

Plugin Repository

And there came the moment where I thought I’d like to share my plugin with the rest of the world :D. Honestly, I just wanted to have it somewhere available for others to use. So my first choice was Bintray. But it is even easier than that! Gradle has it’s own plugin repository available at https://plugins.gradle.org/ and even provides a simple page with instructions for publishing plugins available at https://plugins.gradle.org/docs/publish-plugin. There is a requirement to have an account and use the provided API keys (all described here -> https://plugins.gradle.org/docs/submit). Having everything set up, in order to publish the plugin, it’s only required to invoke gradle publishPlugins and the plugin gets uploaded to the repository. Gradle even provide a page with description on how the uploaded plugin should be used.

buildscript {
  repositories {
    maven {
      url "https://plugins.gradle.org/m2/"
    }
  }
  dependencies {
    classpath "gradle.plugin.com.gmaslowski.gradle.plugin.property:property-gradle-plugin:0.4"
  }
}

apply plugin: "com.gmaslowski.gradle.plugin.property"

That can be found at https://plugins.gradle.org/plugin/com.gmaslowski.gradle.plugin.property

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