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Tinkering around Gradle plugin basics

From time to time, I get to touch something cool and unusual. This time I got to touch Gradle plugins, which I don’t usually do, apart from applying to the project or configuring it by its extensions. I’m pretty sure I am not the only one with this case scenario.

Photo by Danil Shostak on Unsplash

TLDR;

Short story of how can you update / develop / tinker on already created Gradle plugin and bend it to your needs. You’ll find brief intro how to

  • Build / develop Gradle plugin
  • Import locally built plugin into your project with your changes applied
  • Brief overview of important components to know in developing a Gradle plugin

Backstory or just why

Here’s what we were aiming for. We are automating our projects using Appium. And we have decided to run the tests on a device farm. After a bit of investigation, we have decided to use BrowserStack.

But to run the tests on the latest app, we need to upload the APK artifact for the device farm to find. So this is our first challenge — upload the app to an app-automate server.

Luckily enough, there’s already a BrowserStack plugin that does this. However, as most things are too good to be true, we have the same situation here. It does only half of the job. It uploads the app to the app-testing server. But the plugin is open source, which means there’s a possibility to update it ourselves.

Rolling up the sleeves

This is where the fun part begins.

First off, I would like to mention that I did have an opportunity to write a few plugins in my time. However, I would hardly call myself knowing what I’m doing. At the very least most of the time. Just plain trial and error. Even so, I’m pretty sure there are some bits here that will be useful in the future for myself as well.

Integrating the plugin

Updating the plugin is not so different from just applying the plugin regularly. Let’s try that first.

  • First off, we need to provide the path where the plugin is located first. So in {rootProject}/build.gradle file, let’s add a source and a path.
buildscript {
repositories {
maven {
url "https://plugins.gradle.org/m2/"
}
}
dependencies {
classpath "gradle.plugin.com.browserstack.gradle:browserstack-gradle-plugin:3.0.3"
}
}
  • Then we need to apply the plugin. That is normally done in project module — {rootProject}/app/build.gradle file
apply plugin: "com.browserstack.gradle"
  • After integration, I like to check if everything synchronizes and works. My go-to command to check if the plugin has any public commands is ./gradlew tasks, which list all the tasks. Chance is, you’ll probably find a few tasks from the plugin there as well.

Building the plugin

Now that we know the plugin is working let’s try updating it.

My go-to method here is to fork the original project. That way, if we apply our changes, it is easy to contribute those changes back to the main project by creating a pull request.

After we clone the project, we could try to build it first. Generally, to build any project with the Gradle build system is to use ./gradlew build.

You can import the plugin as a regular project into IntelliJ as well. It’ll be easier to develop.

As a side note, there’s one neat trick. If the project does not have a Gradle wrapper, you can generate it by running task gradle wrap if you have a Gradle installed. If you don’t have Gradle installed, you can always use Homebrew to easily install it by running brew install gradle.

Have you managed to build it? Fingers crossed you did 🤞. If not, you can always try

  • Using older / different Gradle version
  • Checking out older branches that buildsb

Integrating a local config

As we have the plugin built on our system, we could try applying it to our project.

After plugin builds, it generates a jar file, which we will use in our app project from before ☝️. Most commonly, after building the plugin, the jar file will be located in {rootPlugin}/build/libs. So in my case, it was {rootPlugin}/build/libs/browserstack-gradle-plugin-3.0.3-u3x.jar.

Now we need to update the project, so that the project would use our locally editable plugin, instead of the official repository.

  • First off, lets change the dependency path to use our local computer path instead of remote path
buildscript {
repositories {
// Original plugin: https://mvnrepository.com/artifact/gradle.plugin.com.browserstack.gradle/browserstack-gradle-plugin/3.0.3
flatDir { dirs "/Users/mm/Projects/random/browserstack-gradle-plugin/build/libs" }
}
dependencies {
classpath ":browserstack-gradle-plugin-3.0.3-u3x"
}
}
  • To specify a directory, we use flatDir
  • We are specifying a directory where .jar file is built after we run a build task on a plugin
  • The classpath part starts with a semicolon - “:”
  • The classpath is a file name of the generated directory without an extension. So if I have a generated file {rootPlugin}/build/libs/browserstack-gradle-plugin-3.0.3-u3x.jar, I would need to provide a :browserstack-gradle-plugin-3.0.3-u3x classpath.
  • To apply the plugin, it is done the same as before. Apply apply plugin: "com.browserstack.gradle" in {rootProject}/app/build.gradle

Updating the plugin

Now that we have everything set up, you can try to update the plugin, build it and run it on a project to see the changes.

Essentially the plugin is a regular project with a bit specific structure in how things are applied. The key points of interest are these.

  • Main plugin class. This is the main class where plugin starts its work. You could find it by searching for a class that implements Plugin<Project> and overrides a public void apply(Project project) method. This is where all work begins.
  • To provide properties from a project, you would need to use a component called extensions. This is a regular POJO class however, to get properties from it, you’ll need to use project.getExtensions()
  • The project will not recognize a plugin unless it has a special property file with essential information. I had a lot of trouble with this, so be sure to check out {rootPlugin}/src/main/resources/META-INF.gradle.plugins/com.browserstack.gradle.properties on a working project. The directory/file naming is important here.
  • Gradle works using tasks. So to find those, you’ll need to keep an eye for extends DefaultTask or something similar. Also, you’ll need to register those tasks to the plugin as well to be recognized - project.getTasks().create("execute" + appVariantName + "TestsOnBrowserstack", EspressoTask.class...

After you change the plugin, build the plugin, then try to build the project you’re using the plugin in. It should synchronize the project and see the changes that you have applied for the plugin.

The end result

After a few tries, I’ve managed to provide a few new features which were essential to our use case. Moreover, I’ve managed to create a pull request to give it back to the open source community. And last but not least, create an example (this blog post) that it is actually not so hard to improve the project, by solving our own pains. In other words, open-source for the win 🚀.

And most important, I have not have had so much fun in a long time 🧁.

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Marius Merkevičius

Marius Merkevičius

Android engineer @ iTo

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