Creating a fat AAR

I recently wrote a new library called NeuroSky Android SDK. It’s used for writing Android apps using signals of the brain waves received from the NeuroSky MindWave Mobile headsets. Probably I’ll write a separate article about it because it’s quite interesting topic. This library uses ThinkGear library, which is distributed by the NeuroSky as a *.jar file, so I couldn’t use it as a Gradle or Maven dependency in my project and I had to put this *.jar file into the lib directory and link it in the build.gradle file. Moreover, I wanted to create a library, which can be added to the project as a single Gradle dependency without messing around with additional *.jar files or custom configuration. Due to this fact, I decided to create a fat *.aar file and deploy it to the Maven Central repository. For those who are not familar with Android, *.aar is an Android version or *.jar file, which can be used as library in the project. I didn’t want to reinvent the wheel, so I searched for the different solutions. Unfortunatey, a few of them didn’t work, but luckilly I’ve found what I wanted. It’s fat AAR Gradle Plugin developed by Mobbeel company.

Here’s how I configured everything:

In my top-level build.gradle file I defined dependency to my custom *.jar file and other dependencies.

ext.deps = [ // other dependencies goes here...
             thinkgear : files('library/libs/ThinkGear.jar')
           ]

Next, I added dependency to the plugin within a buildscript section.

buildscript {
  repositories {
    google()
    jcenter()
    mavenCentral()
    maven {
      url 'https://plugins.gradle.org/m2/'
    }
  }
  dependencies {
   // other plugin dependencies goes here...
   classpath 'gradle.plugin.com.mobbeel.plugin:mobbeel-fataar:1.2.0'
  }
}

After that, in the library/build.gradle file, I could add dependency to the *.jar file.

dependencies {
  api deps.thinkgear
  // other dependencies goes here...
}

Then plugin could be applied.

apply plugin: 'com.mobbeel.plugin'

And I could configure it.

fatAARConfig {
  includeAllInnerDependencies false
}

When we set includeAllInnerDependencies parameter to true, then all transitive dependencies will be included in the fat *.aar. Default value is false and it’s OK for now. I could even skip this step, but sometimes I prefer to define things explicitly.

Now, we can build our library.

./gradlew build

and fat *.aar will be generated in the library/builds/output/ directory as a library-release.aar file. There should be also library-debug.aar. We can unzip this file and notice that it has the following structure:

.
├── AndroidManifest.xml
├── classes.jar
└── libs
    ├── rxandroid.jar
    ├── rxjava-2.2.2.jar
    └── ThinkGear.jar

so we can clearly see that all project dependencies are included in the *.aar file. Library deployed to the Maven Central Repository looks the same and now, users can add library as a single dependency to the project in the build.gradle file.

dependencies {
  implementation 'com.github.pwittchen:neurosky-android-sdk:0.0.2'
}

*.jar library shipped inside *.aar file will be included in the project and everything will just work.