Mill as an Alternative Android Build Tool :: The Mill Build Tool

Mill’s Android support is still young, but it already covers the full build pipeline: resource compilation, manifest merging, packaging, signing, running, and even testing on emulators.

What makes this different from Gradle are control and transparency: every build step is a visible Mill task, easy to run on its own, inspect, check its dependencies, or override, without needing any extra/third party plugins. That means you can debug problems faster, adapt the pipeline to your project’s needs, and extend it without fighting opaque built-in or plugin logic.

If you’re curious, the best way to appreciate this is to try it yourself:

Get the architecture-samples containing the Todo App.

> git clone git@github.com:android/architecture-samples.git
> cd architecture-samples

Install mill

> curl -L https://repo1.maven.org/maven2/com/lihaoyi/mill-dist/1.0.5/mill-dist-1.0.5-mill.sh -o mill
> chmod +x mill
> echo "//| mill-version: 1.0.5" > build.mill
> ./mill version

Configure the mill build

> curl https://raw.githubusercontent.com/com-lihaoyi/mill/bef0194f3eecb4c7938f07e0cfcdf8d741a04468/example/thirdparty/androidtodo/build.mill >> build.mill

Start the emulator and run the app

> ./mill show app.createAndroidVirtualDevice
> ./mill show app.startAndroidEmulator
> ./mill show app.androidInstall
> ./mill show app.androidRun --activity com.example.android.architecture.blueprints.todoapp.TodoActivity

The Android Todo App built with Mill The Todo app built with Mill

Run the instrumented tests and watch the app being tested inside the emulator:

Android Test running inside an emulator

Let’s say you want to know how the apk is built. First, you can check the plan of androidApk, i.e. which tasks it depends on:

$ ./mill plan app.androidApk
[1/1] plan
androidSdkModule0.sdkPath
androidSdkModule0.buildToolsVersion
androidSdkModule0.platformsVersion
androidSdkModule0.remoteReposInfo
androidSdkModule0.installAndroidSdkComponents
androidSdkModule0.buildToolsPath
androidSdkModule0.apksignerPath
androidSdkModule0.zipalignPath
app.mandatoryMvnDeps.super.javalib.JavaModule
app.kotlinVersion

You can use this to visualise the relationships between these tasks and how they feed each other and ultimately the androidApk task:

$ ./mill visualizePlan app.androidApk
[3/3] visualizePlan
[
  ".../architecture-samples/out/visualizePlan.dest/out.dot",
  ".../architecture-samples/out/visualizePlan.dest/out.json",
  ".../architecture-samples/out/visualizePlan.dest/out.png",
  ".../architecture-samples/out/visualizePlan.dest/out.svg",
  ".../architecture-samples/out/visualizePlan.dest/out.txt"
]
[3/3] ============================== visualizePlan app.androidApk ============================== 2s

You can also check the code of each task and what it does exactly inside your IDE: Exploring the Mill Android build tasks in an IDE

In addition, due to Mill’s direct style, you can reason what’s going on with relative ease.

Example: tweak the build in your `build.mill`

import mill._
import mill.androidlib._

object app extends AndroidAppModule {
  def androidApplicationNamespace = "com.example.app"
  def androidApplicationId = "com.example.app"
  def androidCompileSdk = 35

  // Add extra files into the APK
  override def androidPackageableExtraFiles = super.androidPackageableExtraFiles() ++
    Seq(
      AndroidPackageableExtraFile(
          PathRef(moduleDir / "assets/about.txt"),
          os.RelPath("assets/about.txt")
      )
    )

}

Further Exploration

We’d love feedback from the Android community, whether it’s bug reports, feature requests, or success stories. If you’ve ever wished Android builds felt less like a black box, Mill is worth a look.

Example: tweak the build in your build.mill

Further Exploration

Example: tweak the build in your `build.mill`