Adding the potential issues with their resolution steps while building a java based android project using mill

[sushmi-a]

The below are the potential issues that can come up when building a basic java based android project with mill and its resolution instructions

Issue 1: Incorrect or older Mill version used
Root Cause: Some Android features require newer Mill versions with proper support for AndroidAppModule.

Resolution:
You switch to the correct version using:

> ./mill --version
> ./mill -i mill.scalalib.ShowMillVersion
> ./mill -i mill.androidlib.ShowAndroidVersion

or updated your Mill wrapper to a known working version (>= 0.11.5 or custom-built master).

(I have used Mill version 1.0.0-M1 that supports AndroidAppModule)

Issue 2: build.mill having incorrect or incomplete structure
Symptoms: Mill failed to load the build, or Android-related tasks were not available.

Root Cause: build.sc missing required Android plugin imports, or module hierarchy (e.g., AndroidAppModule) is incorrect.

Resolution:
You correct the file to include:

import mill._, mill.scalalib._, mill.androidlib._

object app extends AndroidAppModule {
  def scalaVersion = "2.13.13"
  def manifest = T.source("app/src/main/AndroidManifest.xml")
  def resources = T.sources("app/src/main/res")
  def packageName = "com.helloworld.app"
  def mainClass = Some("com.helloworld.app.MainActivity")
  def minSdkVersion = 19
  def targetSdkVersion = 35
  def platformTarget = "android-35"
}

( Adding the sample build.mill I used in my project )

import mill._, androidlib._, scalalib._, define._, mill.api._

// Android SDK module to provide SDK info to other modules
object androidSdkModule0 extends AndroidSdkModule {
  def buildToolsVersion = "35.0.0"
  def sdkPath = T(Some(PathRef(os.home / "Library" / "Android" / "sdk")))
}

// Main Android application module
object app extends AndroidAppModule {
  def androidSdkModule = mill.define.ModuleRef(androidSdkModule0)
  def androidMinSdk = 19
  def androidCompileSdk = 35
  def androidApplicationId = "com.helloworld.app"
  def androidApplicationNamespace = "com.helloworld.app"

  // Signing config for release APK - replace with your actual secure values before production
  def androidReleaseKeyName = Some("releaseKey.jks")
  def androidReleaseKeyAlias = T(Some("releaseKey"))
  def androidReleaseKeyPass = T(Some("MillBuildTool"))
  def androidReleaseKeyStorePass = T(Some("MillBuildTool"))

  override def androidVirtualDeviceIdentifier = "java-test"

  // Unit tests for the app
  object test extends AndroidAppTests with TestModule.Junit4 {
    def junit4Version = "4.13.2"
  }

  // Instrumented UI tests run on emulator or device
  object it extends AndroidAppInstrumentedTests with AndroidTestModule.AndroidJUnit {
    def androidSdkModule = mill.define.ModuleRef(androidSdkModule0)
    def mvnDeps = super.mvnDeps() ++ Seq(
      mvn"androidx.test.ext:junit:1.2.1",
      mvn"androidx.test:runner:1.6.2",
      mvn"androidx.test.espresso:espresso-core:3.5.1",
      mvn"junit:junit:4.13.2"
    )
  }
}

Issue 3: APK installation fails with certificate error
Symptoms:

> adb install app.unsigned.apk
INSTALL_PARSE_FAILED_NO_CERTIFICATES
Root Cause: Mill produces an unsigned APK by default.

Resolution:
You manually sign the APK using apksigner.

Issue 4: apksigner command not found
Symptoms:

zsh: command not found: apksigner

Root Cause: apksigner from Android Build Tools is not in your system PATH.

Resolution:
You added the path:

> export PATH=$PATH:$ANDROID_HOME/build-tools/35.0.0

Issue 5: Unsure of keystore password
Symptoms: Stuck during signing because keystore credentials were unclear.

Resolution: You use the default debug keystore:

Path: ~/.android/debug.keystore

Password: android

Alias: androiddebugkey

Then signed the APK:

> apksigner sign --ks ~/.android/debug.keystore \
               --ks-key-alias androiddebugkey \
               --ks-pass pass:android \
               out/app/androidUnsignedApk.dest/app.unsigned.apk

Final Step: Successful APK install

> adb install out/app/androidUnsignedApk.dest/app.unsigned.apk

Result:
Success

[lefou]

I moved this into a discussion, since it's not really a actionable issue. I also slightly edited your post to make the listings more readable.