Hello world in Rust, for Android

30 June 2025, south of France, it's too hot to turn on a PC, I want to do some Rust hacking directly on my Android tablet. How hard can it be?

Here is a minimal test application based on NativeActivity that just outputs a line into a file.

If you intend to use SlintUI for the user interface, check out the branch with-slint.

Gradle Build on an Android device

This is an updated version of an old comment on a termux github issue

Install termux

To be able to type commands and install packages, install termux from F-Droid (NOT from Google Play!!!) or from github. Ignore the big fat warning about the android version, there’s a small « install anyway » button/link.

pkg upgrade
pkg install git vim rust
pkg install openjdk-17 gradle android-tools

Install cmdline-tools

First, open https://developer.android.com/studio and scroll down to command line tools to find the last release and update the version number into the URL below.

curl -L https://dl.google.com/android/repository/commandlinetools-linux-13114758_latest.zip -o commandlinetools-linux-latest.zip
mkdir -p ~/Android/sdk/cmdline-tools
unzip commandlinetools-linux-latest.zip -d ~/Android/sdk/cmdline-tools
mv ~/Android/sdk/cmdline-tools/cmdline-tools ~/Android/sdk/cmdline-tools/latest

Install android-sdk-tools

Open https://github.com/lzhiyong/android-sdk-tools/releases to find out the version number for the URL below.

curl -L https://github.com/lzhiyong/android-sdk-tools/releases/download/35.0.2/android-sdk-tools-static-aarch64.zip -o tools.zip
unzip tools.zip
# check you can run it (that’s what doesn’t work with the Google Play version of termux)
build-tools/aapt2

rm -rf ~/Android/sdk/build-tools/latest
mkdir -p ~/Android/sdk/build-tools
mv build-tools ~/Android/sdk/build-tools/latest
rm -rf ~/Android/sdk/platform-tools-latest
mkdir -p ~/Android/sdk
mv platform-tools ~/Android/sdk/platform-tools-latest

Set up environment variables

export ANDROID_HOME=~/Android/sdk/
export JAVA_HOME=$PREFIX/lib/jvm/java-17-openjdk
export PATH=$ANDROID_HOME/cmdline-tools/latest/bin:$PATH
export PATH=$ANDROID_HOME/build-tools/latest:$PATH

Install Android Platforms

yes | sdkmanager –licenses
yes | sdkmanager "platforms;android-30"

Force using the ARM version of aapt2

By default gradle will try to run the x86_64 version of aapt2, while we obviously want to run the ARM version.

echo "android.aapt2FromMavenOverride=$ANDROID_HOME/build-tools/latest/aapt2" >> ~/.gradle/gradle.properties

Self-signing on Android

keytool -genkey -v -keystore $HOME/debug.keystore -storepass android -alias androiddebugkey -keypass android -keyalg RSA -keysize 2048 -validity 10000

The file rust-android-hello-world/app/build.gradle defines a signingConfigs section that points to that debug.keystore file.

Building on Android

Now we can finally build this hello world example.

cd rust-android-hello-world
cargo build --target aarch64-linux-android --release
cp target/aarch64-linux-android/release/*.so app/src/main/jniLibs/arm64-v8a/
./gradlew assembleDebug
cp app/build/outputs/apk/debug/*.apk /sdcard/Download/

Tip: create an alias for those last 4 commands separated by &&.

Installing and running

I didn't get termux-open app-debug.apk to work (it gives an unspecific error about the package). What works is to copy it to a folder like Download (as done in the previous section), launching a file manager to install the APK by clicking on it (after Android analyzes it). Finally, it offers to run it.

Logging

This example doesn't use log!() because when building and running the application directly on an Android device, and typing logcat in termux, nothing appeared. It turns out the app runs under a different UID than Termux so logcat in Termux can't see the app's logs for security reasons. This is why logging to a file is used instead.

Gradle Build on a Linux host (not tested)

According to the rust-mobile example I used as inspiration, this should work:

export ANDROID_NDK_HOME="path/to/ndk"
export ANDROID_HOME="path/to/sdk"

rustup target add aarch64-linux-android
cargo install cargo-ndk

cargo ndk -t arm64-v8a -o app/src/main/jniLibs/  build
./gradlew build
./gradlew installDebug

Cargo APK Build on a Linux host (not tested)

The rust-mobile example I used as inspiration, says:

Since this test doesn't require a custom Activity subclass it's optionally possible to build this example with cargo apk.

export ANDROID_NDK_HOME="path/to/ndk"
export ANDROID_SDK_HOME="path/to/sdk"

rustup target add aarch64-linux-android
cargo install cargo-apk

cargo apk build
cargo apk run

Thoughts

Does this mean cargo-ndk should gain support for building on Android? Currently it says You cannot build cargo-ndk _for_ Android. We could write a similar tool that just runs cargo and copies libs to the specified directory maybe...