j4rs-android-activity

An example project demonstrating how to use j4rs with android-activity.

Usage

Just execute:

./gradlew build

You will find the produced apk under app/build/outputs/apk/debug/app-debug.apk and you can install it in a connected Android device (or emulator) issuing:

adb install ./app/build/outputs/apk/debug/app-debug.apk

You may see the logs in logcat:

adb logcat | grep RustStdoutStderr

11-19 16:42:12.159 5921 5678 I RustStdoutStderr: =======================true

Behind the scenes

Gradle builds the rust project, which generates the native library libj4rs_android_activity.so and copies it under app/src/main/jniLibs/arm64-v8a/libj4rs_android_activity.so.

The rust build is done using cargo:

cargo build --target=aarch64-linux-android --release

Important things for using j4rs and android-activity

Rust

• Use the j4rs dependency from the github (until it is publshed - release 0.22.0)

j4rs = {git = "https://github.com/astonbitecode/j4rs"}

• Retrieve the JavaVM and Activity jobject from the AndroidApp and use them to create a j4rs Jvm

use android_activity::AndroidApp;
use j4rs::{InvocationArg, JvmBuilder};
use j4rs::jni_sys::{JavaVM, jobject};

#[no_mangle]
fn android_main(app: AndroidApp) {
    let java_vm: *mut JavaVM = app.vm_as_ptr().cast();
    let activity_obj: jobject = app.activity_as_ptr().cast();
    let jvm = JvmBuilder::new()
        .with_java_vm(java_vm.clone())
        .with_classloader_of_activity(activity_obj.clone())
        .build()
        .unwrap();
}

You will be able to use j4rs as normal after this.

Note that after the initial Jvm creation, new Jvm instances (eg. in new threads) can be created by just attaching to the thread:

std::thread::spawn(move || {
    let other_jvm = Jvm::attach_thread().unwrap();
}

• Build for the architecture you need. Eg., for arm v8a:

cargo build --target=aarch64-linux-android --release

Gradle

• Include the j4rs dependency in your build.gradle:

dependencies {
    implementation 'io.github.astonbitecode:j4rs:0.21.0'
}

• Define the name of the native library in AndroidManifest.xml:

<activity
    android:name="android.app.NativeActivity"
    android:label="@string/app_name"
    android:exported="true">
    <!-- The android:value must contain the name of the rust library (j4rs_android_activity for this example) -->
    <meta-data android:name="android.app.lib_name" android:value="j4rs_android_activity" />
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>