HelloJniLib

This repo is a sample for Android JNI libraries compilation with NativeAOT.

Prerequisites

This sample requires the NativeAOT Runtime and Libraries for Android Systems.
To get them by now you need to build them by your self using https://github.com/MichalStrehovsky/runtime/commit/0e1708375f93f66476179c54f8bb2123a88b901a

How to build it

This process was tested for android-arm64 compilation but may works for android-x64 too.
The following command assumes:

• The working directory is the same of this repo.

• ANDROID_NDK_ROOT envirionment variable: Full path to NDK.

• RUNTIME_REPO envirionment variable: Full path to runtime repo.

• Android NDK version is r21b.

• Android API version target is 28.

• Android API minimal version is 21.

• Target architecture is arm64.

• Host architecture is x64.

  libSystemPath=$RUNTIME_REPO/artifacts/bin/runtime/net7.0-Android-Release-arm64/ \
  RealCppCompilerAndLinker=$ANDROID_NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android21-clang \
  dotnet publish -r android-arm64 -c Release --self-contained \
   /p:CppCompilerAndLinker=./fakeClang \
   /p:SysRoot=$ANDROID_NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
   /p:IlcSdkPath=$RUNTIME_REPO/artifacts/bin/coreclr/Android.arm64.Release/aotsdk/ \
   /p:IlcFrameworkPath=$RUNTIME_REPO/artifacts/bin/runtime/net7.0-Android-Release-arm64/ \
   /p:IlcFrameworkNativePath=$RUNTIME_REPO/artifacts/bin/runtime/net7.0-Android-Release-arm64/ 
  

Parameters

• libSystemPath: Path of libSystem native libraries. This is needed to apply a hack for excluding the missing libraries.
• RealCppCompilerAndLinker: NDK linker tool. This is needed to apply a hack for excluding the missing libraries at linking process.
• CppCompilerAndLinker: Linker. The fakeClang is just an script that removes the missing libraries from linker invocation.
• SysRoot: Sysroot path from NDK. Needed for NDK compilation.
• IlcSdkPath: Path to NativeAOT libs. We can't use the standard libraries from ilcompiler.
• IlcFrameworkPath: Path to NativeAOT runtime (Managed part). We can't use the standard runtime from ilcompiler.
• IlcFrameworkNativePath: Path to NativeAOT runtime (Native part or libSystem). We can't use the standard runtime from ilcompiler.

Considerations

• You can also build this in reflection-free mode, it will significantly reduce the size of the binary.

How to strip an android binary

You can't strip an android binary using strip command from your linux envirionment so you need use from Android NDK.
For example:

$ANDROID_NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-strip bin/Release/net6.0/android-arm64/publish/AndroidHelloJniLib.so

How to test it?

This sample is inspired by https://github.com/android/ndk-samples/tree/main/hello-jni so with some changes you can load this library from that application.

Considerations:

• The package of the calling class: com_example_hellojni
• The name of the calling class: HelloJni.
• The name of the native function in the calling class: stringFromJNI.
• In the above example the name of the JNI library: libhello-jni.so.

Dependencies

The native Android binary produced has a dependency with libc++_shared.so. So you need include it along with the generated binary.
You can get it for arm64 from:

/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android