HelloJniLib

This repo is a sample for Android JNI libraries compilation with NativeAOT. For now, we support two ways to compile this library:

1. Using BFlat tool.
2. Using .NET SDK 8.0

BFlat

In order to optimize the build process we recommend the use of BFlat tool. This tool works on both Windows and Linux, so WSL environment is not needed for android compilation.

Prerequisites

This library depends on Rxmxnx.PInvoke.Extensions, so you need download the latest version of these packages:

• https://www.nuget.org/api/v2/package/Rxmxnx.PInvoke.Extensions

Extract dlls file to BFlatSupport folder.

Build

Use the following command to build library.

    {PATH_TO_BFLAT}bflat build -r BFlatSupport/Rxmxnx.PInvoke.Extensions.dll --no-stacktrace-data --no-globalization --no-exception-messages -Os --no-pie --separate-symbols --os:linux --arch:arm64 --libc:bionic -o:libhello-jni.so

NET SDK 8.0

With new .NET 8.0 SDK we are now able to compile NativeAOT android binaries using linux-bionic RID.

How to build it

This process was tested for android-arm64 (linux-bionic-arm64) compilation but may work for android-x64 ( linux-bionic-x64) too.
The following commands assume:

• ANDROID_NDK_ROOT environment variable: Full path to NDK. Used to preconfigure CppCompilerAndLinker, ObjCopyName and SysRoot.
• Android NDK version is r26b.
• Target architecture is arm64.

• Host architecture is windows, linux or macOS x64.

  dotnet publish -r linux-bionic-arm64 -p:DisableUnsupportedError=true -p:PublishAotUsingRuntimePack=true -p: AssemblyName=libhello-jni -p:RemoveSections=true

Environment Parameters

• ObjCopyName: Path of NDK llvm-objcopy. This is needed in order to use StripSymbols MSBuild parameter.

MSBuild Parameters

• CppCompilerAndLinker: Linker. The android_fake_clang is just a script that invokes the real NDK Clang executable.
• SysRoot: Sysroot path from NDK. Needed for NDK compilation.
• RemoveSections: Hack to remove __init and __fini symbols from .exports file.
• AssemblyName: In order to produce a .so file with given name.
• UseLibCSections: In order to use __libc_init and __libc_fini as exported __init and __fini symbols.

Considerations

• You can also build this in reflection-free mode, it will significantly reduce the size of the binary.
• This branch uses direct JNI calls via PInvoke but does not require the use of unsafe context.
• All rules for JNI call invocations apply to this implementation.
• The Jni namespace contains the structures and delegates necessary to interact with JNI securely.
• If you want to use JNI in a more friendly and familiar way with .NET code, check out the jnetinterface branch.

How to test it?

This sample is inspired by ndk-sample/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.
• On Android, JNI_OnUnload is never called.