Building and using a locally-modified version of JNA

Java Native Access is an important dependency for the Application Services components on Android, as it provides the low-level interface from the JVM into the natively-compiled Rust code.

If you need to work with a locally-modified version of JNA (e.g. to investigate an apparent JNA bug) then you may find these notes helpful.

The JNA docs do have an Android Development Environment guide that is a good starting point, but the instructions did not work for me and appear a little out of date. Here are the steps that worked for me:

Modify your environment to specify $NDK_PLATFORM, and to ensure the Android NDK tools for each target platform are in your $PATH. On my Mac with Android Studio the config was as follows:

export NDK_ROOT="$HOME/Library/Android/sdk/ndk/26.2.11394342"
export NDK_PLATFORM="$NDK_ROOT/platforms/android-25"
export PATH="$PATH:$NDK_ROOT/toolchains/llvm/prebuilt/darwin-x86_64/bin"
export PATH="$PATH:$NDK_ROOT/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/bin"
export PATH="$PATH:$NDK_ROOT/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin"
export PATH="$PATH:$NDK_ROOT/toolchains/x86-4.9/prebuilt/darwin-x86_64/bin"
export PATH="$PATH:$NDK_ROOT/toolchains/x86_64-4.9/prebuilt/darwin-x86_64/bin"

You will probably need to tweak the paths and version numbers based on your operating system and the details of how you installed the Android NDK.

Install the ant build tool (using brew install ant worked for me).
Checkout the JNA source from Github. Try doing a basic build via ant dist and ant test. This won't build for Android but will test the rest of the tooling.
Adjust ./native/Makefile for compatibility with your Android NSK install. Here's what I had to do for mine:
- Adjust the $CC variable to use clang instead of gcc: CC=aarch64-linux-android21-clang.
- Adjust thd $CCP variable to use the version from your system: CPP=cpp.
- Add -landroid -llog to the list of libraries to link against in $LIBS.
Build the JNA native libraries for the target platforms of interest:
- ant -Dos.prefix=android-aarch64
- ant -Dos.prefix=android-armv7
- ant -Dos.prefix=android-x86
- ant -Dos.prefix=android-x86-64
Package the newly-built native libraries into a JAR/AAR using ant dist. This should produce ./dist/jna.aar.

Configure build.gradle for the consuming application to use the locally-built JNA artifact:

// Tell gradle where to look for local artifacts.
repositories {
    flatDir {
        dirs "/PATH/TO/YOUR/CHECKOUT/OF/jna/dist"
    }
}

// Tell gradle to exclude the published version of JNA.
configurations {
    implementation {
        exclude group: "net.java.dev.jna", module:"jna"
    }
}

// Take a direct dependency on the local JNA AAR.
dependencies {
    implementation name: "jna", ext: "aar"
}

Rebuild and run your consuming application, and it should be using the locally-built JNA!

If you're trying to debug some unexpected JNA behaviour (and if you favour old-school printf-style debugging) then you can this code snippet to print to the Android log from the compiled native code:

#ifdef __ANDROID__
#include <android/log.h>
#define HACKY_ANDROID_LOG(...) __android_log_print(ANDROID_LOG_VERBOSE, "HACKY-DEBUGGING-FOR-ALL", __VA_ARGS__)
#else
#define HACKY_ANDROID_LOG(MSG)
#endif

HACKY_ANDROID_LOG("this will go to the android logcat output");
HACKY_ANDROID_LOG("it accepts printf-style format sequences, like this: %d", 42);

Cross-platform Rust Components

Building and using a locally-modified version of JNA