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# Building and using a locally-modified version of JNA
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](https://github.com/java-native-access/jna/blob/master/www/AndroidDevelopmentEnvironment.md)
that is a good starting point, but the instructions did not work for me and appear a little out of date.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](https://github.com/java-native-access/jna) 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);
```