How to run Java standalone app (with JNI) on Android without creating an apk

In this week, I found a great POC to run a pure Java standalone app (command line tool, no apk) on Android. But what about running a standalone application using JNI (with .so files) on Android like this?

Java app with JNI

Imagine there is a Java program that loads the JNI shared native library to run and use some Android APIs :

package com.example;
import android.os.Build;
import android.util.Log;
public class Helloworld {

static { System.loadLibrary("hello"); }
    public static native String stringFromJNI();

public static void main(String[] args) {
        Log.i("@@", "Hello world, " + Build.MANUFACTURER + " "+ Build.MODEL + "!");
        Log.i("@@", stringFromJNI());
        System.out.println(stringFromJNI());
        System.out.println("DONE.");
    }

public static String getBuildVersion() {
        return Build.VERSION.RELEASE;
    }
}

…the JNI source would be like:

// ... emit codes

JNIEXPORT jstring JNICALL
Java_com_example_Helloworld_stringFromJNI(JNIEnv *env,
                                          jobject thiz)
{
  // ... emit codes
  jmethodID versionFunc = (*env)->GetStaticMethodID(env, clz, "getBuildVersion", "()Ljava/lang/String;");

jstring buildVersion = (*env)->CallStaticObjectMethod(env, clz, versionFunc);
  const char *version = (*env)->GetStringUTFChars(env, buildVersion, NULL);

if (!version)
  {
    LOGE("Unable to get version string");
  }
  else
  {
    LOGI("Build Version - %s\n", version);
    (*env)->ReleaseStringUTFChars(env, buildVersion, version);
  }
  (*env)->DeleteLocalRef(env, buildVersion);

return (*env)->NewStringUTF(env,
                              "Hello from JNI !  Compiled with ABI " ABI ".");
}
// ...

The working directory structure

.
├── Helloworld.java
└── hello-jni.c

Compile and deploy

Now we need to compile both the Java and C sources for Android.

Using javac and dx to compile for a jar file which Android can read:

export BUILD_DIR=$PWD/build
export JARFILE=helloworld.jar
export JAVAC_OPTS=-source 1.8 -target 1.8 -cp .:$ANDROID_HOME/platforms/android-30/android.jar
# Compile .java to .class
javac $JAVAC_OPTS -d $BUILD_DIR/classes Helloworld.java
# Convert .class file into a dex file and embedded in a jar file
$ANDROID_HOME/build-tools/30.0.2/dx --output=$BUILD_DIR/$JARFILE --dex ./$BUILD_DIR/classes

Cross-compile the C to Android shared native library via NDK:

# Using the prebuilt toolchain diretly
# See https://developer.android.com/ndk/guides/other_build_systems
export ANDROID_NDK_STANDALONE=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/darwin-x86_64
$ANDROID_NDK_STANDALONE/bin/clang \
    --target=aarch64-none-linux-android21 \
    --gcc-toolchain=$ANDROID_NDK_STANDALONE \
    --sysroot $ANDROID_NDK_STANDALONE/sysroot \
    -L${ANDROID_NDK_STANDALONE}/sysroot/usr/lib \
    -shared -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
    -fstack-protector-strong -no-canonical-prefixes -fno-addrsig -fPIC \
    -Wl,-llog \
    -Wl,-soname,libhello.so \
    -o $BUILD_DIR/libhello.so hello-jni.c

The build directory looks like:

build
├── classes
│   └── com
│       └── example
│           └── Helloworld.class
├── helloworld.jar
└── libhello.so

Using the adb tool to deploy the helloworld.jar and libhello.so to Android device:

adb shell mkdir /data/local/tmp/helloworld
adb push $BUILD_DIR/helloworld.jar $BUILD_DIR/libhello.so /data/local/tmp/helloworld/

Run appliation on Android

Run helloworld.jar via app_process on Android:

adb shell CLASSPATH="/data/local/tmp/helloworld/helloworld.jar" \
    LD_LIBRARY_PATH=/data/local/tmp/helloworld \
    app_process \
    /data/local/tmp/helloworld \
    com.example.Helloworld
# output
Hello from JNI !  Compiled with ABI arm64-v8a.
DONE.

Logging in adb logcat:

07-06 07:57:38.911 21599 21599 D AndroidRuntime: >>>>>> START com.android.internal.os.RuntimeInit uid 2000 <<<<<<
07-06 07:57:38.915 21599 21599 I AndroidRuntime: Using default boot image
07-06 07:57:39.025 21599 21599 D AndroidRuntime: Calling main entry com.example.Helloworld
07-06 07:57:39.027 21599 21599 I @@      : Hello world, vivo V2219A!
07-06 07:57:39.027 21599 21599 I hello-jni: Build Version - 12
07-06 07:57:39.027 21599 21599 I @@      : Hello from JNI !  Compiled with ABI arm64-v8a.
07-06 07:57:39.027 21599 21599 I hello-jni: Build Version - 12
07-06 07:57:39.028 21599 21599 D AndroidRuntime: Shutting down VM

Startup shell script

We can create a startup shell script for this tool:

#!/system/bin/sh
HERE="$(cd "$(dirname "$0")" && pwd)"

export CLASSPATH=$HERE/helloworld.jar
export ANDROID_DATA=$HERE
export LD_LIBRARY_PATH="$HERE"

if [ -f "$HERE/libc++_shared.so" ]; then
    # Workaround for https://github.com/android-ndk/ndk/issues/988.
    export LD_PRELOAD="$HERE/libc++_shared.so"
fi

echo "try com.example.Helloworld with LD_LIBRARY_PATH=${LD_LIBRARY_PATH}"
cmd="app_process $HERE com.example.Helloworld $@"
echo "run: $cmd"
exec $cmd

Push the script to Android device:

adb push helloworld /data/local/tmp/helloworld/
# executable
adb shell chmod a+x /data/local/tmp/helloworld/helloworld

Execute the shell script via adb shell:

adb shell /data/local/tmp/helloworld/helloworld

Makefile

We can bundle the compile and deploy commands into a convenient Makefile :

BUILD_DIR=build

JAVAC_OPTS=-source 1.8 -target 1.8 -cp .:$(ANDROID_HOME)/platforms/android-30/android.jar

APP_PROCESS=app_process
JARFILE=helloworld.jar
ifeq ($(ARCH),arm)
TARGET=--target=armv7-none-linux-androideabi19 -march=armv7-a -mfpu=vfpv3-d16
APP_PROCESS=app_process32
else
TARGET=--target=aarch64-none-linux-android21
endif

$(BUILD_DIR)/$(JARFILE) : Helloworld.java
	test -d $(BUILD_DIR) || mkdir $(BUILD_DIR)	
	$(JAVA_HOME)/bin/javac $(JAVAC_OPTS) -d $(BUILD_DIR)/classes Helloworld.java
	$(ANDROID_HOME)/build-tools/30.0.2/dx --output=$(BUILD_DIR)/$(JARFILE) --dex ./$(BUILD_DIR)/classes

$(BUILD_DIR)/libhello.so : hello-jni.c
	test -d $(BUILD_DIR) || mkdir $(BUILD_DIR)
	$(ANDROID_NDK_STANDALONE)/bin/clang \
		$(TARGET) --gcc-toolchain=$(ANDROID_NDK_STANDALONE) \
		--sysroot $(ANDROID_NDK_STANDALONE)/sysroot \
		-L$(ANDROID_NDK_STANDALONE)/sysroot/usr/lib \
		-shared -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
		-fstack-protector-strong -no-canonical-prefixes -fno-addrsig -fPIC \
		$(CFLAGS) -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libatomic.a \
		-Wl,--build-id -Wl,--warn-shared-textrel \
		-Wl,--no-undefined -Wl,--as-needed \
		$(LINKFLAGS) -Wl,-llog \
		-Wl,-soname,libhello.so \
		-o $(BUILD_DIR)/libhello.so hello-jni.c 

all: $(BUILD_DIR)/$(JARFILE) $(BUILD_DIR)/libhello.so helloworld
.PHONY : clean deploy
deploy : all
	adb shell mkdir /data/local/tmp/helloworld
	adb push --sync $(BUILD_DIR)/$(JARFILE)  $(BUILD_DIR)/libhello.so helloworld /data/local/tmp/helloworld/
	adb shell chmod a+x /data/local/tmp/helloworld/helloworld
	$(info 	adb shell /data/local/tmp/helloworld/helloworld) 
clean :
	test -d $(BUILD_DIR) && rm -rf $(BUILD_DIR) || true
	adb shell -n "test -d /data/local/tmp/helloworld && rm -rf /data/local/tmp/helloworld || true"

Using make for compile all targets:

ANDROID_NDK_STANDALONE=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/darwin-x86_64 \
    make all

Complete codes

You can get the complete codes in my git repo: https://github.com/yrom/sample-android-java-standalone-tool

git clone https://github.com/yrom/sample-android-java-standalone-tool.git
cd sample-android-java-standalone-tool

ANDROID_NDK_STANDALONE=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/$HOST_TAG \
    make all

…the HOST_TAG:

(from https://developer.android.com/ndk/guides/other_build_systems#overview)

Conclusion:

• Compile Java sources to an Android dex jar file via javac and dx
• Compile C (or C++) sources to shared native library via ndk
• Push complete .jar file and .so file(s) to Android device under /data/local/tmp
• Run the main class via app_prcoess on Android device from adb shell

References: