Using .so files in Android Studio

Question

I am new to Android. I have a basic hello-world native code function like this:

    #include <string.h>
    #include <jni.h>
    #include <cassert>
    #include <string>
    #include <iostream>
    #include <fromhere.h>
    using namespace std;

    /* This is a trivial JNI example.
     * The string returned can be used by java code*/
     extern "C"{
    JNIEXPORT jstring JNICALL
        Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz )
    {
    #if defined(__arm__)
      #if defined(__ARM_ARCH_7A__)
        #if defined(__ARM_NEON__)
          #if defined(__ARM_PCS_VFP)
            #define ABI "armeabi-v7a/NEON (hard-float)"
          #else
            #define ABI "armeabi-v7a/NEON"
          #endif
        #else
          #if defined(__ARM_PCS_VFP)
            #define ABI "armeabi-v7a (hard-float)"
          #else
            #define ABI "armeabi-v7a"
          #endif
        #endif
      #else
       #define ABI "armeabi"
      #endif
    #elif defined(__i386__)
       #define ABI "x86"
    #elif defined(__x86_64__)
       #define ABI "x86_64"
    #elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
       #define ABI "mips64"
    #elif defined(__mips__)
       #define ABI "mips"
    #elif defined(__aarch64__)
       #define ABI "arm64-v8a"
    #else
       #define ABI "unknown"
    #endif
        string s = returnit();
        jstring retval = env->NewStringUTF(s.c_str());
        return retval;
    }
    }

Now if I write fromhere.cpp as follows:

#include <string>
using namespace std;
string returnit()
{
    string s="Hello World";
    return s;
}

I can include fromhere.h by writing a fromhere.h file and declaring returnit in that and just including the above file's name in Android.mk's LOCAL_SRC_FILES and "Hello World" appears in the text view I made from java class.

But I want to compile these fromhere.cpp and fromhere.h as prebuilt .so file built ny ndk and use the returnit() function from it. Can someone explain me step by step how to do this in Android Studio to be specific?

Please correct me if I spoke any nonsense.

Answer 1

You said you were using Android Studio, but by default Android Studio currently ignores your Makefiles and use its own auto-generated ones, with no support for native depencies (for now).

If you deactive the built-in support and do calls to ndk-build yourself, by putting something like this inside your build.gradle:

android {
  sourceSets.main {
      jniLibs.srcDir 'src/main/libs' //set libs as .so's location instead of jniLibs
      jni.srcDirs = [] //disable automatic ndk-build call with auto-generated Android.mk
  }
}

Here is the solution, using Makefiles:

Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_SRC_FILES := fromhere.cpp
LOCAL_MODULE := fromhere
LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH) # useless here, but if you change the location of the .h for your lib, you'll have to set its absolute path here.
include $(BUILD_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_SRC_FILES := hello-world.cpp
LOCAL_MODULE := hello-world
LOCAL_SHARED_LIBRARIES := fromhere
include $(BUILD_SHARED_LIBRARY)