Last Updated: September 27, 2021

Android NDK / JNI Quickstart

This is a tutorial about creating a "Hello World" project on Android using the NDK. It assumes that you are using Eclipse-ADT (from Google). It is not meant to detail how JNI works but only to give an example.

Create a new Android project in Eclipse-ADT
Create a folder called "jni" in the project (at the same level as "src" or "res"). This folder will contain all of the C/C++ files

Create "jni/Android.mk" with the following content. It is actually the makefile used by android to compile your native code

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := helloJNI

LOCAL_LDLIBS := -llog
include $(BUILD_SHARED_LIBRARY)

(Optional, but required for instance with OpenCV): Create "jni/Application.mk" with the following content:
```
APP_STL := gnustl_static
APP_CPPFLAGS := -frtti -fexceptions
APP_ABI := all
```

In your Activity (Java file), include the C++ library you gonna create

public class MainActivity extends Activity {
    static {
        System.loadLibrary("helloJNI");
    }
[…]

Declare a native prototype that will be used to call the corresponding C/C++ function in the library "helloJNI", such as:
```
private native void debugMessage();
```

Actually call this function from Java your code, for instance:

@Override
protected void onResume() {
    super.onResume();
    debugMessage();
}

Create the JNI "glue" between the Java code and its C/C++ equivalence:
- Go to <your_project>/bin/classes
- Run the following command: javah -o ../../jni/mainActivity.h com.example.hellojni.MainActivity
You now have a new file - mainActivity.h - in <your_project>/jni (you may need to refresh in Eclipse).

Create <yourproject>/jni/mainActivity.cpp (NOTE: prototypes are already generated in mainActivity.h by javah_)

#include "mainActivity.h"
#include <android/log.h>

//==============================
// Declarations
//==============================
#define LOG_TAG "mainActivity.cpp"
#define DPRINTF(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#define IPRINTF(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define EPRINTF(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)

JNIEXPORT void JNICALL JNICALL Java_com_example_hellojni_MainActivity_debugMessage
(JNIEnv *, jobject)
{
    DPRINTF( "%s\n", __FUNCTION__ );
    DPRINTF( "This is a debug message coming from my C++ code!\n");
}

Add the new cpp file you created to Android.mk:
```
LOCAL_SRC_FILES := mainActivity.cpp
```

Android.mk now looks like:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := helloJNI
LOCAL_SRC_FILES := mainActivity.cpp

LOCAL_LDLIBS := -llog
include $(BUILD_SHARED_LIBRARY)