Writing applications using the Android NDK

I did not find any easy to read tutorials on how to exactly build an Android app with some native code component. Its true that the NDK package comes with some docs, but for Android beginners, this may not be an exciting prospect. However, I still maintain that the docs in the package are a must-read.

The following app simply prints out a small string returned from a C function on the LogView.

1. Create a Normal Android app. Lets call it 'native1'.
2. The main source file should look like this.

package com.earl.native1;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class native1 extends Activity {
   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.main);
     
       Log.i("--EARL--", new NativeLib().getHelloWorld());
   }
}

class NativeLib
{
   public native String getHelloWorld();
 
   static {
       System.loadLibrary("helloworld");
   }

}

3. If you are using Eclipse, the class files built will be in the 'bin' directory.
4. Navigate to it and run 'javah' on the class file that contains the native method.

javah -jni com.earl.native1.NativeLib

5. After this, you will get one header file. This file contains the JNIEXPORTs generated from the java source.
6. Copy this file and paste it in a folder in the 'sources' dir of the Android NDK. I'm calling the folder 'earl'.
7. Now, create the C implementation file - earl.c It should look like this.

#include "com_earl_native1_NativeLib.h"

JNIEXPORT jstring JNICALL Java_com_earl_native1_NativeLib_getHelloWorld
(JNIEnv * env, jobject obj)
{
   return (*env)->NewStringUTF(env, "Greetings from Earlence !");
}

8. Now you have to create the Android.mk file. This file describes your native code module to the build system. This file is created in the same dir as the source file.

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := helloworld
LOCAL_SRC_FILES := earl.c

include $(BUILD_SHARED_LIBRARY)

9. Now, navigate to the 'apps' dir in the NDK home and create a folder of your choice. This will hold the Android Application you built using Eclipse earlier. I am calling it earl_jni. In this folder, create another one and call it 'projects'. In this, copy your ENTIRE eclipse project 'native1'. Then, navigate one level up to earl_jni and create the Application.mk file.

APP_PROJECT_PATH := $(call my-dir)/project
APP_MODULES      := helloworld

10. Now, navigate to the NDK home and run the make command.

$ make APP=earl_jni

11. You should see this output if everything works correctly.

Android NDK: Building for application 'earl_jni'
Compile thumb  : helloworld <= sources/earl/earl.c
SharedLibrary  :
libhelloworld.so
Install        : libhelloworld.so => apps/earl_jni/project/libs/armeabi

12. Now, a folder called 'libs' will be created in apps/earl_jni/projects. This contains the .so file i.e the native library. Copy this into the eclipse project root. Compile and run. You should see something like on the LogView

I/ActivityManager(  564): Starting activity: Intent { action=android.intent.acti
on.MAIN categories={android.intent.category.LAUNCHER} flags=0x10200000 comp={com
.earl.native1/com.earl.native1.native1} }
I/--EARL--(  699): Greetings from Earlence !
I/ActivityManager(  564): Displayed activity com.earl.native1/.native1: 704 ms

Congratulations!!! You have just built your first JNI app for Android.