五道口宅男 - Android NDK and OpenCV Development With Android Studio

http://hujiaweibujidao.github.io/blog/archives/

五道口宅男

http://hujiaweibujidao.github.io/blog/2014/10/22/android-ndk-and-opencv-development-with-android-studio/

—————- If you do NOT know Chinese, you can just skip this part —————-

一直打算将原来的XFace进行改进，最近终于有了些时间可以动手了，改进计划如下：开发上使用Android Studio作为新的开发环境，配上新的构建系统Gradle；应用上将修改原来的UI设计，内部代码也将有很大的变化，可能会用上 ContentProvider和Service等略高级内容；算法上打算让应用扩展性增强以适应不同的算法，并结合强大的Android Studio和Gradle让这个项目变得更加丰富。说了一堆废话，言归正传，本文的重点是介绍如何在Android Studio中进行NDK开发(目前它还不完全支持NDK开发)，难点是NDK中还包含OpenCV的动态库。最后的最后，本文剩下部分将使用英文，因为 它要成为我在StackOverflow上的处女答，么么哒 ~O(∩_∩)O~

—————————- Here is the right stuff you may need ——————————–

This post shows how to develop an Android NDK application with OpenCV included using Android Studio and Gradle. If you’re working on migrating your original Eclipse Project to Android Studio, you may find this post is what exactly you want!

OK，Let’s start!

Section 1: Three things you must know

1.Firstly, if you are not familiar with Android Studio and Gradle, you may find these links useful. (if you already know these well, skip this part)

①Creating a new Project with Android Studio

②Building Your Project with Gradle

③here.

2.Secondly, if your android ndk project is not that complicated(for example, having no opencv included), you may wanna see ph0b ‘s introduction here, it’s quite a nice job with a video recorded! (you can also follow Section 2 in this post to get a simple Android NDK demo application)

3.Thirdly, if those above two do not meet your needs, then I think you may want to customize the Android.mk with Gradle in Android Studio. Thanks to Gaku Ueda , he had made a great job explaining how to achieve that goal. Actually I have found another nicer solution without adding that many codes and also achieve that goal. :-) Find it out in the next sections.

Gaku Ueda’s post: Using custom Android.mk with Gradle/Android Studio

OK, I will cover all above and give another nice solution in the end, have fun!

Section 2: A simple Android NDK demo application

This section shows creating a simple Android NDK demo application, if you already know, you can directly go the section 3.

1.Create a new Android project named NDKDemo with a blank Activity in AS(=Android Studio).

2.Give an id to the TextView in activity_my.xml such as android:id="@+id/textview", then add these codes in MyActivity.java.

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@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_my); TextView textView = (TextView) findViewById(R.id.textview); textView.setText(hello()); } static { System.loadLibrary("hello"); } public native String hello();

3.Create a new directory jni in folder app/src/main, then you have java, jni and res in this folder.

4.This step is very important! You can add a external tool to run the javah command without typing that much code!

Open AS’s Preferences, then find External Tools in IDE Settings, click + to add one tool with the following configurations. (Make sure you have add JDK tools in your system path, if you don’t know how, click here)

With the help of this tool, each time we right click on a class file, then choose Android Tools -> javah to run this tool, it will automatically generate a C head file for us in the target folder $ModuleFileDir$/src/main/jni , in this case, it is app/src/main/jni. Try this on MyActivity.java file now! The console will print out a log like:

/usr/bin/javah -v -jni -d /Users/hujiawei/AndroidStudioProjects/NDKDemo/app/src/main/jni com.android.hacks.ndkdemo.MyActivity [Creating file RegularFileObject[/Users/hujiawei/AndroidStudioProjects/NDKDemo/app/src/main/jni/ com_android_hacks_ndkdemo_MyActivity.h]]

Then you get a com_android_hacks_ndkdemo_MyActivity.h file in jni folder with the following content.

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/* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class com_android_hacks_ndkdemo_MyActivity */ #ifndef _Included_com_android_hacks_ndkdemo_MyActivity #define _Included_com_android_hacks_ndkdemo_MyActivity #ifdef __cplusplus extern "C" { #endif /* * Class: com_android_hacks_ndkdemo_MyActivity * Method: hello * Signature: ()Ljava/lang/String; */ JNIEXPORT jstring JNICALL Java_com_android_hacks_ndkdemo_MyActivity_hello (JNIEnv *, jobject); #ifdef __cplusplus } #endif #endif

5.Write a simple C implementation file named main.c in jni folder

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#include <jni.h> #include "com_android_hacks_ndkdemo_MyActivity.h" JNIEXPORT jstring JNICALL Java_com_android_hacks_ndkdemo_MyActivity_hello (JNIEnv * env, jobject obj){ return (*env)->NewStringUTF(env, "Hello from JNI"); }

6.In the build.gradle file under app module, add the following codes to configure ndk in defaultConfig element, here we just give the uni module a name hello, you can find other configurations in

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defaultConfig { applicationId "com.android.hacks.ndkdemo" minSdkVersion 16 targetSdkVersion 20 versionCode 1 versionName "1.0" ndk{ moduleName "hello" } }

7.In order to let Gradle run ndk-build command (in some task, maybe NdkCompile task), we should configure the ndk.dir in local.properties file in Project root.

1 2	sdk.dir=/Volumes/hujiawei/Users/hujiawei/Android/android_sdk ndk.dir=/Volumes/hujiawei/Users/hujiawei/Android/android_ndk

8.OK, everything is ready, click Run to give it a try, you will see the result like

All right, so what’s happening inside?

Since you have a jni folder, Gradle will consider it as a default native code folder. When Gradle builds the app, it will run ndk-build command(since you have configured ndk.dir, Gradle knows where to find it) with a generated Android.mk file(locates in app/build/intermediates/ndk/debug/Android.mk), after compiling the native codes, it will generate the libs and obj folder into folder app/build/intermediates/ndk/debug/. Gradle will then package the libs into final apk file in folder app/build/outputs/apk/app-debug.apk(you can unarchive this file to check whether libs is contained)

app/build/intermediates/ndk/debug (lib and obj folders)

app/build/outputs/apk/app-debug.apk (and files within it)

Secontion 3: Using OpenCV

If your project do not use OpenCV, then the section 2 is just enough. But what if you wanna use OpenCV to do other stuff? Of course, we want to use OpenCV for Android instead of JavaCV here, and Of course, we need to package OpenCV library for Android into our application’s APK file (then users who use this app does not have to install OpenCV Manager). So, how can we achieve these goals?

The simplest way has been posted by TGMCians on Stack Overflow

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LOCAL_PATH := $(call my-dir) include $(CLEAR_VARS) LOCAL_SRC_FILES := main.c LOCAL_LDLIBS += -llog LOCAL_MODULE := hello include $(BUILD_SHARED_LIBRARY)