Android ndk: Entering the native world

Intel Developers Relations Division
androidndk:Enteringthenativeworld
Eduardo Carrara
Developer Evangelist – Intel
@DuCarrara
+EduardoCarraraDeAraujo

Intel Developers Relations Division 2
Agenda
The Android NDK
Are you using and is not aware?
Be careful with the Binary Interface
Starting with Android Studio + gradle-experimental
Questions?

Androidndk
The Native Development Kit

“The Android NDK is a set of tools that allows the
implementation of parts of your app using native code
languages like C and C++."
- NDK Android Developers Portal

and maybe you are doing that already...
... let’s a few interesting NDK usages.

UsagesofNDK
What can we do with it?

Game Engines
Others+…
*Other names and brands may be claimed as the property of others.

Computer Vision
• Linear and Non-Linear image filtering.
• Images Geometric Transformations.
• Moviments Prediction in Videos.
• Background segmentation on Videos.
• Object Tracking.
• ...
Outras: Cardboard-SDK, Vuforia, Layar, LibCCV,
Wikitude ...
OpenCV.org

Cross Platform Development

Other Usages and Libraries
Realm
Swift
Libpng
Intel TBB
Intel IPP
Libcairo and libpixman
Libsonic
Busybox
Speex
FFMpeg
OpenSSL
Lua
...

So many interesting usages!
Can we use NDK for
everything?!

Understandingthendk

Source: Using the NDK Performantly (Big Android BBQ 2015)
15
Architecture Overview
App Code
Framework Classes
Runtime
System Libraries
Linux Kernel
JNI
Your Libraries
Stable ABI

In practice...
Microchip Icon – Icon Finder.
App
0010011100
0011100111
x86
ARM
MIPS
Your code is:
• Closer to HW
• Platform Dependent
You must be careful about
it!

You APK X-Ray with
NDK usage

3 Golden Rules for NDK Adoption
1
2
3 Legacy Code that must be re-used and/or to expensive to port to Java*.
Specific performance problems.
Features and Experiences that demand performance.

3 (Main) Barriers for NDK Adoption
1
2
3 Perfomance gains are not assured.
Complexity of Implementation and Maintenance.
Platform Dependency and Compatibility.

We can, but we should not use NDK for everything.
So...
Still interested?
Take the “red pill” and we will show you how far this
can go...

usingthendk

Fundamentals: Java Native Interface (JNI)
Defines how Java and native code will interop.
Java C / C++
• Load lib with:
• System.loadLibrary()
• Definition of Native Methods:
• Reserved word native
• Include the JNI Header:
• #include <jni.h>
• Use specific JNI data types:
• jstring, jint, jboolean, etc.
• Special Variables:
• JNIEnv*, JavaVM*

JNI: Mapping Java Methods To C / C++
In C/C++ the function must:
Use and return Java JNI primitives and Objects:
Follows the naming standards:
Or register functions manually:
jint xxx(JNIEnv* env, jobject instance, ...) { ... }
Java_com_example_hellojni_HelloWorldJni_method
JNIEnv->RegisterNatives();

JNI: Example
Java
C / C++
public class HelloWorldJNI {
static {
System.loadLibrary("hello-jni");
}
public native String getHelloWorldStringFromJNI();
}
#include <jni.h>
jstring
Java_com_example_hellojni_HelloWorldJNI_getHelloWorldStringFromJNI(JNIEnv *env, jobject instance) {
return (*env)->NewStringUTF(env, "Hello World!");
}

Old but Gold
Building NDK Apps classic style.
JNICode
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := hello-jni
LOCAL_SRC_FILES := hello-jni.c
include $(BUILD_SHARED_LIBRARY)
APP_ABI := all
jni/Android.mk jni/Application.mk

Old but Gold
ndk_build script
Building NDK Apps classic style.

Old but Gold
Building NDK Apps (almost) classic
style.
• Add libs to jniLibs directory.
• Create java – jni interfaces.
• For libs pre-builts that is all.
• To build from source is a bit more
complicated but is possible as well..

Limitations
• Debug is not possible on AS. It is necessary to use ndk-gdb and/or Eclipse.
• Current support of gradle plugin was deprecated (ouch).

... So where do we go?

Androidstudio+ndk+gradle-experimental

Important Warnings
• The experimental version of android gradle plugin will be used.
• Integration with Android Studio was improved but it stills unstable and has
some bugs.
• On Windows there are issues with the Editor and/or build that prevent its
usage. (Issues 195483 and 204552).
• Documentation is virtually inexistent or incomplete.

AS Integration
NDK Installation
and Configuration
are now integrated
to Android Studio.

AS Integration

gradle-experimental
Configuration
distributionUrl=https://services
.gradle.org/distributions/gradle-
2.10-all.zip
./build.gradle ./gradle/gradle-wrapper/gradle-wrapper.properties
dependencies {
classpath
'com.android.tools.build:gradle-
experimental:0.7.0'
}

gradle-experimental
Configuration: Combining Versions
Android Studio 1.5 2.0 2.1
gradle 2.8 2.10 2.10
gradle plugin 1.5.0 2.0.0 2.1.0
gradle-experimental plugin 0.4.0 0.6.0 0.7.0

gradle-experimental
Configuration: build.gradle DSL
changes.
Always Check:
- NDK Samples
- gradle-experimental docs
apply plugin: 'com.android.model.application'
model {
}
android {
compileSdkVersion = 23
buildToolsVersion = "23.0.3"
defaultConfig.with {
applicationId = "com.example.hellojni"
minSdkVersion.apiLevel = 4
targetSdkVersion.apiLevel = 23
}
}
android.ndk {
moduleName = "hello-jni"
}
android.buildTypes {
release {
minifyEnabled = false
proguardFiles.add(file('proguard-rules.txt'))
}
}

gradle-experimental
Configuration: multiple apks.
model {
}
android.productFlavors {
}
// … standard gradle stuff
// … other gradle stuff
create ("armv7") {
}
create (“x86") {
}
ndk.abiFilters.add("armeabi-v7a")
ndk.abiFilters.add(“x86")
versionCode = calculateVersionCodeFor(
"armeabi-v7a")
versionCode = calculateVersionCodeFor(
“x86")
create (“fat")

gradle-experimental
Configuration: multiple apks.
model {
}
def actualVersionCode = 13;
// … gradle stuff
// … other gradle stuff
def baseVersionCode = 1000000;
def versionCodeABIPrefixes = [
'armeabi':1, 'armeabi-v7a':2, 'arm64-v8a':3,
'mips': 5,'mips64': 6,
'x86': 8, 'x86_64': 9];
def calculateVersionCodeFor = { String abi ->
return versionCodeABIPrefixes.get(abi, 0) *
baseVersionCode + actualVersionCode
}

gradle-experimental
• It is also possible:
• Configure pre-built libs;
• Add build flags (general and by flavor);
• The current documentation generally do not follow the releases, then:
• Keep an eye to NDK Samples for references!

Final Thoughts
When using NDK, direct or inderectly,add
support to as many platform architectures as
possibles.
Use NDK wisely: test, create benchmarks and
assure that it is the right solution to your
problem.
Follow and test the gradle-experimental plugin
for the NDK!

Questions?

References

References
• NDK Development on Intel
• Intel Tools
• Android* NDK Official Documentation
• Android* NDK Gradle Experimental Documentation
• Android* NDK Google* Samples
• Gradle Stable NDK Support - Xavier Hallade
• New Android Studio NDK Support - Xavier Hallade

thanks

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Android ndk: Entering the native world

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