What is Android NDK

1. What is Android NDK A toolset that lets you embed in you app native source code C, C++(recently supported December 2010) and assembly(?) It is supported on android cupcake(1.5)+ It is aimed to Bring native libraries in android (code reusability) Make some parts of the application really fast using code generated for arm-like cpus Most of the time android SDK is prerequisite for NDK Under heavy development

2. When should be used? Not just because we like c++ more than java! Only when its benefits outrages its drawbacks Why? Always increases application complexity Very difficult debugging But OpenGL graphics to work on android in favor of NDK Increases speed Enable us to port most of the libraries

3. How... ? Android provides 2 ways: Writing the application using the standard SDK framework and then using JNI to access the API provided by NDK. Write a native activity using NativeActivity class and native code to implement activity lifecycle callbacks. Available only on Android 2.3+ Services, Content providers must be implemented using NDK.

4. So what exactly is JNI? Tip: Java Native Interface Available years before android appeared Used when something java couldn't do e.g. platform specific features Many library java classes use JNI Can be used to inteface with C, C++ and assembly But also through that C and C++ can call native JAVA functions!

5. JNI example NativeConnection class public class NativeConnection{ public native String stringFromJNI(); // This method loads as soon as the instance of the class is created static { System.loadLibrary("helloFromJNI"); } } HelloFromJNI.cpp JNIEXPORT jstring JNICALL Java_ceid_1ds2_cbox_helloandroid_NativeConnection_stringFromJNI( JNIEnv* env, //environment pointer jobject thiz ) //object pointer { return env->NewStringUTF("Hello from native C++ code using NJI :-D"); //return (*env)->NewStringUTF("Hello from native C code using NJI :-D"); }

6. Native Method Names A native method name is concatenated from the following components: the prefix Java_ a mangled fully-qualified class name an underscore (“_”) separator a mangled method name for overloaded native methods, two underscores (“__”) followed by the mangled argument signature

7. Native Method Names Other characters: _0XXXX: a Unicode character XXXX. _1: the character “_” _2: the character “;” in signatures _3: the character “[“ in signatures Pretty difficult to remember! But there is a tool that can help us! javah !

8. javah Header and Stub File Generator javah [ options ] fully-qualified-classname e.g. javah -nji com.helloandroid.NativeConnection -o helloFromJNI.h Generates all the function prototypes in an automated way! Suports C-like C++

9. JNI strategy Create the native methods in java class Load the library in java class Create the function prototypes using javah in a .h file Create the final native source file! Use your native methods inside Android activity

10. JNI example! NativeConnection class public class NativeConnection{ public native String stringFromJNI(); // This method loads as soon as the instance of the class is created static { System.loadLibrary("helloFromJNI"); } } HelloFromJNI.cpp JNIEXPORT jstring JNICALL Java_ceid_1ds2_cbox_helloandroid_NativeConnection_stringFromJNI( JNIEnv* env, //environment pointer jobject thiz ) //object pointer { return env->NewStringUTF("Hello from native C++ code using NJI :-D"); //return (*env)->NewStringUTF("Hello from native C code using NJI :-D"); }

11. JNI in android The Android NDK is nothing more than a complement to the Android SDK that helps you to: Generate JNI-compatible shared libraries that can run on the Android platform running on ARM CPUs. Copy the generated libraries to a proper location of your application to be included in .apks - A set of cross-toolchains (compilers, linkers, etc..) that can generate native ARM binaries on Linux, OS X and Windows (with Cygwin) All the rest is just JNI!

12. Android NDK strategy Implement the usual application lifecycle callbacks Implement the native methods in a java class Load the library in a java class Create the function prototypes using javah in a .h file(optional) Create the final native source file Use your native methods somewhere inside Android activity Execute the commands from your working directory: <pathToSDK>/tools/android update project -p . -s <pathToNDK>/ndk-build Build the project using eclipse

13. JNI revisited A native cannot return anything to java It has to be one of the JNI defined types: jboolean, jint, jfloat, jdouble, jstring . . . . j<javaType> To interfere with this types there also many many JNI functions To get the native string from javaString: const char *nativeString = env->GetStringUTFChars(javaString, 0); Some time are the same e.g. int data type is 8 bit signed same as jint.

14. JNI revisited For example Java arrays are not guaranteed to have a continuous memory layout like C arrays. void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array, jsize start, jsize len, NativeType *buf); returns a native type array in a continues region JNIEXPORT jint JNICALL Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr) { jint buf[10]; jint i, sum = 0; env->GetIntArrayRegion(arr, 0, 10, buf); for (i = 0; i < 10; i++) { sum += buf[i]; } return sum;}