Introduction to the Android NDK


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Sebastian Mauer's presentation at the Codefest Karlsruhe on February 20th.

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Introduction to the Android NDK

  1. 1. Introduction to the Android NDK Sebastian Mauer GDG Aachen CodeFest Karlsruhe February 20th, 2014
  2. 2. Who am I? Sebastian Mauer GDG Aachen Co-Lead Software Engineer CS Student I don’t work for Google…yet
  3. 3. Part I: NDK? What is that?
  4. 4. Android Platforms MIPS ARM x86
  5. 5. Two kinds of Apps Apps in the DalvikVM (that’s the kind of apps you probably know) Native Apps (created using the NDK)
  6. 6. One VM to rule them all • Dalvik is a Virtual Machine (VM) • A VM is a common abstraction across different hardware platforms • „Translates“ VM Bytecode to platform specific instructions
  7. 7. It just works™ • The DalvikVM is already optimized for the x86 Platform • Apps relying on the Android SDK / Dalvik Bytecode
 will automatically benefit from Platform-specific Optimizations (like SSE & Co.)
  8. 8. From Source to Bytecode Java Sourcecode Java Bytecode (.class) Dalvik Bytecode (.dex) JAR Archive Dalvik VM Java VM
  9. 9. Android VM Dalvik VM App App App App App App Linux Kernel Hardware
  12. 12. Part II: Going native. The NDK.
  13. 13. What’s the NDK? • NDK stands for Native Development Kit • Allows to compile C/C++ code to native (read: platform specific) executables/libraries. • Build scripts/toolkit to incorporate native code in Android apps via the Java Native Interface (JNI) • Has to be compiled for every platform you want to support
  14. 14. But why? • Performance
 e.g., complex algorithms, multimedia applications, games • Differentiation 
 app that takes advantage of direct CPU/HW access
 e.g., using SSSE3 for optimization • Fluid and lag-free animations • Software code reuse
  15. 15. Why not?
  16. 16. What could possibly go wrong? • Performance improvements are not guaranteed • In fact, you could make it worse (read: slower). • Added complexity (Java/C++ Interop, Multiple platforms) • Somewhat harder to debug
  17. 17. What’s an NDK app? It’s an Android application that uses native libraries. ! Libraries are .so files, usually found inside libs/CPU_ABI/. ! These libs can be generated from native sources inside jni folder, game engines, or required by other 3rd party libraries. ! There is no 100% native application. Even an application purely written in C/C++, using native_app_glue.h, will be executed in the context of the Dalvik Virtual Machine.
  18. 18. NDK Development in a Nutshell C/C++Code Makefile ndk-build APP_ABI := all or APP_ABI := x86 SDK APIs Java* calls GDB debug through jni Android* Applications Java Framework Java Application JNI Native Libs Bionic C Library NDK APIs
  19. 19. NDK Anatomy Dalvik VM Dalvik Bytecode Java Native Interface (JNI) NDK compiled binary Native Platform
  20. 20. Compatibility with Standard C/C++ • Bionic C Library:
 Lighter than standard GNU C Library
 Not POSIX compliant
 pthread support included, but limited
 No System-V IPCs
 Access to Android* system properties • Bionic is not binary-compatible with the standard C library • It means you generally need to (re)compile everything using the Android NDK toolchain
  21. 21. Pick One • By default, libstdc++ is used. It lacks: 
 Standard C++ Library support (except some headers)
 C++ exceptions support
 RTTI support • Fortunately, you have other libs available with the NDK: Runtime Exceptions RTTI STL system No No No gabi++ Yes Yes No stlport Yes Yes Yes gnustl Yes Yes Yes
  22. 22. Compile for all the platforms. If you have the source code of your native libraries, you can compile it for several CPU architectures by setting APP_ABI to all in the Makefile “jni/”:! APP_ABI=all Put APP_ABI=all inside Run ndk-build… ARM v7a libs are built ARM v5 libs are built x86 libs are built mips libs are built The NDK will generate optimized code for all target ABIs You can also pass APP_ABI variable directly to ndk-build, and specify each ABI: ndk-build APP_ABI=x86
  23. 23. NDK
  24. 24. NativeActivity • Only native code in the project • android_main() entry point running in its own thread • Event loop to get input data and frame drawing messages /** * This is the main entry point of a native application that is using * android_native_app_glue. It runs in its own thread, with its own * event loop for receiving input events and doing other things. */ void android_main(struct android_app* state);
  25. 25. Add native methods in Java Declare native methods in your Java classes using the ‘native’ keyword:
 public native String stringFromJNI(); ! Provide a native shared library built with the NDK that contains the methods used by your application:
 ! Your application must load the shared library (before use… during class load for example):
 static { System.loadLibrary("MyLib"); }
  26. 26. Variant I: Use „javah“ javah creates JNI header stubs (.h) based on the Java source files from the compiled Java classes files ! Example: > javah –d jni –classpath bin/classes com.example.hellojni.HelloJni ! Generates com_example_hellojni_HelloJni.h file with the following method definition: JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI(JNIEnv *, jobject);
  27. 27. Variant I: Use „javah“ ... { ... tv.setText( stringFromJNI() ); ... } ! public native String stringFromJNI(); static { System.loadLibrary("hello-jni"); } jstring Java_com_example_hellojni_HelloJni_stringFromJNI(JNIEnv* env, jobject thiz ) { return (*env)->NewStringUTF(env, "Hello from JNI !"); }
  28. 28. Variant II: JNI_OnLoad • Proven method • No more surprises after methods registration • Less error prone when refactoring • Add/remove native functions easily • No symbol table issue when mixing C/C++ code • Best spot to cache Java class object references
  29. 29. Variant II: JNI_OnLoad In your library name your functions as you wish and declare the mapping with JVM methods: jstring stringFromJNI(JNIEnv* env, jobject thiz) { return env->NewStringUTF("Hello from JNI !"); } ! static JNINativeMethod exposedMethods[] = { {"stringFromJNI","()Ljava/lang/String;",(void*)stringFromJNI}, } ! ()Ljava/lang/String; is the JNI signature of the Java* method, you can retrieve it using the javap utility: > javap -s -classpath binclasses -p com.example.hellojni.HelloJni Compiled from "“ … public native java.lang.String stringFromJNI(); Signature: ()Ljava/lang/String; …
  30. 30. Variant II: JNI_OnLoad extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved) { JNIEnv* env; if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK) return JNI_ERR; ! jclass clazz = env->FindClass("com/example/hellojni/HelloJni"); if(clazz==NULL) return JNI_ERR; env->RegisterNatives(clazz, exposedMethods, sizeof(exposedMethods)/sizeof(JNINativeMethod)); env->DeleteLocalRef(clazz); } return JNI_VERSION_1_6;
  31. 31. JNI Primitives Java Type Native Type Description boolean jboolean unsigned 8 bits byte jbyte signed 8 bits char jchar unsigned 16 bits short jshort signed 16 bits int jint signed 32 bits long jlong signed 64 bits float jfloat 32 bits double jdouble 64 bits void void N/A
  32. 32. Memory Handling of Java Objects Memory handling of Java* objects is done by the JVM: ! • You only deal with references to these objects • Each time you get a reference, you must not forget to delete it after use • local references are automatically deleted when the native call returns to Java • References are local by default • Global references are only created by NewGlobalRef()
  33. 33. Creating a Java String C: jstring string = (*env)->NewStringUTF(env, "new Java String"); ! C++: jstring string = env->NewStringUTF("new Java String"); Memory is handled by the JVM, jstring is always a reference. You can call DeleteLocalRef() on it once you finished with it.
  34. 34. Convert a jstring into a C/C++ String const char *nativeString = (*env)>GetStringUTFChars(javaString, null); … (*env)->ReleaseStringUTFChars(env, javaString, nativeString);
  35. 35. Call a Java Method On an object instance: jclass clazz = (*env)->GetObjectClass(env, obj); jmethodID mid = (*env)->GetMethodID(env, clazz, "methodName", "(…)…"); if (mid != NULL) (*env)->Call<Type>Method(env, obj, mid, parameters…); ! Static call: jclass clazz = (*env)->FindClass(env, "java/lang/String"); jmethodID mid = (*env)->GetStaticMethodID(env, clazz, "methodName", "(…)…"); if (mid != NULL) (*env)->CallStatic<Type>Method(env, clazz, mid, parameters…); ! • (…)…: method signature • parameters: list of parameters expected by the Java* method • <Type>: Java method return type
  36. 36. Handling Java Exceptions // call to java methods may throw Java exceptions jthrowable ex = (*env)->ExceptionOccurred(env); if (ex!=NULL) { (*env)->ExceptionClear(env); // deal with exception } ! (*env)->DeleteLocalRef(env, ex);
  37. 37. Throw a Java Exception jclass clazz = (*env->FindClass(env, "java/lang/Exception"); ! if (clazz!=NULL) (*env)->ThrowNew(env, clazz, "Message"); The exception will be thrown only when the JNI call returns to Java*, it will not break the current native code execution.
  38. 38. Demo
  39. 39. Q&A
  40. 40. Thanks for your attention.