1. Native code in Android
applications

2. My projects using NDK

3. Yumm
Low memory overhead for image decoding

4. Hudriks Math
cocos2d-x; cross-platform: iOS and Android

5. Secure Video Player
Critical DRM code; cross-platform code (iOS/Android/Linux/OS X/Windows);
high-performance

6. Overview

7. Your Java Application
Java code
/libs/armeabi/
Android Runtime
Core Libraries (Java)
JVM (Dalvik)
Libraries
OpenSL ES
SQLite
OpenGL
Linux Platform
Media

8. Android NDK
•
Documentation
•
Toolchain - ndk-build, gcc/clang, gdbserver, …
•
NDK framework headers
•
Build system

9. NDK frameworks
android-4 (1.6)
•
dl, zlib, math, log
•
OpenGL ES 1.x
android-14 (4.0)
android-5 (2.0)
•
OpenMAX
OpenGL ES 2.0
android-8 (2.2)
•
•
JNI Graphics - java Bitmaps on low level
android-9 (2.3)
•
EGL
•
OpenSL ES - audio library
•
Native Applications - native activity, etc
android-18 (4.3)
•
OpenGL ES 3.0

10. Compilation process
source
.c/.c++
.o
.o
objects
compiler
.o
.o
.o
shared library
linker
.so
static library
.a (archive)
.so
.o
.o
.o

11. Application structure
•
jni/ (source code)
•
Android.mk
•
[Application.mk]
•
module1/
•
•
Android.mk
libs/armeabi/libnative.so - compiled shared library

12. CPU specific
•
Application Binary Interface (ABI):
• armeabi - at least ARMv5TE
• armeabi-v7a - Thumb-2; VFP hardware FPU; NEON
• x86
• mips
•
Application.mk:
• APP_ABI := all
• APP_ABI := armeabi armeabi-v7a

13. Java Native Interface

14. JNI in C and C++
#include <jni.h>
C
struct JNINativeInterface {
jclass
(*FindClass)(JNIEnv*, const char*);
}
typedef const struct JNINativeInterface* JNIEnv;
!
JNIEnv* env;
(*env)->FindClass(env, “classname”);

15. JNI in C and C++
C++
struct _JNIEnv {
jclass FindClass(const char* name)
{ return functions->FindClass(this, name); }
}
!
JNIEnv* env;
env->FindClass(“classname”);

16. Mapping native functions
libnative.so
Java.class
- function_1()
- function_1()
- function_2()
- function_2()
- function_3()
- function_3()
- function_4()
- function_4()
- function_5()
- function_5()

17. Mapping native functions
.java
public native static int testNative(int a, int b);
.c
jint Java_com_example_jnibasics_NativeDemo_testNative(JNIEnv *env, jclass obj, jint a, jint b)
package name
javah
> javah com.example.jnibasics.NativeDemo

18. Mapping native functions
jint RegisterNatives(JNIEnv *env, jclass clazz, const
JNINativeMethod *methods, jint nMethods);
typedef struct {
char *name;
char *signature;
void *fnPtr;
} JNINativeMethod;
jint addVals(JNIEnv *env, jclass obj, jint a, jint b) {…}

19. Mapping native functions
static JNINativeMethod sMethods[] = {
{"testNative", "(II)I", (void*)addVals}
};
jint JNI_OnLoad(JavaVM* vm, void* reserved) {
JNIEnv *env = NULL;
jclass klass;
if((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6) != JNI_OK)
return -1;
!
klass = (*env)->FindClass(env, “com/example/jnibasics/NativeDemo”);
(*env)->RegisterNatives(env, klass, gMethods, 1);
!
return JNI_VERSION_1_6;
}

20. Loading .so from Java
static {
static {
System.loadLibrary("Dependency");
System.loadLibrary("JNIBasics");
System.loadLibrary("JNIBasics");
}
}
JNIBasics
Dependency

21. Demo

22. References
jobject gObject = NULL;
!
void function(JNIEnv* jobject obj) {
gObject = (*env)->NewGlobalRef(env, obj);
}
!
void finish(JNIEnv* env) {
(*env)->DeleteGlobalRef(env, gObject);
}

23. Calling Java methods from
native
jclass clz = (*env)->FindClass(env, “com/example/jnibasics/NativeDemo");
!
jmethodID method = (*env)->GetMethodID(env, clz,
"methodName", “(II)Ljava/lang/String;");
!
jstring result = (*env)->CallObjectMethod(env, obj, method, 5, 6);

24. Accessing java strings
jstring jstr;
const char* str;
!
str = (*env)->GetStringUTFChars(env, jstr, NULL);
!
...
!
(*env)->ReleaseStringUTFChars(env, jstr, str);

25. Further reading
•
Attaching Java threads
•
Creating new Java objects from native code
•
Distinguish between virtual and non virtual methods
•
Exception handling
•
Accessing Java arrays

26. Using native code

27. Performance
•
Most applications don’t need native code!
•
Only for extensive calculations: games, rich media
•
Take advantage of NEON CPU instructions
•
Avoiding Java Garbage Collection

28. Cross-platform architecture
platform dependant
libraries
(network, UI, etc)
Application
(Java/UIKit)
platform independent
code
(no JNI)
external interface
(JNI/Objective-C)

29. Cross-platform examples
VLC

30. Security

31. 1.
2.
3.
4.
Register as a developer (£60 per year)
Add device UUID to dev account
Generate Provisioning Profile
Sign APK with developer’s certificate
!
or Submit to Apple Store
or Jailbreak device
Binary is encrypted
Decryption is on OS level
Self-signed APK
or not-signed at all
Decompiled Objective C:
Decompiled Java:
class structures
assembly code
readable code

32. Demo
DISCLAIMER: For educational purposes only;
I’m not encouraging to hack somebody else’s applications;

33. Summary
•
Always obfuscate Java code!
•
Never save passwords, use session key or hash
instead
•
Never keep encryption keys in clear data in memory
•
Keep all critical code in native

34. Further protection
•
Hide non-public symbols from .so files
•
Strip debug information into separate files
•
Expose only high-level APIs to Java

35. Tips
•
Debugging native code is tricky
•
•
•
Linux or OSX as dev platform
Use ARM DS-5 Community Edition in Eclipse
Android fragmentation
•
separate .so files for different version

36. Questions?
Dmitry Matyukhin
dmitry@fancygames.net