Useful JNI Snippets

Some useful snippets for working with JNI in application development and AOSP developmenta

Simple C++ code

static jint add(JNIEnv *env, jclass clazz, jint a,jint b) {
		return a+b;
}

static jint sub(JNIEnv *env, jclass clazz, jint a,jint b) {
		return a-b;
}


static JNINativeMethod method_table[] = {
		{ "add", "(II)I", (void *) add },
		{ "libSub", "(II)I", (void *) sub },
};



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;
    } else {
    	jclass clazz = env->FindClass("com/example/simpcpp/MainActivity");
    	if (clazz) {
    		jint ret = env->RegisterNatives(clazz, method_table, sizeof(method_table) / sizeof(method_table[0]));
    		env->DeleteLocalRef(clazz);
    		return ret == 0 ? JNI_VERSION_1_6 : JNI_ERR;
    	} else {
    		return JNI_ERR;
    	}
    }
}

The Java code

public static native int add(int a,int b);
public static native int libSub(int a,int b);
	
static
{
	System.loadLibrary("SimpCpp");
}

Using Log

__android_log_print(ANDROID_LOG_DEBUG, "myfile.c", "add(%lld)", n);

// Add to Android.mk:
// LOCAL_LDLIBS += -llog

Thread function:

static void *threadfn(void *p)
{
	jlong res = fib((long)p);
	JNIEnv *env;
	int n=javaVM->AttachCurrentThread(&env,NULL);

	jmethodID m=env->GetStaticMethodID(cl,"printToLog","(J)V");

	env->CallStaticVoidMethod(cl,m,res);

	javaVM->DetachCurrentThread();
	return (void*)res;
}

static jlong fibNR11(JNIEnv *env, jclass clazz, jlong n) {
	pthread_t t1;
	pthread_create(&t1,NULL,threadfn,(void*)n);
	return 0;
}

Using Reflection 

JNIEXPORT jint JNICALL Java_com_example_testjni_Nlib_sub
  (JNIEnv *env, jobject obj, jint a, jint b)
{

	jclass cl=env->GetObjectClass(obj);

	jmethodID m=env->GetMethodID(cl,"getNum","(I)I");

	jint num=env->CallIntMethod(obj,m,a+b);

	return num;

}