MyNativeActivity.java

public class MyNativeActivity extends AppCompatActivity {
  static {
    System.loadLibrary("ndk_example");
    init();
  }
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    int res = test(1, 3);
    Log.d("JNI Result", String.format("Res: %d"), res));
  }
  
  @Override
  protected void onDestroy() {
    terminate();
    super.onDestroy();
  }
  
  public static native void init();
  
  public static native void terminate();
  
  public native int test(int a, int b);
}

ndk_example_jni.cpp

#include <jni.h>
#include <sstream>
#include "assert.h"
#include "android/log.h"

static int c;

extern "C" {
  JNIEXPORT void
  Java_com_my_package_name_MyNativeActivity_init(JNIEnv* env, jclass thiz) {
    // Static Method는 두번째 인자 type이 jclass.
    // jclass는 MyNativeActivity.java를 지칭
    c = 10;
  }
  
  JNIEXPORT void
  Java_com_my_package_name_MyNativeActivity_terminate(JNIEnv* env, jclass thiz) {
    c = 0;
  }
  
  JNIEXPORT jint
  Java_com_my_package_name_MyNativeActivity_init(JNIEnv* env, jobject thiz, jint a, jint b) {
    // 일반 Method는 두번째 인자 type이 jobject.
    // jobject는 MyNativeActivity를 지칭
    // 함수 인자는 뒤이어 등장. int -> jint, boolean -> jboolean 등의 형태.
    return a + b + c;
  }
}