오랜만에 JNI의 감동적인 노트를 만져봤습니다.

도대체 JNI가 뭐예요?

환경 구축

class SampleNative {
  external fun hello(): String

  companion object {
    init {
      System.loadLibrary("sample-native")
    }
  }
}

cmake_minimum_required(VERSION 3.4.1)

add_library(
        sample-native
        SHARED
        sample-native.cpp)

find_library(
        log-lib
        log)

target_link_libraries(
        sample-native
        ${log-lib})

#ifndef __SAMPLE_NATIVE_H_
#define __SAMPLE_NATIVE_H_

#include <jni.h>
#include <android/log.h>
#include <string>

#ifdef __cplusplus
extern "C" {
#endif

JNIEXPORT jstring JNICALL
Java_xxx_xxx_xxx_xxx_jni_SampleNative_hello(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

#ifdef __cplusplus
}
#endif

#endif // __SAMPLE_NATIVE_H_

android {
  defaultConfig {
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
  }
  externalNativeBuild {
      cmake { path "src/main/java/xxx/jni/CMakeLists.txt" }
  }
}

val sample = SampleNative()
sample.hello() // Hello from C++

Tips

#define TAG               "SampleNative"
#define LOGD(...)         __android_log_print(ANDROID_LOG_DEBUG, TAG, __VA_ARGS__)
#define LOGE(...)         __android_log_print(ANDROID_LOG_ERROR, TAG, __VA_ARGS__)
#define LOGI(...)         __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)

// LOGD("numbers %d %d", num1, num2);

#if defined(DEBUG) || defined(_DEBUG)
#define __ASSERT__(e)      assert(e)
#else
#define __ASSERT__(e)
#endif // DEBUG

/**
 * 引数なしコンストラクタを呼び出しクラスオブジェクトを生成<br>
 *
 * @param  env       JNIEnv
 * @param  classPath クラスパス
 * @return           jobject
 */
jobject newObject(JNIEnv *env, const char *classPath)
{
    jclass cls = env->FindClass(classPath);
    __ASSERT__(cls != NULL);
    // コンストラクタ取得
    jmethodID mid = env->GetMethodID(cls, "<init>", "()V");
    __ASSERT__(mid != NULL);
    jobject jobj = env->NewObject(cls, mid);
    // 後片付け
    env->DeleteLocalRef(cls);
    return jobj;
}

/**
 * java.util.ArrayListクラスのインスタンス取得
 *
 * @param  env JNIEnv
 * @return     インスタンス
 */
jobject newArrayList(JNIEnv *env)
{
    return newObject(env, "java/util/ArrayList");
}

바드의 비결

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