如何在JNI NDK Android中正确且简单地使用回调函数

如何解决如何在JNI NDK Android中正确且简单地使用回调函数

我正在尝试让Java在Android中调用C函数，但我遇到的一些警告是，关于回调的文档通常似乎非常不清楚，因此，关于SO的大多数答案都只是在谈论理论并发布没有实际联系的断开链接码。因此，请在您的答案中发布代码。

我正在尝试用Java创建操纵杆并将其发送到NDK以便执行操作。

如果我可以让Java调用C函数并将int传递给它，那会很好。但我认为这行不通，因为C正在运行这是自己的主循环。
也许我需要使用C来调用Java 函数来获取控制器状态并从那里执行代码。

我确实设法找到了一些代码Here，但看起来该代码虽然可以自行编译，但在移至现代代码后似乎无法正常工作。

如何在Android上以Java C关系实现回调？从上面的代码示例中可以看出，正是C调用了Java函数，这些函数需要像getters函数一样使用。

这里有一些代码，我似乎丢失了一些东西，因为我不清楚我应该如何执行这些回调。如果使用Java或C ++（CPP），我将在此代码中标注函数名称及其位置。

我认为有两种方法可以使它工作。

我要调用的Java函数

public int Cat(){
        Log.e("JniHandler","CAT WAS CALLED!!!");
        return 333;
    }

结构信息CPP。不知道是否需要。

#include <string.h>
#include <inttypes.h>
#include <pthread.h>
#include <jni.h>
#include <android/log.h>
#include <assert.h>


typedef struct tick_context {
    JavaVM  *javaVM;
    jclass   jniHelperClz;
    jobject  jniHelperObj;
    jclass   mainActivityClz;
    jobject  mainActivityObj;
    pthread_mutex_t  lock;
    int      done;
} TickContext;
TickContext g_ctx;

本节有效。看起来，一旦启动JVM，它就会初始化。

JNI_OnLoad CPP

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm,void* reserved) {
    JNIEnv* env;
    memset(&g_ctx,sizeof(g_ctx));

    SDL_Log(" DEBUG  -- JNI INIT!!!");
    g_ctx.javaVM = vm;
    if (vm->GetEnv((void**)&env,JNI_VERSION_1_6) != JNI_OK) {
        return JNI_ERR; // JNI version not supported.
    }


    jclass  clz = env->FindClass("org/foo/bar/mainactivity");
    g_ctx.jniHelperClz = (jclass)env->NewGlobalRef(clz); // hack

    jmethodID  jniHelperCtor = env->GetMethodID(g_ctx.jniHelperClz,"<init>","()V");
    jobject    handler = env->NewObject(g_ctx.jniHelperClz,jniHelperCtor);
    g_ctx.jniHelperObj = env->NewGlobalRef(handler);
    //queryRuntimeInfo(env,g_ctx.jniHelperObj); // This when getting version returns null.
    
    g_ctx.done = 0;
    g_ctx.mainActivityObj = NULL;
    return  JNI_VERSION_1_6;
}

queryRuntimeInfo CPP

void queryRuntimeInfo(JNIEnv *env,jobject instance) {
    SDL_Log("DEBUG QUERYRUNTIME INTO");
    // Find out which OS we are running on. It does not matter for this app
    // just to demo how to call static functions.
    // Our java JniHelper class id and instance are initialized when this
    // shared lib got loaded,we just directly use them
    //    static function does not need instance,so we just need to feed
    //    class and method id to JNI
    jmethodID versionFunc = env->GetStaticMethodID(g_ctx.jniHelperClz,"getBuildVersion","()Ljava/lang/String;"); // This returns null.
    if (!versionFunc) {
        SDL_Log("DEBUG versionFunc INTO");
        //LOGE("Failed to retrieve getBuildVersion() methodID @ line %d",__LINE__);
        return;
    }
    SDL_Log("DEBUG BUILD VERSION INTO");
    jstring buildVersion = (jstring)env->CallStaticObjectMethod(g_ctx.jniHelperClz,versionFunc); // hack
    const char *version = env->GetStringUTFChars(buildVersion,NULL);

    if (!version) {
        SDL_Log("DEBUG buildVersion INTO");
        //LOGE("Unable to get version string @ line %d",__LINE__);
        return;
    }
    //LOGI("Android Version - %s",version);
    SDL_Log("DEBUG ANDROID VERSION %s",version);
    env->ReleaseStringUTFChars(buildVersion,version);

    // we are called from JNI_OnLoad,so got to release LocalRef to avoid leaking
    env->DeleteLocalRef(buildVersion);

    // Query available memory size from a non-static public function
    // we need use an instance of JniHelper class to call JNI
    jmethodID memFunc = env->GetMethodID(g_ctx.jniHelperClz,"getRuntimeMemorySize","()J");
    if (!memFunc) {
        //LOGE("Failed to retrieve getRuntimeMemorySize() methodID @ line %d",__LINE__);
        SDL_Log("DEBUG Failed to retrieve memory size");
        return;
    }
    jlong result = env->CallLongMethod(instance,memFunc);
    //LOGI("Runtime free memory size: %" PRId64,result);
    SDL_Log("DEBUG RUN TIME FREE MEMORY SIZE");
    (void)result;  // silence the compiler warning
}

此函数从未调用过，我不确定谁是调用者。 CPP

extern "C" JNIEXPORT void JNICALL InvokeFunction(JNIEnv *env,jobject instance) {
    SDL_Log("JNI CALL START TICKET");
    pthread_t       threadInfo_;
    pthread_attr_t  threadAttr_;

    pthread_attr_init(&threadAttr_);
    pthread_attr_setdetachstate(&threadAttr_,PTHREAD_CREATE_DETACHED);

    pthread_mutex_init(&g_ctx.lock,NULL);

    jclass clz = env->GetObjectClass(instance);
    g_ctx.mainActivityClz = (jclass)env->NewGlobalRef(clz); // hack
    g_ctx.mainActivityObj = env->NewGlobalRef(instance);

    int result  = pthread_create( &threadInfo_,&threadAttr_,UpdateTicks,&g_ctx);
    assert(result == 0);

    pthread_attr_destroy(&threadAttr_);

    (void)result;
}

updateticks CPP //从未真正调用过此代码。

void*  UpdateTicks(void* context) {

    SDL_Log("DEBUG -- UPDATE TICKS IS CALLED");
    TickContext *pctx = (TickContext*) context;
    JavaVM *javaVM = pctx->javaVM;
    JNIEnv *env;

    jint res = javaVM->GetEnv((void**)&env,JNI_VERSION_1_6);
    if (res != JNI_OK) {
        res = javaVM->AttachCurrentThread(&env,NULL);
        if (JNI_OK != res) {
            //LOGE("Failed to AttachCurrentThread,ErrorCode = %d",res);

            SDL_Log("FAILED TO ATTACH THREAD");
            return NULL;
        }
    }

    jmethodID statusId = env->GetMethodID(pctx->jniHelperClz,"updateStatus","(Ljava/lang/String;)V");
    //sendJavaMsg(env,pctx->jniHelperObj,statusId,"TickerThread status: initializing...");

    // get mainActivity updateTimer function
    jmethodID timerId = env->GetMethodID(pctx->mainActivityClz,"updateTimer","()V");

    struct timeval beginTime,curTime,usedTime,leftTime;
    const struct timeval kOneSecond = {
            (__kernel_time_t)1,(__kernel_suseconds_t) 0
    };

    //sendJavaMsg(env,"TickerThread status: start ticking ...");
    while(1) {
        gettimeofday(&beginTime,NULL);
        pthread_mutex_lock(&pctx->lock);
        int done = pctx->done;
        if (pctx->done) {
            pctx->done = 0;
        }
        pthread_mutex_unlock(&pctx->lock);
        if (done) {
            break;
        }
        env->CallVoidMethod(pctx->mainActivityObj,timerId);

        gettimeofday(&curTime,NULL);
        timersub(&curTime,&beginTime,&usedTime);
        timersub(&kOneSecond,&usedTime,&leftTime);
        struct timespec sleepTime;
        sleepTime.tv_sec = leftTime.tv_sec;
        sleepTime.tv_nsec = leftTime.tv_usec * 1000;

        if (sleepTime.tv_sec <= 1) {
            nanosleep(&sleepTime,NULL);
        } else {
            //sendJavaMsg(env,"TickerThread error: processing too long!");
        }
    }

    //sendJavaMsg(env,"TickerThread status: ticking stopped");
    javaVM->DetachCurrentThread();
    return context;
}

如何在JNI NDK Android中正确且简单地使用回调函数

如何解决如何在JNI NDK Android中正确且简单地使用回调函数

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