Android JNI笔记

JNI、java native interface 。可以实现Java和C、C++之间的调用。
在Android开发中是必须要掌握的内容。
在应用开发中，编写JNI代码的注册可分为动态注册和静态注册

动态注册：
声明好方法、注意这些签名 在JNI_OnLoad中进行注册。

static const JNINativeMethod method[] = {
        {"openUrl","(Ljava/lang/String;Ljava/lang/Object;I)I", (void*)playVideo},
        {"InitView","(Ljava/lang/Object;)I", (void*)InitView},
        {"start","()I", (void*) start},
        {"getPauseStatus","()Z", (void*)getPauseStatus},
        {"onRelease","()Z", (void*)onRelease},
        {"stopVideo","()V", (void*)stopVideo},
        {"seekTo","(D)V", (void*)changeSeek},
        {"changeURL","(Ljava/lang/String;)V", (void*)changeURL},
        {"setPause","(Z)V", (void*)setPause},
        {"getSupportProcel","()Ljava/lang/String;", (void*)getSupportInfo}
};

static const char *mClassName = "com/mark/myapplication/player/core/PlayerCore";

int JNI_OnLoad(JavaVM *vm,void  *re){
    g_vm = vm;
    IPlayerPorxy::Get()->Init(vm);
    AndroidHelper::getInstance()->SetVM(g_vm);
    //获得JNIEnv
    JNIEnv *env = 0;
    int r = vm->GetEnv((void**)&env,JNI_VERSION_1_6);
    //小于0失败，等于0成功
    if(r != JNI_OK){
        return -1;
    }

    //获得class对象
    jclass jcls =  env->FindClass(mClassName);
    //动态注册
    env->RegisterNatives(jcls,method, sizeof(method)/ sizeof(JNINativeMethod));
    return JNI_VERSION_1_6;
}

静态注册：

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

JNI函数签名
https://blog.csdn.net/weixin_75102992/article/details/130771941

全局引用&局部引用&弱全局引用
https://blog.csdn.net/huangjinjin520/article/details/123288292

系统中的JNI代码模块是在哪里被加载的呢。
在zygote启动时
frameworks/base/core/jni/AndroidRuntime.cpp

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
	......
	 JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote, primary_zygote) != 0) {
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.注册JNI
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
    ..............
}

注册了framework中使用的native代码，如果自己需要添加framework的JNI，需要在这里进行添加声明

static const RegJNIRec gRegJNI[] = {
        REG_JNI(register_com_android_internal_os_RuntimeInit),
        REG_JNI(register_com_android_internal_os_ZygoteInit_nativeZygoteInit),
        REG_JNI(register_android_os_SystemClock),
        REG_JNI(register_android_util_EventLog),
        REG_JNI(register_android_util_Log),
        REG_JNI(register_android_util_MemoryIntArray),
        REG_JNI(register_android_app_admin_SecurityLog),
        REG_JNI(register_android_content_AssetManager),
        REG_JNI(register_android_content_StringBlock),
        REG_JNI(register_android_content_XmlBlock),
        ..................
        }

例如storage_StorageManager可以在storage_StorageManager.cpp可找到

//跟APP开发中的动态注册类似
static const JNINativeMethod gStorageManagerMethods[] = {
        {"setQuotaProjectId", "(Ljava/lang/String;J)Z",
         (void*)android_os_storage_StorageManager_setQuotaProjectId},
};

const char* const kStorageManagerPathName = "android/os/storage/StorageManager";

int register_android_os_storage_StorageManager(JNIEnv* env) {
    return RegisterMethodsOrDie(env, kStorageManagerPathName, gStorageManagerMethods,
                                NELEM(gStorageManagerMethods));
}

}