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JNIEnvJNIEnv *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));
}}