在Flutter App工程中的AndroidManifest.xml注册了FlutterApplication和MainActivity，跟Android工程一样都是Application作为入口，看看Application的代码。

public class FlutterApplication extends Application {
    private Activity mCurrentActivity = null;

    public FlutterApplication() {
    }

    @CallSuper
    public void onCreate() {
        super.onCreate();
        FlutterMain.startInitialization(this);
    }

    public Activity getCurrentActivity() {
        return this.mCurrentActivity;
    }

    public void setCurrentActivity(Activity mCurrentActivity) {
        this.mCurrentActivity = mCurrentActivity;
    }
}

在onCreate ()中调用了FlutterMain.startInitialization(this);进行初始化。

 public static void startInitialization(Context applicationContext) {
        startInitialization(applicationContext, new FlutterMain.Settings());
    }

    public static void startInitialization(Context applicationContext, FlutterMain.Settings settings) {
        if(Looper.myLooper() != Looper.getMainLooper()) {
            throw new IllegalStateException("startInitialization must be called on the main thread");
        } else if(sSettings == null) {
            sSettings = settings;
            long initStartTimestampMillis = SystemClock.uptimeMillis();
            initConfig(applicationContext);
            initAot(applicationContext);
            initResources(applicationContext);
            System.loadLibrary("flutter");
            long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;
            nativeRecordStartTimestamp(initTimeMillis);
        }
    }

FlutterMain.Settings是日志logTag的设置，接着初始化一些配置、Aot和资源文件和目录，这里不展开了，最后加载了flutter的so库，还调用了native方法记录初始化时间，接下来去C++层看看如何进行下一步初始化操作的。

// This is called by the VM when the shared library is first loaded.
JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved) {
  // Initialize the Java VM.
  fml::jni::InitJavaVM(vm);

  JNIEnv* env = fml::jni::AttachCurrentThread();
  bool result = false;

  // Register FlutterMain.
  result = shell::FlutterMain::Register(env);
  FML_CHECK(result);

  // Register PlatformView
  result = shell::PlatformViewAndroid::Register(env);
  FML_CHECK(result);

  // Register VSyncWaiter.
  result = shell::VsyncWaiterAndroid::Register(env);
  FML_CHECK(result);

  return JNI_VERSION_1_4;
}

InitJavaVM(vm)

jni_util.cc

void InitJavaVM(JavaVM* vm) {
  FML_DCHECK(g_jvm == nullptr);
  g_jvm = vm;
}

JNIEnv* AttachCurrentThread() {
  FML_DCHECK(g_jvm != nullptr)
      << "Trying to attach to current thread without calling InitJavaVM first.";
  JNIEnv* env = nullptr;
  jint ret = g_jvm->AttachCurrentThread(&env, nullptr);
  FML_DCHECK(JNI_OK == ret);
  return env;
}

主要做了以下工作：初始化JavaVM，获取JNIEnv。

知识补充：

• JavaVM，它是虚拟机在JNI层的代表，全进程只有一个JavaVM对象，不论进程中有多少个线程，JavaVM却是独此一份，所以可以保存，并且在任何地方使用都没有问题。
• JNIEnv代表Java环境。 JNIEnv的指针会被JNI传送到本地方法的实现函数中来对Java端的代码进行操作，如，创建Java类得对象，调用Java对象的方法，获取Java对象的属性等。
• JNIEnv是一个与线程相关的对象，不同线程之间不能共享JNIEnv结构体。
• 通过AttachCurrentThread()，就可得到这个线程的JNIEnv结构体。

FlutterMain::Register(env)

flutter_main.cc

bool FlutterMain::Register(JNIEnv* env) {
  static const JNINativeMethod methods[] = {
      {
          .name = "nativeInit",
          .signature = "(Landroid/content/Context;[Ljava/lang/String;Ljava/"
                       "lang/String;Ljava/lang/String;Ljava/lang/String;)V",
          .fnPtr = reinterpret_cast<void*>(&Init),
      },
      {
          .name = "nativeRecordStartTimestamp",
          .signature = "(J)V",
          .fnPtr = reinterpret_cast<void*>(&RecordStartTimestamp),
      },
  };

  jclass clazz = env->FindClass("io/flutter/view/FlutterMain");

  if (clazz == nullptr) {
    return false;
  }

  return env->RegisterNatives(clazz, methods, arraysize(methods)) == 0;
}

动态注册了两个方法nativeInit和nativeRecordStartTimestamp，供Java层调用，依次看看两个方法。

static void RecordStartTimestamp(JNIEnv* env,
                                 jclass jcaller,
                                 jlong initTimeMillis) {
  int64_t initTimeMicros =
      static_cast<int64_t>(initTimeMillis) * static_cast<int64_t>(1000);
  blink::engine_main_enter_ts = Dart_TimelineGetMicros() - initTimeMicros;
}
void FlutterMain::Init(JNIEnv* env,
                       jclass clazz,
                       jobject context,
                       jobjectArray jargs,
                       jstring bundlePath,
                       jstring appStoragePath,
                       jstring engineCachesPath) {
  std::vector<std::string> args;
  args.push_back("flutter");
  for (auto& arg : fml::jni::StringArrayToVector(env, jargs)) {
    args.push_back(std::move(arg));
  }
  auto command_line = fml::CommandLineFromIterators(args.begin(), args.end());

  auto settings = SettingsFromCommandLine(command_line);

  settings.assets_path = fml::jni::JavaStringToString(env, bundlePath);

  // Restore the callback cache.
  // TODO(chinmaygarde): Route all cache file access through FML and remove this
  // setter.
  blink::DartCallbackCache::SetCachePath(
      fml::jni::JavaStringToString(env, appStoragePath));

  fml::paths::InitializeAndroidCachesPath(
      fml::jni::JavaStringToString(env, engineCachesPath));

  blink::DartCallbackCache::LoadCacheFromDisk();

  if (!blink::DartVM::IsRunningPrecompiledCode()) {
    // Check to see if the appropriate kernel files are present and configure
    // settings accordingly.
    auto application_kernel_path =
        fml::paths::JoinPaths({settings.assets_path, "kernel_blob.bin"});

    if (fml::IsFile(application_kernel_path)) {
      settings.application_kernel_asset = application_kernel_path;
    }
  }

  settings.task_observer_add = [](intptr_t key, fml::closure callback) {
    fml::MessageLoop::GetCurrent().AddTaskObserver(key, std::move(callback));
  };

  settings.task_observer_remove = [](intptr_t key) {
    fml::MessageLoop::GetCurrent().RemoveTaskObserver(key);
  };

#if FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
  // There are no ownership concerns here as all mappings are owned by the
  // embedder and not the engine.
  auto make_mapping_callback = [](const uint8_t* mapping, size_t size) {
    return [mapping, size]() {
      return std::make_unique<fml::NonOwnedMapping>(mapping, size);
    };
  };

  settings.dart_library_sources_kernel =
      make_mapping_callback(kPlatformStrongDill, kPlatformStrongDillSize);
#endif  // FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG

  // Not thread safe. Will be removed when FlutterMain is refactored to no
  // longer be a singleton.
  g_flutter_main.reset(new FlutterMain(std::move(settings)));
}

static std::unique_ptr<FlutterMain> g_flutter_main;

FlutterMain& FlutterMain::Get() {
  FML_CHECK(g_flutter_main) << "ensureInitializationComplete must have already "
                               "been called.";
  return *g_flutter_main;
}

std::unique_ptr使用
RecordStartTimestamp()前面说了是用于记录初始化的事件，可以说是Java层到C++层消息传递的初始化时间，此时初始化还未结束。
Init()方法中进行了一些路径的配置、添加task观察、内核资源配置等，最后创建了FlutterMain对象。

PlatformViewAndroid::Register(env)

bool PlatformViewAndroid::Register(JNIEnv* env) {
  if (env == nullptr) {
    FML_LOG(ERROR) << "No JNIEnv provided";
    return false;
  }

  g_flutter_callback_info_class = new fml::jni::ScopedJavaGlobalRef<jclass>(
      env, env->FindClass("io/flutter/view/FlutterCallbackInformation"));
  if (g_flutter_callback_info_class->is_null()) {
    FML_LOG(ERROR) << "Could not locate FlutterCallbackInformation class";
    return false;
  }

  g_flutter_callback_info_constructor = env->GetMethodID(
      g_flutter_callback_info_class->obj(), "<init>",
      "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)V");
  if (g_flutter_callback_info_constructor == nullptr) {
    FML_LOG(ERROR) << "Could not locate FlutterCallbackInformation constructor";
    return false;
  }

  g_flutter_jni_class = new fml::jni::ScopedJavaGlobalRef<jclass>(
      env, env->FindClass("io/flutter/embedding/engine/FlutterJNI"));
  if (g_flutter_jni_class->is_null()) {
    FML_LOG(ERROR) << "Failed to find FlutterJNI Class.";
    return false;
  }

  g_surface_texture_class = new fml::jni::ScopedJavaGlobalRef<jclass>(
      env, env->FindClass("android/graphics/SurfaceTexture"));
  if (g_surface_texture_class->is_null()) {
    FML_LOG(ERROR) << "Could not locate SurfaceTexture class";
    return false;
  }

  static const JNINativeMethod callback_info_methods[] = {
      {
          .name = "nativeLookupCallbackInformation",
          .signature = "(J)Lio/flutter/view/FlutterCallbackInformation;",
          .fnPtr = reinterpret_cast<void*>(&shell::LookupCallbackInformation),
      },
  };

  if (env->RegisterNatives(g_flutter_callback_info_class->obj(),
                           callback_info_methods,
                           arraysize(callback_info_methods)) != 0) {
    FML_LOG(ERROR) << "Failed to RegisterNatives with FlutterCallbackInfo";
    return false;
  }

  g_attach_to_gl_context_method = env->GetMethodID(
      g_surface_texture_class->obj(), "attachToGLContext", "(I)V");

  if (g_attach_to_gl_context_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate attachToGlContext method";
    return false;
  }

  g_update_tex_image_method =
      env->GetMethodID(g_surface_texture_class->obj(), "updateTexImage", "()V");

  if (g_update_tex_image_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate updateTexImage method";
    return false;
  }

  g_get_transform_matrix_method = env->GetMethodID(
      g_surface_texture_class->obj(), "getTransformMatrix", "([F)V");

  if (g_get_transform_matrix_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate getTransformMatrix method";
    return false;
  }

  g_detach_from_gl_context_method = env->GetMethodID(
      g_surface_texture_class->obj(), "detachFromGLContext", "()V");

  if (g_detach_from_gl_context_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate detachFromGlContext method";
    return false;
  }

  return RegisterApi(env);
}

bool RegisterApi(JNIEnv* env) {
  static const JNINativeMethod flutter_jni_methods[] = {
      // Start of methods from FlutterNativeView
      {
          .name = "nativeAttach",
          .signature = "(Lio/flutter/embedding/engine/FlutterJNI;Z)J",
          .fnPtr = reinterpret_cast<void*>(&shell::AttachJNI),
      },
      {
          .name = "nativeDestroy",
          .signature = "(J)V",
          .fnPtr = reinterpret_cast<void*>(&shell::DestroyJNI),
      },
      {
          .name = "nativeRunBundleAndSnapshotFromLibrary",
          .signature = "(J[Ljava/lang/String;Ljava/lang/String;"
                       "Ljava/lang/String;Landroid/content/res/AssetManager;)V",
          .fnPtr =
              reinterpret_cast<void*>(&shell::RunBundleAndSnapshotFromLibrary),
      },
      {
          .name = "nativeGetObservatoryUri",
          .signature = "()Ljava/lang/String;",
          .fnPtr = reinterpret_cast<void*>(&shell::GetObservatoryUri),
      },
      {
          .name = "nativeDispatchEmptyPlatformMessage",
          .signature = "(JLjava/lang/String;I)V",
          .fnPtr =
              reinterpret_cast<void*>(&shell::DispatchEmptyPlatformMessage),
      },
      {
          .name = "nativeDispatchPlatformMessage",
          .signature = "(JLjava/lang/String;Ljava/nio/ByteBuffer;II)V",
          .fnPtr = reinterpret_cast<void*>(&shell::DispatchPlatformMessage),
      },
      {
          .name = "nativeInvokePlatformMessageResponseCallback",
          .signature = "(JILjava/nio/ByteBuffer;I)V",
          .fnPtr = reinterpret_cast<void*>(
              &shell::InvokePlatformMessageResponseCallback),
      },
      {
          .name = "nativeInvokePlatformMessageEmptyResponseCallback",
          .signature = "(JI)V",
          .fnPtr = reinterpret_cast<void*>(
              &shell::InvokePlatformMessageEmptyResponseCallback),
      },

      // Start of methods from FlutterView
      {
          .name = "nativeGetBitmap",
          .signature = "(J)Landroid/graphics/Bitmap;",
          .fnPtr = reinterpret_cast<void*>(&shell::GetBitmap),
      },
      {
          .name = "nativeSurfaceCreated",
          .signature = "(JLandroid/view/Surface;)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SurfaceCreated),
      },
      {
          .name = "nativeSurfaceChanged",
          .signature = "(JII)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SurfaceChanged),
      },
      {
          .name = "nativeSurfaceDestroyed",
          .signature = "(J)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SurfaceDestroyed),
      },
      {
          .name = "nativeSetViewportMetrics",
          .signature = "(JFIIIIIIIIII)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SetViewportMetrics),
      },
      {
          .name = "nativeDispatchPointerDataPacket",
          .signature = "(JLjava/nio/ByteBuffer;I)V",
          .fnPtr = reinterpret_cast<void*>(&shell::DispatchPointerDataPacket),
      },
      {
          .name = "nativeDispatchSemanticsAction",
          .signature = "(JIILjava/nio/ByteBuffer;I)V",
          .fnPtr = reinterpret_cast<void*>(&shell::DispatchSemanticsAction),
      },
      {
          .name = "nativeSetSemanticsEnabled",
          .signature = "(JZ)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SetSemanticsEnabled),
      },
      {
          .name = "nativeSetAccessibilityFeatures",
          .signature = "(JI)V",
          .fnPtr = reinterpret_cast<void*>(&shell::SetAccessibilityFeatures),
      },
      {
          .name = "nativeGetIsSoftwareRenderingEnabled",
          .signature = "()Z",
          .fnPtr = reinterpret_cast<void*>(&shell::GetIsSoftwareRendering),
      },
      {
          .name = "nativeRegisterTexture",
          .signature = "(JJLandroid/graphics/SurfaceTexture;)V",
          .fnPtr = reinterpret_cast<void*>(&shell::RegisterTexture),
      },
      {
          .name = "nativeMarkTextureFrameAvailable",
          .signature = "(JJ)V",
          .fnPtr = reinterpret_cast<void*>(&shell::MarkTextureFrameAvailable),
      },
      {
          .name = "nativeUnregisterTexture",
          .signature = "(JJ)V",
          .fnPtr = reinterpret_cast<void*>(&shell::UnregisterTexture),
      },
  };

  if (env->RegisterNatives(g_flutter_jni_class->obj(), flutter_jni_methods,
                           arraysize(flutter_jni_methods)) != 0) {
    FML_LOG(ERROR) << "Failed to RegisterNatives with FlutterJNI";
    return false;
  }

  g_handle_platform_message_method =
      env->GetMethodID(g_flutter_jni_class->obj(), "handlePlatformMessage",
                       "(Ljava/lang/String;[BI)V");

  if (g_handle_platform_message_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate handlePlatformMessage method";
    return false;
  }

  g_handle_platform_message_response_method = env->GetMethodID(
      g_flutter_jni_class->obj(), "handlePlatformMessageResponse", "(I[B)V");

  if (g_handle_platform_message_response_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate handlePlatformMessageResponse method";
    return false;
  }

  g_update_semantics_method =
      env->GetMethodID(g_flutter_jni_class->obj(), "updateSemantics",
                       "(Ljava/nio/ByteBuffer;[Ljava/lang/String;)V");

  if (g_update_semantics_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate updateSemantics method";
    return false;
  }

  g_update_custom_accessibility_actions_method = env->GetMethodID(
      g_flutter_jni_class->obj(), "updateCustomAccessibilityActions",
      "(Ljava/nio/ByteBuffer;[Ljava/lang/String;)V");

  if (g_update_custom_accessibility_actions_method == nullptr) {
    FML_LOG(ERROR)
        << "Could not locate updateCustomAccessibilityActions method";
    return false;
  }

  g_on_first_frame_method =
      env->GetMethodID(g_flutter_jni_class->obj(), "onFirstFrame", "()V");

  if (g_on_first_frame_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate onFirstFrame method";
    return false;
  }

  g_on_engine_restart_method =
      env->GetMethodID(g_flutter_jni_class->obj(), "onPreEngineRestart", "()V");

  if (g_on_engine_restart_method == nullptr) {
    FML_LOG(ERROR) << "Could not locate onEngineRestart method";
    return false;
  }

  return true;
}

可见，同样是动态注册一些方法供Java层调用，同时还获取Java层类中的方法，实现双向消息传递。

VsyncWaiterAndroid::Register(env)

vsync_waiter_android.cc

bool VsyncWaiterAndroid::Register(JNIEnv* env) {
  static const JNINativeMethod methods[] = {{
      .name = "nativeOnVsync",
      .signature = "(JJJ)V",
      .fnPtr = reinterpret_cast<void*>(&OnNativeVsync),
  }};

  jclass clazz = env->FindClass("io/flutter/view/VsyncWaiter");

  if (clazz == nullptr) {
    return false;
  }

  g_vsync_waiter_class = new fml::jni::ScopedJavaGlobalRef<jclass>(env, clazz);

  FML_CHECK(!g_vsync_waiter_class->is_null());

  g_async_wait_for_vsync_method_ = env->GetStaticMethodID(
      g_vsync_waiter_class->obj(), "asyncWaitForVsync", "(J)V");

  FML_CHECK(g_async_wait_for_vsync_method_ != nullptr);

  return env->RegisterNatives(clazz, methods, arraysize(methods)) == 0;
}

同理，不再分析。

小结

• 通过FlutterApplication的入口分析，进行了原生一些配置，然后通过加载flutter的so库进行C++层的配置，建立了Java和C++的消息传递通道。
• 注意仅仅是搭建起桥梁，并没有触发init()方法，因为此时Java没有调用nativeInit()，此方法触发取决于Java层中的FlutterMain.ensureInitializationComplete()。

另外可知的信息：

1. Flutter在Android运行也是跟原生App一样，Application有一个FlutterActivity，页面运行在FlutterView（继承SurfaceView）上。
2. 并未创建FlutterView或者使程序跑起来，猜测在创建FlutterActivity后会有相关的逻辑。

下一遍，将从FlutterActivity创建开始分析。