Add jni target to ReactCxxPlatform (#51898)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51898

changelog: [internal]

Reviewed By: andrewdacenko

Differential Revision: D76240376

fbshipit-source-id: bbae620dde69261e6d26d088471601e7d7e9a074

packages/react-native/ReactCxxPlatform/react/jni/JniHelper.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <fbjni/Context.h>
+#include <fbjni/fbjni.h>
+
+namespace facebook::react {
+
+jobject getApplication(JNIEnv* env) {
+  auto activityThreadClass = env->FindClass("android/app/ActivityThread");
+  auto currentApplicationMethodID = env->GetStaticMethodID(
+      activityThreadClass, "currentApplication", "()Landroid/app/Application;");
+  return env->CallStaticObjectMethod(
+      activityThreadClass, currentApplicationMethodID);
+}
+
+jni::alias_ref<jni::AContext> getContext() {
+  auto env = facebook::jni::Environment::ensureCurrentThreadIsAttached();
+  auto application = getApplication(env);
+  return facebook::jni::wrap_alias(
+      static_cast<facebook::jni::AContext::javaobject>(application));
+}
+
+} // namespace facebook::react

packages/react-native/ReactCxxPlatform/react/jni/JniHelper.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <fbjni/Context.h>
+#include <fbjni/fbjni.h>
+
+namespace facebook::react {
+
+jobject getApplication(JNIEnv* env);
+
+jni::alias_ref<jni::AContext> getContext();
+
+} // namespace facebook::react