Executors.java->.kt (facebook#45702)

Summary:
Pull Request resolved: facebook#45702

# Changelog:
[Internal] -

As in the title.

Differential Revision: D60282843

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/runtime/internal/bolts/Executors.kt

@@ -0,0 +1,45 @@
+/*
+ * 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.
+ */
+
+package com.facebook.react.runtime.internal.bolts
+
+import com.facebook.react.bridge.UiThreadUtil
+import java.util.concurrent.Executor
+
+/**
+ * This was created because the helper methods in [java.util.concurrent.Executors] do not work as
+ * people would normally expect.
+ *
+ * Normally, you would think that a cached thread pool would create new threads when necessary,
+ * queue them when the pool is full, and kill threads when they've been inactive for a certain
+ * period of time. This is not how [java.util.concurrent.Executors.newCachedThreadPool] works.
+ *
+ * Instead, [java.util.concurrent.Executors.newCachedThreadPool] executes all tasks on a new or
+ * cached thread immediately because corePoolSize is 0, SynchronousQueue is a queue with size 0 and
+ * maxPoolSize is Integer.MAX_VALUE. This is dangerous because it can create an unchecked amount of
+ * threads.
+ */
+internal object Executors {
+  @JvmField public val UI_THREAD: Executor = UIThreadExecutor()
+  @JvmField public val IMMEDIATE: Executor = ImmediateExecutor()
+
+  private class UIThreadExecutor : Executor {
+    override fun execute(command: Runnable) {
+      // Otherwise, post it on the main thread handler
+      UiThreadUtil.runOnUiThread(command)
+    }
+  }
+
+  /**
+   * An [java.util.concurrent.Executor] that schedules tasks to run asynchronously on the UI thread.
+   */
+  private class ImmediateExecutor : Executor {
+    override fun execute(command: Runnable) {
+      command.run()
+    }
+  }
+}