https://source.android.com/ 
/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.annotation;

import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.RetentionPolicy.SOURCE;

import android.content.Context;
import android.os.AsyncTask;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import java.util.concurrent.Executor;


@paramDocCallback and listener events are dispatched through this Executor, providing an easy way to control which thread is used. To dispatch events through the main thread of your application, you can use Context.getMainExecutor(). To dispatch events through a shared thread pool, you can use AsyncTask.THREAD_POOL_EXECUTOR.
@hide
/**
 * @paramDoc Callback and listener events are dispatched through this
 *           {@link Executor}, providing an easy way to control which thread is
 *           used. To dispatch events through the main thread of your
 *           application, you can use {@link Context#getMainExecutor()}. To
 *           dispatch events through a shared thread pool, you can use
 *           {@link AsyncTask#THREAD_POOL_EXECUTOR}.
 * @hide
 */
@Retention(SOURCE)
@Target(PARAMETER)
public @interface CallbackExecutor {
}