Capturing executor for current thread

Question 1

I think I've see some implementation doing that. The basic Idea is roughly

class UiThreadExecutor implements Executor {
    private final Handler mHandler = new Handler(Looper.getMainLooper());

    @Override
    public void execute(Runnable command) {
        mHandler.post(command);
    }
}

You can delegate to run anything in the main thread by passing it to a handler for the main thread.

Edit: https://github.com/square/retrofit/blob/master/retrofit/src/main/java/retrofit/android/MainThreadExecutor.java for example

Edit2: You can configure the handler like e.g. SensorManager#registerListener(..., Handler handler) allows you to do.

class HandlerThreadExecutor implements Executor {
    private final Handler mHandler;
    public HandlerThreadExecutor(Handler optionalHandler) {
        mHandler = optionalHandler != null ? optionalHandler : new Handler(Looper.getMainLooper());
    }

    @Override
    public void execute(Runnable command) {
        mHandler.post(command);
    }
}

The advantage over using the current thread's looper is that it makes it explicit which Looper you use. In your solution you take the Looper of whatever thread calls new ExecuteOnCaller() - and that's often not the thread you run code in later.

I would be happy to know how to achieve the same effect without resorting to the Android-specific stuff like Handler and Looper, just with pure Java.

Looper, Handler and the message queue behind all that logic are made of mostly pure Java. The problem with a generic solution is that you can't "inject" code to run into a thread. The thread must periodically check some kind of task queue to see if there is something to run.

If you write code like

    new Thread(new Runnable() {
        @Override
        public void run() {
            while (!Thread.interrupted()) {
                System.out.println("Hello");
            }
        }
    }).start();

Then there is no way to make that thread do anything else but constantly print "Hello". If you could do that it would be like dynamically inserting a jump to other code into the program code. That would IMO be a terrible idea.

    final BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>();
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                while (true) {
                    Runnable codeToRunInThisThread = queue.take();
                    codeToRunInThisThread.run();
                }
            } catch (InterruptedException ignored) {}
        }
    }).start();

On the other hand is a simple thread that loops forever on a queue. The thread could do other tasks in between but you have to add a manual check into the code.

And you can send it tasks via

    queue.put(new Runnable() {
        @Override
        public void run() {
            System.out.println("Hello!");
        }
    });

There is no special handler defined here but that's the core of what Handler & Looper do in Android. Handler in Android allows you to define a callback for a Message instead of just a Runnable.

Executors.newCachedThreadPool() and similar do roughly the same thing. There are just multiple threads waiting on code in a single queue.

As an extension to this question, is it possible to do same thing, but not just with UI thread, but with any particular thread, where the call to async method is made?

The generic answer is No. Only if there is a way to inject code to run in that thread.

Question 2

Based on asnwer from @zapl, here is my implementation, which also answers the edited (extended) question: https://gist.github.com/RomanIakovlev/8540439

Figured out I'll also put it here, in case if link will rot some day:

package com.example.concurrent;

import android.os.Handler;
import android.os.Looper;

import java.util.concurrent.Executor;

/**
* When the calling thread has a Looper installed (like the UI thread), an instance of ExecuteOnCaller will submit
* Runnables into the caller thread. Otherwise it will submit the Runnables to the UI thread.
*/
public class ExecuteOnCaller implements Executor {

    private static ThreadLocal<Handler> threadLocalHandler = new ThreadLocal<Handler>() {
        @Override
        protected Handler initialValue() {
            Looper looper = Looper.myLooper();
            if (looper == null)
            looper = Looper.getMainLooper();
            return new Handler(looper);
        }
    };

    private final Handler handler = threadLocalHandler.get();

    @Override
    public void execute(Runnable command) {
        handler.post(command);
    }
}

My pattern to use it would be like this:

/**
* in SomeActivity.java or SomeFragment.java
*/
Futures.addCallback(myModel.asyncOperation(param), new FutureCallback<Void>() {
        @Override
        public void onSuccess(Void aVoid) {
            // handle success
        }

        @Override
        public void onFailure(Throwable throwable) {
            // handle exception
        }
    }, new ExecuteOnCaller());

Question 3

Use com.google.android.gms.tasks.TaskExecutors.MAIN_THREAD.

An Executor that uses the main application thread.

Source: Android docs

The tasks APIs are part of Google Play services since version 9.0.0.

Question 4

For Android UI thread executor use:

ContextCompat.getMainExecutor(context)

Question 5

To address your question and extended question to create an Executor that simply runs on the current thread and avoids Android classes:

class DirectExecutor implements Executor {
   public void execute(Runnable r) {
     r.run();
   }
 }

See documentation: https://developer.android.com/reference/java/util/concurrent/Executor